Toyohiro Tsurumaru

Field test of quantum key distribution in the Tokyo QKD Network

A secure communication network with quantum key distribution in a metropolitan area is reported. Six different QKD systems are integrated into a mesh-type network. GHz-clocked QKD links enable us to demonstrate the world-first secure TV conferencing over a distance of 45km. The network includes a commercial QKD product for long-term stable operation, and application interface to secure mobile phones. Detection of an eavesdropper, rerouting into a secure path, and key relay via trusted nodes are demonstrated in this network.

Security proof for quantum-key-distribution systems with threshold detectors

In this paper, we rigorously prove the intuition that in security proofs for the Bennett-Brassard 1984 (BB84) protocol, one may regard an incoming signal to Bob as a qubit state. From this result, it follows that all security proofs for BB84 protocol based on a virtual qubit entanglement distillation protocol, which was originally proposed by Lo and Chau [Science 283, 2050 (1999)] and by Shor and Preskill [Phys. Rev. Lett. 85, 441 (2000)], are all valid even if Bobs actual apparatus cannot distill a qubit state explicitly. As a consequence, especially, the well-known result that a higher bit error rate of 20% can be tolerated for BB84 protocol by using two-way classical communications is still valid even when Bob uses threshold detectors. Using the same technique, we also prove the security of Bennett-Brassard-Mermin 1992 (BBM92) protocol where Alice and Bob both use threshold detectors.

Concise and tight security analysis of the Bennett-Brassard 1984 protocol with finite key lengths

We present a tight security analysis of the Bennett–Brassard 1984 protocol taking into account the finite-size effect of key distillation and achieving unconditional security. We begin by presenting a concise analysis utilizing the normal approximation of the hypergeometric function. Next we show that a similar tight bound can also be obtained by a rigorous argument without relying on any approximation. In particular, for the convenience of experimentalists who wish to evaluate the security of their quantum key distribution systems, we also give the explicit procedures of our key distillation and show how to calculate the secret key rate and the security parameter from a given set of experimental parameters. In addition to the exact values of key rates and security parameters, we also describe how to obtain their rough estimates using the normal approximation.

Quantum key distribution at 1550 nm using a pulse heralded single photon source

Quantum key distribution with pulsed heralded single photon source was performed over 40 km of fiber for the first time to our knowledge. QBER was measured to be 4.23% suggesting security against unconditional attack.

Dual Universality of Hash Functions and Its Applications to Quantum Cryptography

In this paper, we introduce the concept of dual universality of hash functions and present its applications to quantum cryptography. We begin by establishing the one-to-one correspondence between a linear function family F and a code family C, and thereby defining ε-almost dual universal2 hash functions, as a generalization of the conventional universal2 hash functions. Then, we show that this generalized (and thus broader) class of hash functions is in fact sufficient for the security of quantum cryptography. This result can be explained in two different formalisms. First, by noting its relation to the δ-biased family introduced by Dodis and Smith, we demonstrate that Renners two-universal hashing lemma is generalized to our class of hash functions. Next, we prove that the proof technique by Shor and Preskill can be applied to quantum key distribution (QKD) systems that use our generalized class of hash functions for privacy amplification. While Shor-Preskill formalism requires an implementer of a QKD system to explicitly construct a linear code of the Calderbank-Shor-Steane (CSS) type, this result removes the existing difficulty of the construction of a linear code of CSS code by replacing it by the combination of an ordinary classical error correcting code and our proposed hash function. We also show that a similar result applies to the quantum wire-tap channel. Finally, we compare our results in the two formalisms and show that, in typical QKD scenarios, the Shor-Preskill-type argument gives better security bounds in terms of the trace distance and Holevo information than the method based on the δ-biased family.

Exact minimum and maximum of yield with a finite number of decoy light intensities

In this paper, for the decoy state method using a finite number of decoy light intensities, we present improved upper and lower bounds for the asymptotic yield y{sub n} for n-photon states. In particular if all the light intensities are less than or equal to 1, they are not only a lower or upper bound, but in fact are the exact minimum or maximum.

Implementable quantum-bit-string commitment protocol

Quantum-bit-string commitment [A. Kent, Phys. Rev. Lett. 90, 237901 (2003)] (QBSC) is a variant of bit commitment (BC). In this paper, we propose a QBSC protocol that can be implemented using currently available technology and prove its security under the same security criteria as discussed by Kent. QBSC is a generalization of BC, but has slightly weaker requirements, and our proposed protocol is not intended to break the no-go theorem of quantum BC.

More efficient privacy amplification with less random seeds

We explicitly construct random hash functions for privacy amplification (extractors) that require smaller random seed lengths than the previous literature, and still allow efficient implementations with complexity O(n log n) for input length n. Firstly, we construct two types of hash functions by using the finite-filed. Then, concatenating them, we construct other two types of hash functions. We compare our hash functions with existing hash function in an asymptotic setting under a fixed key generation rate.

Field demonstration of quantum key distribution in the Tokyo QKD Network

A secure communication network with novel quantum key distribution systems in a metropolitan area (very center of Tokyo) is reported. Different QKD schemes are integrated to demonstrate key relay for secure TV conferencing over a distance 45 km, stable long-term operation.

High-Speed Search System for PGP Passphrases

We propose a high-speed passphrase-search system for PGP using FPGA for the purpose of evaluating PGPs passphrase-based security. In order to implement a high-speed search circuit on a single FPGA, we manage to surmount three major hurdles in PGP. The first one, multiprecision arithmetics which arises a problem of speed, is cleared by reducing the number of arithmetics needed. The second one, heavy iteration of hashing which also lowers the search speed, is settled by pipelining the hash function. The last one, candidate passphrase generation which cannot be implemented on hardware, is treated by combining a PC with the FPGA. We thereby achieve a throughput of 56 Gbps per FPGA that amounts to 1.1 ×105 passphrases per second. Compared with a fully software-based search, it shows 38 times faster the speed. We also propose to use an embedded FPGA system and to have part of software such as passphrase generation, to be run on a CPU inside the FPGA. We expect the search system to be more self-contained in an FPGA and thus to have a lower risk of data bus bottleneck between PCs and FPGAs especially in a massive parallel system where many FPGAs are connected to one PC.