Published papers

Generalized measurements (POVMs) are important tools in quantum information. So far, there have been extensive analyses to derive optimal POVMs in various contexts. On the other hand, another emerging topic is the implementation of POVMs with restricted physical tools such as linear optics. Recently, general criteria were proposed to decide whether one can implement a given projection measurement via linear optics [1]. In this approach, implementing the projection operators {∏̂i|π i〉〈πi|} is regarded equivalent to the exact discrimination of the orthogonal states {|πi|〉}. Measurement process can be divided by discrete partial measurement steps and, to perform an exact discrimination, the conditional states after each measurement step must remain orthogonal. The formulae to check whether one can implement such a step via linear optics and photon counting was derived. In this paper, we investigate an expansion of this approach to the limit of continuous photon counting [2]. We discuss the limit that the coupling between the signal and measurement apparatus is infinitesimal at each step and the number of the step is infinitely large. As an example, we consider a specific case where the sates to be discriminated are single-mode states and any nonclassical ancillas are excluded. This scenario is important since if a perfect discrimination is possible in this setup, one can approximate it in practical discrete setup without any expensive ancillas. We show that, only in the continuous limit, one can implement the measurement attaining the minimum error discrimination of binary coherent states {|α〉,|-α〉}, and the projection of photon number qubit signals onto the superposition of zero- and one-photon states {(|0〉±|1〉)/√2} via linear optics without any nonclassical ancillas. The former scheme was firstly proposed by Dolinar (see references in [2]). We will also discuss the implementability of more general binary state discrimination via the same setup. © 2005 IEEE.

The photon subtraction operation is an effective method to enhance an entanglement of the two-mode squeezed vacuum state. We show that it can be well approximated to the two-mode squeezed state with an enhanced squeezing although it is basically a mixed and non-Gaussian state. Such property can be directly observed by the current technology. We also study the continuous variable quantum dense coding with the photon subtracted two-mode squeezed state. It is shown that, in particular squeezing region, its error probability performance can be greatly improved from the original two-mode squeezed state. Since quantum dense coding is the protocol to transmit classical information, our analysis provides an alternative operational meaning of the entanglement involved in the on-off photon subtracted state, compared to the previous results, such as teleportation or the nonlocality, that could help to quantify the entanglement of mixed and non-Gaussian states.

We theoretically and experimentally investigate the optimal conditions for the Bell experiment using spontaneous parametric down-conversion (SPDC) sources. In theory, we show that a relatively large average photon number (typically ∼0.5) is desirable to observe the maximum violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality. Moreover, we show that the violation converges to 0.27 in the limit of large average photon number. In experiment, we perform the Bell experiment without postselection using polarization-entangled photon pairs at 1550-nm telecommunication wavelength generated from SPDC sources. While the violation of the CHSH inequality is not directly observed due to the overall detection efficiencies of our system, the experimental values agree well with those obtained by the theory with experimental imperfections. Furthermore, in the range of small average photon numbers (≤0.1), we propose and demonstrate a method to estimate the ideal CHSH value intrinsically contained in the tested state from the lossy experimental data without assuming the input quantum state.

Modelocked thermal frequency combs (MTCs) are generated by employing spectrally narrowed amplified spontaneous emission (ASE) seeded into an electro-optic frequency comb generator. The MTC emits 2-ps duration ultrashort pulses at a repetition rate of 10 GHz. Autocorrelation of the MTC pulses confirms a reduced coherence time, (Formula presented.) ps, aligning with the narrowed bandwidth of the ASE seed. Intensity correlations of optically gated MTC pulses at a repetition rate of 250 MHz reveal nearly ideal thermal photon statistics with an experimental (Formula presented.), yielding an intrinsic (Formula presented.) after background noise removal. As a practical application, second harmonic generation (SHG) is performed utilizing the optically gated MTC pulses as a pump and experimental intensity correlations, (Formula presented.), are examined for the SH photons. An entire transition in (Formula presented.), continuously changing from six to two by increasing the pump strength, agrees with the single-mode analytical model. Furthermore, time-resolved pulse height correlations allow to simultaneously acquire power variations in SHG and third harmonic generation against the pump. With the maximum peak intensity, (Formula presented.), realized in a periodically poled (Formula presented.) waveguide for SHG, the demonstration highlights the potential for various applications in chaotic quantum optics experiments that necessitate ultrashort, high-intensity, single-spatiotemporal-mode thermal pulses.

Suprecontinuum (SC) light contains complex spectral noise structure and its accurate characterization is important for fundamental understanding of its physics as well as for its applications. Several experimental and theoretical noise characterizations have been performed so far. However, none of them takes into account the quantum mechanical properties. Here, we demonstrate experimental characterisation of quantum noise and its spectral correlations formed in the SC light generated from a photonic crystal fiber. Moreover, by applying an appropriate basis transformation to these correlations, we demonstrate that the SC noise amplitude can be squeezed below the shot-noise limit in some bases, even in the presence of excessively large nonlinearities.

Distribution and distillation of entanglement over quantum networks is a basic task for Quantum Internet applications. A fundamental question is then to determine the ultimate performance of entanglement distribution over a given network. Although this question has been extensively explored for bipartite entanglement-distribution scenarios, less is known about multipartite entanglement distribution. Here we establish the fundamental limit of distributing multipartite entanglement, in the form of GHZ states, over a quantum network. In particular, we determine the multipartite entanglement distribution capacity of a quantum network, in which the nodes are connected through lossy bosonic quantum channels. This setting corresponds to a practical quantum network consisting of optical links. The result is also applicable to the distribution of multipartite secret key, known as common key, for both a fully quantum network and trusted-node based quantum key distribution network. Our results set a general benchmark for designing a network topology and network quantum repeaters (or key relay in trusted nodes) to realize efficient GHZ state/common key distribution in both fully quantum and trusted-node-based networks. We show an example of how to overcome this limit by introducing a network quantum repeater. Our result follows from an upper bound on distillable GHZ entanglement introduced here, called the "recursive-cut-and-merge" bound, which constitutes major progress on a longstanding fundamental problem in multipartite entanglement theory. This bound allows for determining the distillable GHZ entanglement for a class of states consisting of products of bipartite pure states.

We propose practical near-optimal quantum receivers for the binary phase shift keyed coherent signals to overcome the homodyne limit in all areas of the signal power. Our schemes consist of Gaussian operations and photon detection and do not reqiure realtime electrical feedback. We numerically show that the scheme is feasible with the current technology. © 2009 American Institute of Physics.

We experimentally demonstrate a new scheme for the discrimination of binary-encoded optical coherent states. We show that the average error probability of our approach beats the ones obtained by standard receiver schemes for any signal amplitude. © 2008 Optical Society of America.

We study various non-Gaussian states generated by photon subtrastion from a squeezed light source. The source is a cw beam generated by optical parametric oscillator. The photon subtraction is made by tapping a small fraction of the squeezed light source and by guiding it into two Si-APDs, which enable the subtraction of one to two photons. Trigger photon clicks specify a certain temporally localized mode in the remaining squeezed beam. By filtering the remaining squeezed beam through an appropriate mode function, one can generate a variety of non-Gaussian states. This includes single and two photon states, the NOON state (N = 2), Schrödinger kitten states of both odd and even parities, and their arbitrarily desired superposition.

We present the latest results on two kinds of photon detectors: single photon detectors (SPDs) and photon number resolving detector (PNRD). We developed high speed and low noise SPDs using superconducting nano-wire (abbreviated by SNSPD) and semiconductor (InGaAs) avalanche photodiode (APD). The SNSPD system has totally four channels all of which have the detection eciency higher than 16% at 100Hz dark count rate. The InGaAs APD system also has four channels and the best performance is represented by the after-pulse probability of 0.61%, the dark count probability of 0.71×10-6 (∼1kHz), and the detection eciency of 10.9%. Both systems were applied to wavelength division multiplexing quantum key distribution (WDM-QKD) operated at 1.2GHz repetition rate in a eld environment. The PNRD is made of superconducting transition edge sensor. It was applied to the implementation of quantum receiver which could beat the homodyne limit of the bit error rate of binary coherent states. We discuss future perspective of quantum communications with those photon detection technologies, including multi-user QKD networks and low-power high capacity communications. © 2012 SPIE.

We numerically evaluate the deep-space communication performance in a broadband lossy channel of coherent pulse position modulation (PPM) with on/off receiver. We also consider quadrature amplitude modulation (QAM) signal and phase-shift keying (PSK) signal with dyne-type detections. We find that for an extremely weak signal input power, multilevel PPM system is good strategy, while for an extremely strong signal, multilevel QAM system is recommended. © 2011 IEEE.

We numerically evaluate the deep-space communication performance in a broadband lossy channel of coherent pulse position modulation (PPM) with an on/off receiver, single-symbol square root detection, and Holevo information. We also consider quadrature amplitude modulation (QAM) signals and phase-shift keying signals with dyne-type detections. We show the quantitative gap between these detection strategies in terms of the capacity, particularly in the quantum-limited region where the quantum noise seriously limits the transmission rate. In particular, we find that for an extremely weak signal input power, use of a multilevel PPM system is a good strategy, whereas for an extremely strong signal, use of a multilevel QAM system is recommended. © 2011 Optical Society of America.

In a new branch of quantum computing, information is encoded into coherent states, the primary carriers of optical communication. To exploit it, quantum bits of these coherent states are needed, but it is notoriously hard to make superpositions of such continuous-variable states. We have realized the complete engineering and characterization of a qubit of two optical continuous-variable states. Using squeezed vacuum as a resource and a special photon-subtraction technique, we could with high precision prepare an arbitrary superposition of squeezed vacuum and a squeezed single photon. This could lead the way to demonstrations of coherent state quantum computing. © 2010 The American Physical Society.

We consider the phase sensing via weak optical coherent state at quantum limit precision. A new detection scheme for the phase estimation is proposed which is inspired by the suboptimal quantum measurement in coherent optical communication. We theoretically analyze a performance of our detection scheme, which we call the displaced-photon counting, for phase sensing in terms of the Fisher information and show that the displaced-photon counting outperforms the static homodyne and heterodyne detections in wide range of the target phase. The proof-of-principle experiment is performed with linear optics and a superconducting nanowire single photon detector. The result shows that our scheme overcomes the limit of the ideal homodyne measurement even under practical imperfections.

We consider the phase sensing via a weak optical coherent state at quantum limit precision. A detection scheme for the phase estimation is proposed, which is inspired by the suboptimal quantum measurement in coherent optical communication. We theoretically analyze a performance of our detection scheme, which we call the displaced-photon counting, for phase sensing in terms of the Fisher information and show that the displaced-photon counting outperforms the static homodyne and heterodyne detections in a wide range of the target phase. The proof-of-principle experiment is performed with linear optics and a superconducting nanowire single-photon detector. The result shows that our scheme overcomes the limit of the ideal homodyne measurement, even under practical imperfections.