Finer Atomic Matchmaking by Radio Frequency Tuning

The image shows, in the sequence of green arrows, how a pair of ultracold gas atoms collides, briefly forms a molecule, and flies apart, in the presence of an external magnetic field (not shown) that influences this process. By adding RF radiation (lightning bolts) of the right frequency, the atoms can experience being in many different molecular states (red arrows), providing even more extensive and detailed control of the collision. The size of the yellow bursts indicate the amount of absorption/emission of RF radiation. [Image Credit: Eite Tiesinga, Joint Quantum Institute, University of Maryland/ National Institute of Standards and Technology]

In a paper accepted for publication in Physical Review A, a team of scientists at the Joint Quantum Institute (JQI) of the National Institute of Standards and Technology (NIST) and the University of Maryland have reported that properly tuned radio-frequency waves can influence how much the atoms attract or repel one another, opening up new ways to control their interactions.

While investigating mysterious data in ultracold gases of rubidium atoms, they found that the radio-frequency (RF) radiation could serve as a second "knob," in addition to the more traditionally used magnetic fields, for controlling how atoms in an ultracold gas interact. Just as it is easier to improve reception on a home radio by both electronically tuning the frequency on the receiver and mechanically moving the antenna, having two independent knobs for influencing the interactions in atomic gases could produce richer and more exotic arrangements of ultracold atoms than ever before.

Previous experiments with ultracold gases, including the creation of Bose-Einstein condensates, have controlled atoms by using a single knob—traditionally, magnetic fields. These fields can tune atoms to interact strongly or weakly with their neighbors, pair up into molecules, or even switch the interactions from attractive to repulsive. Adding a second control makes it possible to independently tune the interactions between atoms in different states or even between different types of atoms.

Such greater control could lead to even more exotic states of matter. A second knob, for example, may make it easier to create a weird three-atom arrangement known as an Efimov state, whereby two neutral atoms that ordinarily do not interact strongly with one another join together with a third atom under the right conditions [Read past 2Physics report on Efimov state]

For many years, researchers had hoped to use RF radiation as a second knob for atoms, but were limited by the high power required. The new work shows that, near magnetic field values that have a big effect on the interactions, significantly less RF power is required, and useful control is possible.

In the new work, the JQI/NIST team examined intriguing experimental data of trapped rubidium atoms taken by the group of David Hall at Amherst College in Massachusetts. This data showed that the RF radiation was an important factor in tuning the atomic collisions. To explain the complicated way in which the collisions varied with RF frequency and magnetic field, NIST theorist Thomas Hanna developed a simple model of the experimental arrangement.

"The model reconstructed the energy landscape of the rubidium atoms and explained how RF radiation was changing the atoms' interactions with one another. In addition to providing a roadmap for rubidium, this simplified theoretical approach could reveal how to use RF to control ultracold gases consisting of other atomic elements", Hanna says.

Reference
"Radio-frequency dressing of multiple Feshbach resonances"
A.M. Kaufman, R.P. Anderson, T.M. Hanna, E. Tiesinga, P.S. Julienne, and D.S. Hall,
To appear in Physical Review A.
Abstract: We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in 87Rb using radiofrequency radiation (rf). We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.

[We thank NIST for materials used in this posting]

Quantum Fingerprints of Chaos

Poul Jessen [photo courtesy: College of Optical Sciences, University of Arizona]

In a recent article in the journal Nature, Prof. Poul Jessen and his group in the College of Optical Sciences at the University of Arizona have reported some interesting outcomes of a series of experiments showing clear fingerprints of classical-world chaos in a quantum system designed to mimic a textbook example of chaos known as the "kicked top."

Chaotic behavior is the rule, not the exception, in the world we experience through our senses, the world governed by the laws of classical physics. Even tiny, easily overlooked events can completely change the behavior of a complex system, to the point where there is no apparent order to most natural systems we deal with in everyday life.

The weather is one familiar case, but other well-studied examples can be found in chemical reactions, population dynamics, neural networks and even the stock market. Scientists who study "chaos" - which they define as extreme sensitivity to infinitesimally small tweaks in the initial conditions - have observed this kind of behavior only in the deterministic world described by classical physics.

The University of Arizona team (front row, L to R) Worawarong Rakreungdet, Souma Chaudhury, Brian Anderson and (back, L to R) Aaron Smith, Enrique Montano, Jae Hoon Lee, and Poul Jessen [photo courtesy: College of Optical Sciences, University of Arizona]

Until now, no one has produced experimental evidence that chaos occurs in the quantum world, the world of photons, atoms, molecules and their building blocks. This is a world ruled by uncertainty: An atom is both a particle and a wave, and it's impossible to determine its position and velocity simultaneously.

And that presents a major problem. If the starting point for a quantum particle cannot be precisely known, then there is no way to construct a theory that is sensitive to initial conditions in the way of classical chaos.

Yet quantum mechanics is the most complete theory of the physical world, and therefore should be able to account for all naturally occurring phenomena.

"The problem is that people don't see [classical] chaos in quantum systems," said Poul Jessen. "And we believe quantum mechanics is the fundamental theory, the theory that describes everything, and that we should be able to understand how classical physics follows as a limiting case of quantum physics."

Now, however, Jessen and his group in UA's College of Optical Sciences have performed a series of experiments that show just how classical chaos spills over into the quantum world. They studied the quantum version of a spinning top with this laboratory apparatus.

The quantum version of the top is the "spin" of individual laser-cooled cesium atoms that Jessen's team manipulate with magnetic fields and laser light, using tools and techniques developed over a decade of painstaking laboratory work.

"Think of an atom as a microscopic top that spins on its axis at a constant rate of speed," Jessen said. He and his students repeatedly changed the direction of the axis of spin, in a series of cycles that each consisted of a "kick" and a "twist". Because spinning atoms are tiny magnets, the "kicks" were delivered by a pulsed magnetic field. The "twists" were more challenging, and were achieved by subjecting the atom to an optical-frequency electric field in a precisely tuned laser beam.

They imaged the quantum mechanical state of the atomic spin at the end of each kick-and-twist cycle with a tomographic technique that is conceptually similar to the methods used in medical ultrasound and CAT scans. The end results were pictures and stop-motion movies of the evolving quantum state, showing that it behaves like the equivalent classical system in some significant ways.

One of the most dramatic quantum signatures the team saw in their experiments was directly visible in their images: They saw that the quantum spinning top observes the same boundaries between stability and chaos that characterize the motion of the classical spinning top. That is, both quantum and classical systems were dynamically stable in the same areas, and dynamically erratic outside those areas.

Jessen's experiment revealed a new signature of chaos for the first time. It is related to the uniquely quantum mechanical property known as "entanglement" which is best known from a famous thought experiment proposed by Albert Einstein, in which two light particles, or photons, are emitted with polarizations that are fundamentally undefined but nevertheless perfectly correlated. Later, when the photons have traveled far apart in space, their polarizations are both measured at the same instant in time and found to be completely random but always at right angles to each other.

"It's as though one photon instantly knows the result for the other and adjusts its own polarization accordingly," Jessen said. By itself, Einstein's thought experiment is not directly related to quantum chaos, but the idea of entanglement has proven useful, Jessen added.

Theorists have speculated that the onset of chaos will greatly increase the degree to which different parts of a quantum system become entangled. Jessen took advantage of atomic physics to test this hypothesis in his laboratory experiments. The total spin of a cesium atom is the sum of the spin of its valence electron and the spin of its nucleus, and those spins can become quantum correlated exactly as the photon polarizations in Einstein's example. In Jessen's experiment, the electron and nuclear spins remained unentangled as a result of stable quantum dynamics, but rapidly became entangled if the dynamics were chaotic.

Entanglement is a buzzword in the science community because it is the foundation for quantum cryptography and quantum computing. However, Jessen clarified,"Our work is not directly related to quantum computing and communications. It just shows that this concept of entanglement has tendrils in all sorts of areas of quantum physics because entanglement is actually common as soon as the system gets complicated enough."

Reference
"Quantum signatures of chaos in a kicked top"
S. Chaudhury, A. Smith, B. E. Anderson, S. Ghose & P. S. Jessen,
Nature 461, 768-771 (2009). Abstract.

Colorful Quantum Entanglement

Paulo Nussenzveig (left) and Marcelo Martinelli in their lab, in Brazil.



[This is an invited article based on recently published works of the authors and their collaborators -- 2Physics.com]




Authors: Paulo Nussenzveig and Marcelo Martinelli

Affiliation: Experimental Physics Department, Instituto de Fisica -- USP,
Sao Paulo, Brazil
Link to the Laboratory of Coherent Manipulation of Atoms and Light (LMCAL) >>

Quantum entanglement has just become more colorful. In a recent experiment, three bright beams of light, all of different wavelengths, were entangled [1]. Physicists have been playing around with this mind-boggling concept since 1935, but recently they have acquired enormous control over quantum systems. Entanglement is now viewed as a valuable resource to enable sophisticated information tasks. Indeed, the field of quantum information science relies heavily on entanglement in order to perform quantum computing, teleportation, and communication. A quantum internet is envisaged as a dream to be pursued, with information being conveyed among its nodes via quantum teleportation [2].

Since quantum hardware is still composed of different physical systems, which do not always share common resonances for interaction with light, one faces challenges to exchange quantum information among them. By entangling light beams of different wavelengths, this is no longer a problem. This is what was achieved, with one beam in the visible portion of the spectrum (532.251 nm) and two in the near infrared (1062.102 nm and 1066.915 nm). Research was performed by a group at the University of São Paulo, in Brazil, with participation of two researchers (former students in Brazil) from the new Max Planck Institute for the Science of Light, in Germany.

Entanglement in continuous variable (CV) systems, such as bright beams of light, is generated by means of nonlinear optical processes [3]. The lowest nonlinear order is two, corresponding to processes in which three fields are coupled. Examples are second harmonic generation, sum- and difference-frequency generation, and parametric down-conversion. This latter process is used for the generation of twin photons, a well-known way of producing entangled qubits (e.g. polarization-entangled photons). A nonlinear crystal is pumped by a laser, generating spontaneously emitted pairs of photons. In each fundamental process, a pump photon is annihilated and a pair of lower-frequency photons is created. If the crystal is placed inside a cavity, resonant for all three fields involved, photons are emitted in occupied modes (stimulated emission). The resulting gain can overcome losses and the system oscillates, similarly to a laser. This optical parametric oscillator (OPO), as sketched in Fig. 1, was used by the researchers to generate the three-color entanglement.

Fig. 1 : Sketch of an OPO. A nonlinear optical crystal is placed within two mirrors, forming a cavity. Green incident light is down-converted into twin beams of infrared light.

In order to measure entanglement, researchers had to cool the crystal, to reduce thermal vibrations (phonons), which were responsible for generating unwelcome phase noise in the optical fields. In a tripartite Gaussian state, there is a necessary and sufficient criterion to check for entanglement, due to Simon [4] and extended by Werner and Wolf [5]. By measuring the full covariance matrix of the three-field system, researchers could check that the lowest symplectic eigenvalue under partial transposition with respect to each beam was smaller than one, demonstrating full inseparability (Fig. 2).

Fig. 2: Full tripartite inseparability. Symplectic eigenvalues corresponding to transposition by the pump (green), signal (red) and idler (blue) are lower than one for a broad range of values of the pump power relative to the threshold power (from ref. [1]).

Three-color entangled beams can be useful for communications. Since quantum resources are in general very fragile, the robustness of the entanglement against losses was studied. The researchers observed a subtle quantum property hitherto only witnessed in few-particle systems, called entanglement sudden death [6]. Entanglement was lost for partial attenuation, in certain situations. However, in others the researchers showed that entanglement can be kept alive. The states that were generated have different sensitivity to losses, warranting further investigations.

Entanglement implies a certain “familiarity” among the constituents of a system composed of different parts. The Brazilian experiment generates entanglement among the pump and the twins to which it gives birth: one can think of it as “quantum genealogy”, since it is shown that the twins are entangled to their “mother”.

References
[1] "Three-Color Entanglement", A. S. Coelho, F. A. S. Barbosa, K. N. Cassemiro, A. S. Villar, M. Martinelli, and P. Nussenzveig, Science Express, DOI: 10.1126/science.1178683 (September 17, 2009). Abstract.
[2] "The Quantum Internet", H. J. Kimble, Nature 453, 1023 (2008). Abstract.
[3] "Quantum information with continuous variables", S. L. Braunstein and P. van Loock, Rev. Mod. Phys. 77, 513 (2005). Abstract.
[4] "Peres-Horodecki Separability Criterion for Continuous Variable Systems", R. Simon, Phys. Rev. Lett. 84, 2726 (2000). Abstract.
[5] "Bound Entangled Gaussian States", R. F. Werner and M. M. Wolf, Phys. Rev. Lett. 86, 3658 (2001). Abstract.
[6] "Environment-Induced Sudden Death of Entanglement", M. P. Almeida, F. de Melo, M. Hor-Meyll, A. Salles, S. P. Walborn, P. H. Souto Ribeiro, and L. Davidovich, Science 316, 579 (2007). Abstract.

Physics Nobel 2009 : The Masters of Light

Charles K. Kao [photo courtesy: Chinese University of Hong Kong]

This year's Nobel Prize in Physics is awarded for two scientific achievements that have helped to shape the foundations of today’s networked societies. They have created many practical innovations for everyday life and provided new tools for scientific exploration.

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2009 with one half to Charles K. Kao of Standard Telecommunication Laboratories, Harlow, UK, and Chinese University of Hong Kong "for groundbreaking achievements concerning the transmission of light in fibers for optical communication", and the other half jointly to Willard S. Boyle and George E. Smith of Bell Laboratories, Murray Hill, NJ, USA "for the invention of an imaging semiconductor circuit – the CCD sensor".

In 1966, Charles K. Kao made a discovery that led to a breakthrough in fiber optics. He carefully calculated how to transmit light over long distances via optical glass fibers. With a fiber of purest glass it would be possible to transmit light signals over 100 kilometers, compared to only 20 meters for the fibers available in the 1960s. Kao's enthusiasm inspired other researchers to share his vision of the future potential of fiber optics. The first ultrapure fiber was successfully fabricated just four years later, in 1970.

Willard S Boyle [photo courtesy: RMC Club of Canada]

Today optical fibers make up the circulatory system that nourishes our communication society. These low-loss glass fibers facilitate global broadband communication such as the Internet. Light flows in thin threads of glass, and it carries almost all of the telephony and data traffic in each and every direction. Text, music, images and video can be transferred around the globe in a split second.

If we were to unravel all of the glass fibers that wind around the globe, we would get a single thread over one billion kilometers long – which is enough to encircle the globe more than 25 000 times – and is increasing by thousands of kilometers every hour.

A large share of the traffic is made up of digital images, which constitute the second part of the award. In 1969 Willard S. Boyle and George E. Smith invented the first successful imaging technology using a digital sensor, a CCD (Charge-Coupled Device). The CCD technology makes use of the photoelectric effect, as theorized by Albert Einstein and for which he was awarded the 1921 year's Nobel Prize. By this effect, light is transformed into electric signals. The challenge when designing an image sensor was to gather and read out the signals in a large number of image points, pixels, in a short time.

George E. Smith [photo courtesy: IEEE]

The CCD is the digital camera's electronic eye. It revolutionized photography, as light could now be captured electronically instead of on film. The digital form facilitates the processing and distribution of these images. CCD technology is also used in many medical applications, e.g. imaging the inside of the human body, both for diagnostics and for microsurgery.

Digital photography has become an irreplaceable tool in many fields of research. The CCD has provided new possibilities to visualize the previously unseen. It has given us crystal clear images of distant places in our universe as well as the depths of the oceans.

BOSS – A New Kind of Search for Dark Energy

David Schlegel, principal investigator of BOSS, shows one of the numerous “plug plates” used to map and select hundreds of galaxies for each exposure. Light from each galaxy enters a hole in the plate and is carried to the CCD camera by its own optical fiber [Image Courtesy: Lawrence Berkeley National Laboratory]

On the night of September 14 the largest program in the Sloan Digital Sky Survey-III, the Baryon Oscillation Spectroscopic Survey (BOSS) achieved 'first light' with an upgraded spectrographic system across the entire focal plane of the Sloan Foundation 2.5-meter telescope at Apache Point Observatory in New Mexico.

BOSS is the most ambitious attempt yet to map the expansion history of the Universe using the technique known as baryon acoustic oscillation (BAO). It is the largest of four surveys in SDSS-III, with 160 participants from among SDSS-III’s 350 scientists and 42 institutions.

“Baryon oscillation is a fast-maturing method for measuring dark energy in a way that’s complementary to the proven techniques of supernova cosmology,” says David Schlegel, Principal Investigator of BOSS. “The data from BOSS will be some of the best ever obtained on the large-scale structure of the Universe.”

The distribution of visible mass in the universe

“Baryon” (meaning protons and neutrons and other relatively massive particles) is shorthand for ordinary matter. For almost the first 400,000 years, the universe was so dense that particles of matter were thoroughly entangled with particles of light (photons), the whole a vast, quivering, liquid-like blob where density variations caused sound waves (pressure waves) to move spherically outward at over half the speed of light.

Suddenly the expanding universe cooled enough for light and matter to “decouple.” Photons shot through transparent space unimpeded; the speed of sound plummeted. What had been variations in the density of the liquid universe left two marks in the now-transparent sky.

Variations in the temperature of the radiation that filled the early universe have descended to us as anisotropies in the cosmic microwave background (CMB). Variations in the density of matter persist in the clustering of galaxies, as baryon acoustic oscillations (BAO). The two scales, the roughly one-degree anisotropy of the CMB and the 500-million-light-year clustering of BAO, are closely related; the standard ruler of the universe measured from BAO can be calculated from the CMB for any epoch since decoupling.

Anisotropies in the cosmic microwave background, originating when the universe was less than 400,000 years old, are directly related to variations in the density of galaxies as observed today [Image courtesy: Lawrence Berkeley National Laboratory]

Schlegel and his colleague Nikhil Padmanabhan, who came to Berkeley Lab from Princeton in late 2006, first used the SDSS telescope to complete the largest three-dimensional map of the universe ever made until then: 8,000 square degrees of sky out to a distance of 5.6 billion light years, determining the clustering of 60,000 luminous red galaxies. This program, part of SDSS-II, measured galactic distances to a redshift of z = 0.35 and detected the 500-million-light-year scale of BAO.

Measuring baryon oscillations

Baryon oscillations began as pressure waves propagated through the hot plasma of the early universe, creating regions of varying density that can be read today as temperature variations in the cosmic microwave background. The same density variations left their mark as the Universe evolved, in the periodic clustering of visible matter in galaxies, quasars, and intergalactic gas, as well as in the clumping of invisible dark matter.

Comparing these scales at different eras makes it possible to trace the details of how the Universe has expanded throughout its history – information that can be used to distinguish among competing theories of dark energy.

BOSS will measure 1.4 million luminous red galaxies at redshifts up to 0.7 (when the Universe was roughly seven billion years old) and 160,000 quasars at redshifts between 2.0 and 3.0 (when the Universe was only about three billion years old). BOSS will also measure variations in the density of hydrogen gas between the galaxies. The observation program will take five years.

“BOSS will survey the immense volume required to obtain percent-level measurements of the BAO scale and transform the BAO technique into a precision cosmological probe,” says survey scientist Martin White. “The high precision, enormous dynamic range, and wide redshift span of the BOSS clustering measurements translate into a revolutionary data set, which will provide rich insights into the origin of cosmic structure and the contents of the Universe.”

The spectrum of one of the quasars captured in the BOSS "first light" exposure (image: Vaishali Bhardwaj, David Hogg, Nic Ross - click for best resolution)

Existing SDSS spectrographs were upgraded to include new red cameras more sensitive to the red portion of the spectrum, featuring CCDs designed and fabricated at Berkeley Lab, with much higher efficiency than standard astronomical CCDs in the near infrared.

“Visible light emitted by distant galaxies arrives at Earth redshifted into the near-infrared, so the improved sensitivity of these CCDs allows us to look much further back in time,” says BOSS instrument scientist Natalie Roe.

To make these measurements BOSS will craft two thousand metal plates to fit the telescope’s focal plane, plotting the precise locations of two million objects across the northern sky. Each morning astronomers begin plugging optical fibers into a thousand tiny holes in each of the “plug plates” to carry the light from each specific target object to an array of spectrographs.

Steering each optical fiber to the right CCD was no trivial task, says Schlegel. “The new BOSS fiber cartridges are snake pits of a thousand fibers each. It would be a disaster if you didn’t know which one went where.”

One of the BOSS cartridges containing 1,000 optical fibers, which guide light from specific target galaxies and quasars to the spectrograph; Sloan Foundation telescope in background (photo by Dan Long, Apache Point Observatory - click on image for best resolution)

With a thousand holes in each plug plate, stopping to seek out specific holes to plug a fiber into, or tracing where each fiber ends up, would take an impossibly long time. Instead a computer assigns the correct target identity to each fiber as a fiber-mapping laser beam moves over the plugged-in fibers and records where the light from each emerges.

Fast and simple – but not quite foolproof. “In our first test images it looked like we’d just taken random spectra from all over,” Schlegel says. After some hair-pulling, the problem turned out to be simple. “After we flipped the plus and minus signs in the program, everything worked perfectly.”

Now BOSS is on its way to generating data of unprecedented precision on two million galaxies and quasars, and density variations in the intergalactic gas. The SDSS tradition of releasing data to the public will continue, with the first release from SDSS-III planned for December 2010.

More about BOSS:
[1] BOSS homepage
[2] "BOSS: The Baryon Oscillation Spectroscopic Survey", Nikhil Padmanabhan, David Schlegel, Natalie Roe, Martin White, Daniel Eisenstein and David Weinberg (a white paper describing the BOSS experiment).
[3] "Baryon Acoustic Oscillations (BAO) at LBNL", David Schlegel (a presentation at Lawrence Berkeley National Laboratory Physics Division Review, Nov 2006).
[4] The Sloan Digital Sky Survey III homepage.

[We thank Lawrence Berkeley National Laboratory for materials used in this posting]