arXiv subjects

Toshihiko Sasaki

Publications and source records attributed to Toshihiko Sasaki.

33 records · Page 2Linked to original sources

arXiv2017

Rigorous calibration method for photon-number statistics

Characterization of photon statistics of a light source is one of the most basic tools in quantum optics. Although the outcome from existing methods is believed to be a good approximation when the measured light is sufficiently weak, there is no rigorous quantitative bounds on the degree of the approximation. As a result, they fail to fulfill the demand arising from emerging applications of quantum information such as quantum cryptography. Here, we propose a calibration method to produce rigorous bounds for a photon-number probability distribution by using a conventional Hanbury-Brown-Twiss setup with threshold photon detectors. We present a general framework to treat any number of detectors and non-uniformity of their efficiencies. The bounds are conveniently given as closed-form expressions of the observed coincidence rates and the detector efficiencies. We also show optimality of the bounds for light with a small mean photon number. As an application, we show that our calibration method can be used for the light source in a decoy-state quantum key distribution protocol. It replaces the a priori assumption on the distribution that has been commonly used, and achieves almost the same secure key rate when four detectors are used for the calibration.

quant-ph↗

arXiv2017

Information-theoretic security proof of differential-phase-shift quantum key distribution protocol based on complementarity

We show the information-theoretic security proof of the differential-phase-shift (DPS) quantum key distribution (QKD) protocol based on the complementarity approach [arXiv:0704.3661 (2007)]. Our security proof provides a slightly better key generation rate compared to the one derived in the previous security proof in [arXiv:1208.1995 (2012)] that is based on the Shor-Preskill approach [Phys. Rev. Lett. ${\bf 85}$, 441 (2000)]. This improvement is obtained because the complementarity approach can employ more detailed information on Alice's sending state in estimating the leaked information to an eavesdropper. Moreover, we remove the necessity of the numerical calculation that was needed in the previous analysis to estimate the leaked information. This leads to an advantage that our security proof enables us to evaluate the security of the DPS protocol with any block size. This paper highlights one of the fundamental differences between the Shor-Preskill and the complementarity approaches.

quant-ph↗

arXiv2017

A security proof of the round-robin differential phase shift quantum key distribution protocol based on the signal disturbance

The round-robin differential phase shift (RRDPS) quantum key distribution (QKD) protocol is a unique quantum key distribution protocol whose security has not been understood through an information-disturbance trade-off relation, and a sufficient amount of privacy amplification was given independently of signal disturbance. Here, we discuss the security of the RRDPS protocol in the asymptotic regime when a good estimate of the bit error rate is available as a measure of signal disturbance. The uniqueness of the RRDPS protocol shows up as a peculiar form of information-disturbance trade-off curve. When the length of a block of pulses used for encoding and the signal disturbance are both small, it provides a significantly better key rate than that from the original security proof. On the other hand, when the block length is large, the use of the signal disturbance makes little improvement in the key rate. Our analysis will bridge a gap between the RRDPS protocol and the conventional QKD protocols.

quant-ph↗

arXiv2017

Finite-key analysis for quantum key distribution with weak coherent pulses based on Bernoulli sampling

An essential step in quantum key distribution is the estimation of parameters related to the leaked amount of information, which is usually done by sampling of the communication data. When the data size is finite, the final key rate depends on how the estimation process handles statistical fluctuations. Many of the present security analyses are based on the method with simple random sampling, where hypergeometric distribution or its known bounds are used for the estimation. Here we propose a concise method based on Bernoulli sampling, which is related to binomial distribution. Our method is suitable for the BB84 protocol with weak coherent pulses, reducing the number of estimated parameters to achieve a higher key generation rate compared to the method with simple random sampling. We also applied the method to prove the security of the differential-quadrature-phase-shift (DQPS) protocol in the finite-key regime. The result indicates that, the advantage of the DQPS protocol over the phase-encoding BB84 protocol in terms of the key rate, which was previously confirmed in the asymptotic regime, persists in the finite-key regime.

quant-ph↗

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

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

A comparison of X-ray stress measurement methods \\based on the fundamental equation

Stress measurement methods using X-ray diffraction (XRD methods) are based on so-called fundamental equations. The fundamental equation is described in the coordinate system that best suites the measurement situation, and, thus, making a comparison between different XRD methods is not straightforward. However, by using the diffraction vector representation, the fundamental equations of different methods become identical. Furthermore, the differences between the various XRD methods are in the choice of diffraction vectors and the way of calculating the stress from the measured data. The stress calculation methods can also be unified using the general least-squares method, which is a common least-squares method of multivariate analysis. Thus, the only difference between these methods turns out to be in the choice of the set of diffraction vectors. In light of these ideas, we compare three commonly used XRD methods: the sin^2 psi method, the XRD^2 method, and the cos alpha method using the estimation of the measurement errors.

cond-mat.mtrl-sci↗

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↗

arXiv2013

Universal Separability and Entanglement in Identical Particle Systems

Entanglement is known to be a relative notion, defined with respect to the choice of physical observables to be measured (i.e., the measurement setup used). This implies that, in general, the same state can be both separable and entangled for different measurement setups, but this does not exclude the existence of states which are separable (or entangled) for all possible setups. We show that for systems of bosonic particles there indeed exist such universally separable states: they are i.i.d. pure states. In contrast, there is no such state for fermionic systems with a few exceptional cases. We also find that none of the fermionic and bosonic systems admits universally entangled states.

quant-ph↗

arXiv2013

Complex Probability Measure and Aharonov's Weak Value

We present a complex probability measure relevant for double (pairs of) states in quantum mechanics, as an extension of the standard probability measure for single states that underlies Born's statistical rule. When the double states are treated as the initial and final states of a quantum process, we find that Aharonov's weak value, which has acquired a renewed interest as a novel observable quantity inherent in the process, arises as an expectation value associated with the probability measure. Despite being complex, our measure admits the physical interpretation as mixed processes, i.e., an ensemble of processes superposed with classical probabilities.

quant-ph↗

arXiv2012

Outcome Independence of Entanglement in One-Way Computation

We show that the various intermediate states appearing in the process of one-way computation at a given step of measurement are all equivalent modulo local unitary transformations. This implies, in particular, that all those intermediate states share the same entanglement irrespective of the measurement outcomes, indicating that the process of one-way computation is essentially unique with respect to local quantum operations.

quant-ph↗

arXiv2010

Entanglement of Indistinguishable Particles

We present a general criterion for entanglement of N indistinguishable particles decomposed into arbitrary s subsystems based on the unambiguous measurability of correlation. Our argument provides a unified viewpoint on the entanglement of indistinguishable particles, which is still unsettled despite various proposals made mainly for the s = 2 case. Even though entanglement is defined only with reference to the measurement setup, we find that the so-called i.i.d. states form a special class of bosonic states which are universally separable.

quant-ph↗

arXiv2010

Separability of N-particle Fermionic States for Arbitrary Partitions

We present a criterion of separability for arbitrary s partitions of N-particle fermionic pure states. We show that, despite the superficial non-factorizability due to the antisymmetry required by the indistinguishability of the particles, the states which meet our criterion have factorizable correlations for a class of observables which are specified consistently with the states. The separable states and the associated class of observables share an orthogonal structure, whose non-uniqueness is found to be intrinsic to the multi-partite separability and leads to the non-transitivity in the factorizability in general. Our result generalizes the previous result obtained by Ghirardi et. al. [J. Stat. Phys. 108 (2002) 49] for the s = 2 and s = N case.

quant-ph↗

arXiv2009

Entanglement Measures for Intermediate Separability of Quantum States

We present a family of entanglement measures R_m which act as indicators for separability of n-qubit quantum states into m subsystems for arbitrary 2 \leq m \leq n. The measure R_m vanishes if the state is separable into m subsystems, and for m = n it gives the Meyer-Wallach measure while for m = 2 it reduces, in effect, to the one introduced recently by Love et al. The measures R_m are evaluated explicitly for the GHZ state and the W state (and its modifications, the W_k states) to show that these globally entangled states exhibit rather distinct behaviors under the measures, indicating the utility of the measures R_m for characterizing globally entangled states as well.

quant-ph↗

arXiv2008

Exchange Symmetry and Multipartite Entanglement

Entanglement of multipartite systems is studied based on exchange symmetry under the permutation group S_N. With the observation that symmetric property under the exchange of two constituent states and their separability are intimately linked, we show that anti-symmetric (fermionic) states are necessarily globally entangled, while symmetric (bosonic) states are either globally entangled or fully separable and possess essentially identical states in all the constituent systems. It is also shown that there cannot exist a fully separable state which is orthogonal to all symmetric states, and that full separability of states does not survive under total symmetrization unless the states are originally symmetric. Besides, anyonic states permitted under the braid group B_N should also be globally entangled. Our results reveal that exchange symmetry is actually sufficient for pure states to become globally entangled or fully separable.

quant-ph↗