arXiv subjects

Masato Koashi

Publications and source records attributed to Masato Koashi.

At least 55 records · Page 3Linked to original sources

arXiv2016

Experimental demonstration of robust entanglement distribution over reciprocal noisy channels assisted by a counter-propagating classical reference light

We experimentally demonstrate a proposal [Phys. Rev. A 87, 052325 (2013)] of a scheme for robust distribution of polarization entangled photon pairs over collective noisy channels having the reciprocity. Although the scheme employs the robustness of two qubit decoherence-free subspace, by utilizing the forward propagation of one half of the entangled photons and the backward propagation of a classical reference light, it achieves an entanglement-sharing rate proportional to the transmittance of the quantum channel for the signal photon. We experimentally observed the efficient sharing rate while keeping a highly entangled state after the transmission. We also show that the protection method is applicable to transmission of arbitrary polarization state of a single photon.

quant-ph↗

arXiv2016

Heralded single excitation of atomic ensemble via solid-state-based telecom photon detection

Telecom photonic quantum networks with matter quantum systems enable a rich variety of applications, such as a long distance quantum cryptography and one-way quantum computing. Preparation of a heralded single excitation (HSE) in an atomic ensemble by detecting a telecom wavelength photon having a correlation with the atomic excitation is an important step. Such a system has been demonstrated with a quantum frequency conversion (QFC) to telecom wavelength employing a Rb atomic cloud. However the limited wavelength selection prevents the next step. Here we for the first time demonstrate HSE with a solid-state-based QFC and a detector for a telecom wavelength that will have a great advantage of the utility of mature telecom technologies. We unambiguously show that the demonstrated HSE indicates a non-classical statistics by the direct measurement of the autocorrelation function.

quant-ph↗

arXiv2016

All-optical production of a superfluid Bose-Fermi mixture of $^6$Li and $^7$Li

We report the first all-optical production of a superfluid Bose-Fermi mixture with two spin states of $^6$Li (fermion) and one spin state of $^7$Li (boson) under the resonant magnetic field of the s-wave Feshbach resonance of the fermions. Fermions are cooled efficiently by evaporative cooling and they serve as coolant for bosons. As a result, a superfluid mixture can be achieved by using a simple experimental apparatus and procedures, as in the case of the all-optical production of a single Bose-Einstein condensate (BEC). We show that the all-optical method enables us to realize variety of ultracold Bose-Fermi mixtures.

cond-mat.quant-gas↗

arXiv2016

Quantum key distribution protocols with slow basis choice

Many quantum key distribution (QKD) protocols require random choice of measurement basis for each pulse or each train of pulses. In some QKD protocols, such as the Round-Robin Differential Phase Shift (RRDPS) QKD protocol, this requirement is a bit challenging as randomly choosing hundreds of settings for every, say, 100 pulses may be too fast with current technologies. In this paper, we solve this issue by proving the security of QKD protocols with slow basis choice without compromising the secret key rate. We also show that the random choice of the bases for the state preparation can be made slow if the signals do not leak any information on the basis. Examples of QKD protocols that our technique can apply include the RRDPS protocol and BB84-type protocols, and our technique relaxes demands for the implementation of QKD systems.

quant-ph↗

arXiv2016

Frequency-domain Hong-Ou-Mandel interference

Hong-Ou-Mandel (HOM) interference unveils a distinct behavior of identical particles which cannot be distinguished from each other. Especially for bosons, two separated identical particles passing through a beamsplitter always go together into one of the output ports, but that is not the case with other particles including fermions or classical ones. So far many elemental properties of quantum physics and information have been discovered through the concatenated HOM effects, which has been demonstrated in photons and recently in plasmons, atoms and phonons. However, all demonstrations in optical region employed two particles in different spatial modes. Here we first report the HOM interference between two photons in a single spatial mode with different frequencies (energies) by using a partial frequency conversion. The demonstrated frequency-domain interferometer allows us to replace spatial optical paths by optical frequency multiplexing, which opens up a distinct architecture of the quantum interferometry.

quant-ph↗

arXiv2016

Security of differential quadrature phase shift quantum key distribution

One of the simplest methods for implementing quantum key distribution over fiber-optic communication is the Bennett-Brassard 1984 protocol with phase encoding (PE-BB84 protocol), in which the sender uses phase modulation over double pulses from a laser and the receiver uses a passive delayed interferometer. Using essentially the same setup and by regarding a train of many pulses as a single block, one can carry out the so-called differential quadrature phase shift (DQPS) protocol, which is a variant of differential phase shift (DPS) protocols. Here we prove the security of the DQPS protocol based on an adaptation of proof techniques for the BB84 protocol, which inherits the advantages arising from the simplicity of the protocol, such as accommodating the use of threshold detectors and simple off-line calibration methods for the light source. We show that the secure key rate of the DQPS protocol in the proof is eight thirds as high as the rate of the PE-BB84 protocol.

quant-ph↗

arXiv2015

Experimental quantum key distribution without monitoring signal disturbance

Since the invention of Bennett-Brassard 1984 (BB84) protocol, many quantum key distribution (QKD) protocols have been proposed and some protocols are operated even in field environments. One of the striking features of QKD is that QKD protocols are provably secure unlike cryptography based on computational complexity assumptions. It has been believed that, to guarantee the security of QKD, Alice and Bob have to monitor the statistics of the measurement outcomes which are used to determine the amount of the privacy amplification to generate a key. Recently a new type of QKD protocol, called round robin differential phase shift (RRDPS) protocol, was proposed, and remarkably this protocol can generate a key without monitoring any statistics of the measurement outcomes. Here we report an experimental realization of the RRDPS protocol. We used a setup in which Bob randomly chooses one from four interferometers with different pulse delays so that he could implement phase difference measurements for all possible combinations with five-pulse time-bin states. Using the setup, we successfully distributed keys over 30 km of fiber, making this the first QKD experiment that does not rely on signal disturbance monitoring.

quant-ph↗

arXiv2015

Extracting an entangled photon pair from collectively decohered pairs at a telecommunication wavelength

We experimentally demonstrated entanglement extraction scheme by using photons at the telecommunication band for optical-fiber-based quantum communications. We generated two pairs of non-degenerate polarization entangled photons at 780~nm and 1551~nm by spontaneous parametric down-conversion and distributed the two photons at 1551~nm through a collective phase damping channel which gives the same amount of random phase shift on the two photons. Through local operation and classical communication, we extracted an entangled photon pair from two phase-disturbed photon pairs. An observed fidelity of the extracted photon pair to a maximally entangled photon pair was 0.73 $\pm$ 0.07 which clearly shows the recovery of entanglement.

quant-ph↗

arXiv2014

Optimal two-qubit tomography based on local and global measurements: Maximal robustness against errors as described by condition numbers

We present an error analysis of various tomographic protocols based on the linear inversion for the reconstruction of an unknown two-qubit state. We solve the problem of finding a tomographic protocol which is the most robust against errors in terms of the lowest value (i.e., equal to 1) of a condition number, as required by the Gastinel-Kahan theorem. In contrast, standard tomographic protocols, including those based on mutually unbiased bases, are nonoptimal for determining all 16 elements of an unknown two-qubit density matrix. Our method is based on the measurements of the 16 generalized Pauli operators, where twelve of them can be locally measured, and the other four require nonlocal Bell measurements. Our method corresponds to selectively measuring, one by one, all of the real and imaginary elements of an unknown two-qubit density matrix. We describe two experimentally feasible setups of this protocol for the optimal reconstruction of two photons in an unknown polarization state using conventional detectors and linear-optical elements. Moreover, we define the operators for the optimal reconstruction of the states of multiqubit or multilevel (qudit) systems.

quant-ph↗

arXiv2014

Generating a state $t$-design by diagonal quantum circuits

We investigate protocols for generating a state $t$-design by using a fixed separable initial state and a diagonal-unitary $t$-design in the computational basis, which is a $t$-design of an ensemble of diagonal unitary matrices with random phases as their eigenvalues. We first show that a diagonal-unitary $t$-design generates a $O(1/2^N)$-approximate state $t$-design, where $N$ is the number of qubits. We then discuss a way of improving the degree of approximation by exploiting non-diagonal gates after applying a diagonal-unitary $t$-design. We also show that it is necessary and sufficient to use $O(\log_2 t)$-qubit gates with random phases to generate a diagonal-unitary $t$-design by diagonal quantum circuits, and that each multi-qubit diagonal gate can be replaced by a sequence of multi-qubit controlled-phase-type gates with discrete-valued random phases. Finally, we analyze the number of gates for implementing a diagonal-unitary $t$-design by {\it non-diagonal} two- and one-qubit gates. Our results provide a concrete application of diagonal quantum circuits in quantum informational tasks.

quant-ph↗

arXiv2014

A low-noise frequency down-conversion to the telecommunication band for a quantum communication based on NV centers in diamond

We demonstrate a low-noise frequency down-conversion of photons at 637 nm to the telecommunication band at 1587 nm by the difference frequency generation in a periodically-poled lithium niobate. An internal conversion efficiency of the converter is estimated to be 0.44 at the maximum which is achieved by a pump power of 430 mW, whereas a rate of internal background photons caused by the strong cw pump laser is estimated to be 9 kHz/mW within a bandwidth of about 1 nm. By using the experimental values related to the intrinsic property of the converter, and using the intensity correlation and the average photon number of a 637-nm input light pulse, we derive the intensity correlation of a converted telecom light pulse. Then we discuss feasibility of a single-photon frequency conversion to the telecommunication band for a long-distance quantum communication based on NV centers in diamond.

quant-ph↗

arXiv2014

Universal gates for transforming multipartite entangled Dicke states

We determine the minimal number of qubits that it is necessary to have access to in order to transform Dicke states into other Dicke states. In general, the number of qubits in Dicke states cannot be increased via transformation gates by accessing only a single qubit, in direct contrast to other multipartite entangled states such as GHZ, W and cluster states. We construct a universal optimal gate which adds spin-up qubits or spin-down qubits to any Dicke state by minimal access. We also show the existence of a universal gate which transforms any size of Dicke state as long as it has access to at least the required number of qubits. Our results have important consequences for the generation of Dicke states in physical systems such as ion traps, all-optical setups and cavity-QED settings where they can be used for a variety of quantum information processing tasks.

quant-ph↗

arXiv2013

Observation of two output light pulses from a partial wavelength converter preserving phase of an input light at a single-photon level

We experimentally demonstrate that both a converted and an unconverted light pulses after wavelength conversion with various conversion efficiencies preserve phase information of an input light at a single-photon level. In our experiment, we converted temporally-separated two coherent light pulses with average photon numbers of $\sim$ 0.1 at 780 nm to light pulses at 1522 nm by using difference-frequency generation in a periodically-poled lithium niobate waveguide. We observed a single-photon interference between temporally-separated two modes for both the converted and the unconverted light pulses at various values of the conversion efficiency. We observed interference visibilities greater than 0.88 without suppressing the background noises for any value of the conversion efficiency the wavelength converter achieves. At a conversion efficiency of $\sim$ 0.5, the observed visibilities are 0.98 for the unconverted light and 0.99 for the converted light.

quant-ph↗

arXiv2013

Robustness of quantum communication based on a decoherence-free subspace using a counter-propagating weak coherent light pulse

We study distribution schemes for a polarization entangled photon pair based on a decoherence-free subspace over lossy quantum channels and propose an efficient scheme that is robust against not only collective phase noises but also general collective noises for two qubits. While the proposed scheme employs two photons to build the decoherence-free subspace, the success probability is proportional to the channel transmittance of a single photon with the aid of a counter-propagating weak coherent light pulse. The key ingredient in the scheme is found to be the reciprocity of the channel. The proposed scheme shares the rest of the properties with the previously proposed schemes, i.e., it can be realized by linear optical elements and it is robust against the fluctuations in the optical circuits used by the two parties.

quant-ph↗

arXiv2013

Non-classical two-photon interference between independent telecom light pulses converted by difference-frequency generation

We experimentally demonstrated the Hong-Ou-Mandel (HOM) interference between two photons after visible-to-telecommunication wavelength conversion. In the experiment, we prepared a heralded single photon by using spontaneous parametric down-conversion and the other photon from a weak laser source at 780 nm. We converted the wavelength of both photons to the telecommunication wavelength of 1522 nm by using difference-frequency generation in a periodically-poled lithium niobate, and then observed the HOM interference between the photons. The observed visibility is $0.76\pm 0.12$ which clearly shows the non-classical interference of the two photons. The high-visibility interference is an important step for fiber-based quantum communications of photons generated from visible photon emitters.

quant-ph↗

arXiv2013

Does "quantum nonlocality without entanglement" have quantum origin?

Quantum separable operations are defined as those that cannot produce entanglement from separable states, and it is known that they strictly surpass local operations and classical communication (LOCC) in a number of tasks, which is sometimes referred to as "quantum nonlocality without entanglement." Here we consider a task with such a gap regarding the trade-off between state discrimination and preservation of entanglement. We show that this task along with the gap has an analogue in a purely classical setup, indicating that the quantum properties are not essential in the existence of a nonzero gap between the separable operations and LOCC.

quant-ph↗

arXiv2012

Fundamental limit to Qubit Control with Coherent Field

The ultimate accuracy as regards controlling a qubit with a coherent field is studied in terms of degradation of the fidelity by employing a fully quantum mechanical treatment. While the fidelity error accompanied by pi/2 pulse control is shown to be inversely proportional to the average photon number in a way similar to that revealed by the Gea-Banacloche's results. Our results show that the error depends strongly on the initial state of the qubit. When the initial state of the qubit is in the ground state, the error is about 20 times smaller than that of the control started from the exited state, no matter how large N is. This dependency is explained in the context of an exact quantum mechanical description of the pulse area theorem. By using the result, the error accumulation tendency of successive pulse controls is found to be both non-linear and initial state-dependent.

quant-ph↗

arXiv2012

Unconditional security of coherent-state-based differential phase shift quantum key distribution protocol with block-wise phase randomization

We prove the unconditional security of coherent-state-based differential phase shift quantum key distribution protocol (DPSQKD) with block-wise phase randomization. Our proof is based on the conversion of DPSQKD to an equivalent entanglement-distillation protocol where the estimated phase error rate determines the amount of the privacy amplification. The generated final key has a contribution from events where the sender emits two or more photons, indicating the robustness of DPSQKD against photon-number-splitting attacks.

quant-ph↗