Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V33: Quantum Cryptography and Quantum Communication II |
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Sponsoring Units: GQI Chair: Christopher Fuchs, Bell Labs, Alcatel-Lucent Room: Colorado Convention Center 403 |
Thursday, March 8, 2007 11:15AM - 11:27AM |
V33.00001: Photonic quantum memories for quantum repeaters Christoph Simon, Nicolas Sangouard, Hugues de Riedmatten, Mikael Afzelius, Hugo Zbinden, Nicolas Gisin We analyze a quantum memory protocol for photons based on the controlled reversible broadening of absorption lines. Based on an explicit solution of the equations of motion, we have studied the dependence of the memory efficiency on the optical depth of the medium and on the characteristics of the spectral distribution of the absorbers. Our group is particularly interested in the implementation of the described protocol using crystals doped with rare-earth ions. We describe how quantum memories based on this principle could be used to realize effective quantum repeaters. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V33.00002: Investigations of an optical memory based on stimulated photon echoes Wolfgang Tittel, Matthias Staudt, Sara Simon-Hastings, Mikael Afzelius, Valerio Scarani, Nicolas Gisin By interfering photon echoes produced in a Erbium-doped LiNbO3 waveguiding structure, we investigated the preservation of information encoded into the relative phase and amplitudes of optical pulses during storage and retrieval in an optical memory based on stimulated photon echo. Our findings are of particular interest for future long-distance quantum communication protocols, which rely on the reversible transfer of quantum states between light and atoms with high fidelity. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V33.00003: Numerical Modeling and Optimization of Type-I Entangled-Photon Sources Gleb Akselrod, Joseph Altepeter, Michael Goggin, Jaime Valle, Joseph Yasi, Paul Kwiat A numerical model of double-crystal entangled-photon sources based on type-I down conversion is presented. The purity of such sources is reduced by spectral and spatial decoherence resulting from crystal birefringence, imperfect phase-matching conditions, collection optics, and pump beam bandwidth and spatial modes. By accounting for these experimental phenomena, we simulated the total two-photon quantum state for sources employing uniaxial or biaxial nonlinear crystals. To verify the model, the predicted state was compared with experimentally obtained quantum state tomography data, showing good agreement. Furthermore, the model was used to design spatial and temporal phase compensation crystals to reduce the phase decoherence and improve the brightness and purity of our sources. This code will be freely available to the quantum optics community as a resource for designing and characterizing optimized entangled-photon sources. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V33.00004: Hyperentangled Bell-state analysis Tzu-Chieh Wei, Julio Barreiro, Paul G. Kwiat It is known that it is impossible to unambiguously distinguish the four Bell states of two photons using linear optics. However, hyperentanglement, the simultaneous entanglement in more than one degree of freedom, has been shown to assist the complete Bell measurement of the four Bell states (given a fixed state of the other degrees of freedom). Yet introducing other degrees of freedom also enlarges the number of Bell-like states. We investigate the limits for unambiguously distinguishing a subset of all Bell-like states. In particular, we show that the optimal number is 7 out of the 16 Bell-like states, which are composed of polarization and one additional qubit-like degree of freedom. The implications for quantum communication are also discussed. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V33.00005: Security aspects of the authentication used in quantum key growing Jan-{\AA}ke Larsson, J\"orgen Cederl\"of Unconditionally secure message authentication is an important part of Quantum Cryptography (QC). We analyze security effects of using a key obtained from QC for authentication purposes in later rounds of QC. In particular, the eavesdropper gains partial knowledge on the key in QC that may have an effect on the security of the authentication in the later round. Our initial analysis indicates that this partial knowledge has little effect on the authentication part of the system, in agreement with previous results on the issue. However, when taking the full QC protocol into account, the picture is different. By accessing the quantum channel used in QC, the attacker can change the message to be authenticated. This together with partial knowledge of the key does incur a security weakness of the authentication. The underlying reason for this is that the authentication used, which is insensitive to such message changes when the key is unknown, becomes sensitive when used with a partially known key. We suggest a simple solution to this problem, and stress usage of this or an equivalent extra security measure in QC. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V33.00006: Unconditional security at a low cost Xiongfeng Ma, Hoi-Kwong Lo In this talk, I will discuss several post-processing schemes for quantum key distribution (QKD). I will compare QKD with and without decoy states. By simulating four QKD experiments and analyzing one decoy-state QKD experiment, we compare two data postprocessing schemes based on security against individual attacks by L\"{u}tkenhaus, and unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our results show that these two schemes yield close performances. In QKD, since unconditional security is highly sought after, we conclude that one is better off considering unconditional security, rather than restricting to individual attacks. This work is appeared in Ref.~[X. Ma, Phys.~Rev.~A \emph{74}, 052325, (2006)]. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V33.00007: Decoy State Quantum Key Distribution: Theory and Practice Yi Zhao, Hoi-Kwong Lo, Xiongfeng Ma, Bing Qi, Kai Chen, Li Qian Decoy state quantum key distribution (QKD) has been proposed as a novel approach to improve dramatically both the security and the performance of practical QKD set-ups. We proved its security, and proposed the first practical decoy state QKD protocols, including the one-decoy protocol, the weak+vacuum protocol, and the general two-decoy protocol. Our further study shows that the two-way communication can effectively improve the performance of decoy state QKD. We performed the first experiments of decoy state QKD. Two protocols -- the one-decoy protocol and the weak+vacuum protocol -- were implemented with a maximum transmission distance of 60km. We implemented the decoy state method by adding commercial acousto-optic modulator to a commercial QKD system. Our theoretical and experimental studies show explicitly the power and the feasibility of decoy method, and brings it to our real- life. Our works are published in [1-5]. [1] H. -K. Lo, X. Ma, and K. Chen, Phys. Rev. Lett. 94 230504 (2005) [2] X. Ma \emph{et. al.}, Phys. Rev. A 72, 012326 (2005) [3] Y. Zhao \emph{et. al.}, Phys. Rev. Lett., 96, 070502 (2006) [4] Y. Zhao \emph{et. al.}, in \emph{Proceedings of IEEE ISIT} (IEEE, 2006) pp. 2094-2098 [5] X. Ma \emph{et. al.}, Phys. Rev. A 74, 032330 (2006) [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V33.00008: What kind of assistance enables quantum cloning? Koji Azuma, Masato Koashi, Nobuyuki Imoto We investigate the cases where a set $S$ of states $\{ |\psi_i \rangle \}$ cannot be cloned by itself, but is clonable with the help of another system prepared in state $\hat{\rho}_i$. When $S$ is pair-wise nonorthogonal, it is known that one can generate the copy from $\hat{\rho}_i$ alone, with no interaction with the original system. Here we show that a set containing orthogonal pairs exhibits a property forming a striking contrast; For any such set, there is a choice of $\hat {\rho}_i$ that enables cloning only when the two systems are interacted in a purely quantum manner that is not achievable via classical communication. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V33.00009: Stochastic Local Distinguishability Somshubhro Bandyopadhyay, Anirban Roy, Jonathan Walgate We pose the question, ``when is globally available information is also locally available?'', formally as the problem of local state discrimination, and show that the deep qualitative link between local distinguishability and entanglement lies at the level of stochastic rather than deterministic local protocols. We restrict our attention to sets of mutually orthogonal pure quantum states. We define a set of states $\{ |\psi_{i}\rangle \}$ as being\emph{stochastically locally distinguishable} if and only if there is a LOCC protocol whereby the parties can conclusively identify a member of the set with some nonzero probability. If a set is stochastically locally distinguishable, then the complete global information is potentially locally available. If not, the physical information encoded by the system can never be completely locally exposed. Our results are proved true for all orthogonal quantum states regardless of their dimensionality or multipartiality. First, we prove that entanglement is a necessary property of any system whose total global information can never be locally accessed. Second, entangled states that form part of an orthogonal basis can never be locally singled out. Completely entangled bases are, always stochastically locally indistinguishable. Third, we prove that any set of three orthogonal states, is stochastically locally distinguishable. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V33.00010: Detector dead-time effects in high-speed quantum key distribution Daniel Rogers, Joshua Bienfang, Anastase Nakassis, Charles W. Clark Recent advances in quantum key distribution (QKD) have given rise to systems that operate at transmission periods significantly shorter than the dead times of their component single-photon detectors. As systems continue to increase to multi-gigahertz transmission rates, the effects of detectors with dead times on the order of 50 ns become progressively more important. We demonstrate that security constraints in the presence of finite dead times create the seemingly-counterintuitive situation where an increase in transmission rate results in a decrease in sifted bit rate. We present an analytic model of this condition and propose novel and efficient methods to mitigate its effects. We further show that there exists an optimal transmission rate to achieve maximum secret key production for given detectors and link loss and verify that analysis with a Monte Carlo simulation. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V33.00011: Wigner Function in Phase Coding QKD Martin Suda Phase coding systems in quantum cryptography use two largely separated fiber glass Mach-Zehnder (MZ) interferometers which belong to Alice and Bob, respectively. If both partners adjust their phase shifters accordingly, the time pulses behind the second MZ interferometer exhibit interference effects which can be applied to the encoding-decoding procedure in quantum key distribution. Using Gaussian wave packets, the energy and time spectra of those interference effects are analyzed by means of the Wigner function taking into account the wave length distribution, chromatic dispersion, absorption and the dimensions of the interferometric set-up. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V33.00012: Characteristics of the near field diffraction of Laguerre-Gaussian modes using computer generated holograms. Ayman Sweiti, Warner Miller There has been interest in quantum communication devices that utilize the higher dimensions afforded by the orbital angular momentum (OAM) of photons. We present experimental and computational results on the production and superposition of OAM states. In particular, we present here the near field diffraction of the orbital angular momentum states with gray scale diffraction gratings. These patterns exhibit different fork-like structures characteristic of OAM states. These are modeled using Fresnel-Kirchhoff approximation. Also the phase structure of the diffracted field has been identified. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V33.00013: Quantum Communication with a Twist: QKD using Orbital Angular Momentum Photons Warner Miller, Ayman Sweiti We briefly outline our progress in developing a programmable-optics QKD system that utilizes the orbital angular momentum (OAM) eigenstates of photons. A photon can be prepared in a state that exhibits both its polarization as well as OAM. A single photon with polarization can communicate one bit of information. However a single photon from an appropriate set of axial eigenstates (OAM or ``twisted photon'') can, in principle, transmit many bits. While there is no improvement in bandwidth over conventional spin-based QKD devises, the use of an OAM eigenmodes in an n-state QKD system can substantially reduce the system's optical fidelity requirements. We outline the relative strengths and weaknesses in using OAM states verses polarization states regarding the (1) state preparation, (2) state propagation and (3) state detection. An essential element of any QKD system is the generation, propagation and sorting of mutually unbiased (MUB) quantum states. We demonstrate here the diffractive optics generation of MUB states built from the superpositions in an n-dimensional Hilbert space of OAM photons. In particular, we show the generation of a MUB state utilizing a liquid crystal spatial light modulator. [Preview Abstract] |
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