Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session J1: Thesis Prize Session |
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Chair: J. Shertzer, College of the Holy Cross Room: TELUS Convention Centre Macleod BC |
Thursday, June 7, 2007 1:30PM - 2:06PM |
J1.00001: Real-time Manipulation of Entanglement between Remote Atomic Ensembles for a Scalable Quantum Network Invited Speaker: Entanglement, a uniquely quantum mechanical property of correlations among various components of a physical system, has been recognized as a critical resource in quantum information science. Besides deterministic approaches, entanglement can be created probabilistically by way of quantum interference. It is essential that the success of entanglement creation is heralded unambiguously (by a ``trigger'') so that the resulting entangled state is available for subsequent operations. In addition, quantum memory is required to store the entangled states until they are needed for the protocol at hand. Combined, the ``trigger'' and quantum memory can lead to exponential speedup for protocols exploiting multiple components. We report the initial observation of measurement-induced entanglement between excitations stored in remote cold atomic ensembles. The resulting entangled state is heralded and stored in quantum memories. The heralded nature and quantum memory for certain quantum states are exploited to implement real-time control of the states of atomic ensembles and significantly improve the success rates of two quantum information protocols for scalable quantum networks. In one protocol, we observe the interference of two single photons from two ensembles and characterize their indistinguishability. In the other, we first prepare two pairs of entangled ensembles shared between two remote sites. The ensembles are then exploited to generate polarization-entangled photon pairs at the two remote sites, with the entanglement verified by the violation of a Bell's inequality. The photon pairs have potential applications for entanglement-based long-distance quantum communication protocols, such as quantum key distribution and quantum teleportation. [Preview Abstract] |
Thursday, June 7, 2007 2:06PM - 2:42PM |
J1.00002: Quantum Networking with Atomic Ensembles Invited Speaker: Quantum communication networks enable secure transmission of information between remote sites. However, at present, photon losses in the optical fiber limit communication distances to less than 150 kilometers. The quantum repeater idea allows extension of these distances. In practice, it involves the ability to store quantum information for a long time in atomic systems and coherently transfer quantum states between matter and light. Previously known schemes involved atomic Raman transitions in the UV or near-infrared and suffered from severe loss in optical fiber that precluded long-distance quantum communication. In this work a practical quantum telecommunication scheme based on cascade atomic transitions is proposed, with particular reference to cold alkali metal ensembles. Within this proposal, essential building blocks for a quantum network architecture are demonstrated experimentally, including storage and retrieval of single photons transmitted between remote quantum memories, collapses and revivals of quantum memories, deterministic generation of single photons via conditional quantum evolution, quantum state transfer between atomic and photonic qubits, entanglement of atomic and photonic qubits, entanglement of remote atomic qubits, and entanglement of a pair of 1530 nm and 780 nm photons. These results pave the way for construction of a realistic quantum repeater for long distance quantum communication. [Preview Abstract] |
Thursday, June 7, 2007 2:42PM - 3:18PM |
J1.00003: Experimental realization of BCS-BEC crossover physics with a Fermi gas of atoms Invited Speaker: In my talk I will present experiments performed with a strongly interacting Fermi gas of $^{40}$K atoms. These experiments, along with the work of groups studying the fermion $^6$Li, pioneered many new techniques for the study of ultracold Fermi gases and culminated in the observation of fermion pairing and superfluidity. As a first step in this work Feshbach resonances between $^{40}$K atoms were found and characterized. At these scattering resonances, we had the unique ability to arbitrarily tune the fermion-fermion interaction, and we discovered how to convert a large fraction of our fermionic atoms into bosonic molecules. By adiabatically converting a low entropy Fermi gas to such molecules, we created one of the first molecular Bose-Einstein condensates. Even more importantly I will describe how we were able to observe a phase transition near the peak of the Feshbach resonance through condensation of fermionic atom pairs. The pairs here have some properties of Cooper pairs and some properties of diatomic molecules. The physics is thus in the crossover between BCS superconductivity and Bose-Einstein condensation (BEC) of tightly bound pairs. Lastly, I will discuss an example of what we learned from subsequent studies of BCS-BEC crossover physics. [Preview Abstract] |
Thursday, June 7, 2007 3:18PM - 3:54PM |
J1.00004: Quantum Information Processing in Artificial Molecules Invited Speaker: Isolated atomic and molecular systems are known for their robust coherence properties. Further, their quantum states can now be controlled with exquisite precision, which provides an excellent starting point for implementing fundamental ideas from quantum information science. In this talk, we describe recent progress in developing techniques for quantum control of artificial molecules composed of coupled semiconductor quantum dots. We first focus on the electron spin degrees of freedom associated with such systems and show that the coherence properties of electron spins are determined by hyperfine interactions with large ensembles of lattice nuclear spins. Next we determine that the fine-structure states of quantum dot molecules provides a mechanism for robust manipulation of electron spins, while coupling to nuclei can be mitigated by using local, electrical control of the system. We further consider possible applications of such systems in quantum information science. We conclude with a discussion of the long-term prospects and fault tolerance properties of semiconductor quantum dots for large scale quantum information processing. [Preview Abstract] |
Thursday, June 7, 2007 3:54PM - 4:30PM |
J1.00005: High-temperature superfluidity in an ultracold Fermi gas Invited Speaker: Fermionic superfluidity occurs in a wide variety of physical systems, ranging from superconductors and helium-3 to distant neutron stars. Its realization in ultracold atomic Fermi gases provides us with a unique model system for the study of strongly interacting fermions in a clean and highly controllable environment. We have observed superfluidity in a gas of fermionic lithium-6. Strong interactions between the fermions are induced via a Feshbach resonance. This leads to the highest transition temperature relative to the Fermi temperature ever reported for a fermionic superfluid or superconductor. Scaled to the density of electrons in a metal, the superfluid transition temperature would lie far above room temperature. By varying the interatomic interaction, we can explore the crossover between two limiting cases of fermionic superfluidity: A Bose-Einstein condensate (BEC) of tightly bound molecules and a Bardeen-Cooper-Schrieffer (BCS) superfluid of long-range Cooper pairs. Condensates of up to 10 million fermion pairs are observed in a regime where pairing is purely a many-body effect, the pairs being stabilized by the presence of the surrounding particles. Superfluidity and phase coherence is directly demonstrated throughout the BEC-BCS crossover via the observation of long-lived, ordered vortex lattices in rotating Fermi gases. Further, we establish superfluidity in Fermi mixtures with imbalanced spin populations, addressing a long-standing debate on the ground state of these systems. We observe the separation of the trapped gas into a superfluid region at equal spin densities, surrounded by a shell at unequal densities. Above a certain critical imbalance, a phase transition to the normal state is identified, the Chandrasekhar-Clogston limit of fermionic superfluidity. [Preview Abstract] |
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