Using the Uncertainty Principle to Detect Entanglement of One Photon Shared Among Four Locations

Members of the Caltech team, from left to right, undergraduate Garrett Drayna, postdoctoral scholar Hui Deng, graduate student Kyung Soo Choi, and postdoctoral scholar Scott Papp.

A team of physicists at the California Institute of Technology has demonstrated a new method to detect entanglement in the form of one photon shared among four optical paths. Their work is reported in the May 8 issue of the journal Science [1]. In their experiments, led by H. Jeff Kimble, entanglement is detected using quantum uncertainty relations for the regime of discrete variables, in which photons are taken one by one. Their approach builds on the famous Heisenberg uncertainty principle that places a limit on the precision with which the momentum and position of a particle can be known simultaneously.

Link to Professor Jeff Kimble's Quantum Optics group at Caltech >>
Past 2Physics Articles on the work of this group>>

Entanglement, which lies at the heart of quantum physics, is a state in which the parts of a composite system are more strongly correlated than is possible for any classical counterparts, regardless of the distances separating them. Entanglement in a system with more than two parts, or multipartite entanglement, is a critical tool for diverse applications in quantum information science, such as for quantum metrology, computation, and communication. In the future, a ‘quantum internet’ will rely on entanglement for the teleportation of quantum states from place to place [2].

For some time physicists have studied bipartite entanglement, and techniques for classifying and detecting the entanglement between two parts of a composite system are well known. But that isn’t the case for multipartite states. Their classification is much richer, and detecting their entanglement is extremely challenging.

In the Caltech experiment, a pulse of light was generated containing a single photon—a massless bundle, with both wave-like and particle-like properties, that is the basic unit of electromagnetic radiation. The team split the single photon to generate an entangled state of light in which the quantum amplitudes of the photon propagate among four distinct paths, all at once. This so-called W state plays an important role in quantum information science.

To enable future applications of multipartite W states, the entanglement contained in them must be detected and characterized. This task is complicated by the fact that entanglement in W states can be found not only among all the parts, but also among a subset of them. To distinguish between these two cases in real-world experiments, collaborators Steven van Enk and Pavel Lougovski from the University of Oregon developed a novel approach to entanglement detection based on the uncertainty principle. (See also the recent theoretical article by van Enk, Lougovski, and the Caltech group [3].)

The new approach to entanglement detection used in the Caltech experiments makes use of non-local measurements of a photon propagating through all four paths. The measurements indicate whether a photon is present, but not which path it takes. From this information the scientists can estimate the level of correlation in the photon’s paths. Correlations above a certain level signify entanglement among all the paths – even partially entangled W states do not attain a similar level of correlation. A key feature of this approach is that only a relatively small number of measurements must be performed.

Due to their fundamental structure, the entanglement of W states persists even in the presence of some sources of noise. This is an important feature of W states for real-world applications conducted in noisy environments. The Caltech experiments have directly tested this property by disturbing the underlying correlations of the entangled state. When the correlations are purposely weakened, the Caltech team detects a reduction in the number of paths of the optical system that are entangled. Yet, as predicted by the structure of W states, the entanglement amongst a subset of the paths still remains.

The work was funded by the Intelligence Advanced Research Projects Activity, the National Science Foundation, and Northrop Grumman Space Technology.

References
[1] “Characterization of Multipartite Entanglement for One Photon Shared Among Four Optical Modes”
S. B. Papp, K. S. Choi, H. Deng, P. Lougovski, S. J. van Enk, and H. J. Kimble, Science 324, 764 (2009). Abstract.
[2] “The Quantum Internet” H. J. Kimble, Nature 453, 1023 (2008). Abstract.
[3] “Verifying multi-partite mode entanglement of W states”
P. Lougovski, S. J. van Enk, K. S. Choi, S. B. Papp, H. Deng, and H.J. Kimble at http://xxx.lanl.gov/abs/0903.0851.

Long-Distance Teleportation between Two Atoms

Figure 1: Teleportation Team (rear from left: Christopher Monroe, Dzmitry Matsukevich; front from left: Peter Maunz, Steven Olmschenk, David Hayes) (Photo credit: Jonathan Mizrahi)


Quantum teleportation is the faithful transfer of quantum states between systems. A team from the Joint Quantum Institute (JQI) at the University of Maryland (UMD) and the University of Michigan has succeeded in teleporting a quantum state directly from one atom to another over a distance of one meter [Figure 1]. In the Jan. 23 issue of the journal Science [1], the scientists report that, by using their protocol, atom-to-atom teleported information can be recovered with perfect accuracy about 90% of the time.


>>Link to `Trapped Ion Quantum Information Group' led by Christopher Monroe, University of Maryland
>>Link to past 2Physics article on the work of this group

Teleportation works because of a remarkable quantum phenomenon, called “entanglement.” Once two objects are put in an entangled state, their properties are inextricably entwined. Although those properties are inherently unknowable until a measurement is made, measuring either one of the objects instantly determines the characteristics of the other, no matter how far apart they are.

The JQI team set out to entangle the quantum states of two individual ytterbium ions so that information embodied in the condition of one could be teleported to the other. Each ion was isolated in a separate high-vacuum trap. [Figure 2] The researchers identified two readily discernible ground (lowest energy) states of the ions that would serve as the alternative “bit” values of an atomic quantum bit, or qubit.

Figure 2: Experimental setup. Single photons from each of two ions in separate traps interact at a beamsplitter. If both detectors record a photon simultaneously, the ions are entangled. At that point, Ion A is measured, revealing exactly what operation must be performed on Ion B in order to teleport Ion A’s information. (Image Credit: Curt Suplee, JQI)

At the start of the experimental process, each ion (designated A and B) is initialized in a given ground state. Then ion A is irradiated with a specially tailored microwave burst from one of its cage electrodes, placing the ion in some desired superposition of the two qubit states – in effect writing into memory the information to be teleported.

Immediately thereafter, both ions are excited by a picosecond laser pulse. The pulse duration is so short that each ion emits only a single photon as it sheds the energy gained from the laser pulse and falls back to one or the other of the two qubit ground states. Depending on which one it falls into, each ion emits a photon whose color is perfectly correlated with the two atomic qubit states. It is this entanglement between each atomic qubit and its photon that will eventually allow the atoms themselves to become entangled.

The emitted photons are captured by lenses, routed to separate strands of fiber-optic cable, and carried to a 50-50 beamsplitter where it is equally probable for either photon to pass straight through the splitter or to be reflected. On either side of the beamsplitter are detectors that can record the arrival of a single photon. Because of the quantum interference of the two photons [2], a simultaneous detection at both output ports of the beamsplitter occurs only if the photons are in a particular quantum state. Since state of the photons was initially correlated with the state of the atomic qubits this measurement leaves atomic qubits in an entangled state [3]. The simultaneous detection of photons at the detectors does not occur often, so the laser stimulus and photon emission process has to be repeated many thousands of times per second. But when a photon appears in each detector, it is an unambiguous signature of entanglement between the ions.

Figure 3: Quantum state of the atom is teleported by 1 meter. (Image credit: N.R. Fuller, National Science Foundation)

When an entangled condition is identified, the scientists immediately take a measurement of ion A. The act of measurement forces it out of superposition and into a definite condition: one of the two qubit states. But because ion A’s state is irreversibly tied to ion B’s, the measurement of A also forces B into a complementary state. Depending on which state ion A is found in, the researchers now know precisely what kind of microwave pulse to apply to ion B in order to recover the exact information that had originally been stored in ion A. Doing so results in the accurate teleportation of the information.

This method combines the unique advantages of both photons and atoms. Photons are ideal for transferring information fast over long distances, whereas atoms offer a valuable medium for long-lived quantum memory. The combination represents an attractive architecture for a ‘quantum repeater,’ that would allow quantum information to be communicated over much larger distances than can be done with just photons.

The work reported in Science was supported by the Intelligence Advanced Research Project Activity program under U.S. Army Research Office contract, the National Science Foundation (NSF) Physics at the Information Frontier Program, and the NSF Physics Frontier Center at JQI. This report is written by Curt Suplee.

References
[1] "Quantum Teleportation between Distant Matter Qubits," S. Olmschenk, D. N. Matsukevich, P. Maunz, D. Hayes, L.-M. Duan, and C. Monroe, Science 323, 486 (2009). Abstract.
[2] “Measurement of subpicosecond time intervals between two photons by interference,” C. K. Hong, Z. Y. Ou, L. Mandel, Phys. Rev. Lett. 59, 2044 (1987). Abstract.
[3] “Robust Long-Distance Entanglement and a Loophole-Free Bell Test with Ions and Photons,”
C. Simon, W. T. M. Irvine, Phys. Rev. Lett. 91, 110405 (2003). Abstract.

Quantum Data Buffering

Alberto Marino

In a paper published in Feb. 12 issue of the journal Nature [1], a team of researchers at the Joint Quantum Institute (JQI) of the University of Maryland and National Institute of Standards and Technology (NIST) demonstrated the development of a "quantum buffer," a technique that could be used to control the data flow inside a quantum computer.

This new work follows up on the researchers' landmark creation in 2008 [2] of pairs of multi-pixel quantum images (past posting in 2Physics). A pair of quantum images is "entangled" which means that their properties are linked in such a way that they exist as a unit rather than individually. In this work, each quantum image is carried by a light beam and consists of up to 100 "pixels." A pixel in one quantum image displays random and unpredictable changes say, in intensity, yet the corresponding pixel in the other image exhibits identical intensity fluctuations at the same time, and these fluctuations are independent from fluctuations in other pixels. This entanglement can persist even if the two images are physically disconnected from one another.

[Image credit: A. Marino, JQI] Closeup of two "quantum images" created with the help of a "pump" laser beam. The two images are "entangled," so that if there is a change in the intensity in one region ("pixel") of the image, there would be an identical change in the intensity in the corresponding pixel in the second image. In this experiment, one of the images is delayed on its arrival to a detector, so that the correlations between the two images can be out of sync by up to 27 nanoseconds, something that is potentially useful for managing data to a future "quantum computer."

"If you want to set up some sort of communications system or a quantum information-processing system, you need to control the arrival time of one data stream relative to other data streams coming in," says JQI's Alberto Marino, lead author of the paper. "We can accomplish the delay in a compact setup, and we can rapidly change the delay if we want, something that would not be possible with usual laboratory apparatus such as beamsplitters and mirrors," he says.

By using a gas cell to slow down one of the light beams to 500 times slower than the speed of light, the group has demonstrated that they could delay the arrival time of one of the entangled images at a detector by up to 27 nanoseconds. The correlations between the two entangled images still occur—but they are out of sync. A flicker in the first image would have a corresponding flicker in the slowed-down image up to 27 nanoseconds later.

While such "delayed entanglement" has been demonstrated before, it has never been accomplished in information-rich quantum images. Up to now, the "spooky action at a distance" has usually been delayed in single-photon systems.

"What gives our system the potential to store lots of data is the combination of having multiple-pixel images and the possibility of each pixel containing 'continuous' values for properties such as the intensity," says co-author Raphael Pooser.

To generate the entanglement, the researchers use a technique known as four-wave mixing, in which incoming light waves are mixed with a "pump" laser beam in a rubidium gas cell to generate a pair of entangled light beams. In their experiment, the researchers then send one of the entangled light beams through a second cell of rubidium gas where a similar four-wave mixing process is used to slow down the beam. The beam is slowed down as a result of the light being absorbed and re-emitted repeatedly in the gas. The amount of delay caused by the gas cell can be controlled by changing the temperature of the cell (by modifying the density of the gas atoms) and also by changing the intensity of the pump beam for the second cell.

[Image credit: A. Marino, JQI] In this simplified representation of the experimental setup for a ‘quantum buffer,’ a cell containing rubidium gas is used to produce a pair of information-rich entangled images. One of the images goes through a second rubidium gas cell and slows down, which is potentially useful for feeding data at properly timed intervals to future quantum computers. The delay can be controlled such that, during the time it takes one image to travel a centimeter, the other image can travel up to 8 meters. The twisted loops illustrate the entanglement between the images.

This demonstration shows that this type of quantum buffer could be particularly useful for quantum computers, both in its information capacity and its potential to deliver data at precisely defined times. Quantum computers could potentially speed up or expand present capabilities in decrypting data, searching large databases, and other tasks.

References
[1] "Tunable Delay of Einstein-Podolsky-Rosen Entanglement"
A.M. Marino, R.C. Pooser, V. Boyer, and P.D. Lett, Nature, 457, 859-862 (2009), Abstract.
[2] "Entangled Images from Four-Wave Mixing"
V. Boyer, A. Marino, R. Pooser, and P. Lett, Science, 321, 544 - 547 (2008), Abstract.

[We thank NIST for materials used in this article, and Institut de Ciències Fotòniques, Barcelona for Alberto Marino's photo]

Adding and Subtracting Photons for Fundamental Tests of Physics and for Optical Quantum Technologies

(from left to right) Alessandro Zavatta, Marco Bellini, and Valentina Parigi

Author: Marco Bellini

Affiliation: Istituto Nazionale di Ottica Applicata – CNR
and
European Laboratory for Non-linear Spectroscopy (LENS), Florence, Italy.
>>Link to the Group Homepage.

[This is an invited article based on recent work of the author and his colleagues -- 2Physics.com]

Imagine a magician’s hat containing some rabbits whose precise number is unknown, but whose number probability distribution is well defined, so that, for a large ensemble of identical hats with the same probability distribution, one may define an average number of rabbits.

Now, if the magician puts one more rabbit in each hat, the mean rabbit number will, quite naturally, increase by one, while it will decrease by one if he takes one away (unless, of course, the hat was initially empty, in which case he would not be able to extract anything). Moreover, whatever the initial distribution, if the magician performs the two actions in a sequence, by first adding one rabbit and then taking one away, he/she will end up with exactly the same distribution for the number of rabbits remaining in the hat.

What would happen if, instead of normal rabbits, the magician used a microscopic hat containing quantum rabbits?

According to quantum physics, an electromagnetic field is composed of photons, which are so small that even a laser pointer with a typical power of 1mW emits a few millions of billions of them each second. Pure single photons are the ideal means to carry and encode information in emerging quantum technologies, but generating and manipulating them is still a very challenging task.

If the rabbits were identical quantum particles, one could assimilate them to photons in a radiation field (the hat), and would naturally use the so-called creation and annihilation operators to perform the addition and the subtraction of quantum rabbits to/from the hat. Indeed, as undergraduate physics students know, the photon creation operator acts on a state with a well-defined number of photons (also called a Fock state) by increasing this number by one. Conversely, when the photon annihilation operator acts on the same state, it subtracts a quantum of excitation, thus reducing the number of photons in the state by exactly one.

However, the situation becomes completely different as soon as one starts dealing with general superpositions or mixtures of Fock states. If the magician were using a distribution of quantum rabbits, the operation of adding one animal to the hat by a “rabbit creation operator” and then, immediately after, subtracting another by a “rabbit annihilation operator”, would lead to a final probability distribution of rabbits in the hat completely different from the initial one. Furthermore, the reverse sequence of operations would lead to a third outcome, different from both, i.e. the two operations do not commute.

This is the manifestation of one of the most profound laws in quantum physics. Indeed, the non-commutativity of particular quantum operations leads to many of the counterintuitive and fascinating aspects of quantum mechanics, including the famous Heisenberg uncertainty principle.

In 2007 our team (A. Zavatta, V. Parigi, and M. Bellini) at the Istituto Nazionale di Ottica Applicata – CNR (Florence, Italy), in collaboration with M. S. Kim from the Queen’s University (Belfast, UK), succeeded in performing the first direct tests of this fundamental principle of quantum physics in a laboratory [1]. We chose to use photons (which are much easier to manipulate than rabbits) and applied sequences of the creation and annihilation operators to an ordinary light pulse by making use of beam-splitters [2] and non-linear crystals [3]. As non-commutativity predicts, we found that the order of the operations makes a big difference to the outcome.

Figure 2: Setups to conditionally subtract (a) and add (b) a single photon from/to a light field. BS is a low-reflectivity beam-splitter; PDC is a nonlinear crystal where parametric down-conversion takes place; the two white boxes denote on/off photodetectors that herald the success of the corresponding quantum operation on the initial field state.

During those experiments we also found that the quantum operations behave so unusually that, under particular conditions, subtracting a photon changed the quantum state of the light pulse to the extent that its mean number of photons increased instead of diminishing. Taking a quantum rabbit away from the hat could actually increase the mean number of the remaining ones!

In one of our recent works [4] we decided to verify this behavior in a systematic way for some paradigmatic states of light. By applying photon annihilation to a Fock state with a well-defined number of photons we confirmed the intuitive decrease of the photon number by exactly one unit. Surprises appeared when we subtracted a single photon from a thermal state, the most common state of light (both the sun and ordinary light bulbs emit chaotic thermal light). We found that the mean number of photons in the pulse after subtraction was the double of the initial one.

Figure 3: Experimental density matrices and Wigner functions for a thermal state (left panel) and for the same state after a single-photon subtraction (right panel). The photon-subtracted state has a broader Wigner and photon number distribution than the original one.

Finally, when we tried to subtract a photon from a coherent state (the most classical, wave-like, state of light) we found that nothing changed in the state. In other words, we performed the first experimental demonstration that coherent states are invariant under photon annihilation. Since their introduction by Nobel laureate Roy Glauber in the 60’s, coherent states have been a cornerstone in the quantum description of light, but their definition as eigenstates of the annihilation operator had never been verified so directly in an experiment.

Figure 4: Experimental density matrices and Wigner functions for a coherent state (left panel) and for the same state after a single-photon subtraction (right panel). Photon annihilation does not modify a coherent state.

Although counterintuitive, the strange behavior of quantum operations is not unphysical and does not put energy conservation at stake: most of its weirdness simply derives from the misleading implicit assumption that a deterministic addition and subtraction of particles can be represented by the creation and annihilation operators which, on the contrary, work in a probabilistic way (i.e., the probability of extracting a particle from the hat scales with the number of particles already there) [5].

Apart from providing some beautiful demonstrations of the inner working of quantum mechanics, the techniques used in these experiments could in principle be used to arbitrarily engineer light at the most accurate levels by the appropriate sequence of photon additions and subtractions. This capability will open the way to “tailor-made” quantum light for future technologies, like the secure exchange of information through quantum cryptography or the development of novel protocols for quantum-enhanced measurements and communications.

For further info, please contact: Dr. Marco Bellini, Email: bellini@inoa.it

References
[1] “Probing Quantum Commutation Rules by Addition and Subtraction of Single Photons to/from a Light Field”, V. Parigi, A. Zavatta, M.S. Kim, and M. Bellini, Science, 317, 1890-1893 (2007). Abstract.
[2] “Non-Gaussian Statistics from Individual Pulses of Squeezed Light”, J. Wenger, R. Tualle-Brouri, and P. Grangier, Phys. Rev. Lett. 92, 153601 (2004). Abstract.
[3] “Quantum-to-classical transition with single-photon-added coherent states of light”, A. Zavatta, S. Viciani and M. Bellini, Science, 306, 660-662 (2004). Abstract.
[4] “Subtracting photons from arbitrary light fields: experimental test of coherent state invariance by single-photon annihilation”, A. Zavatta, V. Parigi, M. S. Kim, and M. Bellini, New Journal of Physics, 10, 123006 (2008). Abstract.
[5] “Recent developments in photon-level operations on travelling light fields”, M. S. Kim, J. Phys. B 41, 133001 (2008). Abstract.

Squeezing of Quantum Noise successfully used to develop First Tunable, ‘Noiseless’ Amplifier

Konrad Lehnert [Photo Courtesy: JILA, Boulder]

By significantly reducing the uncertainty in delicate measurements of microwave signals, a team of researchers from the National Institute of Standards and Technology (NIST) and Joint Institute of Laboratory Astrophysics (JILA) could successfully develop the first tunable “noiseless” amplifier which could boost the speed and precision of quantum computing and communications systems.

Conventional amplifiers add unwanted “noise,” or random fluctuations, when they measure and boost electromagnetic signals. Amplifiers that theoretically add no noise have been demonstrated before, but the JILA/NIST technology offers better performance and is the first to be tunable, operating between 4 and 8 GHz, according to JILA group leader Konrad Lehnert. It is also the first amplifier of any type ever to boost signals sufficiently to overcome noise generated by the next amplifier in a series along a signal path, Lehnert says, a valuable feature for building practical systems.

Noisy amplifiers force researchers to make repeated measurements of, for example, the delicate quantum states of microwave fields—that is, the shape of the waves as measured in amplitude (or power) and phase (or point in time when each wave begins). The rules of quantum mechanics say that the noise in amplitude and phase can’t both be zero, but the JILA/NIST amplifier exploits a loophole stipulating that if you measure and amplify only one of these parameters—amplitude, in this case—then the amplifier is theoretically capable of adding no noise. In reality, the JILA/NIST amplifier adds about half the noise that would be expected from measuring both amplitude and phase.

The JILA/NIST amplifier could enable faster, more precise measurements in certain types of quantum computers—which, if they can be built, could solve some problems considered intractable today—or quantum communications systems providing “unbreakable” encryption. It also offers the related and useful capability to “squeeze” microwave fields, trading reduced noise in the signal phase for increased noise in the signal amplitude. By combining two squeezed entities, scientists can “entangle” them, linking their properties in predictable ways that are useful in quantum computing and communications. Entanglement of microwave signals, as opposed to optical signals, offer some practical advantages in computing and communication such as relatively simple equipment requirements, Lehnert says.

[Image Credit: M. Castellanos-Beltran/JILA] In the JILA/NIST “noiseless” amplifier, a long line of superconducting magnetic sensors (beginning on the right in this photograph) made of sandwiches of two layers of superconducting niobium with aluminum oxide in between, creates a 'metamaterial' that selectively amplifies microwaves based on their amplitude rather than phase.

The new amplifier is a 5-millimeter-long niobium cavity lined with 480 magnetic sensors called SQUIDs (superconducting quantum interference devices). The line of SQUIDs acts like a “metamaterial,” a structure not found in nature that has strange effects on electromagnetic energy. Microwaves ricochet back and forth inside the cavity like a skateboarder on a ramp. Scientists tune the wave velocity by manipulating the magnetic fields in the SQUIDs and the intensity of the microwaves. An injection of an intense pump tone at a particular frequency, like a skateboarder jumping at particular times to boost speed and height on a ramp, causes the microwave power to oscillate at twice the pump frequency. Only the portion of the signal which is synchronous with the pump is amplified.

Reference
"Amplification and squeezing of quantum noise with a tunable Josephson metamaterial",
M.A. Castellanos-Beltran, K.D. Irwin, G.C. Hilton, L.R. Vale and K.W. Lehnert,
Nature Physics, published online: 5 Oct. 5 2008; doi:10.1038/nphys1090. Abstract

[We thank Media Relation, NIST for materials used in this posting]

Entangling the Spatial Properties of Laser Beams

Image 1: Physicists Jiri Janousek, Hongxin Zou and Kate Wagner (left to right) control the entanglement experiment at the Australian National University.

The Quantum Imaging team (K. Wagner, J. Janousek, H. Zou, C. C. Harb, and H-A. Bachor) of the ARC Centre of Excellence for Quantum-Atom Optics (ACQAO) at the Australian National University has experimentally demonstrated entanglement of the spatial properties (position and momentum) of two laser beams. This research has been done in collaboration with the Laboratoire Kastler Brossel (J. F. Morizur, N. Treps) in France.

The scientists have achieved spatialy entangled beams by combining a TEM₀₀ reference beam with a squeezed TEM₁₀ beam, and then entangling this beam with another TEM₁₀ squeezed beam. For each entangled beam, a measurement can be made on the TEM₁₀ component in order to find the beam position (real part) or the transverse beam momentum (imaginary part).

A direct measurement of the correlations between the two beams allows a calculation of the degree of inseparability. The two beams are entangled if these correlations are stronger than can be attained by classical means. The EPR (Einstein, Podolsky and Rosen) entanglement is measured by making predictions on what will be measured on one beam, based on a measurement of the other beam, and this is quantified by the degree of EPR paradox. An inseparability measurement of 0.51 and a degree of EPR paradox of 0.62 have been achieved, showing a genuine proof of the entanglement of position and momentum of two laser beams.

Image 2: No laser beam can have a fixed position or momentum. Spatial entanglement manifests itself as a strong quantum correlation between the position and direction of two beams, A (blue) and B (red). On the left, this illustration shows the fluctuating directions θ_A and θ_B of two beams, which are correlated, and on the right, the positions X_A and X_B, which are anti-correlated. For perfectly entangled beams the differences (θ_A-θ_B) and (X_A+X_B) would both be zero. Real entangled beams have a small residual differential movement. The variances V(X_A+X_B) and V(θ_A-θ_B) are calibrated against their respective quantum noise limit (QNL), which corresponds to the differential movement of two laser beams with independent quantum noise. A good measure of entanglement is the Inseparability, which for a symmetric system is the product I = V(X_A+X_B) V(θ_A-θ_B). This is shown as the area of the filled rectangles in the centre of this figure. Each slice of the tower represents one measurement and the comparison of the area with the QNL (the green box) shows directly the degree of inseparability.

This is the first time optical multi-mode entanglement has been created, and this is a very clear demonstration of the original ideas of Einstein, Podolsky and Rosen, applied to the position and momentum of continuous laser beams. The technology developed by the Quantum Imaging team at ACQAO can be used to make high precision optical measurements, or as a resource for new quantum information applications, particularly those that require multi-mode entanglement.

Reference
[1] "Entangling the Spatial Properties of Laser Beams",
Katherine Wagner, Jiri Janousek, Vincent Delaubert, Hongxin Zou, Charles Harb, Nicolas Treps, Jean François Morizur, Ping Koy Lam, Hans A. Bachor,
Science, v.321. no. 5888, pp. 541 - 543 (2008). Abstract.
[2] Wikipedia page on EPR paradox.

The Frontier of Quantum Communication is the Space

Paolo Villoresi

Author: Paolo Villoresi

Affiliation: Department of Information Engineering,
University of Padua, Italy

[This is an invited article based on recent work of the author and his collaborators. - 2Physics.com]

As the advancements in the implementation of single light-quanta exchange in the quantum channels is constantly progressing and refining, one may think that the quantum communications (QC) will soon widespread in everyday life. The appeals of this perspective may be synthesised by choosing the quality versus the quantity or to encode information in the quantum state of a single particle instead of sending a large bunch of photons to express just one bit.

But quality has its price. Each photon that is carrying information has to be clearly sorted out from these wandering around as general background, and its quantum state has also to be kept unblemished along its propagation until the receiver. As expected, QC had its cradle in the research labs, where effective countermeasures against decoherence and background photons are relatively easy to adopt. Significant steps were done in quantum communication along optical fibres, for which already viable technologies were proposed for the distribution of cryptographic keys over legs of several tens of kilometres. In the free space counterpart, where a beam with the train of quanta is aimed toward a received with no needs of infrastructures in between, the difficulties are stronger. The intense backlights, the atmospheric turbulence, the diffusion and absorption of light are some of the issues to fight against in order to implement QC. Beside, the Earth curvature set a final limit to the leg length. The actual limit is represented by a quantum channel in which the nature of quantum entanglement has been demonstrated between two parties separated by 144 km. The experiment was done between two islands of Canary archipelagos, with the stations located quite high in the mountains. But the further extension of the free-space QC has a natural direction: going in space and communicate with the Earth.

Indeed, in our experiment we aimed to establish a link between an orbiting source of single photons and a ground telescope. Our team was set up with my coworkers at the LUXOR Labs at DEI, University of Padova in Italy and colleagues in Austria, of the group of Anton Zeilinger at the University of Vienna and Academy of Science of Austria, of the group of Cesare Barbieri at the Astronomy Department of University of Padova, Italy and of Giuseppe Bianco of Italian Space Agency in Matera. The realization of this link is the first step in the communication space-ground or space-space and based on the coding of the bits of information in the quantum state of a photon, or qubit. The experiment also demonstrated that present technology is mature enough for this purpose, and the crucial crossing from the theoretical predictions and the experimental demonstration was possible. On the other hand, the experiment required a combined effort from different expertises, from classical Optics, to satellite laser-ranging for Geodesy, to Quantum Optics, to advanced electronics. Our team synthesized these points of view and succeeded in the single photon link.

More in detail, in this experiment we have essentially simulated a quantum communication source onboard a satellite, and showed how the very dim signals could be detected. Such a quantum source has to fulfill the particular requirement, that only one single photon per pulse is emitted. In this work this is realized by sending a rapid sequence of weak laser pulses (outward pulses in the figure) towards a Japanese satellite equipped with retroflectors (Ajisai) at about 1600 km of slant distance. There is a very small probability that the photons hit the satellite and are reflected back to ground, therefore this is just as if we would have a suitable quantum source on the satellite. The main challenge was to detect the very view reflected photons amongst a huge background signal which is exactly the same situation is would be if we had the real quantum communication system. The detector is an avalance-photon-detector (APD) connected to a timing circuit. the orbital data of the satellite were used to identify the returned photons out of the background.

The next step will be to board a quantum sender on a satellite. This will allow quantum physics experiments over distances impossible on ground. In particular, it will push the limits of fundamental physics tests addressing experimentally questions as if there is a spatial limit to the entanglement, the "spooky" action? Beside, technologies as the quantum key distribution may be implemented on a global scale. And a real economic impact of the quantum communication from satellite may be expected, to be based on the cryptography, on novel paradigms as quantum teleportation, on advanced atmospheric monitoring, based on the modification of the optical signal during the downlink. There could also be impact in the global distribution of temporal information, as in the case of the so called “legal time”, and advanced methods for the clock synchronization using entangled photon pairs.

The study of the quantum satellite is ongoing, under the auspices of Italian Space Agency as well also of the European Space Agency, and we really hope that the quantum satellite will soon be on its way, that is along an orbit some hundred kilometres above us.

References
[1] "Experimental verification of the feasibility of a quantum channel between space and Earth",
P Villoresi, T Jennewein, F Tamburini, M Aspelmeyer, C Bonato, R Ursin, C Pernechele, V Luceri, G Bianco, A Zeilinger and C Barbieri,
New J. Phys., v10, 033038 (March, 2008) [IOP select paper], Abstract Link.
[2] "Ground to satellite secure key exchange using quantum cryptography",
Rarity J G, Tapster P R, Gorman P M and Knight P,
New J. Phys., v4, 82 (2002), Abstract Link.
[3] European Quantum Roadmap: http://qist.ect.it/
[4] "The Physics of Quantum Information",
D Bouwmeester, A Ekert, A Zeilinger, (Springer, 2000).

Silicon Photonics for Optical Quantum Technologies

Jeremy O’Brien

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Authors: Jeremy O’Brien and Alberto Politi

Affiliation: Centre for Quantum Photonics, Department of Physics and Department of Electrical & Electronic Engineering, University of Bristol

Quantum information science has shown that quantum mechanical effects can dramatically improve performance for certain tasks in communication, computation and measurement. Single particles of light – photons – are an excellent choice for quantum technologies because they are relatively noise free; information can be moved around quickly – at the speed of light; and manipulating single photons is easy. For these reasons photons have been widely used in quantum communication, quantum metrology, and quantum lithography settings, as well as quantum bits (or qubits) for quantum information processing [1].

The fact that photons see low noise during the propagation is a great advantage, but, at the same time, makes two photons interact with a negligible probability. Two photons interaction is a fundamental task for quantum information processing, and it is at the heart of the Controlled-NOT (CNOT) gate –one of the building block of a future quantum computer. A 2 photon CNOT gate was demonstrated experimentally back in 2003 [2].

A conspicuous number of different experimental realizations of this CNOT gate have since been developed in recent years by different groups, but all the realizations performed so far are based on bulk optical elements on an optical table and photon propagation in air. This approach is useful for proof of principle quantum logic operation, however, photonic quantum technologies will require scalable, miniaturized gates, with improved performances.

The Team at Bristol University has designed and measured integrated optical devices on a chip, with dimensions measured in millimetres [3]. This impressive miniaturisation was permitted thanks to the silica-on-silicon technology used in commercial devices for modern optical telecommunications, which guides light on a chip in the same way as in optical fibers.

For the first time, the feasibility of integrated quantum information was demonstrated, by achieving the key element of all quantum optics experiments, namely non-classical interference. This effect appears when two indistinguishable photons arrive at the different inputs of a beam-splitter (a half refractive mirror) at the same time. In this case, contrary to the classical analysis, the two photons always exit together from one of the two ports, and they never exit different outputs. The simplest integrated analogous of a free space bream-splitter is a directional coupler, (illustrated in Figure 1). When two waveguides are close one to each other there is a non-zero overlap between the modes of the waveguides. By choosing the waveguide separation and the length of the coupling region, it is possible to choose the amount of power that goes to one waveguide to the other (coupling ratio).


Figure 1 shows a directional coupler on a chip, the integrated analogue of a beam splitter.

Sending pairs of single photons in the two inputs of the directional coupler, it was possible to demonstrate the quantum interference effect, with a very high visibility of the quantum behaviour.

Using the same technology and various directional couplers with different coupling ratios it is possible to realise a CNOT gate, schematically represented in Figure 1. With this scheme it was possible to achieve a fidelity of the CNOT operation of more than 94%.

Figure 2 shows the schematic representation of an integrated CNOT gate. The “1/2” and “1/3” numbers indicate the coupling ratio of the different couplers that compose the CNOT gate.

The experimental characterisation of the quantum chips also proved that one of the strangest phenomena of the quantum world, namely “quantum entanglement”, was achieved on-chip. Quantum entanglement of two particles means that the state of either of the particles is not defined, but only their collective state.

This on-chip entanglement has important applications in quantum metrology. Last year Dr O’Brien and his collaborator Professor Takeuchi and co-workers at Hokkaido University reported such a quantum metrology measurement with four photons [4].

The results achieved using integrated chips show that it is possible to realize sophisticated photonic quantum circuits on a silicon chip, which will be of benefit to future quantum technologies based on photons as well as the next generation of fundamental studies in quantum optics.

References
[1] “Optical Quantum Computing”
Jeremy L. O’Brien,
Science 318, 1567 (2007), Abstract.
[2] “Demonstration of an all-optical quantum controlled-NOT gate”
J. L. O'Brien, G. J. Pryde, A. G. White, T. C. Ralph, D. Branning,
Nature 426, 264 (2003), Abstract.
[3] “Silica-on-Silicon Waveguide Quantum Circuits”
A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O'Brien,
Science, Vol. 320. no. 5876, pp. 646 - 649 (May 2, 2008)
Published Online March 27, 2008 (10.1126/science.1155441)
Abstract, Link to Full text in the website of Bristol Centre for Quantum Photonics.
[4] "Beating the Standard Quantum Limit with Four-Entangled Photons"
T. Nagata, R. Okamoto, J. L. O'Brien, K. Sasaki, S. Takeuchi,
Science 316, 726 (2007), Abstract.

Entangled Memory

Jeff Kimble [Photo courtesy: Caltech]

In a paper published in today's issue of the journal Nature, Caltech's Valentine Professor of Physics H. Jeff Kimble and his colleagues have laid the groundwork for a crucial step in quantum information science. They demonstrate for the first time an important capability required for the control of quantum information and quantum networks, namely the coherent conversion of photonic entanglement into and out of separated quantum memories.

Entanglement lies at the heart of quantum physics, and is a state where parts of a composite system are more strongly correlated than is possible for any classical counterparts regardless of the distance separating them. Entanglement is a critical resource for diverse applications in quantum information science, such as for quantum metrology, computation, and communication. Quantum networks rely on entanglement for the teleportation of quantum states from place to place.

Entanglement lies at the heart of quantum physics, and is a state where parts of a composite system are more strongly correlated than is possible for any classical counterparts regardless of the distance separating them. Entanglement is a critical resource for diverse applications in quantum information science, such as for quantum metrology, computation, and communication. Quantum networks rely on entanglement for the teleportation of quantum states from place to place.

In a quest to turn these abstract ideas into real laboratory systems and to distribute entanglement to remote locations (even on a continental scale), Kimble explains that quantum physicists have studied ways to propagate photonic information into and out of quantum memory using a system called a quantum repeater, invented in 1998 by H. Briegel, J.I. Cirac, and P. Zoller at the University of Innsbruck. Until now, work in Kimble's group on the realization of a quantum repeater with atomic ensembles relied upon the probabilistic creation of entanglement. In this setting entanglement between two clouds of atoms was generated probabilistically but with an unambiguous heralding event.

While such systems hold the potential for scalable quantum networks, it has been difficult for Kimble's Quantum Optics Group to apply such schemes to certain protocols necessary for quantum networks, such as entanglement connection. Now, with the new protocol and future improvements, "We can push a button and generate entanglement," says physics graduate student Kyung Soo Choi, one of four authors of the Caltech experiment.

[Image Courtesy: Nature]

In the Caltech experiment, a single photon is first split, generating an entangled state of light with quantum amplitudes for the photon to propagate two distinct paths, taking both at once. The Caltech team in turn transcribed, or mapped, the entanglement onto distinct atomic ensembles separated by one millimeter. To create the interface between the light and matter, the team employed laser-cooled cesium atoms whose atomic states interact with a control laser to create destructive quantum interference, making the atomic ensembles either invisible or highly opaque to the input light. Called Electromagnetically Induced Transparency and pioneered by S. Harris at Stanford University, the mechanism manipulates the speed of the light for the incoming entangled photon and that kicks off the entire procedure.

In this experiment, the photonic entanglement was mapped into the atomic ensembles in a time ~ 20 nanoseconds and then stored in the atomic ensembles for one microsecond, with storage times extendable up to 10 microseconds. The photonic entanglements of the input and output of the quantum interface were explicitly quantified with a conversion efficiency of 20 percent. However, the researchers emphasize, real-world realization of a quantum network remains far out of reach even with these parameters and the state-of-the-art of quantum controls. Choi comments, "Further improvements in quantum control and storage capabilities in matter-light interfaces will lead to fruitful and exciting discoveries in Quantum Information Science, including for the realization of quantum networks."

In addition to Kimble and Choi, other authors are Hui Deng, a postdoctoral scholar at the Center for the Physics of Information; and Julien Laurat, a former Caltech physics postdoctoral scholar who is now an associate professor at Laboratoire Kastler Brossel (Universite P. et M. Curie, Ecole Normale Superieure and CNRS) in Paris, France.

Reference
"Mapping photonic entanglement into and out of a quantum memory"
K. S. Choi, H. Deng, J. Laurat & H. J. Kimble,
Nature 452, 67-71 (6 March 2008), Abstract Link

[We thank Caltech Media Relations for materials used in this posting]

Observation of the Spin Hall Effect of Light

Onur Hosten (left) and Paul Kwiat (right) [Photo credit: L. Brian Stauffer]

Physicists Onur Hosten and Paul G. Kwiat, at the University of Illinois at Urbana-Champaign showed that light exhibits a Spin Hall effect, analogous to the Spin Hall effect in electronic systems, showing the universality of the effect for particles of different nature. The researchers used a novel technique from quantum weak measurements to enhance the tiny Spin Hall displacements prior to observation.

Whenever the propagation direction of a beam of light changes due to a variation in the refractive index of the medium (in the experiment, refraction at an air-glass interface serves this purpose), the beam center experiences a spin-dependent (or circular polarization-dependent) displacement perpendicular both to the initial propagation direction and the change in the propagation direction, i.e. a lateral shift. Two different spin components (parallel and anti-parallel to the propagation direction) acquire opposite displacements. This is the spin Hall effect as it applies to light. Therefore, when a beam of linearly polarized light (which is an equal combination of spin parallel and anti-parallel to the propagation direction) changes direction, the beam slightly splits into two beams, each containing different spin states.

Figure 1: Spin Hall effect of Light. (Click on figure to watch it) In the animation, a beam of linearly polarized light incident on an air-glass interface slightly splits into its two spin components -- spin parallel or anti-parallel to the propagation direction (or right and left circular polarizations) -- upon refraction at the interface. [Animation credit: Onur Hosten]

The effect takes place due to conservation of angular momentum (spin plus orbital). Due to the rotational symmetry around the axis perpendicular to the interface (z-axis), the total angular momentum of light around this axis has to be conserved. Assume that, initially the spin angular momentum of light is either parallel or anti-parallel to the propagation direction, and has a certain component along the z-axis. When light refracts at the interface, the spin still remains either parallel or anti-parallel to the new propagation direction. But this time the spin makes a different angle with the z-axis, therefore the spin angular momentum component along the z-axis changes. The spin Hall effect compensates for this change in the angular momentum component, and light acquires an orbital angular momentum by shifting itself laterally from the z-axis.

In the experiment a linearly polarized laser beam was incident on a glass prism at an angle. Upon refraction, the two different spin components acquired opposite displacements out of the plane of incidence. Because the separation between these two beams was only on the order of nanometers, and the beam widths themselves on the order of millimeters, the two beams overlap to a great extent. The researchers measured the separation between the two beams using a novel metrological method (quantum weak measurements in pre- and post-selected systems) to measure the miniscule effect.

Essentially, the spin Hall effect performs a weak measurement of the spin state of the photons. If the measurement were to be strong, the beams corresponding to different spin states would completely separate from each other, and one would be able to tell the spin state by looking at the beam position. But, in the University of Illinois experiment, the spin state measurement was a weak measurement, because the beams were still overlapping to a great extent and one could tell only very little about the spin state by looking at the position of the beam.

When the researchers made a particular pre- and post-selection on the polarization state of the photons before and after the weak measurement (i.e., the spin Hall effect), due to an interference effect between the two beams, there resulted an enhancement of the original displacement by a factor of ten thousand. This pre- and post-selection step experimentally amounts to sending the photons through two calcite polarizers, one before and one after the spin Hall effect, oriented at angles almost perpendicular to each other. Therefore, for instance, an angstrom displacement was enhanced to a micron displacement. Then the enhanced displacement was read by a position-sensitive photodiode (a photodiode split into two halves – the difference signal is proportional to the beam displacement). The researchers were thus able to characterize the Spin Hall effect of light with angstrom resolution.

The measurement technique holds further promise for achieving better resolutions. In particular, the researchers believe that by incorporating standard signal modulation and lock-in detection techniques, a resolution of picometers can be achieved. Moreover, the technique is not limited to position measurements; similar tricks in the appropriate experimental conditions will enhance any kind of signal, e.g., position or momentum of any particle, intensity (e.g. photon number) or amplitude (e.g. electric field) of a field.

The researchers think that it would be interesting to demonstrate the case when the index of refraction varies continuously (as opposed to observing the effect at a discrete air-glass interface), which is the analogous case for the spin Hall effect in semiconductors. The researchers are also theoretically looking for systems where they can separate the spin states into two completely separate beams, and use this for both quantum and classical optical information processing applications.

Reference:
"Observation of the Spin Hall Effect of Light via Weak Measurements"
O. Hosten and P. Kwiat,
Science 319, 787 (2008); published online 10 January 2008 (10.1126/Science.1152697).
Link to Abstract
Link to Full text in the website of Kwiat Quantum Information Group

A Single-Photon Transistor using Nanoscale Surface Plasmons

Author: Darrick Edward Chang

Affiliation: Physics Department, Harvard University

[This is an invited article based on a recent work done by the author and his collaborators and published in 'Nature']

Finding ways to make pulses of light interact with each other has been an active area of research for several decades. In fact, the study of “nonlinear optics” has led to countless breakthroughs and technological advances in fields as diverse as imaging, spectroscopy, laser physics, communications, and signal processing [1]. Interactions between pulses of light are achieved by their common interaction with some material medium. However, because such processes are generally very weak, optical nonlinearities typically become significant only when very large light intensities are used.

The ability to achieve nonlinear interactions at low optical powers would enable a new generation of devices that consume much less power than their predecessors and enable new applications as well. The ultimate limit would be to achieve nonlinear interactions between individual photons, the constituent particles that comprise light. Recently, there has been great interest in this area in part because of potential applications in quantum computing and quantum information science [2,3].

The interaction strength between matter and light can be increased by confining the light in space to very small dimensions, which causes the associated optical fields to become very intense. In normal dielectric media, light cannot be confined to regions smaller than an optical wavelength. However, the situation changes dramatically when light is coupled to the free electrons in a conductor. The unique properties of these coupled excitations of light and charge (known as surface plasmons) [4] allow them to be confined to arbitrarily small dimensions.

Image: An illustration of how a single atom near a nanowire can prevent light from propagating past it

Recently, we proposed [5] and experimentally investigated [6] the strong interaction between single atoms (or other optical emitters) and individual surface plasmons tightly confined to a conducting nanowire. The strong coupling causes the nanowire to act as a “super-lens” that directs the majority of emission into the surface plasmon modes. More recently, we have theoretically shown that such a system also leads to remarkable nonlinear optical effects [7]. In particular, the confinement of the surface plasmons is so strong that when a single surface plasmon (i.e., a single photon) is incident on a single emitter, the two must interact, and this interaction prevents the photon from being transmitted past the emitter. However, because the emitter cannot interact with more than a single photon at a time, its response to a second incident photon becomes fundamentally different and transmission is now much more likely. In this sense, the single emitter behaves as an efficient, single-photon switch.

One can gain even further control over the nonlinear optical interactions in this system by using techniques from quantum optics to coherently manipulate the emitter. In fact, we have shown that the system can behave as a single-photon transistor, where the presence or absence of a single photon in a “gate” field can prevent or allow the propagation of a whole stream of “signal” photons. In analogy to the role that electronic transistors play in electronic computing devices, a single-photon transistor would open the door to optical computing devices and many other possibilities.

Our experimental efforts to explore the nonlinear properties of this system are just beginning, and considerable work remains to be done before large-scale, integrated quantum plasmonic devices can be practically realized. More broadly, however, work such as this suggests the great promise of merging the tools of quantum optics with plasmonics and the many other novel optical materials that have recently arisen. Ultimately this merger may help us to achieve unprecedented control over the interactions of light quanta.

This work was done in collaboration with Mikhail Lukin and Eugene Demler, both in the Physics Dept. at Harvard University, and Anders Sorensen in the Physics Dept. at the Niels Bohr Institute, Copenhagen, Denmark.

References:
[1] R.W. Boyd, Nonlinear Optics (Academic, New York, 1992).
[2] L.-M. Duan and H.J. Kimble, Phys. Rev. Lett. 92, 127902 (2004) Abstract.
[3] M.D. Lukin and A. Imamoglu, Phys. Rev. Lett. 84, 1419 (2000) Abstract.
[4] H.A. Atwater, Sci. Am. 296, 53 (2007).
[5] D.E. Chang, A.S. Sorensen, P.R. Hemmer, M.D. Lukin, Phys. Rev. Lett. 97, 053002 (2006) Abstract.
[6] A.V. Akimov et al., accepted by Nature (2007).
[7] D.E. Chang, A.S. Sorensen, E.A. Demler, and M.D. Lukin, Nature Physics advance online publication, doi:10.1038/nphys708 (2007) Abstract

Quantum Entanglement between Single Atoms One Meter Apart !

Christopher Monroe in his lab in Department of Physics, University of Michigan (photo credit: Mary Monroe)

A team of physicists has exploited one of the most mysterious phenomena in nature to make a major advance toward the long-sought goal of super-fast quantum computing. Christopher Monroe and colleagues at the University of Michigan and now at the University of Maryland established a spooky, intimate quantum-mechanical condition called “entanglement” between two completely unconnected individual atoms a meter apart in separate enclosures by carefully manipulating photons emitted by the atoms. As a result, even though the atoms have never come in physical contact, their properties are entangled: inextricably linked and giving precisely corresponding values if measured.

“This type of long-distance entanglement generation is a radically new way to propagate quantum information and perform quantum computations over long distances,” Monroe says. “It should be possible to scale it up to networks of many interconnected components that will eventually be necessary for a general-purpose quantum internet.” The team reported its results in the September 6 issue of the journal Nature [1].

Entangled entities are doubly strange. First, they have a property peculiar to the atomic-scale world of quantum mechanics: Each exists in a “superposition” of different states at the same time – like a coin with sides that are neither heads nor tails, but somehow both at once – and remains that way until a measurement forces it to take on a specific state.

This aspect of quantum mechanics makes it very attractive for potential information processing. Conventional computer bits are stored in tiny capacitors, each of which can have only one of two values (on or off, 0 or 1) as determined by a measurable electrical charge. But thanks to superposition, a quantum bit, or “qubit,” can be a 0, a 1, or both at the same time. If arranged into a computer, qubits could exponentially increase the speed at which certain kinds of problems can be solved.

Quantum entanglement allows the “wiring” of qubits together and is the key to such massive parallelism. Even though they are physically unconnected, entangled qubits always have complementary characteristics. If the state of first is known, then the state of the second is known as well – even when the second state is not measured. In the coin analogy, an entangled pair would work like this: If one coin were flipped and came up heads, then the other would always come up tails, even if it were simultaneously flipped 10,000 miles away.

Figure 1: Two trapped atomic ions (glowing "stars" at center of each disc), with entanglement depicted by the ambiguous perspective of the discs: when one is seen a particular way, so is the other. (picture credit: Boris Blinov, Univ. Washington)

The problem with all this is that entangled quantum superpositions are incredibly fragile – if any part of the entangled system interacts with its environment or gets measured, then the quantum nature of the entire system is generally lost in a process called “decoherence.”

Before the experiment reported today, entanglement of distant individual atoms had never been achieved. “Atoms make ideal qubits,” Monroe says, “because they can be trapped and maintained in the same condition for long time periods. But photons are the ideal medium for transferring and controlling information. This work shows how to combine the best of both systems.”

The researchers began by confining two ytterbium ions, one in each of two chambers separated by about 1 meter. Once trapped, the ions stay in their positions for several days. The atomic qubits are realized as stable states of electron and nuclear spin within each ion. The team excited the ions simultaneously with precisely tuned laser pulses so brief that each ion emitted at most a single photon as it fell back to one of the qubit states. The color of each photon became entangled with its parent atomic qubit, and the photons were guided through optical fibers. The emerging photons were combined on a beamsplitter, and quantum interference of the photons [2] ensures that whenever photons are simultaneously detected behind each port of the beamsplitter, the atomic qubits become entangled.

This entanglement was verified by probing the trapped ions with specially tuned laser beams that directly measured the state of each qubit. Not only were correlations in the qubit states clearly visible, but the correlations persisted after each qubit state was scrambled in a particular way before measurement – a proof of entanglement.

Figure 2: Schematic of experiment to entangle two remote trapped ions. Laser pulses simultaneously excite the two atoms, and their emitted photons are guided by fibers onto a beamsplitter. Whenever two photons emerge from the beamsplitter and are detected in coincidence, the trapped ions are entangled.

Because of the numerous sources of light-gathering inefficiencies (imperfect laser pulse excitation of the ions, imperfections in the optical fibers, inefficiencies in the filters and detectors, and so forth), the team observed the telltale signature of an entanglement event only about three times in every billion pulses. That’s about once every few minutes. But “it’s okay that it almost never works,” says David Moehring, the lead graduate student on the project, and now a Research Fellow at the Max Planck Institute for Quantum Optics near Munich, Germany. “Once we receive the clicks from the detectors, we know right then that the two ions are entangled and ready for use.”

The entanglement events were measured with extremely high efficiency, and detected with sufficient fidelity to demonstrate the phenomenon and constitute a proof of concept for controlling future quantum networks. The group has identified a number of ways to increase the yield in subsequent experiments, and improvements have recently been implemented.

[We thank Curt Suplee for writing this piece]

References:
[1] D. L. Moehring, P. Maunz, S. Olmschenk, K. C. Younge, D. N. Matsukevich, L.-M. Duan, and C. Monroe, "Entanglement of single-atom quantum bits at a distance," Nature 449, 68 (2007). Abstract
[2] C. K. Hong, Z. Y. Ou, and L. Mandel, “Measurement of subpicosecond time intervals between two photons by interference,” Phys. Rev. Lett. 59, 2044 (1987). Abstract

Entanglement and One-Way Quantum Computing

Prof. Anton Zeilinger [photo credit: Jacqueline Godany]

Authors: Robert Prevedel and Anton Zeilinger

Affiliation: Faculty of Physics, University of Vienna
and
Institute of Quantum Optics and Quantum Information, Austrian Academy of Science.

[This is an invited article based on some recent publications by the authors in various journals referred below.
-- 2Physics.com Team]

According to Erwin Schrödinger [1], one of the founding fathers of quantum mechanics, entanglement is the essence of quantum physics. It inspires fundamental questions about the principles of nature and is also the basis for emerging technologies [2] of quantum information processing such as quantum cryptography [3], quantum teleportation [4,5] and quantum computation [6].

Entangled particles possess correlations stronger than those allowed by classical physics, and can only be described by their joint behaviour, no matter how far the two particles are located from each other. Imagine two ‘entangled coins’, placed at opposite ends of a galaxy, each of which, when thrown individually, yields a random results (head or tails). However, owing to entanglement between the coins, the throw of the second coin always leads to an outcome that is fully determined by the result of rolling the first one, independent of their spatial separation or the time ordering of the throws.

This feature can be used to transmit information securely between two parties (quantum cryptography) or, when generalized to many particles, to process information faster and more efficiently than possible by any classical means. The latter has sparked the now increasingly growing research field of quantum computation [6]. Owing to the unique features of quantum mechanics, such as superposition and entanglement, quantum computers have the potential to perform tasks utterly intractable on any conceivable classical computing hardware.

While different theoretical approaches of how to realize these quantum computers in the laboratory exist, one particular model excites the community because of its simplicity for experiments. It is an entanglement based scheme widely known as “one-way” quantum computing [7]. In this scheme the computation or information processing is performed by measuring particles that are part of a large entangled network. As with the coins, the state of all the particles in the network will be influenced according to the outcome of the measurement at a particular site. This allows to manipulate large arrays of quantum particles (usually called “cluster states”) and therefore to utilize their powerful features to execute algorithms in a speed and fashion impossible with a classical processor. Depending on which and how the individual particles are measured, the remaining particles will occupy different states [7]. Reading out this state will provide the user with the output of his computation. Because of the irreversibility of the measurement process (it collapses the quantum superposition to a definite state) the term “one-way” quantum computer was introduced.

In the experiments, we employ the polarization degree of freedom of single photons as our information carriers. A photon that carries horizontal polarization thus represents a qubit which is in the logical “0” state, while vertical polarization embodies “1”. However, we can also prepare photons whose polarization is along 45 degrees, thus representing “0+1”, i.e. it exists in a superposition of both basis states.

After entangling a certain number of these photons (qubits), we start to run our one-way quantum computer by measuring the photons polarizations in a certain order and fashion [8]. By doing so, the input information, represented by the initial state of some photons, is altered and processed due to the measurement task. Reading out the quantum state of the remaining photons yields the output of the computation. The larger the initial, entangled state, the more measurements can be performed and the more complex and powerful the computation.

In proof-of-principle experiments, small versions of these quantum computers have already been built [8,9]. In the most successful realization, a process called spontaneous parametric down-conversion is employed in order to create photon pairs (also known as Bell states) which are entangled in their polarization state. To generate large networks of entangled states, these entangled photon pairs are further subject to operations that combine the individual Bell states to yield large cluster states.

Using single photons as qubits has the advantage that their quantum state remains basically unaltered during the experiment, owing to the typically weak coupling of photons to their environment. Moreover, single photons can be conveniently controlled with standard optical components.

In the actual experiment, we utilized two Bell states to create a cluster state made up of four photons. Depending on the type and order of measurements on this cluster, different one-qubit and two-qubit operations could be realized, therefore demonstrating the working principles of such one-way quantum computers and the potential to perform even more complex computations. With this set of operations, the successful realization of a quantum algorithm has also been shown. Quantum algorithms [6] can execute certain tasks such as searching in an unsorted database with less elementary steps than classically possible. Take e.g. a database with N elements. A classical algorithm needs to look, on average, N/2 times into the database to find a desired entry. Grover’s quantum algorithm [10] masters this search tasks with only steps. In our experiment, the database consists of 4 entries, each embodied by a photon. Intriguingly, in this case, Grover’s algorithm finds the corresponding entry with unit fidelity after only a single run [8-10]. This has been verified in the experiment and our demonstrations will hopefully pave the way for the realization of even more complex quantum algorithms, such as Shor’s algorithm [11] for the efficient factorization of large integers into prime numbers – a method on which most of today’s encryption protocols rely.

References:
[1] Schrödinger, E. Die gegenwärtige Situation der Quantenmechanik. Die Naturwissenschaften 49, 823-828 (1935).
[2] Prevedel, R. et al. Photonic entanglement as a resource in quantum computation and quantum communication. J. Opt. Soc. Am. B 24, 241-248 (2007).
[3] Ekert, A. K. Quantum Cryptography Based on Bell's Theorem. Phys. Rev. Lett. 67, 661-663 (1991).
[4] Bennett, C. H. et al. Teleporting an unknown quantum state via dual classical and EPR channels. Phys. Rev. Lett. 70, 1895-1899 (1993).
[5] Bouwmeester, D. et al. Experimental Quantum Teleportation. Nature 390, 575 (1997).
[6] Deutsch, D. & Ekert, E. Quantum computation. Phys. World 11, 47-52 (1998).
[7] Raussendorf R. & Briegel, H. J. A one-way quantum computer. Phys. Rev. Lett. 86, 5188-5191 (2001).
[8] Walther, P. et al. Experimental one-way quantum computing. Nature 434, 169-176 (2005).
[9] Prevedel, R. et al. High-speed linear optics quantum computing using active feed-forward. Nature 445, 65-69 (2007).
[10] Grover, L. K. Quantum mechanics helps in search for a needle in a haystack. Phys. Rev. Lett. 79, 325-328 (1997).
[11] Shor, P. W. Algorithms for quantum computation: Discrete logarithms and factoring. Proceedings of the 35th Annual Symposium on Foundations of Computer Science. Los Alamitos: IEEE Computer Society Press 124-134 (1994).