Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A29: Quantum Communication, Theoretical Entanglement, and Cryptography |
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Sponsoring Units: GQI Chair: Jan-Ake Larsson, Linkoping University Room: C148 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A29.00001: Intrinsic Quantum Correlations of Weak Coherent States for Quantum Communication Yong Meng Sua, Erin Scanlon, Travis Beaulieu, Viktor Bollen, Kim Fook Lee Intrinsic quantum correlations of weak coherent states are observed between two parties, which can be used as a supplement to the existence decoy-state BB84 and differential phase-shift quantum key distribution protocols. In a proof-of-principle experiment, we generate bi-partite correlations of weak coherent states using weak local oscillator fields in two spatially separated balanced homodyne detections. We employ non-linearity of post-measurement method to obtain the bi-partite correlations from two single-field interferences at individual homodyne measurement. This scheme is then used to demonstrate bits correlations in a transmission fiber over a distance of 10 km. We believe that the scheme can add another physical layer of security to these protocols for quantum key distribution and implement linear optics quantum computing with weak coherent states. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A29.00002: Achieving the physical limits of the bounded-storage model Prabha Mandayam, Stephanie Wehner The security of most cryptographic systems in use today is based on the premise that certain computational problems are hard to solve for the adversary. However, recent cryptographic models such as the bounded-storage model and the noisy-storage model, are based on more physical assumptions regarding the two parties' resources and allow us to obtain security without relying on any additional hardness results. In the bounded-storage model, where the adversary's quantum storage is limited, it is known that security can be achieved if the adversary can store strictly less then half of the qubits transmitted during the protocol. It has been an open question whether security can still be achieved if the adversary's storage were any larger. Here, we answer this question positively and demonstrate a two-party protocol which is secure as long as the adversary cannot store even a small fraction of the transmitted pulses. This not only settles the question, but also highlights the sharp contrast to classical bounded storage, where it is known that security can only be obtained if the adversary's classical storage is at most quadratic in the storage required by the honest players. In the more general setting of the noisy-storage model, where the adversary's memory is simply assumed to be imperfect, we show that our protocol extends security to a larger class of noisy quantum memories. (Reference: arXiV - quant-ph 1009.1596) [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A29.00003: High-speed single-photon signaling for daytime QKD Joshua Bienfang, Alessandro Restelli, Charles Clark The distribution of quantum-generated cryptographic key at high throughputs can be critically limited by the performance of the systems' single-photon detectors. While noise and afterpulsing are considerations for all single-photon QKD systems, high-transmission rate systems also have critical detector timing-resolution and recovery time requirements. We present experimental results exploiting the high timing resolution and count-rate stability of modified single-photon avalanche diodes (SPADs) in our GHz QKD system operating over a 1.5 km free-space link that demonstrate the ability to apply extremely short temporal gates, enabling daytime free-space QKD with a 4{\%} QBER.\footnote{A. Restelli, J.C. Bienfang A. Mink, and C.W. Clark, \textit{IEEE J. Sel. Topics in Quant. Electron} \textbf{16}, 1084 (2010).} We also discuss recent advances in gating techniques for InGaAs SPADs that are suitable for high-speed fiber-based QKD. We present afterpulse-probability measurements that demonstrate the ability to support single-photon count rates above 100 MHz with low afterpulse probability. These results will benefit the design and characterization of free-space and fiber QKD systems. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A29.00004: Security Proof for QKD Using Qudits and Finite Key Length Analysis of Protocols Lana Sheridan, Thinh Le, Valerio Scarani It is advantageous to use $d$-dimensional quantum systems for QKD because each signal carries $\log d>1$ bits, allowing a larger amount of information to be sent per transmission through the channel, and moreover, studies have indicated that the resistance to noise of the protocols increases when the dimension is increased. We provide a security bound against coherent attacks that takes into account finite-key effects for two families of protocols: two-basis protocols, the natural generalization of the Bennett-Brassard 1984 protocol for qubits, and $(d+1)$-basis protocols, the generalization of the six-state protocol for qubits. In the asymptotic limit, our bound vindicates the previous partial results concerning the higher resistance to noise. We also show that for finite key lengths the key rate corrections vary little with $d$ for $2 \leq d\leq 20$ indicating the protocol can be effective in realistic conditions. Finally, we consider some other finite key techniques for more general protocols. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A29.00005: Quantum Spread Spectrum Communication Travis Humble Spread spectrum techniques are widely used in classical contexts, including sensing and communication, for establishing low probability of intercept, resistance to narrowband jamming, and multiuser access protocols. In SS, the spectrum of the signal is spread much larger than the minimal information bandwidth to yield a boost in channel capacity. In this contribution, we apply SS modulation to the transmission and detection of the single-photon spectral probability amplitude (as opposed to SS of the field). We draw upon previous methods for coherently dilating single-photon spectral states to motivate our ideas. Techniques for direct modulation of the spectral amplitude, modulation via pumped single-photon up-conversion, and modulation via spread spectral teleportation are developed as particular modulation schemes for quantum spread spectrum communication. We quantify QSSC performance using the channel capacity and process gain expressed in terms of the spread bandwidth, and we investigate its behavior for a frequency-selective fading model. We conclude by discussing the potential for QSSC to underlie a QKD multiuser access control (MAC) protocol. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A29.00006: Remote Semi-State Preparation as SuperDense Quantum Teleportation Herbert J. Bernstein Recent advances in experimental technique make SuperDense Teleportation (SDT) possible. The effect uses remote state preparation to send more state-specifying parameters per bit than ordinary quantum teleportation (QT) can transmit. SDT uses a maximal entanglement to teleport the relative phases of an \textbf{n}-dimensional equimodular state. This means that one can send only \textbf{n}-1 of the total (2\textbf{n}-2) parameters -- comprising the relative phases and amplitudes -- of a general state. Nevertheless, for \textbf{n}$\ge $ 3, SDT sends more of these state-specifying parameters than QT for a given number of classical bits. In the limit of large \textbf{n} the ratio is 2 to 1, hence the nomenclature Bennett suggested, SDT, by analogy with Super Dense Coding. Alice's measurements and Bob's transformations are simpler than in QT. The roles of Charles the state chooser, and Diana who deploys it, are different than in QT. I briefly review possible experimental realizations, including two that are under consideration at the present time by an experimental group leading in higher-dimension entanglement work. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A29.00007: Entanglment assisted zero-error codes William Matthews, Laura Mancinska, Debbie Leung, Maris Ozols, Aidan Roy Zero-error information theory studies the transmission of data over noisy communication channels with strictly zero error probability. For classical channels and data, much of the theory can be studied in terms of combinatorial graph properties and is a source of hard open problems in that domain. In recent work, we investigated how entanglement between sender and receiver can be used in this task. We found that entanglement-assisted zero-error codes (which are still naturally studied in terms of graphs) sometimes offer an increased bit rate of zero-error communication even in the large block length limit. The assisted codes that we have constructed are closely related to Kochen-Specker proofs of non-contextuality as studied in the context of foundational physics, and our results on asymptotic rates of assisted zero-error communication yield non-contextuality proofs which are particularly `strong' in a certain quantitive sense. I will also describe formal connections to the multi-prover games known as pseudo-telepathy games. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A29.00008: Extreme Spin Squeezing Beyond Spin-1/2 Ensembles Collin Trail, Leigh Norris, Ivan Deutsch We consider a protocol for squeezing the collective spin of a cold atomic ensemble through coherent control of the spin and light-polarization interactions. By retro-reflecting a short pulse of light through the ensemble followed by a quantum eraser and phase matching, we achieve exponential scaling of the squeezing with optical density. We show how these results can be extended using state preparation and mapping techniques for s>1/2 systems, and extend our model of photon-atom scattering to account for decoherence in the higher dimensional case. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A29.00009: Engineered optical nonlinearity for a quantum light source Agata Branczyk, Alessandro Fedrizzi, Tom Stace, Tim Ralph, Andrew White Many applications in optical quantum information processing benefit from careful spectral shaping of single-photon wave-packets. By engineering the nonlinearity profile of a poled crystal, we were able to tailor the joint spectral wave-function of photons created in parametric down-conversion. We designed a crystal with an approximately Gaussian nonlinearity profile and confirmed successful wave-packet shaping by two-photon interference experiments. To further explore the underlying spectral correlations in the spectral amplitude, we also measured spatial quantum beating patterns. We numerically show how our method can be applied for attaining one of the currently most important goals of single-photon quantum optics, the creation of pure single photons without spectral correlations. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A29.00010: Disappearance of entanglement: a topological point of view Dong Zhou, Robert Joynt, Gia-Wei Chern, Jianjia Fei We give a topological classification of the evolution of entanglement, particularly the different ways the entanglement can disappear. Four categories exhaust all possibilities given the initial quantum state is entangled and the final one is not. Exponential decay of entanglement, entanglement sudden death and sudden birth can all be understood and visualized in the associated geometrical picture - the polarization vector representation. The entanglement evolution categories of any model are determined by the topology of the state space, the limiting state and the memory effect of the environment. Transitions between these types of behaviors as a function of physical parameters are also possible. These transitions are thus of topological nature. We illustrate the general concepts with a visualizable model. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A29.00011: Optimal Entanglement Transformations Among N-qubit W-Class States Wei Cui, Eric Chitambar, Hoi-Kwong Lo We investigate the physically allowed probabilities for transforming one $N$-partite W-class state to another by means of local operations assisted with classical communication (LOCC). Recently, Kinta\c{s} and Turgut have obtained an upper bound for the maximum probability of transforming two such states [1]. Here, we provide a simple sufficient and necessary condition for when this upper bound can be satisfied and thus when optimality of state transformation can be achieved. Our discussion involves obtaining lower bounds for the transformation of arbitrary W-class states and showing precisely when this bound saturates the bound of [1]. Finally, we consider the question of transforming symmetric W-class states and find that in general, the optimal one-shot procedure for converting two symmetric states requires a non-symmetric filter by all the parties. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A29.00012: Negativity Fonts in Four qubit Maximally Entangled States Santosh Shelly Sharma, Naresh Kumar Sharma Recently, we introduced negativity fonts as the basic units of multipartite entanglement in pure states. We show that the relation between global negativity of partial transpose of N- qubit state and linear entropy of reduced single qubit state yields an expression for global negativity in terms of determinants of negativity fonts. Transformation equations for determinants of negativity fonts under local unitaries (LU's) are used to construct N-qubit LU invariant and N-tangle (an entanglement monotone). The difference of squared negativity and N-tangle is an N qubit invariant which contains information on entanglement of the state caused by quantum coherences that are not annihilated by removing a single qubit. Entanglement monotones that detect the entanglement of specific parts of a four qubit state are also constructed. It is shown that these entanglement monotones bring out distinct features of several states which have been proposed to be the maximally entangled four qubit states. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A29.00013: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 10:36AM - 10:48AM |
A29.00014: Density matrix renormalization group study of the Kitaev honeycomb lattice model Zhenyue Zhu, Steven White The Kitaev model on the honeycomb lattice can be solved exactly through mapping into free majorana fermions with a Z$_2$ gauge field. As a benchmark for DMRG on this two dimensional system, we have simulated this model with a cylindrical geometry with varying widths. The ground state energy and degeneracy match well with theoretical predictions. The different degenerate ground states exhibit the same short range spin-spin correlation patterns. The von Newmann entanglement entropy and its spectrum are evaluated. We show that the entropy of the Kitaev model satisfies the area law, with the entropy being more specifically proportional to the number of bonds cut at the boundary between the two different regions. The degeneracy of entanglement spectrum can also be determined by the number of dangling majorana fermions at the cut. The above results hold for both the gapped and gapless phase. The non-Abelian phase obtained by applying a magnetic field, which is not exactly solvable, will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A29.00015: Towards entanglement of very high orbital angular momentum Robert Fickler, Radek Lapkiewicz, Christoph Schaeff, Peizhe Li, Sven Ramelow, Marcin Wiesniak, Anton Zeilinger Orbital angular momentum (OAM) of single photons has become an often used tool to realize entanglement in higher dimensions [1,2]. Laguerre-Gaussian modes of light with their helical phase structure carry photonic OAM and thus can be used to define an infinitely dimensional discrete Hilbert. However, the creation of photonic OAM entanglement using the well known spontaneous parametric downconversion process is limited by the strongly reduced efficiency for higher momenta [3]. We investigate novel methods to create this entanglement between two photons with a very high difference in their OAM quantum number and momentum respectively. Furthermore we explore hybrid entanglement of photons in these spatial modes and polarization degree of freedom.\\[4pt] [1] G. Molina-Terriza, J. P. Torres, L Torner, Nature Physics 3, 305 (2007)\\[0pt] [2] A. Mair, A. Vaziri, G. Weihs, A. Zeilinger, Nature 412, 313 (2001)\\[0pt] [3] B. Jack, J. Leach, H. Ritsch, S. M. Barnett, M. J. Padgett, S. Franke-Arnold, NJP 11, 103024 (2009) [Preview Abstract] |
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