Masahiro Takeoka

Entanglement distillation from Gaussian input states

Hiroki Takahashi, 2 Jonas S. Neergaard-Nielsen, Makoto Takeuchi, Masahiro Takeoka, Kazuhiro Hayasaka, Akira Furusawa, and Masahide Sasaki 1 National Institute of Information and Communications Technology (NICT), 4-2-1 Nukui-kitamachi, Koganei, Tokyo 184-8795, Japan 2 Department of Applied Physics and Quantum Phase Electronics Center, School of Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656, Japan (Dated: July 13, 2009)

Generation of Large-Amplitude Coherent-State Superposition via Ancilla-Assisted Photon Subtraction

We propose and demonstrate a novel method to generate a large-amplitude coherent-state superposition (CSS) via ancilla-assisted photon subtraction. The ancillary mode induces quantum interference of indistinguishable processes in an extended space, widening the controllability of quantum superposition at the conditional output. We demonstrate this by a simple time-separated two-photon subtraction from continuous wave squeezed light. We observe the largest CSS of traveling light ever reported without correcting any imperfections, which will enable various quantum information applications with CSS states.

Fundamental rate-loss tradeoff for optical quantum key distribution

Since 1984, various optical quantum key distribution (QKD) protocols have been proposed and examined. In all of them, the rate of secret key generation decays exponentially with distance. A natural and fundamental question is then whether there are yet-to-be discovered optical QKD protocols (without quantum repeaters) that could circumvent this rate-distance tradeoff. This paper provides a major step towards answering this question. Here we show that the secret key agreement capacity of a lossy and noisy optical channel assisted by unlimited two-way public classical communication is limited by an upper bound that is solely a function of the channel loss, regardless of how much optical power the protocol may use. Our result has major implications for understanding the secret key agreement capacity of optical channels-a long-standing open problem in optical quantum information theory-and strongly suggests a real need for quantum repeaters to perform QKD at high rates over long distances.

A coherent all-solid-state laser array using the Talbot effect in a three-mirror cavity

Lateral mode coupling in a diode-pumped Nd:YAG microchip laser array is demonstrated with a Talbot cavity for the first time. The relatively low laser gains of solid-state lasers compared with diode lasers and CO/sub 2/ lasers, to which the Talbot cavity has already been applied successfully, are solved by employing a novel auxiliary Talbot cavity configuration. A brighter twin-peak far-field pattern indicating an out-of-phase array mode, whose spot is 9.3 times smaller than that obtained by incoherent superposition of the individual microchip laser outputs, is obtained from the phase-locked microchip laser array with a mode-selecting slit. Without the mode-selecting slit, a far-field pattern with a single narrow peak is obtained, showing that the array is locked in an in-phase mode, presumably because of multiple reflections in the auxiliary Talbot cavity.

Discrimination of the binary coherent signal: Gaussian-operation limit and simple non-Gaussian near-optimal receivers

We address the limit of the Gaussian operations and classical communication in the problem of quantum state discrimination. We show that the optimal Gaussian strategy for the discrimination of the binary phase shift keyed (BPSK) coherent signal is a simple homodyne detection. We also propose practical near-optimal quantum receivers that beat the BPSK homodyne limit in all areas of the signal power. Our scheme is simple and does not require realtime electrical feedback. PACS numbers: 03.67.Hk, 42.50.Dv

Demonstration of Near-Optimal Discrimination of Optical Coherent States

The optimal discrimination of nonorthogonal quantum states with minimum error probability is a fundamental task in quantum measurement theory as well as an important primitive in optical communication. In this work, we propose and experimentally realize a new and simple quantum measurement strategy capable of discriminating two coherent states with smaller error probabilities than can be obtained using the standard measurement devices: the Kennedy receiver and the homodyne receiver.

Quantum receiver beyond the standard quantum limit of coherent optical communication.

The most efficient modern optical communication is known as coherent communication, and its standard quantum limit is almost reachable with current technology. Though it has been predicted for a long time that this standard quantum limit could be overcome via quantum mechanically optimized receivers, such a performance has not been experimentally realized so far. Here we demonstrate the first unconditional evidence surpassing the standard quantum limit of coherent optical communication. We implement a quantum receiver with a simple linear optics configuration and achieve more than 90% of the total detection efficiency of the system. Such an efficient quantum receiver will provide a new way of extending the distance of amplification-free channels, as well as of realizing quantum information protocols based on coherent states and the loophole-free test of quantum mechanics.

Discrimination of binary coherent states using a homodyne detector and a photon number resolving detector

We investigate quantum measurement strategies capable of discriminating two coherent states probabilistically with significantly smaller error probabilities than can be obtained using nonprobabilistic state discrimination. We apply a postselection strategy to the measurement data of a homodyne detector as well as a photon number resolving detector in order to lower the error probability. We compare the two different receivers with an optimal intermediate measurement scheme where the error rate is minimized for a fixed rate of inconclusive results. The photon number resolving (PNR) receiver is experimentally demonstrated and compared to an experimental realization of a homodyne receiver with postselection. In the comparison, it becomes clear that the performance of the PNR receiver surpasses the performance of the homodyne receiver, which we prove to be optimal within any Gaussian operations and conditional dynamics.

Experimental demonstration of quantum teleportation of a squeezed state

Quantum teleportation of a squeezed state is demonstrated experimentally. Due to some inevitable losses in experiments, a squeezed vacuum necessarily becomes a mixed state which is no longer a minimum uncertainty state. We establish an operational method of evaluation for quantum teleportation of such a state using fidelity and discuss the classical limit for the state. The measured fidelity for the input state is 0.85{+-}0.05, which is higher than the classical case of 0.73{+-}0.04. We also verify that the teleportation process operates properly for the nonclassical state input and its squeezed variance is certainly transferred through the process. We observe the smaller variance of the teleported squeezed state than that for the vacuum state input.

Quadrature phase shift keying coherent state discrimination via a hybrid receiver

We propose and experimentally demonstrate a near-optimal discrimination scheme for the quadrature phase shift keying (QPSK) protocol. We show in theory that the performance of our hybrid scheme is superior to the standard scheme—heterodyne detection—for all signal amplitudes and underpin the predictions with our experimental results. Furthermore, our scheme provides hitherto the best performance in the domain of highly attenuated signals. The discrimination is composed of a quadrature measurement, a conditional displacement and a threshold detector.We propose and experimentally demonstrate a near-optimal discrimination scheme for the quadrature phase shift keying protocol (QPSK). We show in theory that the performance of our hybrid scheme is superior to the standard scheme heterodyne detection for all signal amplitudes and underpin the predictions with our experimental results. Furthermore, our scheme provides the hitherto best performance in the domain of highly attenuated signals. The discrimination is composed of a quadrature measurement, a conditional displacement and a threshold detector. PACS numbers: 03.67.Hk, 03.65.Ta, 42.50.Ex ar X iv :1 20 4. 08 88 v2 [ qu an tph ] 2 6 Ju n 20 12 QPSK coherent state discrimination via a hybrid receiver 2