Published papers

The presented probabilistic scheme for discrimination of optical coherent states consists of an optimized displacement followed by postselection of a photon number resolving measurement. The scheme outperforms the homodyne receiver in theory and experiment. © 2010 Optical Society of America.

The presented probabilistic scheme for discrimination of optical coherent states consists of an optimized displacement followed by postselection of a photon number resolving measurement. The scheme outperforms the homodyne receiver in theory and experiment. © 2010 Optical Society of America.

The quantum receiver is an important tool for overcoming the standard quantum limit (SQL) of discrimination errors in optical communication. We theoretically study the quantum receivers for discriminating ternary and quaternary phase-shift-keying coherent states in terms of average error rate and mutual information. Our receiver consists of on-off-type photon detectors and displacement operations without electrical feedforward operations. We show that for the ternary signals, the receiver shows a reasonable gain from the SQL even without feedforward. This scheme is realizable with the currently available technology. For the quaternary signals feedforward operation is crucial to overcome the SQL with imperfect devices. We also analytically examine the asymptotic limit of the performance of the proposed receiver with respect to the number of feedforward steps. © 2012 American Physical Society.

A novel, wide-range, high-speed, and tunable wavelength conversion scheme, '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. In order to get a large frequency difference between two carrier frequencies, this fiber Raman process is cascaded twice. However, since the common external pump laser can be used in two cascaded Raman processes as long as phase- matching conditions are attained, the entire configuration is still simple. We numerically demonstrate that wide-range wavelength conversion from 1.31 micrometer to 1.55 micrometer for optical fiber communication is feasible at up to 5 Gbit/s. ©2004 Copyright SPIE - The International Society for Optical Engineering.

We study the measurement-induced non-Gaussian operation on the Gaussian squeezed vacuum state with beam splitters and on-off detectors, and apply it to the dense coding scheme as an operational entanglement measure.

We study the measurement-induced non-Gaussian operation on the single- and two-mode Gaussian squeezed vacuum states with beam splitters and on-off type photon detectors, with which mixed non-Gaussian states are generally obtained in the conditional process. It is known that the entanglement can be enhanced via this non-Gaussian operation on the two-mode squeezed vacuum state. We show that, in the range of practical squeezing parameters, the conditional outputs are still close to Gaussian states, but their second order variances of quantum fluctuations and correlations are effectively suppressed and enhanced, respectively. To investigate an operational meaning of these states, especially entangled states, we also evaluate the quantum dense coding scheme from the viewpoint of the mutual information, and we show that non-Gaussian entangled state can be advantageous compared with the original two-mode squeezed state. © 2005 The American Physical Society.

Efficient generation and detection of indistinguishable twin photons are at the core of quantum information and communications technology (Q-ICT). These photons are conventionally generated by spontaneous parametric down conversion (SPDC), which is a probabilistic process, and hence occurs at a limited rate, which restricts wider applications of Q-ICT. To increase the rate, one had to excite SPDC by higher pump power, while it inevitably produced more unwanted multi-photon components, harmfully degrading quantum interference visibility.Here we solve this problem by using recently developed 10 GHz repetition-rate-tunable comb laser, combined with a group-velocity-matched nonlinear crystal, and superconducting nanowire single photon detectors. They operate at telecom wavelengths more efficiently with less noises than conventional schemes, those typically operate at visible and near infrared wavelengths generated by a 76 MHz Ti Sapphire laser and detected by Si detectors. We could show high interference visibilities, which are free from the pump-power induced degradation. Our laser, nonlinear crystal, and detectors constitute a powerful tool box, which will pave a way to implementing quantum photonics circuits with variety of good and low-cost telecom components, and will eventually realize scalable Q-ICT in optical infra-structures.

We demonstrate a down-conversion-based twin photon source pumped by a 10 GHz repetition-rate-tunable comb laser at 1553 nm wavelength. We show high Hong-Ou-Mandel interference visibilities, which are free from the pump-power induced degradation.

We demonstrate a down-conversion-based twin photon source pumped by a 10 GHz repetition-rate-tunable comb laser at 1553 nm wavelength. We show high Hong- Ou-Mandel interference visibilities, which are free from the pump-power induced degradation. © 2014 Optical Society of America.

Efficient generation and detection of indistinguishable twin photons are at the core of quantum information and communications technology (Q-ICT). These photons are conventionally generated by spontaneous parametric down conversion (SPDC), which is a probabilistic process, and hence occurs at a limited rate, which restricts wider applications of Q-ICT. To increase the rate, one had to excite SPDC by higher pump power, while it inevitably produced more unwanted multi-photon components, harmfully degrading quantum interference visibility. Here we solve this problem by using recently developed 10 GHz repetition-rate-tunable comb laser, combined with a group-velocity-matched nonlinear crystal, and superconducting nanowire single photon detectors. They operate at telecom wavelengths more efficiently with less noises than conventional schemes, those typically operate at visible and near infrared wavelengths generated by a 76 MHz Ti Sapphire laser and detected by Si detectors. We could show high interference visibilities, which are free from the pump-power induced degradation. Our laser, nonlinear crystal, and detectors constitute a powerful tool box, which will pave a way to implementing quantum photonics circuits with variety of good and low-cost telecom components, and will eventually realize scalable Q-ICT in optical infra-structures.

Finding joinable tables in data lakes is key procedure in many applications such as data integration, data augmentation, data analysis, and data market. Traditional approaches that find equi-joinable tables are unable to deal with misspellings and different formats, nor do they capture any semantic joins. In this paper, we propose PEXESO, a framework for joinable table discovery in data lakes. We embed textual values as high-dimensional vectors and join columns under similarity predicates on high-dimensional vectors, hence to address the limitations of equi-join approaches and identify more meaningful results. To efficiently find joinable tables with similarity, we propose a block-and-verify method that utilizes pivot-based filtering. A partitioning technique is developed to cope with the case when the data lake is large and the index cannot fit in main memory. An experimental evaluation on real datasets shows that our solution identifies substantially more tables than equi-joins and outperforms other similarity-based options, and the join results are useful in data enrichment for machine learning tasks. The experiments also demonstrate the efficiency of the proposed method.

Nitrogen-Vacancy (NV) centers in diamond are promising solid-state quantum emitters that can be utilized for photonic quantum applications. Various diamond nanophotonic devices have been fabricated for efficient extraction of single photons emitted from NV centers to a single guided mode. However, for constructing scalable quantum networks, further efficient coupling of single photons to a guided mode of a single-mode fiber (SMF) is indispensable and a difficult challenge. Here, we propose a novel efficient hybrid system between an optical nanofiber and a cylindrical-structured diamond nanowire. The maximum coupling efficiency as high as 75% for the sum of both fiber ends is obtained by numerical simulations. The proposed hybrid system will provide a simple and efficient interface between solid-state quantum emitters and a SMF suitable for constructing scalable quantum networks.

We investigate the practical scheme of engineering a qubit of two optical continuous-variable states, based on squeezed vacuum and coherently displaced photon subtraction. A fully multimode model is developed to describe a continuous wave (cw) setting of the experiment, which has been successfully implemented to generate highly pure non-classical states. The model agrees well with the experimental results and the numerical results show that the intrinsic imperfection due to the cw nature of the source is relatively small. Our photon subtraction technique could also be useful for preparing optical Schrodinger's cat-like states in arbitrary superposition, which is a basic qubit resource for linear optics coherent state quantum computing. © 2011 Taylor & Francis.

Entanglement distillation is an essential protocol for long-distance quantum communications1, typically for extending the range of quantum key distribution. In the field of continuous variable quantum information processing, quantum as well as classical information is encoded in the light field quadratures, often in the form of Gaussian states. However, distillation from Gaussian input states has not yet been accomplished. It is made difficult by a prominent no-go theorem stating that no Gaussian operation can distill Gaussian states2-4. Here we demonstrate, for the first time, such distillation from Gaussian input states, realized by the implementation of non-Gaussian operations. By subtracting one or two photons, a large gain of entanglement was observed. For two photons, Gaussian-like entanglement was also improved. Other than quantum key distribution, this distilled entanglement can also be used for downstream applications such as high-fidelity quantum teleportation and a loophole-free Bell test6,7. © 2010 Macmillan Publishers Limited. All rights reserved.

We consider the problem of evaluating the entanglement of non-Gaussian mixed states generated by photon subtraction from entangled squeezed states. The entanglement measures we use are the negativity and the logarithmic negativity. These measures possess the unusual property of being computable with linear algebra packages even for high-dimensional quantum systems. We numerically evaluate these measures for the non-Gaussian mixed states which are generated by photon subtraction with on/off photon detectors. The results are compared with the behavior of certain operational measures, namely the teleportation fidelity and the mutual information in the dense coding scheme. It is found that all of these results are mutually consistent, in the sense that whenever the enhancement is seen in terms of the operational measures, the negativity and the logarithmic negativity are also enhanced. © 2006 The American Physical Society.

We show that the Fisher information associated with entanglement-assisted coding has a monotonic relationship with the logarithmic negativity, an important entanglement measure, for certain classes of continuous variable (CV) quantum states of practical significance. These are the two-mode squeezed states and the non-Gaussian states obtained from them by photon subtraction. This monotonic correspondence can be expressed analytically in the case of pure states. Numerical analysis shows that this relationship holds to a very good approximation even in the mixed state case of the photon-subtracted squeezed states. The Fisher information is evaluated by the CV Bell measurement in the limit of weak signal modulation. Our results suggest that the logarithmic negativity of certain sets of non-Gaussian mixed states can be experimentally accessed without homodyne tomography, leading to significant simplification of the experimental procedure.