Published papers

Heralded single photons (HSPs) and entangled photon pairs (EPPs) via spontaneous parametric down-conversion are essential tools for the development of photonic quantum information technologies. In this paper, we report a novel ultra-high-rate nonclassical light source realized by developing 50 GHz-repetition-rate mode-locked pump pulses and multiplexed superconducting nanowire single-photon detectors. The presence of the single-photon state in the heralded photons with our setup was indicated by the second-order intensity correlation below 1/2 at the heralding rate over 20 Mcps. Even at the rate beyond 50 Mcps, the nonclassicality was still observed with the intensity correlation below unity. Moreover, our setup is also applicable to the polarization-EPP experiment, where we obtained the maximum coincidence rate of 1.6 Mcps with the fidelity of 0.881 ± (0.254 × 10 ) to the maximally entangled state. Our versatile source could be a promising tool to explore various large-scale quantum-photonic experiments with low success probability and heavy attenuation. −3

We demonstrate the first continuous variable quantum key distribution (CV-QKD) system with fully digital phase and polarization tracking, enabling the first wavelength division multiplexing experiment for CV-QKD. We transmit 194 × 0.5 GHz channels with 25 GHz channel spacing, demonstrating an aggregate estimated asymptotic secret key rate of 172.6 Mbit/s over 25 km.

Quantum key distribution (QKD) can offer communication with unconditional security and is a promising technology to protect next generation communication systems. For QKD to see commercial success, several key challenges have to be solved, such as integrating QKD signals into existing fiber optical networks. In this paper, we present experimental verification of QKD co-propagating with a large number of wavelength division multiplexing (WDM) coherent data channels. We show successful secret key generation over 24 h for a continuous-variable QKD channel jointly transmitted with 100 WDM channels of erbium doped fiber amplified polarization multiplexed 16-ary quadrature amplitude modulation signals amounting to a datarate of 18.3 Tbit/s. Compared to previous co-propagation results in the C-band, we demonstrate more than a factor of 10 increase in the number of WDM channels and more than 90 times higher classical bitrate, showing the co-propagation with Tbit/s data-carrying channels.

Broadband light sources play essential roles in diverse fields, such as high-capacity optical communications, optical coherence tomography, optical spectroscopy, and spectrograph calibration. Although a nonclassical state from spontaneous parametric down-conversion may serve as a quantum counterpart, its detection and characterization have been a challenging task. Here we demonstrate the direct detection of photon numbers of an ultrabroadband (110 nm FWHM) squeezed state in the telecom band centred at 1535 nm wavelength, using a superconducting transition-edge sensor. The observed photon-number distributions violate Klyshko's criterion for the nonclassicality. From the observed photon-number distribution, we evaluate the second- and third-order correlation functions, and characterize a multimode structure, which implies that several tens of orthonormal modes of squeezing exist in the single optical pulse. Our results and techniques open up a new possibility to generate and characterize frequency-multiplexed nonclassical light sources for quantum info-communications technology.

By employing a microstructured fiber of which zero-dispersion wavelength is 820 nm, significantly large photon-number squeezing of -4.6 dB was obtained using a 100-fs femtosecond laser pulse. The quantum nonlinear Schrödinger equation was solved to explain these experiments. © 2000 Optical Society of America.

We address the problem of unambiguous discrimination among oracle operators. The general theory of unambiguous discrimination among unitary operators is extended with this application in mind. We prove that entanglement with an ancilla cannot assist any discrimination strategy for commuting unitary operators. We also obtain a simple, practical test for the unambiguous distinguishability of an arbitrary set of unitary operators on a given system. Using this result, we prove that the unambiguous distinguishability criterion is the same for both standard and minimal oracle operators. We then show that, except in certain trivial cases, unambiguous discrimination among all standard oracle operators corresponding to integer functions with fixed domain and range is impossible. However, we find that it is possible to unambiguously discriminate among the Grover oracle operators corresponding to an arbitrarily large unsorted database. The unambiguous distinguishability of standard oracle operators corresponding to totally indistinguishable functions, which possess a strong form of classical indistinguishability, is analysed. We prove that these operators are not unambiguously distinguishable for any finite set of totally indistinguishable functions on a Boolean domain and with arbitrary fixed range. Sets of such functions on a larger domain can have unambiguously distinguishable standard oracle operators, and we provide a complete analysis of the simplest case, that of four functions. We also examine the possibility of unambiguous oracle operator discrimination with multiple parallel calls and investigate an intriguing unitary superoperator transformation between standard and entanglement-assisted minimal oracle operators. © 2007 IOP Publishing Ltd.

In this paper, we consider the generalized measurement where one particular quantum signal is unambiguously extracted from a set of non-commutative quantum signals and the other signals are filtered out. Simple expressions for the maximum detection probability and its POVM are derived. We applyl such unambiguous quantum state filtering to evaluation of the sensing of decoherence channels. The bounds of the precision limit for a given quantum state of probes and possible device implementations are discussed.

The measurement scenario of unambiguous quantum-state filtering where one particular signal is unambiguously extracted from a set of non-orthogonal signals is discussed. The maximum detection probability and its corresponding measurement operators are given by simple expressions although the formalism includes mixed state signals. As an application, the sensing of decoherence in quantum channels is considered.

In this paper, we consider a generalized measurement where one particular quantum signal is unambiguously extracted from a set of noncommutative quantum signals and the other signals are filtered out. Simple expressions for the maximum detection probability and its positive operator valued measure are derived. We apply such unambiguous quantum state filtering to evaluation of the sensing of decoherence channels. The bounds of the precision limit for a given quantum state of probes and possible device implementations are discussed. © 2003 The American Physical Society.

We consider quantum key distribution (QKD) and entanglement distribution using a single-sender multiple-receiver pure-loss bosonic broadcast channel. We determine the unconstrained capacity region for the distillation of bipartite entanglement and secret key between the sender and each receiver, whenever they are allowed arbitrary public classical communication. A practical implication of our result is that the capacity region demonstrated drastically improves upon rates achievable using a naive time-sharing strategy, which has been employed in previously demonstrated network QKD systems. We show a simple example of a broadcast QKD protocol overcoming the limit of the point-to-point strategy. Our result is thus an important step toward opening a new framework of network channel-based quantum communication technology.

Bosonic channels are important in practice as they form a simple model for free-space or fiber-optic communication. Here we consider a single-sender multiple-receiver pure-loss bosonic broadcast channel and determine the unconstrained capacity region for the distillation of bipartite entanglement and secret key between the sender and each receiver, whenever they are allowed arbitrary public classical communication. We show how the state merging protocol leads to achievable rates in this setting, giving an inner bound on the capacity region. We also determine an outer bound on the region and find that the outer bound matches the inner bound in the infinite-energy limit, thereby establishing the unconstrained capacity region for such channels. Our result could provide a useful benchmark for implementing a broadcasting of entanglement and secret key through such channels. An important open question relevant to practice is to determine the capacity region in both this setting and the single-sender single-receiver case when there is an energy constraint on the transmitter.

Quantum cryptographic technology (QCT) is expected to be a fundamental technology for realizing long-term information security even against as-yet-unknown future technologies. More advanced security could be achieved using QCT together with contemporary cryptographic technologies. To develop and spread the use of QCT, it is necessary to standardize devices, protocols, and security requirements and thus enable interoperability in a multi-vendor, multi-network, and multi-service environment. This report is a technical summary of QCT and related topics from the viewpoints of 1) consensual establishment of specifications and requirements of QCT for standardization and commercialization and 2) the promotion of research and design to realize New-Generation Quantum Cryptography.

Electromagnetic induced transparency (EIT) in condensed media, especially in a semiconductor, is attractive for nonlinear applications, because of its high density, compactness, and flexibility in artificial design. However, fast relaxation time and fermionic character of electrons obstructs the realization of EIT in semiconductor materials. We propose a resonant parametric conversion scheme which is applicable to the semiconductor quantum well (QW) microcavity (MC) exciton system. Because of its large oscillator strength, the semiconductor QW MC excitons show the large vacuum Rabi splitting which can be larger than the phase relaxation broadening of the media. It is found that when the coupling strength of the MC mode and the medium transition is larger than any other Rabi frequency and relaxation rate, the nonlinear susceptibility term becomes dominant compared to the linear susceptibility term.

We report a wavelength division multiplexed time-bin entangled photon pair source in telecom wavelength using a 10 μm radius Si ring resonator. This compact resonator has two add ports and two drop ports. By pumping one add port by a continuous laser, we demonstrate an efficient generation of two-wavelength division multiplexed time-bin entangled photon pairs in the telecom C-band, which come out of one drop port, and are then split into the signal and idler photons via a wavelength filter. The resonator structure enhances four-wave mixing for pair generation. Moreover, we demonstrate the double-port pumping where two counter propagating pump lights are injected to generate entanglement from the two drop ports simultaneously. We successfully observe the highly entangled outputs from both two drop ports. Surprisingly, the count rate at each drop port is even increased by twice that of the single-port pumping. Possible mechanisms of this observation are discussed. Our technique allows for the efficient use of the Si ring resonator and widens its functionality for variety of applications.

A wavelength conversion scheme based on a fiber Raman converter is proposed, in which an externally injected high power pump laser and the associated Stokes laser are used to assist the Raman conversion process of signal light coded with optical information. Because the Raman gain spectrum in fibers is extremely broad, a wavelength conversion device with wide-range tunability is feasible. We numerically demonstrate that wavelength conversion from 1.31 to 1.42 μm can be realized using a fiber Raman converter at up to 10 Gb/s with an efficiency of 18%. It is also demonstrated that wide range conversion from 1.31 to 1.55 μm for optical fiber communication is feasible at up to 5 Gb/s when the fiber Raman converters are cascaded twice.