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David Tong talks with ScienceWatch.com and answers a few questions about this month's Fast Moving Front in the field of Physics. 
Tong Article: Local blockade of Rydberg excitation in an ultracold gas
Authors: Tong, D;Farooqi, SM;Stanojevic, J;Krishnan, S;Zhang, YP;Cote, R;Eyler, EE;Gould, PL
Journal: PHYS REV LETT, 93 (6): art. no.-063001, AUG 6 2004
Addresses: Univ Connecticut, Dept Phys, Storrs, CT 06269 USA.
Univ Connecticut, Dept Phys, Storrs, CT 06269 USA.

 Why do you think your paper is highly cited?

The possibility of a computer that operates on the principle of quantum mechanics has attracted growing interest across many research fields. Just within the past decade, we have seen many advances in the field of quantum information. Numerous implementations to build quantum phase gates have been proposed, one of which is using trapped neutral atoms which offer many advantages: there are already well-developed techniques for the cooling and trapping of neutral atoms, such as far-off-resonance traps (FORT), microtraps, optical lattices, and so on. The ground states of neutral atoms hardly interact with the environment, giving rise to very long coherence times, and the strong interactions of highly excited Rydberg states allows information to be exchanged quickly before decoherence sets in.

On the other hand, creating quantum phase gates from microscopic samples of these neutral atoms and addressing these microscopic qubits individually pose extreme experimental challenges. An exceptional degree of control over submicron systems would be necessary to address a single qubit. Our paper demonstrates a new experimental technique to work around these challenges.

 Does it describe a new discovery, methodology, or synthesis of knowledge?

"Our work exhibits a new method to create and address quantum phase gates using trapped neutral atoms."

Through this work, we offer the experimental realization of a theoretical work developed in 2001 (MD Lukin, et al., Phys. Rev. Lett. 87, 037901). Our work exhibits a new method to create and address quantum phase gates using trapped neutral atoms.

By exploiting the strong interactions of the high Rydberg states, we are able to show a coherent manipulation of quantum information stored in these collective excitations of mesoscopic many-atom ensembles. The strong interactions shift the Rydberg levels of other nearby atoms and prevent (or blockade) them from being excited. This blockading mechanism causes the mesoscopic ensemble to behave as a single super-atom and thus, many stringent requirements for the implementation of quantum gates can be relaxed.

 Would you summarize the significance of your paper in layman's terms?

Quantum computers would rely on the operation of quantum gates, which pose experimental challenges since extremely high control over the size of an atom would be required. Our work alleviates many of these experimental difficulties. The blockading mechanism allows us to address many atoms at once, with the end result being as if we were to address one of them individually. We can now work with many atoms, rather than having to address a single atom, yet we can still expect the behavior of a single atom.

 How did you become involved in this research and were there any particular problems encountered along the way?

This work was done under the supervision of three Principal Investigators in the Department of Physics at the University of Connecticut: Dr. Phillip Gould and Dr. Edward Eyler on the experimental side, and Dr. Robin Côté giving support on the theoretical side. I primarily work on the experimental side as part of my Ph.D. research. This work relies on the observation of suppression in signal size and thus, we spend an elaborate amount of time and effort to ensure that we are free from any saturation effect.

We also spend considerable amounts of time choosing and calibrating our detection mechanism to enable us to detect small signals with confidence, apart from all the background/noise we have in the system. Once we are confident that we have the detection mechanism we need and that our signals are free from any possible saturation effect, we are able to complete the entire experiment within a relatively short period of time.

 Where do you see your research leading in the future?

The blockading mechanism is part of the work leading towards having quantum phase gates based on mesoscopic samples of neutral atoms. There are still other things that need to be done, e.g., making sure that we have uniform density in our sample, making sure that we are able to detect a signal from a single atom, etc.

 Do you foresee any social or political implications for your research?

Probably not with this particular work, but there will be some implications within the field of quantum information as a whole. Even though quantum computation is still in its early phases, we should already make preparations and adjustments towards the realization of quantum computation. For example, the RSA encryption and decryption, which is based on the factorization of large numbers, have been proven to be good enough as the best algorithm available on the current classical computers, yet can only solve the factorization problem in exponential time.

However, the quantum algorithm (e.g., as developed by Peter Shor, the Morss Professor of Applied Mathematics at MIT) can efficiently factor large numbers (with more than 100 digits or so) in polynomial time, a very significant improvement. The current strongest encryption and decryption may prove to be insufficient if quantum computation were available today.

David Tong, Ph.D. Candidate
Physics Department
University of Connecticut
Storrs, CT, USA

Keywords: quantum mechanics, quantum information, quantum computation, quantum phase gates, far-off-resonance traps, microtraps, optical lattices, highly excited rydberg states, high rydberg states, the quantum algorithm, peter shor.

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2008 : November 2008 - Fast Moving Fronts : David Tong