A Current-Driven Single-Atom Memory

Top row: (left to right): Christian Schirm, Manuel Matt, Fabian Pauly. Bottom row (left to right): Elke Scheer, Juan Carlos Cuevas, Peter Nielaba.

Authors: Christian Schirm¹, Manuel Matt¹, Fabian Pauly¹, Juan Carlos Cuevas², Peter Nielaba¹, Elke Scheer¹

Affiliation:
¹Department of Physics, University of Konstanz, Germany.
²Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Spain.

Building functional devices at the atomic scale is a central vision of nanotechnology. Recently we demonstrated a two-state electrical switch with basically a single atom acting as moving part [1]. The switch consists of an aluminum atomic contact that is connected to electrodes at each side. Applying positive or negative currents above a certain threshold will change the state. Lower currents can be used to read out the switching status by simply measuring the resistance. This qualifies the device for future potential applications in high-density solid-state memories.

The single-atom switch was analyzed in close collaboration between experimental and theoretical physicists. Quantum mechanical transport channels of the electrical conductance served as an important link between experiment and theory. (The conductance is the inverse of the electrical resistance.) On the one hand these channels could be determined experimentally by measuring non-linear current-voltage characteristics in the superconducting state at very low temperatures [2]. On the other hand the channels could be calculated through a combination of molecular dynamics simulations, electronic structure theory, and transport theory expressed in terms of non-equilibrium Green's functions [4]. The theory enabled us to identify atomic configurations for both states of the experimentally realized switches. The comparison showed that in some cases the observed switching effect can be attributed to the geometrical rearrangement of a single atom.

Fig 1: Aluminum bridge with a 100 nm constriction, fabricated by electron beam lithography (see left panel). Due to the use of an elastic substrate, the aluminum structure can be stretched by carefully bending the sample. Since we measure the conductance continuously, we can stop the breaking procedure at a constriction narrowed to a single atom (see sketch in the right panel).

In the experiment we created an atomic contact of aluminum by the mechanically controllable break-junction technique using a lithographically fabricated bridge with a 100 nm wide constriction [3]. Fig. 1 shows a scanning electron micrograph (in false colors) of such an aluminum sample on an elastic substrate and illustrates the breaking process to reach an atomic-sized contact. Next, as shown in the graph in Fig. 2, we applied a slowly increasing electrical current and measured the conductance simultaneously. At some threshold current an electromigration process takes place, which can be observed as a jump in the conductance. The current is now decreased until the next jump is detected, then increased and so on. After some rearrangements the system may reversibly switch between two conductance states. A comparison of the conduction channel signatures verifies that these states remain exactly the same.

Fig 2: Applying a careful electromigration protocol when an atom-size contact has formed, a two-state atomic switch may develop. For this the current through the contact is increased slowly with time. When a jump in the conductance is detected, the current is reversed. After some time the system may switch reversibly between two conductance states. The inset shows the rectangular conductance-current hysteresis of a successfully generated two-level switch. The unit of conductance is G₀=2e²/h, also called the “quantum of conductance”.

In molecular dynamics simulations we performed a stretching procedure similar to the preparation of the contact in the experiment. The simulations generate realistic contact geometries and transport data as we verified by comparing experimental and theoretical conductance histograms [4,5]. Fig. 3 shows two atomic geometries as obtained in a stretching process. The calculated total conductance and individual conductance channels were in good agreement with several experimentally realized switches.

Fig 3: Molecular dynamics simulation of geometries corresponding to conductance values and channel signatures found in the experiment. The primary difference between both geometries is a relocation of the central atom and the breaking of two bonds to move from state “1” to state “0” (symbolized by the red scissors). The conductance and the individual channels are given in units of G₀.

The project demonstrates how theory and experiment can work hand in hand to advance nanotechnology. The atomic switch fulfills several technological requirements. For instance, it is relatively easy to fabricate due to the two-terminal configuration. Our work can be considered as a feasibility study of a one-bit atomic-size memory [6]. Potentially, an array of such switches can be implemented in a cross-bar architecture of solid-state memories when further technological issues, such as the stable operation at room temperature, can be solved.

References:
[1] C. Schirm, M. Matt, F. Pauly, J. C. Cuevas, P. Nielaba, E. Scheer, "A current-driven single-atom memory", Nature Nanotechnology 8, 645–648 (2013). Abstract. 
[2] E. Scheer, P. Joyez, D. Esteve, C. Urbina, M. H. Devoret, “Conduction channel transmissions of atomic-size aluminum contacts”, Physical Review Letters, 78, 3535–3538 (1997). Abstract. 
[3] J. M. van Ruitenbeek, A. Alvarez, I. Piñeyro, C. Grahmann, P. Joyez, M. H. Devoret, D. Esteve, C. Urbina, "Adjustable nanofabricated atomic size contacts", Review of Scientific Instruments, 67, 108 (1996). Abstract. 
[4] F. Pauly, M. Dreher, J. K. Viljas, M. Häfner, J. C. Cuevas, P. Nielaba, "Theoretical analysis of the conductance histograms and structural properties of Ag, Pt, and Ni nanocontacts", Physical Review B, 74, 235106 (2006). Abstract. 
[5] See supplementary information in reference [1].
[6] Sense Jan van der Molen, "Single-atom switches: Toggled with electrical current", Nature Nanotechnology, 8, 622 (2013). Abstract. 

Atom Interferometry in a 10 Meter Atomic Fountain

[From left to right] Mark Kasevich; the 10 m fountain team in 2013: Alex Sugarbaker, Tim Kovachy, Jason Hogan, Susannah Dickerson, Sheng-wey Chiow; and Dave Johnson

Authors: Alex Sugarbaker, Susannah M. Dickerson, Jason M. Hogan, David M. S. Johnson, and Mark A. Kasevich

Affiliation: Department of Physics, Stanford University, USA

Link to Kasevich Group Website >>

The equivalence principle states that all objects fall with the same acceleration under the influence of gravity. It is the conceptual foundation of Einstein’s general relativity, but is it true exactly? If not, there are profound implications for our understanding of gravity and the nature of the universe. It is therefore important to continue to test the equivalence principle as precisely as we can.

Galileo reportedly tested it by dropping spheres from the Leaning Tower of Pisa. Apollo astronauts tested it by dropping a hammer and a feather on the moon. More recent measurements have shown that the accelerations of two falling objects differ by no more than one part in 10¹³ [1, 2]. We aim to test the equivalence principle to one part in 10¹⁵ by dropping atoms of two different isotopes of rubidium in a 10 meter tower.

We will precisely measure acceleration differences between the two isotopes using atom interferometry. According to quantum mechanics, atoms are waves. Just as in optical interferometry, it is possible to split and recombine them to form an interference pattern [3, 4, 5]. In our interferometer, we send each atom along two different paths through space – each is in two places at once. When the atom waves are brought back together, the interference pattern depends on the phase difference between the two paths taken.

This phase difference in turn depends sensitively on the forces that act differently on the two parts of the atom while they are separated. This sensitivity to forces is what makes atom interferometry so useful. Compact atom interferometers have been made that can precisely measure rotation and acceleration, which can aid in navigation, mineral exploration, and geophysics. Atom interferometers have also measured the gravitational and fine-structure constants [6, 7]. They could also be used to search for gravitational waves [8].

The sensitivity of an atom interferometer increases with longer interferometer durations. Therefore, as recently described in Physical Review Letters [9, 10, 11], we have built an atom interferometer in which ⁸⁷Rb atoms are separated for 2.3 seconds before being recombined and interfered (Fig. 1). Three times longer than previous records [12], this multiple-second duration is well into the range of macroscopic, human-perceivable timescales. Furthermore, the two halves of each atom are separated by 1.4 centimeters before recombination – that's enough for you to swing your hand between them!

Fig. 1 Photograph of the 10 meter atomic fountain in a pit in the basement of the physics building at Stanford University.

How do we make a long-duration atom interferometer? We prepare a cloud of atoms at the bottom of a 10 meter vacuum tower and then launch them to the top. The interferometry is performed while the atoms rise up and fall back down to the bottom of the tower. The atoms are in free-fall, isolated from the noisy environment.

The cloud of atoms used must be very cold – a few billionths of a degree above absolute zero. At room temperature, the atoms in a gas move at speeds of hundreds of meters per second. Room-temperature rubidium atoms would collide with the walls of our vacuum chamber long before they fell back to the bottom. We therefore cool a few million rubidium atoms to a few nanokelvin before launching them into the tower. (The cooling process builds upon the same techniques used to generate Bose-Einstein condensates [13].)

Even at a few nanokelvin, individual atoms follow slightly different trajectories through the interferometer (like the droplets in a fountain of water), experiencing different position- and velocity-dependent forces. This yields a spatially-dependent phase, which in turn yields a spatial variation in the output atom density distribution that we can observe directly with a CCD camera (Fig. 2). This might at first appear undesirable, but it actually reveals rich details about the forces that generate the spatial interference pattern. Similar spatial fringe patterns have been used to great benefit in optical interferometers for centuries, but it is only recently that the effect has been leveraged in atom interferometry.

Fig. 2 Atomic interference patterns observed at the output of the interferometer. The images are sorted by phase, which can be measured for each experimental shot.

The long drift time of our interferometer enables it to have an acceleration sensitivity of 7 X 10^-12 g for each experimental shot, a hundredfold improvement over previous limits [14]. This is roughly the same as the gravitational attraction you would feel towards a person 10 meters away from you. We have used the sensitive interferometer and the spatial fringe patterns mentioned above to make precise measurements of Earth's rotation [9, 10].

The high sensitivity of the interferometer also holds great promise for our design goal – testing the equivalence principle (as mentioned above). By averaging more measurements or implementing advanced interferometry techniques, we can achieve the desired 10^-15 g sensitivity. Adding a simultaneous ⁸⁵Rb interferometer and comparing the results for the two isotopes will then enable us to make a new precision test of the equivalence principle. This will probe the fundamental assumptions of our current theory of gravity.

References:
[1] S. Schlamminger, K.Y. Choi, T.A. Wagner, J.H. Gundlach, and E.G. Adelberger, “Test of the Equivalence Principle Using a Rotating Torsion Balance”, Physical Review Letters, 100, 041101 (2008). Abstract.
[2] James G. Williams, Slava G. Turyshev, Dale H. Boggs, “Progress in Lunar Laser Ranging Tests of Relativistic Gravity”, Physical Review Letters, 93, 261101 (2004). Abstract.
[3] Mark Kasevich and Steven Chu, “Atomic interferometry using stimulated Raman transitions”, Physical Review Letters, 67, 181 (1991). Abstract.
[4] Alexander D. Cronin, Jörg Schmiedmayer, David E. Pritchard, “Optics and interferometry with atoms and molecules”, Reviews of Modern Physics, 81, 1051 (2009). Abstract.
[5] We focus on light-pulse atom interferometry, where pulses of laser light are used to split, recombine, and interfere the atoms.
[6] G. Lamporesi, A. Bertoldi, L. Cacciapuoti, M. Prevedelli, and G.M. Tino, “Determination of the Newtonian Gravitational Constant Using Atom Interferometry”, Physical Review Letters, 100, 050801 (2008). Abstract.
[7] Rym Bouchendira, Pierre Cladé, Saïda Guellati-Khélifa, François Nez, and François Biraben, “New Determination of the Fine Structure Constant and Test of the Quantum Electrodynamics”, Physical Review Letters, 106, 080801 (2011). Abstract.
[8] Jason Hogan, “A new method for detecting gravitational waves”, SPIE Newsroom, 6 May (2013). Article.
[9] Susannah M. Dickerson, Jason M. Hogan, Alex Sugarbaker, David M. S. Johnson, Mark A. Kasevich, “Multiaxis Inertial Sensing with Long-Time Point Source Atom Interferometry”, Physical Review Letters, 111, 083001 (2013).  Abstract.
[10] Alex Sugarbaker, Susannah M. Dickerson, Jason M. Hogan, David M. S. Johnson, Mark A. Kasevich, “Enhanced Atom Interferometer Readout through the Application of Phase Shear”, Physical Review Letters, 111, 113002 (2013). Abstract.
[11] P. Bouyer, “Viewpoint: A New Starting Point for Atom Interferometry”, Physics, 6, 92 (2013). Article.
[12] H. Müntinga, H. Ahlers, M. Krutzik, A. Wenzlawski, S. Arnold, D. Becker, K. Bongs, H. Dittus, H. Duncker, N. Gaaloul, C. Gherasim, E. Giese, C. Grzeschik, T. W. Hänsch, O. Hellmig, W. Herr, S. Herrmann, E. Kajari, S. Kleinert, C. Lämmerzahl, W. Lewoczko-Adamczyk, J. Malcolm, N. Meyer, R. Nolte, A. Peters, M. Popp, J. Reichel, A. Roura, J. Rudolph, M. Schiemangk, M. Schneider, S. T. Seidel, K. Sengstock, V. Tamma, T. Valenzuela, A. Vogel, R. Walser, T. Wendrich, P. Windpassinger, W. Zeller, T. van Zoest, W. Ertmer, W. P. Schleich, E. M. Rasel, “Interferometry with Bose-Einstein Condensates in Microgravity”, Physical Review Letters, 110, 093602 (2013). Abstract.
[13] “Bose-Einstein condensate”. Past 2Physics Article.
[14] Holger Müller, Sheng-wey Chiow, Sven Herrmann, Steven Chu, Keng-Yeow Chung, “Atom-Interferometry Tests of the Isotropy of Post-Newtonian Gravity”, Physical Review Letters, 100, 031101 (2008). Abstract.

Laser Cooling to Quantum Degeneracy

The SrBEC team in October 2012. From left to right: Slava M. Tzanova^1,2, Benjamin Pasquiou¹, Rudolf Grimm^1,2, Simon Stellmer^1,2,3 (author), Florian Vogl^1,2, Florian Schreck¹ (author), Alex Bayerle^1,2

Authors: Simon Stellmer^1,2,3 and Florian Schreck¹

Affiliation:
¹Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, 6020 Innsbruck, Austria
²Institute for Experimental Physics and Center for Quantum Physics, University of Innsbruck, 6020 Innsbruck, Austria
³Institute for Atomic and Subatomic Physics, Vienna University of Technology, 1020 Vienna, Austria

Link to the Sr BEC project homepage >>           
Link to the “Ultracold Atoms and Quantum Gases” group in Innsbruck >>

Laser cooling is a very elegant and versatile technique, as it permits to cool atoms, molecules, ions, and even mechanical objects from room temperature down to temperatures as low as one millionth of a degree above absolute zero [1,2]. At these low temperatures, we can enter into the world of quantum mechanics. One of the fascinating phenomena associated with this ultracold regime is the appearance of quantum degeneracy in atomic gases [2]. Since the early days of laser cooling, the question has been asked if the quantum degenerate regime could be reached using laser cooling as the only cooling process. Despite significant experimental and theoretical effort to overcome the limitations of laser cooling this goal has been elusive.

Past 2Physics article by Florian Schreck:
November 29, 2009: "Bose-Einstein Condensation of Strontium"

In 1995, the combination of laser cooling with a subsequence stage of evaporative cooling led to the attainment of quantum degeneracy in bosonic alkali-metal gases [2,3]. These quantum gases, called “Bose-Einstein condensates” (BECs), have opened an extraordinary window for the exploration of the quantum world.

To create a BEC, the phase-space density of the gas has to be increased beyond a critical value by lowering the temperature and increasing the density of the gas. Laser cooling was so far unable to reach quantum degeneracy because the photons used to cool the gas have negative side effects, which limit the achievable density and destroy a BEC.

In our experiment we overcome these side effects and create a BEC of strontium by laser cooling [4]. Furthermore, our method creates a BEC immersed in a laser cooled cloud of atoms, which opens a simple path towards the construction of a truly continuous atom laser.

Our scheme relies on the combination of three techniques, favored by the properties of strontium. The first technique is called “narrow line cooling”. Strontium possesses a narrow laser cooling transition of only 7 kHz width. Operating a magneto-optical trap (MOT) on this narrow line, we can cool a gas of strontium atoms down to temperatures of about 800 nK. This is already more than an order of magnitude colder than conventional MOTs of alkali atoms!

The second technique is a separation of our cold gas into two spatial regions (see Fig. 1): one large region, in which about 10 million atoms are trapped in a “reservoir” optical dipole trap and continuously cooled by laser light, and a small region, in which about 1 million atoms are confined at a much higher density by a steep confining potential, often called a “dimple”. This is the region where the BEC will be created.

Figure 1: Three absorption images of 10 million strontium atoms trapped in a dipole trap and cooled to about 800nK by laser light. All images are taken on the narrow cooling transition. The left image shows the reservoir of atoms, which is used to dissipate heat. For the central image, we have applied the transparency beam, such that atoms located within this beam cannot absorb photons from the cooling light. The density in this region is greatly enhanced by a dimple beam, as can be seen on the right image, where the transparency beam has been turned off.

The third technique allows us to overcome the negative side effects of laser cooling photons in the dimple region. We protect atoms in this region from those photons by the help of an extra laser beam, which we call the “transparency beam”. This beam acts like a cap of invisibility, as it modifies the energy states of the atoms in the dimple region such that they cannot absorb laser cooling photons (see Fig. 1). Importantly, the atoms are not only transparent to the cooling laser beam, but also to laser cooling photons scattered towards the dimple region from atoms in the laser cooled reservoir.

Now we have two different regions: the outside “reservoir” region, in which atoms are gently cooled by laser light and a central dimple region, in which the BEC will form. A connection between the two regions is established through the elastic scattering between atoms: in this way, heat can be transferred very rapidly (on timescales of a few 10 ms) from the dimple into the reservoir, where the heat is dissipated. To maximize this heat transfer, we place the dimple right into the center of the reservoir, as can be seen in Fig. 1.

Once the system is prepared in this configuration, it takes only about 60 ms for a BEC to appear, and after a little over 100 ms, the BEC has reached its final size of about 100 000 atoms (see Fig. 2).

Figure 2: Absorption images, taken 24 ms after release from the trap. On the left image, the BEC is faintly visible as an elliptic density increase in the center. For the right image, we have removed all atoms from the reservoir just before the release from the trap, and the BEC stands out clearly.

An important property of our system is that laser cooling constantly provides strong dissipation, removing entropy from the gas. Even if we destroy our BEC by local heating of the dimple region, it will quickly reform after the heating process is switched off, as long as enough atoms are contained in the dipole trap.

We believe that our scheme can be adapted to other elements. We expect it to work with all species that possess a narrow cooling transition and have a reasonable scattering behavior. These criteria are fulfilled by a selection of elements, most prominently the lanthanides. The range of suitable candidates can be increased substantially by going one step further: sympathetic cooling between strontium and another element. This element would not need to feature a narrow cooling transition. Instead, it would be trapped in the dimple region and sympathetically cooled through collisions with the strontium atoms in the reservoir. We have recently implemented this sympathetic laser cooling scheme in a mixture of rubidium and strontium. The rubidium gas is cooled very efficiently by thermal contact with laser cooled strontium atoms, delivering ideal starting conditions for the creation of quantum degenerate Rb-Sr mixtures by evaporative cooling [5]. By using a Rb specific dipole trap as a dimple, it should also be possible to create a Rb BEC without evaporative cooling.

Our scheme also paves a relatively simple path towards a truly continuous atom laser. Here, a thermal source of atoms would be converted into a coherent beam of atoms, constantly outcoupled from the dimple region. The dimple would be continuously fed by the reservoir region, which in turn would be replenished by pre-cooled atoms from a MOT. Such truly continuous atom lasers are highly desired in various schemes of precision measurements.

References :
[1] Proceedings of the International School of Physics "Enrico Fermi", Course CXVIII, Varenna, 9-19 July 1991, Laser Manipulation of Atoms and Ions, edited by E. Arimondo, W. D. Phillips, and F. Strumia (North Holland, Amsterdam, 1992).
[2] Physics 2000, BEC homepage. Link.
[3] Proceedings of the International School of Physics ‘‘Enrico Fermi’’, Course CXI, Varenna, 7-17 July 1998, Bose-Einstein Condensation in Atomic Gases, edited by M. Inguscio, S. Stringari, and C. E. Wieman (North Holland, Amsterdam, 1999).
[4] Simon Stellmer, Benjamin Pasquiou, Rudolf Grimm, and Florian Schreck, "Laser Cooling to Quantum Degeneracy", Physical Review Letters, 110, 263003 (2013). Abstract.
[5] Benjamin Pasquiou, Alex Bayerle, Slava M. Tzanova, Simon Stellmer, Jacek Szczepkowski, Mark Parigger, Rudolf Grimm, and Florian Schreck, "Quantum degenerate mixtures of strontium and rubidium atoms", Physical Review A, 88, 023601 (2013). Abstract. 

Storage and Control of Optical Photons

Authors of the paper in Physical Review Letters (reference [1] ).
Left to Right: 
(top row) D. Maxwell, D. J. Szwer, D. Paredes-Barato, 
(middle row) H. Busche, J. D. Pritchard, A. Gauguet, 
(bottom row) K. J. Weatherill, M. P. A. Jones, C. S. Adams.



Authors: 
David Szwer and Hannes Busche

Affiliation:
Joint Quantum Centre (JQC) Durham-Newcastle, Department of Physics, Durham University, UK.





Quantum communication and information processing rely on robust carriers of quantum information (qubits), and optical photons are prime candidates. Light can easily be transmitted using optical technologies such as fibre cables, and barely interact with other photons or the environment. To process the information they carry however, controllable interactions between the single photons are needed. Physicists at Durham University in the UK have combined two advanced techniques of quantum optics with a microwave synthesiser, to control the interactions between individual photons [1,2]. The photons were stored in a cloud of rubidium atoms as so-called Rydberg polaritons. Due to long-ranged interactions between them, only a single photon may be stored in a volume of a few micrometres, limiting the number of stored photons to about three. The microwaves manipulated the photons while they were stored, forcing them to interact in ways whose details are still not fully understood. The ability to not only induce interactions on the level of single optical photons, but also to control them using microwave fields, could be an important new angle for future quantum technologies.

A beam of light is one of the least substantial things you'll ever encounter. It is the very essence of something fast and untrappable - so much so that Einstein invented relativity specifically because he couldn't imagine what light would be like if it was stationary. And although light reflects off mirrors and scatters off other objects, we never consider that two light beams could bounce off each other.

However, in specially tuned systems physicists can not only stop light, but make light interact with light. Our work achieves both these effects, making the interaction work at the single-photon level and even controlling it with microwaves.

The loophole that allows us to do these things is that light isn't always the pure electromagnetic wave that it is in vacuum. When light travels through matter, like glass or water, the electric field of the light slightly distorts the atoms in the matter, and they tug back on the light. This slows the light down (to half-speed or slower), and is ultimately responsible for lenses, optical fibres, rainbows and other phenomena.

Almost always, matter responds linearly to light so that the distortion from one beam does not affect another. At very high intensities, nonlinear effects can appear that do allow light to affect light. However, the dependence is so weak that it only manifests at very high powers. We care about the quantum nature of light, which only becomes apparent at the level of single photons. Photons carry a tiny amount of energy: a normal red laser pointer (emitting 1mW of power) produces over 3 million billion photons every second. Hence photons almost never interact.

To slow photons to a crawl, bring them to a full stop and make them interact, we therefore need to deliberately engineer our own medium. We start from an extremely thin and cold gas of the element rubidium (it’s normally a solid metal) created by laser cooling and trapping techniques [3]. The gas is about a million times more dilute than normal air (number of atoms per unit volume), so the experiment is done in a vacuum chamber. The gas needs to be extremely cold, just 100 μK (a ten-thousandth of a degree above absolute zero), because if the gas atoms move too fast they’ll mix up the light they’re storing.

Figure 1: Relevant energy levels of rubidium, showing the 780 nm (red, signal) and 480 nm (blue, control) spectral lines, and the Rydberg-Rydberg microwaves. 

 Gases like rubidium are useful because they have strong resonance lines – specific frequencies of light they interact with strongly and correspond to transitions between the discrete energy levels of the atom. We use two lines in rubidium, of wavelength 780 nm (less than a thousandth of a millimetre, a dim red colour) and 480 nm (bright blue). The red light is the signal we store – it excites normal, ground-state atoms to a particular excited energy level. The blue light takes atoms from that excited state to an even higher level called a Rydberg state. It affects how the rubidium atoms respond to the red light, not only preventing them from scattering the signal, but also slowing it down greatly because it is mixed so strongly with atomic distortion. If we turn the blue light off while signal is travelling through the cloud, the red photons are stopped and transformed completely into atomic distortion: a specific pattern of Rydberg excitations called a polariton.

Unlike photons, Rydberg states do interact with each other extremely strongly, causing shifts in the resonance frequencies needed to excite nearby atoms to Rydberg states. As a single Rydberg excitation shifts the remaining atoms out of resonance with our blue light, a blockade effect occurs preventing multiple Rydberg excitations within a volume of a few micrometres. If photons are mixed with Rydberg states, like in our cloud, the photons gain this ability to interact strongly and the blockade applies to them, imposing a minimum separation between photons. Photons that are too close together are scattered out of the cloud. If the cloud is dense enough, this leads to a continuous laser beam being filtered into a series of individual photons, as demonstrated recently by a group at MIT [4]. Alternatively we can stop light in the cloud, and allow the blockade to work while the photons are stored – this was done not only in our experiment, but also by a group at Georgia Tech where a single photon source was implemented [5]. Reliable single-photon sources like this are important for many other quantum experiments, as well as for quantum cryptography [6].

Our cloud is large enough for about three polaritons to sit in a line, each containing a single photon shared among many atoms. Due to the blockade, they are separated by a distance called the blockade radius, which measures the strength of interaction between two Rydberg atoms. This interaction is called the Van der Waals interaction, and it’s the same force that sticks molecules to each other in most solids around you.

Once we have some photons stored, our next step is to beam microwaves at the cloud, tuned to a transition between two different Rydberg states. One of these is the state we stored a photon in (called “S”), and the other state was previously empty (called “P”). Our blue light is only tuned to interact with the S state, not the P, so it will only release a stored photon if the polariton is in S. In an isolated Rydberg polariton, this would drive regular sine-wave “Rabi” oscillations between the two states. So if we apply microwaves for just long enough to transfer to P, and then turn the blue beam on, we don’t retrieve the photon.

Figure 2: Diagram of the experiment, showing the cloud of atoms with three stored polaritons. The two single-photon detectors shown are used to prove that the number of stored photons is limited, via a Hanbury Brown & Twiss experiment [7.8].

Instead of a single polariton, we’re most interested in what happens when the polaritons aren’t isolated, but have neighbours to interact with. The microwaves make them oscillate at the same rate. For a start, photons are only retrieved from the cloud when all polaritons are in S. This leads to the Rabi oscillations (shown in Figure 3) having flattened bottoms and sharper peaks when we scan the microwaves, and the exact shape helps us estimate the number of polaritons we have. Also, while the polaritons are half-way between S and P, they have large electric dipole moments – (the static electricity equivalent of a tiny bar magnet). The original van der Waals interaction is replaced with resonant dipole-dipole interactions – this makes them interact much more strongly than before, and at longer distances. Blockade was caused by the atoms within a polariton interacting – but now the polaritons interact with each other. We see that at low microwave powers, these interactions completely prevent the stored photons being retrieved, scattering them out of the cloud in other directions. But stronger microwaves can overwhelm the dipole-dipole interactions, and we see oscillations again.

Figure 3: The microwave-induced Rabi oscillations between two Rydberg states. Microwave power increases from left to right. The dashed line shows the transition from low power (dominated by Rydberg-Rydberg interactions) to high power (dominated by the microwaves).

This is a subtle system, and not yet fully understood. There is plenty more experimental and theoretical work to do. But we can already imagine applications. By controlling strong interactions between individual photons, it may be possible to build quantum logic gates that could be used in super-fast quantum computers, or super-secure quantum cryptography. There are also similarities between the way that specific Rydberg states can “hop” between polaritons, and the hopping of excitations along light harvesting complexes in plants. Experiments like these may help model photosynthesis, perhaps even helping scientists design more efficient solar cells.

Bibliography
[1] D. Maxwell, D. J. Szwer, D. Paredes-Barato, H. Busche, J. D. Pritchard, A. Gauguet, K. J. Weatherill, M. P. A. Jones, C. S. Adams, "Storage and Control of Optical Photons Using Rydberg Polaritons". Physical Review Letters , 110, 103001 (2013). Abstract.
[2] Antoine Browaeys, "Viewpoint: Catch and Release of Photons". Physics , 6, 25 (2013). Article.
[3] Nobel Prize in Physics 1997: Link.
[4] Thibault Peyronel, Ofer Firstenberg, Qi-Yu Liang, Sebastian Hofferberth, Alexey V. Gorshkov, Thomas Pohl, Mikhail D. Lukin, Vladan Vuletić, "Quantum nonlinear optics with single photons enabled by strongly interacting atoms". Nature, 488, 57-60 (2012). Abstract.
[5] Y.O. Dudin,  A. Kuzmich, "Strongly Interacting Rydberg Excitations of a Cold Atomic Gas. Science". 336, 887-889 (2012). Abstract.
[6] Alexios Beveratos, Rosa Brouri, Thierry Gacoin, André Villing, Jean-Philippe Poizat, and Philippe Grangier, "Single Photon Quantum Cryptography". Physical Review Letters, 89, 187901 (2002). Abstract.
[7] Michel Orrit, "Photon Statistics in Single Molecule Experiments". Single Molecules, 3, 255-265 (2002). Abstract.
[8] R. Hanbury Brown, R.Q.  Twiss, "Correlation between Photons in two Coherent Beams of Light". Nature, 177, 27-29 (1956). Abstract.

“Dressing” Atoms with Laser Allows High Angular Momentum Scattering : Could Reveal Ways to Observe Majorana Fermions

Ian Spielman (photo courtesy: Joint Quantum Institute, USA)

Scientists at the Joint Quantum Institute (JQI, a collaborative enterprise of the 'National Institute of Standards and Technology' and the University of Maryland) have for the first time engineered and detected the presence of high angular momentum collisions between atoms at temperatures close to absolute zero. Previous experiments with ultracold atoms featured essentially head-on collisions. The JQI experiment, by contrast, is able to create more complicated collisions between atoms using only lasers that dramatically influences their interactions in specific ways.

Such light-tweaked atoms can be used as proxies to study important phenomena that would be difficult or impossible to study in other contexts. Their most recent work, appearing in Science [1] demonstrates a new class of interactions thought to be important to the physics of superconductors that could be used for quantum computation.

Particle interactions are fundamental to physics, determining, for example, how magnetic materials and high temperature superconductors work. Learning more about these interactions or creating new “effective” interactions will help scientists design materials with specific magnetic or superconducting properties.Because most materials are complicated systems, it is difficult to study or engineer the interactions between the constituent electrons. Researchers at JQI build physically analogous systems using supercooled atoms to learn more about how materials with these properties work.

The key to the JQI approach is to alter the atoms’ environment with laser light. They “dress” rubidium atoms by bathing them in a pair of laser beams, which force the atoms to have one of three discrete values of momentum. In the JQI experiment, rubidium atoms comprise a Bose-Einstein condensate (BEC). BECs have been collided before. But the observation of high-angular-momentum scattering at such low energies is new.

The paper in 'Science Express' [1] includes a variety of technical issues which require some explanation:

Collisons

One of the cardinal principles of quantum science is that matter must be simultaneously thought of as both particles and waves. When the temperature of a gas of atoms is lowered, the wavelike nature of the atom emerges, and the idea of position becomes fuzzier. While an atom at room temperature might spread over a hundredth of a nm, atoms at nano-kelvin temperatures have a typical wavelength of about 100 nm. This is much larger than the range of the force between atoms, only a few nm. Atoms generally collide only when they meet face to face.

However, to study certain interesting quantum phenomena, such as searching for Majorana particles---hypothetical particles that might provide a robust means of encoding quantum information---it is desirable to engineer inter-atomic collisions beyond these low-energy, head-on type. That’s what the new JQI experiment does.

Partial Waves

Scattering experiments date back to the discovery of the atomic nucleus 100 years ago, when Ernest Rutherford shot alpha particles into a foil of gold. Since then other scattering experiments have revealed a wealth of detail about atoms and sub-atomic matter such as the quark substructure of protons.

A convenient way of picturing an interaction between two particles is to view their relative approach in terms of angular momentum. Quantized angular momentum usually refers to the motion of an electron inside an atom, but it necessarily pertains also to the scattering of the two particles, which can be thought of as parts of a single quantum object.

If the value of the relative angular momentum is zero, then the scattering is designated as “s-wave” scattering. If the pair of colliding particles has one unit of angular momentum, the scattering is called p-wave scattering. Still more higher-order scattering scenarios are referred to by more letters: d-wave, f-wave, g-wave, and so on. This model is referred to as the partial waves view.

In high energy scattering, the kind at accelerators, these higher angular-momentum scattering scenarios are important and help to reveal important structure information about the particles. In atomic scattering at low temperatures, the s-wave interactions completely swamp the higher-order scattering modes. For ultralow-temperature s-wave scattering, when two atoms collide, they glance off each other (back to back) at any and all angles equally. This isotropic scattering doesn’t reveal much about the nature of the matter undergoing collision; it’s as if the colliding particles were hard spheres.

This has changed now. The JQI experiment is the first to create conditions in which d-wave and g-wave scattering modes in an ultracold experiment could be seen in otherwise long-lived systems.

Quantum Collider

Ian Spielman and his colleagues at the National Institute for Standards and Technology (NIST) chill Rb atoms to nano-kelvin temperatures. The atoms, around half a million of them, have a density about a millionth that of air at room temperature. Radiofrequency radiation places each atom into a superposition of quantum spin states. Then two (optical light) lasers impart momentum (forward-going and backward-going motion) to the atoms.

Schematic drawing of collision between two BECs (the gray blobs) that have been “dressed” by laser light (brown arrows) and an additional magnetic field (green arrow). The fuzzy halo shows where atoms have been scattered. The non-uniform projection of the scattering halo on the graph beneath shows that some of the scattering has been d-wave and g-wave [image courtesy: JQI]

If this were a particle physics experiment, we would say that these BECs-in-motion were quantum beams, beams with energies that came in multiples of the energy kick delivered by the lasers. The NIST “collider” in Gaithersburg, Maryland is very different for the CERN collider in Geneva, Switzerland. In the NIST atom trap the particles have kinetic energies of a hundred pico-electron-volts rather than the trillion-electron-volt energies used at the Large Hadron Collider.

At JQI, atoms are installed in their special momentum states, and the collisions begin. Outward scattered atoms are detected after the BEC clouds are released by the trap. If the atoms hadn’t been dressed, the collisions would have been s-wave in nature and the observed scattered atoms would have been seen uniformly around the scattering zone.

The effect of the dressing is to screen the atoms from s-wave scattering in the way analogous to that in some solid materials, where the interaction between two electrons is modified by the presence of trillions of other electrons nearby. In other words, the laser dressing effectively increased the range of the inter-atom force such that higher partial wave scattering was possible, even at the lowest energies.

In the JQI experiment, the observed scattering patterns for atoms emerging from the collisions was proof that d-wave and g-wave scattering had taken place. “The way in which the density of scattered atoms is distributed on the shell reflects the partial waves,” said Ian Spielman. “A plot of scattered-density vs. spherical polar angles would give the sort of patterns you are used to seeing for atomic orbitals. In our case, this is a sum of s-, p-, and d- waves.”

Simulating Solids Using Gases

Ultracold atomic physics experiments performed with vapors of atoms are excellent for investigating some of the strongly-interacting quantum phenomena usually considered in the context of condensed matter physics. These subjects include superconductivity, superfluids, the quantum Hall effect, and topological insulators, and some things that haven’t yet been observed, such as the “Majorana” fermions.

Several advantages come with studying these phenomena in the controlled environment of ultracold atoms. Scientists can easily manipulate the landscape in which the atoms reside using knobs that adjust laser power and frequency. For example, impurities that can plague real solids can be controlled and even removed, and because (as in this new JQI experiment) the scattering of atoms can now (with the proper “dressing”) reveal higher-partial-wave effects. This is important because the exotic quantum effects mentioned above often manifest themselves under exactly these higher angular-momentum conditions.

“Our technique is a fundamentally new method for engineering interactions, and we expect this work will stimulate new directions of research and be of broad interest within the physics community, experimental and theoretical,” said Spielman. “We are modifying the very character of the interactions, and not just the strength, by light alone.”

On To Fermions

The JQI team, including Nobel Laureate William Phillips, is truly international, with scientists originating in the United Kingdom (lead author Ross Williams), Canada (Lindsay LeBlanc), Mexico (Karina Jiménez-García), and the US (Matthew Beeler, Abigail Perry, William Phillips and Ian Spielman).

The researchers now will switch from observing bosonic atoms (with a total spin value of 1) to fermion atoms (those with a half-integral spin). Combining the boson techniques demonstrated here with ultracold fermions offers considerable promise for creating systems which are predicted to support the mysterious Majorana fermions. “A lot of people are looking for the Majorana fermion,” says lead author and JQI postdoctoral fellow Ross Williams. “It would be great if our approach helped us to be the first.”

Reference
[1] R. A. Williams, L. J. LeBlanc, K. Jiménez-García, M. C. Beeler,A. R. Perry, W. D. Phillips, I. B. Spielman, "Synthetic partial waves in ultracold atomic collisions”, Science Express, (December 7, 2011). DOI: 10.1126/science.1212652. Abstract.

Towards the Ideal Quantum Measurement

Juergen Volz

Author: Juergen Volz

Affiliation: Laboratoire Kastler Brossel de l'E.N.S., Paris, France

Measurement lies at the heart of quantum physics and gives rise to many of the counter-intuitive aspects of the theory. In a classical world, a measurement can in principle be performed with arbitrary precision without disturbing the system. In contrast, a quantum mechanical measurement inevitably projects the system into one of its basis states, although initially the system may have been in a superposition of these states. For example, it is possible to prepare an atom in a superposition of two internal states. However, a state measurement will always yield the result that the atom is either in one or the other state, which is typically referred to as 'collapse of the wave function'. This collapse during the measurement process constitutes the unavoidable back-action of the measurement on the measured system and gives rise to phenomena as for example Heisenberg's uncertainty relation[1].

While theoretically well described, experimental realizations typically fall short of these predictions, always causing a back-action on the system being measured -- with orders of magnitude larger than required for an ideal measurement. This additional back-action typically results in a energy transfer to the quantum system and heating which is a major drawback for many experiments.

The modern field of quantum optics and quantum information, relies on the accurate readout of quantum information stored in so-called quantum bits, i.e. single quantum objects that for example can be realized using the internal states of single atoms or ions. The most efficient detection method for internal atomic states is the fluorescence detection[2]. Here an atom with two stable ground states is subject to an incident light field resonant to a transition of one of the two ground states to an excited state. If the atom is in the resonant state, it will be repeatedly excited and - after spontaneously emitting a photon - decay back into the original state, while an atom in the off-resonant state is not affected by light field. The presence (or absence) of fluorescence photons then indicates the atomic state. However, this requires the atom to undergo a large amount of spontaneous emissions events, leading to an inevitable energy exchange between atom and light field.






















Fig 1: (a) Simplified level scheme of Rubidium. (b) Principle of the resonator measurement where the transmission properties of the resonator yield information on the atomic state (all light reflected: atom is in the resonant state, all light transmitted: atom is in the off-resonant state). (c) Schematic view of the experimental setup with the fiber-resonator implemented on an atomchip.

Theoretically, an ideal atomic state measurement only projects the atom onto its basis states and requires no spontaneous scattering. Therefore, our research team at the Ecole Normale Supérieure in Paris decided to investigate if we can reach a regime where we can perform a measurement with significantly less than one spontaneous emission event[3]. In our experiments we use as measurement device a so-called Fabry-Perot resonator which consists of two highly reflecting mirrors facing each other. This allows to keep light inside the resonator for approximately 38000 round trips before it is lost. With the help of a novel fiber-optical technology, the mirrors are directly imprinted on the tips of two optical fibers. In this way we can produce miniaturized resonators with a small enough volume so that a single atom placed between its mirrors is enough to shift the resonance frequency by a sizeable amount. As long as the atom is in the off resonant state it does not affect the resonator and a laser tuned to the empty cavity’s resonance is fully transmitted through the cavity. If, however, the atom is in the resonant state the resonance frequency changes and nearly all of the incident light is reflected without ever entering the resonator, thereby avoiding spontaneous scattering.






















Fig 2: Experimental results for the detection process. The blue data points are the residual error of the atomic state measurement, plotted as function of average number of scattered photons. The green curve is the minimum error possible for our resonator and the grey area corresponds to the regime accessible without resonator.

To investigate the exact amount of residual scattering in our experiment, we determine the state of the atom after each measurement from which we can deduce that our measurement allows us to infer the atomic state with a fidelity of more than 90% while scattering only 0.2 photons on average. The main limit of our measurement scheme is the small probability to finally detect the incident photons. In order to measure how our system would perform under ideal circumstances, we also directly analyzed the fundamental measurement back-action on the atom using the quantum Zeno effect[4]. This effect states that permanently measuring a physical system will stop its temporal evolution and freeze it in its current state. In our experiment, we apply a microwave pulse to transfer the atomic state to the other. At the same time we perform our state detection which permanently projects the atom into its initial state an thereby prevents the transfer. This allows us to directly measure the projection rate of the atom into its basis states, from which we conclude that three photons incident on the cavity are enough for a full collapse of the atomic state.

These results demonstrate that during the measurement (nearly) no spontaneous emission occurred and the back-action on the atom approaches the fundamental limit given by the uncertainty principle. Besides giving a compelling illustration of the quantum measurement process, these results have important consequences for quantum information applications with atoms or ions, allowing internal state readout without any heating, hereby allowing much higher cycling rates. In addition, this new detection scheme promises the development of efficient optical detectors for complex quantum systems as e.g. single molecules.

References:
[1] Maximilian Schlosshauer, "Decoherence, the measurement problem, and interpretations of quantum mechanics", Rev. Mod. Phys. 76, 1267 (2005). Abstract.
[2] A. H. Myerson, et al., "High-Fidelity Readout of Trapped-Ion Qubits", Phys. Rev. Lett 100, 200502 (2008). Abstract.
[3] Jürgen Volz, Roger Gehr, Guilhem Dubois, Jérôme Estève and Jakob Reichel, "Measurement of the internal state of a single atom without energy exchange", Nature 475, 210 (2011). Abstract.
[4] Wayne M. Itano, "Perspectives on the quantum Zeno paradox.", arXiv:quant-ph/0612187v1 (2006).

Quantum Quirk: Packing Atoms Together to Prevent Collisions in Atomic Clock

Jun Ye [photo courtesy: JILA/University of Colorado]

In a paradox typical of the quantum world, JILA scientists have eliminated collisions between atoms in an atomic clock by packing the atoms closer together. The surprising discovery, described in the Feb. 3 issue of Science Express [1], can boost the performance of experimental atomic clocks made of thousands or tens of thousands of neutral atoms trapped by intersecting laser beams.

[JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder. Once upon a time, JILA was the Joint Institute for Laboratory Astrophysics. These days, that name doesn't encompass the breadth of science conducted at JILA. So, after extended discussion in 1994, JILA's fellows decided to keep the word JILA but drop the meaning.]

JILA scientists demonstrated the new approach using their experimental clock made of about 4,000 strontium atoms. Instead of loading the atoms into a stack of pancake-shaped optical traps as in their previous work, scientists packed the atoms into thousands of horizontal optical tubes. The result was a more than tenfold improvement in clock performance because the atoms interacted so strongly that, against all odds, they stopped hitting each other. The atoms, which normally like to hang out separately and relaxed, get so perturbed from being forced close together that the ensemble is effectively frozen in place.

The idea was proposed by JILA theorist Ana Maria Rey and demonstrated in the lab by Ye's group.

Ana Maria Rey [photo courtesy: JILA/University of Colorado]

"The atoms used to have the whole dance floor to move around on and now they are confined in alleys, so the interaction energy goes way up," says NIST/JILA Fellow Jun Ye, leader of the experimental team.

How exactly does high interaction energy—the degree to which an atom's behavior is modified by the presence of others—prevent collisions? The results make full sense in the quantum world. Strontium atoms are a class of particles known as fermions. If they are in identical energy states, they cannot occupy the same place at the same time—that is, they cannot collide. Normally the laser beam used to operate the clock interacts with the atoms unevenly, leaving the atoms dissimilar enough to collide [Read past 2Physics report dated May 2, 2009] But the interaction energy of atoms packed in optical tubes is now higher than any energy shifts that might be caused by the laser, preventing the atoms from differentiating enough to collide.

Intersecting laser beams create "optical tubes" to pack atoms close together, enhancing their interaction and the performance of JILA's strontium atomic clock.[Image Credit: Baxley/JILA]

Given the new knowledge, Ye believes his clock and others based on neutral atoms will become competitive in terms of accuracy with world-leading experimental clocks based on single ions (electrically charged atoms). The JILA strontium clock is currently the best performing experimental clock based on neutral atoms and, along with several NIST ion and neutral atom clocks, a possible candidate for a future international time standard. The devices provide highly accurate time by measuring oscillations (which serve as "ticks") between the energy levels in the atoms.

In addition to preventing collisions, the finding also means that the more atoms in the clock, the better. "As atom numbers increase, both measurement precision and accuracy increase accordingly," Ye says.

To trap the atoms in optical tubes, scientists first use blue and red lasers to cool strontium atoms to about 2 microKelvin in a trap that uses light and magnetic fields. A vertical lattice of light waves is created using an infrared laser beam that spans and traps the atom cloud. Then a horizontal infrared laser beam is turned on, creating optical tube traps at the intersection with the vertical laser.

Reference:
[1] Matthew D. Swallows, Michael Bishof, Yige Lin, Sebastian Blatt, Michael J. Martin, Ana Maria Rey, Jun Ye, "Suppression of collisional shifts in a strongly interacting lattice clock", Science Express (Posted online Feb.3, 2011). DOI: 10.1126/science.1196442. Abstract.

[We thank National Institute of Standards and Technology for materials used in this report]

“Spin Doctors” Build a Better Magnetometer

(From left to right) Mikhail Balabas, Todor Karaulanov, Dmitry Budker, and Micah Ledbetter in Budker's laboratory in University of California, Berkeley [Photo credit: Damon English, Lawrence Berkeley National Laboratory]

Magnetometers come in many shapes and sizes – an ordinary hand-held compass is the simplest – but alkali-vapor magnetometers are extrasensitive devices that measure magnetic fields using light and atoms. They can detect archaeological remains and mineral deposits underground by their faint magnetic signatures, among a host of other scientific applications.

Researchers from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, the University of California at Berkeley, and the Vavilov State Optical Institute in St. Petersburg, Russia, have now made sensitive measurements of magnetic fields by maintaining the spin polarization of atoms in an alkali-vapor magnetometer for more than 60 seconds at room temperature – a two-orders-of-magnitude improvement in this important measurement parameter over the best previous performance [1].

In a spin-polarized population of atoms, more than half the atoms are oriented in the same direction. An alkali-vapor magnetometer polarizes a vapor of alkali-metal atoms, for example potassium, rubidium, or cesium, inside a glass cell using a circularly-polarized “pump” laser beam.

Because the spinning atoms have a magnetic moment (with north and south magnetic poles, like a bar magnet), an outside magnetic field will tilt the axis of spin and cause it to precess like a spinning top that’s been pushed off the vertical. Changes in the outside field’s strength or direction can be detected using a probe laser to repeatedly measure the vapor’s average spin orientation.

In a vapor-cell magnetometer, the spin of a population of atoms is first polarized, as indicated by the vertical red arrow, by a pump laser that is itself circularly polarized. When a magnetic field is applied, the spin vector is rotated, as indicated by the tilted red arrow. (The magnetic field is perpendicular to the plane of this diagram.) The probe laser’s own plane polarization is rotated by the atom’s spin, and the degree of rotation is measured at the detector In a vapor-cell magnetometer, the spin of a population of atoms is first polarized, as indicated by the vertical red arrow, by a pump laser that is itself circularly polarized. When a magnetic field is applied, the spin vector is rotated, as indicated by the tilted red arrow. (The magnetic field is perpendicular to the plane of this diagram.) The probe laser’s own plane polarization is rotated by the atom’s spin, and the degree of rotation is measured at the detector. [Image courtesy: Lawrence Berkeley National Laboratory]

“The fundamental sensitivity of the measurement depends on a number of variables,” says Dmitry Budker of Berkeley Lab’s Nuclear Science Division, a professor of physics at UC Berkeley. “These include the number of atoms in the sample and, most important, the spin relaxation time of the polarized atoms.”

Spin relaxation is the loss of polarization, the return of the population of atoms to random orientations, which happens faster as atoms collide with other atoms, or if the external magnetic field varies.

How to keep ‘em spinning

“When an alkali-metal atom bounces off a glass wall, it tends to stick for a little while,” says Budker. “During its stay it is subject to fluctuating magnetic fields, which cause it to lose polarization. So one way to maintain polarization is to keep the atoms away from the wall, or to make their sojourns on the wall shorter.”

One approach is to fill the cell with an inert buffer gas like helium or neon, at a density high enough that the alkali atoms constantly bump into the buffer gas atoms instead of colliding with the walls. The resulting slow diffusion keeps many of the polarized atoms away from the wall for a long time. Nevertheless, collisions with the buffer gas atoms eventually relax the polarization of the metal atoms.

(From left to right) Mikhail Balabas, Todor Karaulanov, Micah Ledbetter, and Dmitry Budker with the antirelaxation-coated vapor cell. Inset shows the rubidium reservoir and the lock (red) that can open or close off the interaction area in the bulb [Photo credit: Damon English, Lawrence Berkeley National Laboratory]

A better way to keep spin coherence high is to coat the interior of the glass vapor cell with an “antirelaxation” coating. The goal is to increase the number of bounces an atom can survive before losing its polarization.

“It’s important to reduce magnetic fluctuations by avoiding any heavy atoms in the coating,” Budker says. Compounds of light carbon and hydrogen atoms are the choice; state-of-the-art antirelaxation coatings are paraffins, known chemically as alkanes. A polarized atom can hit a paraffin coating 10,000 times before losing its polarization.

But Budker and his long-time colleague Mikhail Balabas of St. Petersburg’s Vavilov State Optical Institute have worked to extend relaxation times using different coatings. Contrary to conventional wisdom, Balabas suggested substituting a different kind of hydrocarbon known as an alkene, or olefin. Alkenes are similar to alkanes but, instead of being saturated (all single bonds), have one carbon double bond in the molecule. The researchers’ experiments with rubidium vapor cells subsequently showed that a polarized rubidium atom could bounce off an alkene coating a million times before losing its polarization.

Fine-tuning the experiment

“The coating material is not all there is to prolonging polarization, however,” Budker says. “One way polarization is lost is when polarized rubidium atoms in the cell get in contact with uncoated surfaces in the cell’s rubidium reservoir – the sidearm that contains a droplet of the solid metal.”
Balabas devised a simple lock – a sliding glass plug that, merely by rotating the cell assembly, opens or closes the stem between the reservoir and the interaction region where the atoms are polarized and measured.

Finally, the researchers slowed spin relaxation due to collisions among the rubidium atoms inside the interaction area of the cell by modifying a technique called SERF (for “spin exchange, relaxation-free” [2]). The physics of SERF were developed by William Happer and applied to magnetometry by Michael Romalis, both of Princeton University. SERF normally uses buffer gas to reduce the number of alkali atoms hitting the cell wall, while at the same time paradoxically stepping up collisions among the alkali atoms themselves, heating the cell to some 150 degrees Celsius and increasing the density of the atomic vapor.

SERF works only for very weak magnetic fields, where precession is slow. Since atoms collide many times during any period of precession, the multiple collisions frequently exchange spin states among the atoms and keep the average polarization high. To extend the relaxation time still further, the Berkeley and Vavilov Institute collaboration used their “super” antirelaxation coating instead of the usual buffer gas.

The experimental setup was built in Budker’s laboratory by Micah Ledbetter and Todor Karaulanov, and was designed to maintain fine control over the shape of magnetic fields inside the experimental chamber. The vapor cell was shielded from Earth’s magnetic field by four layers of mu metal, an alloy of nickel and iron that shunts magnetic fields around the shielded area, plus a cylinder of ceramic ferrite.

The experimental assembly was gimbaled so the vapor cell could be rotated, letting the sliding plug lock the neck of the flask or unlock it to allow rubidium vapor into the reaction region. Then a circularly polarized pump beam traversed the axis of the experiment to polarize the atomic vapor, while a probe beam passing through the cell from side to side recorded the spin state of the rubidium vapor by measuring how the probe beam’s own linear polarization was rotated.

Three cells were tested, which differed either in construction or in the rubidium isotopes they contained. Relaxation times in two of the cells were about 15 seconds, already a significant extension, but in one, using the most common isotope of rubidium, 85Rb, the relaxation time stretched to over a minute. In contrast to the usual SERF setup, this very long relaxation time was achieved at room temperature instead of extreme heat.

“We have demonstrated two orders of magnitude improvement over the best paraffin coatings, and at room temperature – but at a relatively low magnetic field,” Budker says. “The next challenge is to use this technique in stronger magnetic fields – as strong as Earth’s magnetic field, for example, where many of the practical applications are.”

At the same time, Budker and his colleagues intend to explore the application of their new coatings, and the other tricks they used to achieve long relaxation times, to devices other than magnetometers. Among the candidates are atomic clocks, quantum memory devices, and other scientific gadgets that likewise depend on long-lived spin polarization of atoms.

An early exploration of how vapor-cell wall coatings increase spin relaxation, “Relaxation of optically pumped Rb atoms on paraffin-coated walls,” by French physicists Marie-Anne Bouchiat and Jean Brossel, was published in 1966 in Physical Review [3]. A more recent “Investigation of anti-relaxation coatings for alkali-metal vapor cells using surface science techniques” was conducted by Budker and colleagues including Balabas, Bouchiat, Karaulanov, Alex Pines, and others, and is available online [4].

Reference:
[1] Mikhail Balabas, Todor Karaulanov, Micah Ledbetter, Dmitry Budker, "Polarized Alkali-Metal Vapor with Minute-Long Transverse Spin-Relaxation Time", Phys. Rev. Lett, 105, 070801 (2010). Abstract. arXiv:1005.1617.
[2] Wikipedia article on SERF technique and its history.
[3] M. A. Bouchiat and J. Brossel, "Relaxation of Optically Pumped Rb Atoms on Paraffin-Coated Walls", Phys. Rev. 147, 41–54 (1966). Abstract.
[4] S. J. Seltzer, D. J. Michalak, M. H. Donaldson, M. V. Balabas, S. K. Barber, S. L. Bernasek, M.-A. Bouchiat, A. Hexemer, A. M. Hibberd, D. F. Jackson Kimball, C. Jaye, T. Karaulanov, F. A. Narducci, S. A. Rangwala, H. G. Robinson, D. L. Voronov, V. V. Yashchuk, A. Pines, D. Budker, "Investigation of Anti-Relaxation Coatings for Alkali-Metal Vapor Cells Using Surface Science Techniques", arXiv:1002.4417.

[This article is written by Paul Preuss of Lawrence Berkeley National Laboratory, USA]

Watching An Atom's Electrons Move in Real Time

Stephen Leone [photo courtesy: University of California, Berkeley]

An international team of scientists led by groups from the Max Planck Institute of Quantum Optics (MPQ) in Garching, Germany, and from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley has used ultrashort flashes of laser light to directly observe the movement of an atom’s outer electrons for the first time.

Through a process called attosecond absorption spectroscopy, researchers were able to time the oscillations between simultaneously produced quantum states of valence electrons with great precision. These oscillations drive electron motion.

“With a simple system of krypton atoms, we demonstrated, for the first time, that we can measure transient absorption dynamics with attosecond pulses,” says Stephen Leone of Berkeley Lab’s Chemical Sciences Division, who is also a professor of chemistry and physics at UC Berkeley. “This revealed details of a type of electronic motion – coherent superposition – that can control properties in many systems.”

Image 1: A classical diagram of a krypton atom (background) shows its 36 electrons arranged in shells. Researchers have measured oscillations of quantum states (foreground) in the outer orbitals of an ionized krypton atom, oscillations that drive electron motion.

Leone cites recent work by the Graham Fleming group at Berkeley on the crucial role of coherent dynamics in photosynthesis as an example of its importance, noting that “the method developed by our team for exploring coherent dynamics has never before been available to researchers. It’s truly general and can be applied to attosecond electronic dynamics problems in the physics and chemistry of liquids, solids, biological systems, everything.”

The team’s demonstration of attosecond absorption spectroscopy began by first ionizing krypton atoms, removing one or more outer valence electrons with pulses of near-infrared laser light that were typically measured on timescales of a few femtoseconds (a femtosecond is 10^-15 second, a quadrillionth of a second). Then, with far shorter pulses of extreme ultraviolet light on the 100-attosecond timescale (an attosecond is 10^-18 second, a quintillionth of a second), they were able to precisely measure the effects on the valence electron orbitals.

The results of the pioneering measurements performed at MPQ by the Leone and Krausz groups and their colleagues are reported in the August 5 issue of the journal Nature.

Parsing the fine points of valence electron motion

Valence electrons control how atoms bond with other atoms to form molecules or crystal structures, and how these bonds break and reform during chemical reactions. Changes in molecular structures occur on the scale of many femtoseconds and have often been observed with femtosecond spectroscopy, in which both Leone and Krausz are pioneers.

Zhi-Heng Loh of Leone’s group at Berkeley Lab and UC Berkeley worked with Eleftherios Goulielmakis of Krausz’s group to perform the experiments at MPQ. By firing a femtosecond pulse of infrared laser light through a chamber filled with krypton gas, atoms in the path of the beam were ionized by the loss of one to three valence electrons from their outermost shells.

Image 2: Femtosecond-scale pulses were fired to ionize krypton atoms (wide beam). Separately created attosecond-scale pulses (narrow beam) were absorbed by the krypton atoms. Spectroscopy mapped the precise timing of the oscillation between quantum states thus created.

The experimenters separately generated extreme-ultraviolet attosecond pulses (using the technique called “high harmonic generation”) and sent the beam of attosecond probe pulses through the krypton gas on the same path as the near-infrared pump pulses.

By varying the time delay between the pump pulse and the probe pulse, the researchers found that subsequent states of increasing ionization were being produced at regular intervals, which turned out to be approximately equal to the time for a half cycle of the pump pulse. (The pulse is only a few cycles long; the time from crest to crest is a full cycle, and from crest to trough is a half cycle.)

“The femtosecond pulse produces a strong electromagnetic field, and ionization takes place with every half cycle of the pulse,” Leone says. “Therefore little bursts of ions are coming out every half cycle.”

Although expected from theory, these isolated bursts were not resolved in the experiment. The attosecond pulses, however, could precisely measure the production of the ionization, because ionization – the removal of one or more electrons – leaves gaps or “holes,” unfilled orbitals that the ultrashort pulses can probe.

The attosecond pulses do so by exciting electrons from lower energy orbitals to fill the gap in krypton’s outermost orbital – a direct result of the absorption of the transient attosecond pulses by the atoms. After the “long” femtosecond pump pulse liberates an electron from the outermost orbital (designated 4p), the short probe pulse boosts an electron from an inner orbital (designated 3d), leaving behind a hole in that orbital while sensing the dynamics of the outermost orbital.

In singly charged krypton ions, two electronic states are formed. A wave-packet of electronic motion is observed between these two states, indicating that the ionization process forms the two states in what’s known as quantum coherence.

Says Leone, “There is a continual ‘orbital flopping’ between the two states, which interfere with each other. A high degree of interference is called coherence.” Thus when the attosecond probe pulse clocks the outer valence orbitals, it is really clocking the high degree of coherence in the orbital motion caused by ionization.

Image 3: In krypton’s single ionization state, quantum oscillations in the valence shell cycled in a little over six femtoseconds. Attosecond pulses probed the details (black dots), filling the gap in the outer orbital with an electron from an inner orbital, and sensing the changing degrees of coherence between the two quantum states thus formed (below).


Indispensable attosecond pulses

“When the bursts of ions are made quickly enough, with just a few cycles of the ionization pulse, we observe a high degree of coherence,” Leone says. “Theoretically, however, with longer ionization pulses the production of the ions gets out of phase with the period of the electron wave-packet motion, as our work showed.”

So after just a few cycles of the pump pulse, the coherence is washed out. Thus, says Leone, “Without very short, attosecond-scale probe pulses, we could not have measured the degree of coherence that resulted from ionization.”

The physical demonstration of attosecond transient absorption by the combined efforts of the Leone and Krausz groups and their colleagues will, in Leone’s words, “allow us to unravel processes within and among atoms, molecules, and crystals on the electronic timescale” – processes that previously could only be hinted at with studies on the comparatively languorous femtosecond timescale.

Reference
Eleftherios Goulielmakis, Zhi-Heng Loh, Adrian Wirth, Robin Santra, Nina Rohringer, Vladislav Yakovlev, Sergey Zherebtsov, Thomas Pfeifer, Abdallah Azzeer, Matthias Kling, Stephen Leone, and Ferenc Krausz, “Real-time observation of valence electron motion,” Nature, 466, 739-743 (5 August 2010). Abstract.

[This report is written by Paul Preuss of Lawrence Berkeley National Laboratory]