Keiji Matsumoto

Quantum Merlin-Arthur Proof Systems: Are Multiple Merlins More Helpful to Arthur?

This paper introduces quantum “multiple-Merlin”-Arthur proof systems in which Arthur uses multiple quantum proofs unentangled with each other for his verification. Although classical multi-proof systems are obviously equivalent to classical single-proof systems, it is unclear whether quantum multi-proof systems collapse to quantum single-proof systems. This paper presents a necessary and sufficient condition under which the number of quantum proofs is reducible to two. It is also proved that using multiple quantum proofs does not increase the power of quantum Merlin-Arthur proof systems in the case of perfect soundness, and that there is a relativized world in which co-NP (actually co-UP) does not have quantum Merlin-Arthur proof systems even with multiple quantum proofs.

Quantum cloning machines for equatorial qubits

Quantum cloning machines for equatorial qubits are studied. For the case of a one to two phase-covariant quantum cloning machine, we present the networks consisting of quantum gates to realize the quantum cloning transformations. The copied equatorial qubits are shown to be separable by using Peres-Horodecki criterion. The optimal one to M phase-covariant quantum cloning transformations are given.

Oracularization and Two-Prover One-Round Interactive Proofs against Nonlocal Strategies

This paper presents three results on the power of two-prover one-round interactive proof systems based on oracularization under the existence of prior entanglement between dishonest provers. It is proved that the two-prover one-round interactive proof system for PSPACE by Cai, Condon, and Lipton [JCSS 48:183-193, 1994] still achieves exponentially small soundness error in the existence of prior entanglement between dishonest provers (and more strongly, even if dishonest provers are allowed to use arbitrary no-signaling strategies). It follows that, unless the polynomial-time hierarchy collapses to the second level, two-prover systems are still advantageous to single-prover systems even when only malicious provers can use quantum information. It is also shown that a dummy question may be helpful when constructing an entanglement-resistant multi-prover system via oracularization. This affirmatively settles a question posed by Kempe et al. [FOCS 2008, pp. 447-456] and every language in NEXP is proved to have a two-prover one-round interactive proof system even against entangled provers, albeit with exponentially small gap between completeness and soundness. In other words, it is NP-hard to approximate within an inverse-polynomial the value of a classical two-prover one-round game against entangled provers. Finally, both for the above proof system for NEXP and for the quantum two-prover one-round proof system for NEXP proposed by Kempe et al., it is proved that exponentially small completeness-soundness gaps are best achievable unless soundness analysis uses the structure of the underlying system with unentangled provers.

General-Purpose Parallel Simulator for Quantum Computing

With current technologies, it seems to be very difficult to implement quantum computers with many qubits. It is therefore of importance to simulate quantum algorithms and circuits on the existing computers. However, for a large-size problem, the simulation often requires more computational power than is available from sequential processing. Therefore, simulation methods for parallel processors are required.We have developed a general-purpose simulator for quantum algorithms/ circuits on the parallel computer (Sun Enterprise4500). It can simulate algorithms/circuits with up-to 30 qubits. In order to test efficiency of our proposed methods, we have simulated Shors factorization algorithm and Grovers database search, and we have analyzed robustness of the corresponding quantum circuits in the presence of both decoherence and operational errors. The corresponding results, statistics and analyses are presented in this paper.

Using Entanglement in Quantum Multi-prover Interactive Proofs

The central question in quantum multi-prover interactive proof systems is whether or not entanglement shared among provers affects the verification power of the proof system. We study for the first time positive aspects of prior entanglement and show how it can be used to parallelize any multi- prover quantum interactive proof system to a one-round system with perfect completeness, soundness bounded away from 1 by an inverse polynomial in the input size, and one extra proven Alternatively, we can also parallelize to a three-turn system with the same number of provers, where the verifier only broadcasts the outcome of a coin flip. This public-coin property is somewhat surprising, since in the classical case public-coin multi-prover interactive proofs are equivalent to single prover ones.

Entanglement cost of antisymmetric states and additivity of capacity of some quantum channels

We study the entanglement cost of the states in the antisymmetric space, which consists of (d − 1) d-dimensional systems. The cost is always log2(d − 1) ebits when the state is divided into bipartite . Combined with the arguments in [6], additivity of channel capacity of some quantum channels is also shown.

Asymptotic performance of optimal state estimation in qubit system

We derive an asymptotic bound for the error of state estimation when we are allowed to use the quantum correlation in the measuring apparatus. It is also proven that this bound can be achieved in any statistical model in the qubit system. Moreover, we show that this bound cannot be attained by any quantum measurement with no quantum correlation in the measuring apparatus except for several specific statistical models. That is, in such a statistical model, the quantum correlation can improve the accuracy of the estimation in an asymptotic setting.

Quantum cloning machines of a d-level system

Optimal N to M (M>N) quantum cloning machines for a d-level system are presented. Unitary cloning transformations achieve the bound of the fidelity.

Universal distortion-free entanglement concentration

We propose a new protocol of textit{universal} entanglement concentration, which converts many copies of an textit{unknown} pure state to an textit{% exact} maximally entangled state. The yield of the protocol, which is outputted as a classical information, is probabilistic, and achives the entropy rate with high probability, just as non-universal entanglement concentration protocols do. Our protocol is optimal among all similar protocols in terms of wide varieties of measures either up to higher orders or non-asymptotically, depending on the choice of the measure. The key of the proof of optimality is the following fact, which is a consequence of the symmetry-based construction of the protocol: For any invariant measures, optimal protocols are found out in modifications of the protocol only in its classical output, or the claim on the product. We also observe that the classical part of the output of the protocol gives a natural estimate of the entropy of entanglement, and prove that that estimate achieves the better asymptotic performance than any other (potentially global) measurements.

A study of LOCC-detection of a maximally entangled state using hypothesis testing

We study how well we can answer the question Is the given quantum state equal to a certain maximally entangled state? using LOCC, in the context of hypothesis testing. Under several locality and invariance conditions, optimal tests will be derived for several special cases by using basic theory of group representations. Some optimal tests are realized by performing quantum teleportation and checking whether the state is teleported. We will also give a finite process for realizing some optimal tests. The performance of the tests will be numerically compared.