Published papers

We determine a fundamental upper bound on the performance of any adaptive protocol for discrimination or estimation of a channel which has an unknown parameter encoded in the state of its environment. Since our approach relies on the principle of data processing, the bound applies to a variety of discrimination measures, including quantum relative entropy, hypothesis testing relative entropy, Rényi relative entropy, fidelity, and quantum Fisher information. We apply the upper bound to thermal (amplifier) channels with a known transmissivity (gain) but unknown excess noise. In these cases, we find that the upper bounds are achievable for several discrimination measures of interest, and the method for doing so is non-adaptive, employing a highly squeezed two-mode vacuum state at the input of each channel use. Estimating the excess noise of a thermal channel is of principal interest for the security of quantum key distribution, in the setting where a fiber-optic cable has a known transmissivity but a tampering eavesdropper alters the excess noise on the channel, so that estimating the excess noise as precisely as possible is desirable. Finally, we outline a practical strategy which can be used to achieve these limits.

A numerical study of the dependence of initial pulse shape on the spectral filtering photon-number squeezing of ultrashort optical pulses in nonlinear optical fibers was performed. An adaptive system for the optimization of intrapulse quantum structures was presented. Calculations were performed by solving the quantum nonlinear Schrödinger equation (QNLSE). The evolutions of the pulse shape and squeezing were found to depend on dispersion and nonlinearity of the fiber, and the initial waveform of the pulse.

We propose an adaptive waveform-control approach to optimize photon-number squeezing by the nonlinear fiber spectral filtering method. More than -8-dB squeezing of the sech envelope pulse is numerically predicted when the spectral phase is modulated before the pulse is sent to a fiber. The quantum cross correlations of the photon-number fluctuation become uniformly negative when the pulse is optimally shaped. © 2001 Optical Society of America.

We numerically study the performance of the displacement based quantum receiver for the discrimination of weak 3- and 4-phase-shift keyed (PSK) coherent state signals. We show that due to the nontrivial asymmetry of the receiver structure, optimization of the prior probability increases the mutual information and achieves sub-shot-noise limit discrimination. Moreover, we estimate the cutoff rate for a 4-PSK signal and confirm that the prior probability optimization shortens the code length for a given decoding error criterion. Such consideration for the asymmetric channel matrix is essential in a study of the compassable quantum receiver. © 2013 Elsevier B.V.

To the best of our knowledge, we demonstrate for the first time the generation of photon number squeezing by spectral filtering for ultrabroadband light generated by microstructure fibers at 800 nm. A maximum squeezing of 4.6 dB is observed, corresponding to 10.3 dB after correcting for detection losses. We numerically analyzed the quantum dynamics of ultrashort laser pulse propagation through optical fibers by solving a nonlinear quantum Schrödinger equation that included Raman scattering, especially for the quantum correlation of photon number fluctuation among frequency modes in broadband pulses. © 2005 The American Physical Society.

Quantum correlation in broadband spectra generated with a microstructure fiber is experimentally and theoretically studied. We experimentally obtained the highest squeezing of -4.6 dB using a 100-fs femtosecond pulse. Quantum correlation among the spectral components is analyzed with QNLSE including Raman effect.

Quantum correlation in broadband spectra generated with a microstructure fiber(MF) is experimentally and theoretically studied. We experimentally obtained the highest squeezing of -4.6 dB using a 100-fs femtosecond pulse. Quantum correlation among the spectral components is analyzed with QNLSE including Raman effect.

We review our most recent results on application of the photon subtraction technique for optical quantum information processing primitives, in particular entanglement distillation and generation of squeezed qubit states. As an introduction we provide a brief summary of other experimental accomplishments in the field. © 2011 National Institute of Informatics.

Characteristics of photon-number squeezing in a soliton-like Raman Stokes component split from a main pulse during propagation in a microstructure fiber are studied. Dependence of squeezing on the incident power and fiber length is obtained.

To observe the loophole-free violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality between distant two parties, i.e. the CHSH value > 2, the main limitation of distance stems from the loss in the transmission channel. The entanglement swapping relay (ESR) is a simple way to amplify the signal and enables us to evade the impact of the transmission loss. Here, we experimentally test the heralded nonlocality amplifier protocol based on the ESR. We observe that the obtained probability distribution is in excellent agreement with those expected by the numerical simulation with experimental parameters which are precisely characterized in a separate measurement. Moreover, we experimentally estimate the nonlocality of the heralded state after the transmission of 10 dB loss just before final detection. The estimated CHSH value is S = 2.113 > 2, which indicates that our final state possesses nonlocality even with transmission loss and various experimental imperfections. Our result clarifies an important benchmark of the ESR protocol, and paves the way towards the long-distance realization of the loophole-free CHSH-violation as well as device-independent quantum key distribution.

We determine the optimal method of discriminating and comparing quantum states from a certain class of multimode Gaussian states and their mixtures when arbitrary global Gaussian operations and general Gaussian measurements are allowed. We consider the so-called constant-p displaced states, which include mixtures of multimode coherent states arbitrarily displaced along a common axis. We first show that no global or local Gaussian transformations or generalized Gaussian measurements can lead to a better discrimination method than simple homodyne measurements applied to each mode separately and classical postprocessing of the results. This result is applied to binary state comparison problems. We show that homodyne measurements, separately performed on each mode, are the best Gaussian measurement for binary state comparison. We further compare the performance of the optimal Gaussian strategy for binary coherent states comparison with these of non-Gaussian strategies using photon detections.

We experimentally demonstrate programmable multimode phase-sensitive amplification multiplexed in the frequency domain for flexible control of parallelly generated squeezed states. We utilize the unique phase-matching condition of a type-II periodically poled potassium titanyl phosphate (PPKTP) crystal and pulse shaping technique to fully control the frequency-domain parallel generation of squeezed states in the optical telecommunication band. We experimentally verify that the independent programmability of phase-sensitive optical parametric amplification (OPA) for the modes corresponding to different frequency bands can be achieved by shaping the pump laser pulse from optical parametric gain measurements using a coherent probe light generated by a degenerate synchronously pumped optical parametric oscillator.

Free-space optical (FSO) links offer a practical approach to realize quantum key distribution (QKD) in a global scale. However, when one wants to further extend the distance from the geostationary orbit to the ground, currently known QKD schemes cannot realize practical key rates mainly due to the diffraction losses of a laser beam. If the facts that the FSO links are highly directional and must be used in the line-of-sight (LoS) condition are taken into account, one may impose some physical restrictions on an eavesdropping model to explore longer-distance QKD. In this paper, we propose a novel FSO secret key agreement scheme, line-of-sight QKD (LoS-QKD), based on a quantum wiretap channel. In our model, an eavesdropper can tap only a limited fraction of the FSO signal beam but perform any physically allowable operations on the tapped signals. Fading effects which are significant in the FSO links are fully taken into account. We provide a security proof for the differential phase shift (DPS) keying scheme in terms of the metric which meets the composability. We investigate numerically the performances of LoS-QKD with DPS keying, including finite-length analysis, showing that our proposed scheme can realize high-speed and long-distance secret key agreement with information-theoretic security.

The quantum key distribution (QKD) network with Vernam's one-time pad encryption and secret sharing are powerful security tools to realize an information theoretically secure (ITS) distributed storage system. In the work of Fujiwara et al., a single-password-authenticated secret sharing (SPSS) scheme based on the QKD network and Shamirs secret sharing was experimentally demonstrated; it confirmed ITS data transmission, storage, authentication, and integrity. To achieve data integrity, an ITS message authentication code (MAC) tag is employed, and a data owner of the secret sharing performs both the MAC tag generation and verification. However, for a scenario in which the data owner and end users are different entities, the above approach may not work, since the data owner can cheat the end users. In this article, we resolve this problem by proposing an ITS integrity protection scheme employing a third-party verification with time-stamp. The ITS integrity protection is realized by two steps: integrity check by the data owner at data reconstruction and data integrity certification by the data owner, the end user, and the third-party verifier using a MAC based on universal2 hash function and random number provided from the QKD network. In addition to introducing the third-party verifier, we institute 'a trusted calculator,' which computes shares of the data and MAC tags and sends MAC tags to the third-party verifier. The random number used in calculation is stored in the trusted calculator. We implement this scheme on the SPSS system installed in the Tokyo QKD Network.