Published papers

We study a different scheme for generating photon number correlation and squeezing for two copropagating pulses, a soliton and a trapped pulse, in an optical fiber. When the center wavelength of a trapped pulse is close to that of a soliton pulse, the two pulses interact with each other through the third-order optical nonlinear process and exchange photons between the two pulses. The soliton pulse exhibits photon number squeezing. When the center wavelengths of the two pulses are sufficiently separated and no photon-number exchange takes place, the strong negative correlation in the photon number between the parts of the trapped pulse and the soliton pulse is formed via cross-phase modulation. By measuring the photon number of the negatively correlated part of the trapped pulse, we can obtain the photon number of the soliton pulse with a variance less than the shot-noise limit. © 2007 The American Physical Society.

Quantum correlation in broadband spectra generated with a microstructure fiber is experimentally and theoretically studied. Synthesis of quantum correlation among the spectral modes in ultra-broadband pulses through manipulating nonlinear optics in fibers is discussed.

Quantum correlation in broadband spectra generated by ultrafast laser pulses propagated through a microstructure fiber is experimentally and theoretically studied. We experimentally obtained the highest squeezing of -4.6 dB with a certain band-pass filter. Quantum correlation among the spectral components is analyzed with QNLSE including Raman effect.

The last two decades has witnessed the emergence a new paradigm in information theory based on quantum theory. We review a fundamental element of quantum information theory, source coding, which entails the compression of quantum data. We also briefly outline a recent experiment that demonstrates this fundamental principle. © Springer-Verlag Berlin Heidelberg 2003.

We numerically evaluate the wavelength division multiplexing (WDM) data transmission of coherent phase-shift keying (PSK) and quadrature amplitude modulation (QAM) signals in optical fiber communication and deep-space communication channels with conventional homodyne-based (dyne-type) detections and various quantum detection strategies. We show the quantitative gap between these detection strategies and especially in the quantum-limited region where the quantum noise seriously limits the transmission rate. For an extremely weak signal input power, there is a crucial gap between the capacity limit and the transmission rates of the WDM system with dyne-type detections. We show that this gap is filled by applying a collective square root detection (SRD) only for each channel, not necessary for quantum collective decoding among WDM channels. © 2010 IEEE.

We numerically evaluate the transmission rates of a single fiber with the wavelength-division-multiplexing (WDM) transmission of coherent signals with conventional homodyne-based (dyne-type) detections and various quantum detection strategies. We reveal the quantitative gap between these detection strategies especially in the quantum-limited region where the quantum noise seriously limits the transmission rate. For an extremely weak signal input power, there is a crucial gap between the capacity limit and the transmission rates of the WDM system with dyne-type detections. We show that this gap is filled by applying a collective square root detection (SRD) only for each channel, not necessary for quantum collective decoding among WDM channels. © 2010 Optical Society of America.

We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.

The locking effect is a phenomenon that is unique to quantum information theory and represents one of the strongest separations between the classical and quantum theories of information. The Fawzi-Hayden- Sen locking protocol harnesses this effect in a cryptographic context, whereby one party can encode n bits into n qubits while using only a constant-size secret key. The encoded message is then secure against any measurement that an eavesdropper could perform in an attempt to recover the message, but the protocol does not necessarily meet the composability requirements needed in quantum key distribution applications. In any case, the locking effect represents an extreme violation of Shannon's classical theorem, which states that information-theoretic security holds in the classical case if and only if the secret key is the same size as the message. Given this intriguing phenomenon, it is of practical interest to study the effect in the presence of noise, which can occur in the systems of both the legitimate receiver and the eavesdropper. This paper formally defines the locking capacity of a quantum channel as the maximum amount of locked information that can be reliably transmitted to a legitimate receiver by exploiting many independent uses of a quantum channel and an amount of secret key sublinear in the number of channel uses. We provide general operational bounds on the locking capacity in terms of other well-known capacities from quantum Shannon theory.We also study the important case of bosonic channels, finding limitations on these channels' locking capacity when coherent-state encodings are employed and particular locking protocols for these channels that might be physically implementable.

Current information and communications technology was designed based on physical laws established in the 19th century, and issues regarding transmission capacity and security of encryptions are prompting concern that we will reach the limits of these laws in the future. To overcome such limitations, NICT is conducting R&D on quantum information and communication technology, new technology based on quantum mechanics, the ultimate physical laws, and applications of this technology. We give an overview of this work below.

Quantum network sensing shows potential to enhance the estimation precision for functions of spatially distributed parameters beyond the shot noise limit. The key resource required for this task is possibly multi-partite quantum entanglement. The photonic entanglement is the most natural for this task; however, distributing it over long distances presents significant difficulties, mainly because of unavoidable loss in communication channels. In this research, we analyze a quantum network sensing protocol based on a recently proposed, efficient GHZ state distribution scheme. In comparison to conventional methods based on entanglement distribution , our protocol shows the decreasing loss-induced estimation error of certain functions of distributed parameters including their arbitrary linear combinations.

We present practical GHz-clocked QKD systems, next generation entanglement QKD technologies, and QKD platform to manage the secure keys and to support a variety of applications. We then show the intrinsic limit of QKD, i.e., a key rate bound, and discuss how to realize the provable (information theoretic) security with a larger secrecy capacity over longer distances. In particular, we present a basic theory of physical layer cryptography, which characterizes the secrecy capacity, and engineers the tradeoff between the efficiency of reliable transmission and secrecy of communication. We introduce a concept to unify these schemes in photonic network, referred to as quantum photonic network. Future issues for realizing this new network paradigm are discussed.

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. © 2011 American Physical Society.

We propose quantum receivers with optical squeezing and photon-number-resolving detectors (PNRDs) for the near-optimal discrimination of quaternary phase-shift-keyed coherent state signals. The basic scheme is similar to the previous proposals [e.g., Izumi, Phys. Rev. APLRAAN1050-294710. 1103/PhysRevA.86.042328 86, 042328 (2012)] in which displacement operations, on-off detectors, and electrical feedforward operations were used. Here we study two types of receivers, one of which installs optical squeezings and the other uses PNRDs instead of on-off detectors. We show that both receivers can attain lower error rates than that in the previous scheme. In particular, we show the PNRD-based receiver has a significant gain when the ratio between the mean photon number of the signal and the number of the feedforward steps is relatively high, in other words, when the probability of detecting two or more photons at each detector is not negligible. Moreover, we show that the PNRD-based receiver can suppress the errors due to dark counts, which the receiver with the on-off detector cannot do with a small number of feedforwards. © 2013 American Physical Society.