Published papers

Diamond nanowires are fabricated on a bulk, single crystalline diamond near an edge of aluminum coating using inductively coupled plasma reactive ion etching. Two different density areas are simultaneously appeared where the dense area has 9 times higher density than that of the sparse area while keeping the size of nanowires almost uniform in these areas. The nanowire sizes realized in the dense (sparse) area are 858 ± 22nm (876 ± 25nm) in height and 126 ± 6 nm (124 ± 7 nm) in diameter, which is suitable for applications in optical quantum information processing.

We extended a scheme that evaluates delocalized bipartite single-photon entanglement to multipartite entanglement that reconstructs a partial density matrix by preparing weak local oscillators, which is simpler and more loss-tolerant than the conventional scheme. We experimentally demonstrated the utility of our scheme by reconstructing the density matrix of several 3-mode W-states, after confirming the mode overlap between measured photons and local oscillator photons by Hong-Ou-Mandel interference.

Hong-Ou-Mandel (HOM) interference between independent photon sources (HOMI-IPS) is the fundamental block for quantum information processing. All the previous HOMI-IPS experiments were carried out in time-domain, however, the spectral information during the interference was omitted. Here, we investigate the HOMI-IPS in spectral domain using the recently developed fast fiber spectrometer, and demonstrate the spectral distribution during the HOM interference between two heralded single-photon sources, and two thermal sources. This experiment not only can deepen our understanding of HOMI-IPS from the viewpoint of spectral domain, but also presents a tool to test the theoretical predictions of HOMI-IPS using spectrally engineered sources.

Hong-Ou-Mandel (HOM) interference between independent photon sources (HOMI-IPS) is the fundamental block for quantum information processing, such as quantum gate, Shor's algorithm, Boson sampling, etc. All the previous HOMI-IPS experiments were carried out in time-domain, however, the spectral information during the interference was lost, due to technical difficulties. Here, we investigate the HOMI-IPS in spectral domain using the recently developed fast fiber spectrometer, and demonstrate the spectral distribution during the HOM interference between two heralded single-photon sources, and two thermal sources. This experiment can not only deepen our understanding of HOMI-IPS in the spectral domain, but also be utilized to improve the visibility by post-processing spectral filtering.

We define the squashed entanglement of a quantum channel as the maximum squashed entanglement that can be registered by a sender and receiver at the input and output of a quantum channel, respectively. A new subadditivity inequality for the original squashed entanglement measure of Christandl and Winter leads to the conclusion that the squashed entanglement of a quantum channel is an additive function of a tensor product of any two quantum channels. More importantly, this new subadditivity inequality, along with prior results of Christandl, Winter, et al., establishes the squashed entanglement of a quantum channel as an upper bound on the quantum communication capacity of any channel assisted by unlimited forward and backward classical communication. A similar proof establishes this quantity as an upper bound on the private capacity of a quantum channel assisted by unlimited forward and backward public classical communication. This latter result is relevant as a limitation on rates achievable in quantum key distribution. As an important application, we determine that these capacities can never exceed log((1 + η)=(1 - η)) for a pure-loss bosonic channel for which a fraction η of the input photons make it to the output on average. The best known lower bound on these capacities is equal to log(1=(1 - η)). Thus, in the high-loss regime for which η ≪ 1, this new upper bound demonstrates that the protocols corresponding to the above lower bound are nearly optimal. © 2014 IEEE.

We demonstrate a sub-shot-noise-limit discrimination of on-off keyed coherent signals by an optimal displacement quantum receiver in which a superconducting transition edge sensor is installed. Use of a transition edge sensor and a fiber beam splitter realizes high total detection efficiency and high interference visibility of the receiver and the observed average error surpasses the shot-noise-limit in a wider range of the signal power. Our technique opens up a new technology for the sub-shot-noiselimit detection of coherent signals in optical communication channels. © 2010 Optical Society of America.

Suppression of chaotic instability arising in a single-mode Nd:YVO4 microchip laser subject to frequency-shifted optical feedback is numerically demonstrated by sinusoidally modulating the population inversion. Many modes of stable periodic oscillation can be obtained from one chaotic oscillation simply by selecting appropriate modulation parameters.

We study non-Gaussian states generated by two-photon subtraction from a continuous-wave (cw) squeezed light source. In a cw scheme, one can subtract two photons from the source with a designated time separation and can generate temporally multiplexed superposition states of continuous variables. We numerically study the properties of these states in light of bosonic interference in the time domain. In an appropriate temporal mode, an amplified superposition state of continuous variable is produced in a region where the time separation is comparable with the coherence time of squeezed packets. © 2008 The American Physical Society.

Summary form only given. Quantum entanglement and other related effects with bright squeezed fields have attracted a great interest since it does not need the few quanta system. We propose the novel adaptive system for optimization of the initial pulse waveform to obtain desired intermode quantum structures, and numerically evaluated its feasibilities. We show the experimental setup for the proposed system. The (classically) shaped pulses are propagated through a nonlinear fiber and the quantum state information of these pulses is detected after spectral filtering. The detected information is fed back to the pulse shaper with an optimization algorithm, and thus we can find out the optimum initial waveform. for obtaining the desired quantum structure.

This paper defines the squashed entanglement of a quantum channel as the maximum squashed entanglement that can be registered by a sender and receiver at the input and output of a quantum channel, respectively. A new subadditivity inequality for the original squashed entanglement measure of Christandl and Winter leads to the conclusion that the squashed entanglement of a quantum channel is an additive function of a tensor product of any two quantum channels. More importantly, this new subadditivity inequality, along with prior results of Christandl and Winter, establishes the squashed entanglement of a quantum channel as an upper bound on the quantum communication capacity of any channel assisted by unlimited forward and backward classical communication. A similar proof establishes this quantity as an upper bound on the private capacity of a quantum channel assisted by unlimited forward and backward public classical communication. This latter result is relevant as a limitation on rates achievable in quantum key distribution. As an important application, we determine that these capacities can never exceed log((1+η)/(1-η)) for a pure-loss bosonic channel for which a fraction η of the input photons make it to the output on average. The best known lower bound on these capacities is equal to log(1/(1-η)). Thus, in the high-loss regime for which η ≪ 1, this new upper bound demonstrates that the protocols corresponding to the above lower bound are nearly optimal. © 2014 IEEE.

A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones. © 2011 OSA.

A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones. © OSA/ CLEO 2011.

A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones. © OSA/CLEO 2011.

A novel secure communication network with quantum key distribution in a metropolitan area is reported. Different QKD schemes are integrated to demonstrate secure TV conferencing over a distance 45km, stable long-term operation, and application to secure mobile phones. © OSA/ CLEO 2011.

Device-independent quantum key distribution (DIQKD) guarantees unconditional security of secret key without making assumptions about the internal workings of the devices used. It does so using the loophole-free violation of a Bell's inequality. The primary challenge in realizing DIQKD in practice is the detection loophole problem that is inherent to photonic tests of Bell's inequalities over lossy channels. We revisit the proposal of Curty and Moroder [Phys. Rev. A 84, 010304(R) (2011)] to use a linear optics-based entanglement-swapping relay (ESR) to counter this problem. We consider realistic models for the entanglement sources and photodetectors; more precisely, (a) polarization-entangled states based on pulsed spontaneous parametric downconversion (SPDC) sources with infinitely higher order multi-photon components and multimode spectral structure, and (b) on-off photodetectors with non-unit efficiencies and non-zero dark count probabilities. We show that the ESR-based scheme is robust against the above imperfections and enables positive key rates at distances much larger than what is possible otherwise.

One of the peculiar features in quantum mechanics is that a superposition of macroscopically distinct states can exist. In optical system, this is highlighted by a superposition of coherent states (SCS), i.e. a superposition of classical states. Recently this highly nontrivial quantum state and its variant have been demonstrated experimentally. Here we demonstrate the superposition of coherent states in quantum measurement which is also a key concept in quantum mechanics. More precisely, we propose and implement a projection measurement onto an arbitrary superposition of two weak coherent states in optical system. The measurement operators are reconstructed experimentally by a novel quantum detector tomography protocol. Our device is realized by combining the displacement operation and photon counting, well established technologies, and thus has implications in various optical quantum information processing applications.