NEWS for March 2009www.jqi.umd.edu

Joint Quantum Institute

Dressing Up Rubidium for Computing

page 2

INSIDE

1

Quantum Buffer: Expect Slight Delays

Meetings,Outreach andMuch More

page 6

GAITHERSBURG, Md.—Pushing the envelope of Albert Einstein's "spooky action at a distance," known as entanglement, JQI researchers at the National Institute of Standards and Technology (NIST) have demonstrated a "quantum buffer," a technique that could be used to control the data flow inside a quantum computer.

Quantum computers could potentially speed up or expand present capabilities in decrypting data, searching large databases, and other tasks. The new research is published in the Feb. 12 issue of Nature.

"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 continued, page 5

J IQJQI

New SQUID Lab Gets in on the Ground Floor

JQI Fellow Fred Wellstood (left)examines the new space with Doug Bensen, coordinator for the Center for Nanophysics and Advanced Materials.

Experiment setup: A cell containing rubidium gas is used to produce a pair of information-rich entangled images. One of them goes through a second gas cell and slows down -- potentially useful for feeding data at timed intervals to future quantum computers. The delay can be controlled such that, during the time it takes one image to travel 1 cm, the other can travelup to 8 meters. Credit: A. Marino/JQI

A JQI-Physics Frontier Center effort to study the use of Josephson junctions as quantum qubits has reached new heights -- about two floors above its prior location. JQI Fellows Fred Wellstood, Chris Lobb, Bob Anderson and Alex Dragt and their research group have been using space in the sub-basement of the Physics

continued, page 4

2

Dressing Up Rubidium for a Leading RoleNeutral atoms—having no net electric charge—usually don’t act very dramatically around a magnetic field. But by “dressing them up” with light, researchers at the Joint Quantum Institute (JQI), a collaborative venture of the National Institute of Standards and Technology (NIST) and the University of Maryland at College Park, have caused ultracold rubidium atoms to undergo a startling transformation. They force neutral atoms to act like pointlike charged particles that can undergo merry-go-round-like “cyclotron” motions just as electrons do when subjected to a suitable magnetic field. This extreme makeover for ultracold atoms promises to give physicists clues on how to achieve an exotic form of computation that would rely upon special “fractionally charged” particles dancing around on a surface.

Just as good theatrical plays provide teachable insights about complex human situations, ultracold atomic gases are ideal proxies for studying complex phenomena in physics. Since it is relatively easy to manipulate the energy levels of ultracold atoms in gases and to control the interactions between them,

scientists can learn important clues about physical phenomena that occur in more complicated and less controllable liquid or solid systems.

Among such complex phenomena are the quantum Hall and fractional quantum Hall effects, the subjects of the 1985 and 1998 Nobel Prizes in physics. In the latter effect, low-temperature electrons, confined to a plane and placed in high magnetic fields, can act as if they form “quasiparticles” carrying a fraction of an electric charge as well as several bundles of magnetism known as “magnetic flux quanta.”

Physicists believe an as-yet-unseen configuration

of such quasiparticles might provide a practical system for achieving “topological quantum computing,” in which quasiparticles on a two-dimensional surface would be able to perform powerful logic operations that obey the particular rules of quantum mechanics.

With this goal in mind, postdoc Yu-Ju Lin, physicist Ian Spielman and the rest of the JQI team have set out to make a gas of neutral atoms behave like electrically charged particles. They couldn’t simply add electric charges to the atoms, or play around with electrons themselves because their mutual electrical repulsion would cause the cloud to fly apart.

In their experiment, they cause a gas of rubidium-87 to form an ultracold state of matter known as a Bose-Einstein condensate. Then, laser light from two opposite directions bathes or “dresses” the rubidium atoms in the gas. The laser light interacts with the atoms, shifting their energy levels in a peculiar momentum-dependent manner. One nifty consequence of this is that the atoms now react to a magnetic field gradient in a way mathematically identical to the reaction of charged particles like electrons to a uniform magnetic field. “We can make our neutral atoms have the same equations of motion as charged particles do in a magnetic field,” says Spielman.

continued, next page

On the Team: Left to right, PhD student Abby Perry and postdoctoral researcher Yu-Ju Lin

Dressed Rubidium, continued from previous page

3

Pitaevskii at NIST/JQI Das Sarma at KITP

On February 25, JQI Fellow Sankar Das Sarma gave a public lecture titled "Quantum Reality" at the Kavli Institute for Theoretical Physics (KITP) in Santa Barbara, CA. He was introduced by institute Director and Nobel laureate David Gross. KITP sponsors three such lectures per year. The online version is available at: http://online.itp.ucsb.edu/plecture/dassarma/.

In this first experiment, Spielman and colleagues have effectively “put an electric charge” on atoms, but haven’t “turned on the field.” In subsequent experiments, they plan to introduce an effective magnetic field and watch “electrified” rubidium atoms go on their merry cyclotron ways, with the goal of revealing new insights about the fractional quantum Hall effect and topological computing.

* Y.-J. Lin, R.L. Compton, A.R. Perry, W.D. Phillips, J.V. Porto and I.B. Spielman, A Bose-Einstein condensate in a uniform light-induced vector potential. Physical Review Letters (forthcoming).

Media Contact: Ben Stein, ben.stein@nist.gov, (301) 975-3097

Celebrated quantum physicist Lev Pitaevskii (below, center) gave a talk at NIST on Mar. 5 titled "The superfluid Fermi liquid in a unitary regime." The co-creator of the Gross-Pitaevskii equation that describes the wavefunction of Bose-Einstein condensates, he was once a student of legendary theorist Lev Landau. Pitaevskii is on the faculty of the Istituto Nazionale per la Fisica della Materia in Trento, Italy.

From left: Claude Fabre, Paul Lett, Victor Yakovenko, Roman Lutchyn, Charles Clark, Lev Pitaevskii, William Phillips, Paul Julienne, Garnett Bryant, Sergio Muniz, Eite Tiesinga. Photo: Kelly Talbott, NIST

On the Team, Part 2: NIST/NRC postdoctoral researcher Rob Compton

New SQUID Lab Gets Underway

4

The current floor plan for new laboratory space on the first floor of the John S. Toll Physics Building at the University of Maryland. The 9-by-9-foot orange box marks the location of the shielded SQUID equipment and dilution refrigerator. The red dotted lines indicate field strength of the high-field magnet located at the center red cross. North is to the right.

JQI Fellow Fred Wellstood and Lorraine DeSalvo, Director of Administrative Services for the UMD Physics Department, discuss the lab space with Doug Bensen.

Department at the University of Maryland. The new lab, on the first floor of the Physics building, will add over 1000 square feet of space. It will house a superconducting quantum interference device (SQUID) and related equipment -- in a highly shielded double-wall enclosure -- as well as a dilution refrigerator needed to cool the qubits to milli-Kelvin temperatures and associated apparatus. In addition, the lab will have space for five workstations and other experimental setups.

Renovation and installation of equipment are expected to extend through several more months, with the lab ready for operation by fall of this year.

Quantum Buffer continued from page 1

5

stream relative to other data streams coming in," says JQI's Alberto Marino. "We can accomplish the delay in a compact setup, and rapidly change the delay if we want, something that would not be possible with usual apparatus such as beamsplitters and mirrors," he says.

This new work follows the researchers' landmark creation in 2008 of pairs of multi-pixel quantum images. 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.

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.

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 "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.

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.

Reference: "Tunable Delay of Einstein-Podolsky-Rosen Entanglement," A.M. Marino, R.C. Pooser, V. Boyer, and P.D. Lett. Nature. Feb. 12, 2009.

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." Credit: A. Marino/JQI

Presentations

Last month JQI Fellows Chris Monroe and Trey Porto gave invited talks at SCALA (SCAlable quantum information with Light and Atoms) European Network Final Review in Cortina, Italy.

Earlier in the month, both had attended a UC-Berkeley, Workshop "The Quantum Emulator: Opportunity at the Nexus of Atomic and Con-densed-Matter Physics." (Other JQI attendees: Ming-Shien Chang, Kihwan Kim, Simcha Ko-renblit and Rajibul Islam). Monroe also gave a seminar at Berkeley.

Steven Olmschenk of Monroe's group gave an invited talk, with a contribution by Ming-Shien Chang, at the Integrated Atomic Systems Work-shop in Seattle. In addition, both Chang and Olmschenck contributed to an invited talk that Monroe gave at SQUINT (Southwestern Quantum Information and Technology) in Seattle.

Robert McFarland, a student of JQI Fellow Bruce Kane, is giving an invited talk at the APS March Meeting about the group's recent work on elec-trons on silicon surfaces.

Publications

Physical Review Letters has accepted "Observation of a 2D Bose-gas: from thermal to quasi-condensate to superfluid" by JQI Fellows Bill Phillips and Kris Helmerson, with col-leagues at NIST and UMD.

JQI Fellow Alan Migdall and colleagues pub-lished "Improved implementation and model-ing of deadtime reduction in an actively multi-plexed detection system" in the Journal of Modern Optics, 20 Jan-10 Feb 2009.

Distinctions

JQI Fellow and Co-Director Steve Rolston has been named an Out-standing Referee by the American Physical Society. The lifetime award is conferred "to recognize scientists who have been exceptionally helpful in assessing manuscripts for publication in the APS journals."

JQI Fellow Paul Lett is serving as General Co-Chair for the International Quantum Electronics Conference to be held May 31 to June 5 in Balti-more. And Chris Monroe is serving as Vice-Chair of the APS Division of Atomic, Molecular and Optical Physics. DAMOP meets in May.

Entangled States

6

JQI is a joint venture of the University of Maryland and the National Institute of Standards and Technology, with support from the Laboratory for Physical Sciences.

Joint Quantum InstituteDepartment of Physics, Univ. of MarylandCollege Park, MD 20742E-mail: jqi_info@squid.umd.eduTelephone: (301) 405-6129