Published papers

The two-photon subtraction from overlappping squeezed packets generates temporally multiplexed superposition states of continuous variables with a designated time separation. In an appropriate mode an amplified superposition state is produced due to bosonic quantum interference. © 2008 Optical Society of America.

We investigated the matching of two continuous fields in a closed four-level system quantum mechanically. When the atomic coherence is prepared in the dressed state by two strong external coupling fields, fluctuations of phase and amplitude between the two probe fields are selectively absorbed, while these fields are mainly transparent in the media. Such selective absorption establishes the phase and amplitude correlation between the two weak probe fields over a wide range of frequencies. However, the correlation remains limited by the standard quantum limit, because of the addition of the anti-phase atomic noise during the four-wave mixing process.

Reconciliation is a key element of continuous-variable quantum key distribution (CV-QKD) protocols, affecting both the complexity and performance of the entire system. During the reconciliation protocol, error correction is typically performed using low-density parity-check (LDPC) codes with a single decoding attempt. In this paper, we propose a modification to a conventional reconciliation protocol used in four-state protocol CV-QKD systems called the multiple decoding attempts (MDA) protocol. MDA uses multiple decoding attempts with LDPC codes, each attempt having fewer decoding iteration than the conventional protocol. Between each decoding attempt we propose to reveal information bits, which effectively lowers the code rate. MDA is shown to outperform the conventional protocol in regards to the secret key rate (SKR). A 10% decrease in frame error rate and an 8.5% increase in SKR are reported in this paper. A simple early termination for the LDPC decoder is also proposed and implemented. With early termination, MDA has decoding complexity similar to the conventional protocol while having an improved SKR.

Physical layer cryptography is the cryptographic technique which realizes the information-theoretically secure message transmission and key establishment by exploiting physical properties of transmission channels. In this paper, we give an outline of fundamental model for physical layer cryptography and present the main result of channel estimation experiment toward the physical layer cryptography in free space optical communication which is carried out by Quantum ICT Advanced Developed Center.

In information and communication networks, transmission capacity and security are main technical demands that are required to be continuously and rapidly upgraded. To use the maximum physical potential of the network, one has to develop "quantum node technology" which enables us to control quantum states of light and matters to achieve the ultimate communication capacity and sensitivity. In this article, we review our recent progress toward this goal, in particular, the technical progress of the quantum control photonic signals and demonstration of new quantum communication protocols by using these technologies.

Transmission of nonclassical quantum states by quantum teleportation of continuous variables is studied. Protocol of quantum teleportation via a two-mode squeezed-vacuum state in a noisy environment is formulated by the Glauber-Sudarshan P-function. Using the nonclassical depth as an estimation parameter of transmission performance, we compare the teleportation scheme with the direct transmission through a noisy channel. The noise model is based on the coupling to the vacuum field. We find that the teleportation channel has better transmission performance than the direct transmission channel in a certain region. The bounds for such region and for obtaining the nonvanished nonclassicality of the teleported quantum states are also discussed. We also mention the required conditions for transmitting nonclassical features in real experiments.

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 CO2 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.

In this paper, we present the Monte-Carlo Compressive Optimization algorithm, a new method to solve a combinatorial optimization problem that is assumed compressible. The method relies on random queries to the objective function in order to estimate generalized moments. Next, a greedy algorithm from compressive sensing is repurposed to find the global optimum when not overfitting to samples. We provide numerical results giving evidences that our methods overcome state-of-the-art dual annealing. Moreover, we also give theoretical justification of the algorithm success and analyze its properties. The practicality of our algorithm is enhanced by the ability to tune heuristic parameters to available computational resources.

To realize the practical implementation of device-independent quantum key distribution (DIQKD), the main difficulty is that its security is solely based on the detection loophole-free violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality i.e. the CHSH value S > 2 [1]. To circumvent the inevitable losses in the transmission channels which create the detection loophole, several probabilistic methods to recover the nonlicality have been proposed. Here, we theoretically and experimentally investigate the optimal strategy for the method utilizing linear optical entanglement swapping relay (ESR) with photon pairs from spontaneous parametric-down conversion (SPDC) sources [2,3]. In the previous methods, it is common to employ the ESR node in Bob's system as shown in Fig. 1(a) (Hereafter, we call this configuration the local-heralding (LH).). We employ another configuration: The middle-heralding (MH) shown in Fig. 1(b), where the ESR node is placed in the middle of Alice and Bob similar to the measurement device independent QKD. We numerically compare S of the above two schemes by using the numerical simulation based on the characteristic-function approach [3]. The results are shown in Fig. 1(c). We see that the larger values of S are expected in the MH scheme when the dark count probabilities (ν) of the detectors are considered. In experiment, we perform the Bell-test experiment using the ESR with SPDC sources. We employ the MH scheme, and measure S changing the transmission losses. The experimental parameters (the average photon numbers and measurement angles) are chosen such that S is maximized under the condition of the experimental imperfections (mode-mismatch, dark count and so on) and the assumption of the unit local detection efficiencies (η = 1). While the overall detection efficiencies of our system (η ∼ 15 %) are not the range of directly observing the violation of the CHSH inequality, S obtained by the experiment well agree with the numerical simulation with experimental parameters. When the overall transmission loss is set to be 10 dB, which corresponds to the transmittance of a 50 km-optical fiber, we observe S = 1.478, while S = 1.487 is expected by the numerical simulation. This result means that our numerical simulation can rigorously model the experiment. This allows us to estimate the nonlocality of the quantum state just before the local detection losses by the numerical simulation. The CHSH value is estimated to be S = 2.104 > 2, which indicates that the quantum state with experimental imperfections possesses the potential to violate the CHSH inequality. exp th exp η=1

We discuss challenges and recent progress on coding, digital signal processing and joint transmission with classical data channels, for quantum key distribution.