Published papers

The effect of injecting sinusoidally frequency-shifted light into microchip solid-state laser was investigated. Periodic and chaotic burst corresponding to the period of the piezoelectric transducer (PZT) vibration at 1 kHz was observed. The chaotic burst was consisted of chaotic sharp pulses at a repetition frequency near the relaxation oscillation frequency of 375 kHz. A time interval between the bunches of chaotic pulses was observed which corresponds to the instantaneous Doppler-shift frequency of the feedback light. The abrupt change in the phase of the laser field induced by the frequency-modulated feedback light caused relaxation oscillation with chaotic amplitude change.

Electromagnetically-induced transparency (EIT) is a phenonemon where the optically resonant transition is rendered nearly transparent by applying a strong coupling laser field. An optical coherent control scheme of the polarization of optical pulses using EIT quantum coherence is proposed for fast optical switching.

We propose a scheme of all-optical polarization switching in a four-level atomic system. In this scheme, the polarization state of an optical pulse is controlled by an another control pulse via a four-wave mixing (FWM) process between orthogonal components of a pulse. Using a sufficiently strong control pulse, a linear absorption loss during FWM is significantly suppressed, therefore the polarization can be controlled with high transmission efficiency. Since our scheme requires only one ground (stable) state, it is applicable to the excitonic systems of semiconductor quantum wells. Our numerical calculation, based on typical parameters of Pb-based inorganic-organic-layered perovskite-type materials, shows that, using ultrashort-pulse trains, polarization switching with a high repetition rate of more than 10 GHz is possible.

We present a framework to deal with a range of large-scale compressive-sensing problems using a quantum subroutine. We apply a quantum approximate optimization algorithm (QAOA) to support detection in a sparse-signal reconstruction algorithm: matching pursuit. The constrained optimization required in this algorithm is difficult to handle when the size of the problem is large, and the constraints usually given by unstructured patterns also become an issue. Our framework utilizes specially designed structured constraints that are easy to manipulate and reduce the optimization problem to the solution of an Ising model which can be found using Ising solvers. In this study, we simulate QAOA for this purpose and evaluate the performances. We observe that our method can outperform reference classical methods in the same context. Our results explore a promising path for applying quantum computers in the compressive-sensing field.

We present a framework to deal with a range of large scale compressive sensing problems using a quantum subroutine. We apply a quantum approximate optimization algorithm (QAOA) to support detection in a sparse signal reconstruction algorithm: matching pursuit. The constrained optimization required in this algorithm is difficult to handle when the size of the problem is large and constraints are given by unstructured patterns. Our framework utilizes specially designed structured constraints that are easy to manipulate and reduce the optimization problem to the solution of an Ising model which can be found using Ising solvers. In this research, we test the performance of QAOA for this purpose on a simulator of quantum computer. We observe that our method can outperform reference classical methods. Our results explore a promising path of applying quantum computers in the compressive sensing field.

A free-space quantum key distribution system is being developed by the National Institute of Information and Communications Technology (NICT) in Koganei, Japan. Quantum cryptography is a new technique for transmitting information where the security is guaranteed by the laws of physics. In such systems, a single photon is used for the quantum information. However, since the transmission distance in optical fibers is limited by the absorption of photons by the fiber, the maximum demonstrated range has been limited to about 100 km. Free-space quantum cryptography between an optical ground station and a satellite is a possible solution to extend the distance for a quantum network beyond the limits of optical fibers. At NICT, a laser communication demonstration between the NICT optical ground station and a low earth orbit satellite was successfully conducted in 2006. The use of free-space quantum key distribution for such space communication links is considered an important future application. This paper presents conceptual designs for the onboard transceivers for satellite quantum cryptography. © 2009 SPIE.

We present a scheme to conditionally generate an arbitrary superposition of a pair of coherent states from a squeezed vacuum by means of the modified photon subtraction where a coherent state ancilla and two on/off type detectors are used. We show that, even including realistic imperfections of the detectors, our scheme can generate a target state with a high fidelity. The amplitude of the generated states can be amplified by conditional homodyne detections. © 2007 The American Physical Society.

We present an experiment where a range of highly non-classical optical states were generated and fully characterized. As a class they can be considered as realizations of a qubit state in a squeezed vacuum/squeezed photon basis. The central technique of displaced photon subtraction will have applications to quantum computing with coherent states. © 2011 American Institute of Physics.

A quantum channel is derived for continuous variable teleportation which is performed by means of an arbitrary entangled state and the standard protocol. When a Gaussian entangled state such as a two-mode squeezed-vacuum state is used, the continuous variable teleportation is equivalent to the thermalizing quantum channel. Continuous variable quantum dense coding is also considered.

The quantum channel consisting of continuous variable quantum teleportation based on the standard protocol is investigated. It is shown that a quantum channel for continuous variable teleportation by an aribitrary entangled state is equivalent to a generalized thermalizing channel. The transmission of nonclassical states by the teleportation channel with a two-mode squeezed-vacuum state in a noisy environment is also studied. The transmisson performance is evaluated by using the Glauber-Sudarshan P-function and the nonclassical depth. Finally, the obtained results are compared to those of the noisy direct transmission channel. It is shown that the teleportation channel has better performance than the direct transmission channel in a certain region.

A quantum channel consisting of continuous variable quantum teleportation based on the standard protocol is investigated. It is shown that a quantum channel for continuous variable teleportation by an aribitrary entangled state is equivalent to a generalized thermalizing channel. We also study the transmission of nonclassical states by a teleportation channel with a two-mode squeezed-vacuum state in a noisy environment. The transmisson performance is evaluated by using the Glauber-Sudarshan P function and the nonclassical depth. Finally, we compare the results obtained to those of a noisy direct transmission channel. It is shown that the teleportation channel performs better than the direct transmission channel in a certain region. © 2003 MAIK "Nauka/Interperiodica".

Novel fiber-based schemes for controlling quantum correlation between frequency bands to generate photon-number squeezing and quadrature two-mode squeezing are studied. The scheme is based on a nonlinear fiber, a pulse shaper, and an adaptive feedback loop. Experimental results of photon-number squeezing are presented. ©2003 Optical Society of America.

Novel fiber-based schemes for controlling quantum correlation between frequency bands to generate photon-number squeezing and quadrature two-mode squeezing are studied. The scheme is based on a nonlinear fiber, a pulse shaper, and an adaptive feedback loop. Experimental results of photon-number squeezing are presented.