arXiv subjects

Keiji Matsumoto

Publications and source records attributed to Keiji Matsumoto.

At least 55 records · Page 3Linked to original sources

arXiv2012

When is an input state always better than the others ?: universally optimal input states for statistical inference of quantum channels

Statistical estimation and test of unknown channels have attracted interest of many researchers. In optimizing the process of inference, an important step is optimization of the input state, which in general do depend on the kind of inference (estimation or test, etc.), on the error measure, and so on. But sometimes, there is a universally optimal input state, or an input state best for all the statistical inferences and for all the risk functions. In the paper, the existence of a universally optimal state is shown for group covariant/contravariant channels, unital qubit channels and some measurement families. To prove these results, theory of "comparison of state families" are used. We also discuss about effectiveness of entanglement and adaptation of input states.

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arXiv2011

An asymptotic theory of cloning of classical state families

Cloning, or approximate cloning, is one of basic operations in quantum information processing. In this paper, we deal with cloning of classical states, or probability distribution in asymptotic setting. We study the quality of the approximate (n,rn)-clone, with n being very large and r being constant. The result turns out to be \parallel N(0,r1)-N(0,1)\paralell_1, where N(μ,Σ) is the Gaussian distribution with mean μ and covariance Σ. Notablly, this value does not depend on the the family of porbability distributions to be cloned. The key of the argument is use of local asymptotic normality: If the curve θ\rightarrow P_{θ} is sufficiently smooth in θ, then, the behavior of P_{θ'}^{\otimes n} where θ'-θ=o(\surd(1/n)), is approximated by Gaussian shift. Using this, we reduce the general case to Gaussian shift model.

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arXiv2010

Reverse Test and Characterization of Quantum Relative Entropy

The aim of the present paper is to give axiomatic characterization of quantum relative entropy utilizing resource conversion scenario. We consider two sets of axioms: non-asymptotic and asymptotic. In the former setting, we prove that the upperbound and the lowerbund of $\mathrm{D}^{Q}(ρ||σ) $ is $\mathrm{D}^{R}(ρ||σ) :=\mathrm{tr}% \,ρ\ln\sqrtρσ^{-1}\sqrtρ$ and $\mathrm{D}(ρ||σ) :=$ $\mathrm{tr}\,ρ(\lnρ-\lnσ) $, respectively. In the latter setting, we prove uniqueness of quantum relative entropy, that is, $\mathrm{D}^{Q}(ρ||σ) $ should equal a constant multiple of $\mathrm{D}(ρ||σ) $. In the analysis, we define and use reverse test and asymptotic reverse test, which are natural inverse of hypothesis test.

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arXiv2010

Test of Purity by LOCC

Given n-copies of unknown bipartite (possiblly mixed) state, our task is to test whether the state is a pure state of not. Allowed to use the global operations, optimal one-sided error test is the projection onto the symmetric subspace, obviously. Is it possible to approximate the globally optimal measurement by LOCC when n is large?

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arXiv2010

Reverse test and quantum analogue of classical fidelity and generalized fidelity

The aim of the present paper is to give axiomatic characterization of quantum relative entropy utilizing resource conversion scenario. We consider two sets of axioms: non-asymptotic and asymptotic. In the former setting, we prove that the upperbound and the lowerbund of D^{Q}(ρ||σ) is D^{R}(ρ||σ):=trρlnσ^{1/2}ρ^{-1}σ^{1/2} and D(ρ||σ):= trρ(lnρ-lnσ), respectively. In the latter setting, we prove uniqueness of quantum relative entropy, that is, D^{Q}(ρ||σ) should equal a constant multiple of D(ρ||σ). In the analysis, we define and use reverse test and asymptotic reverse test, which are natural inverse of hypothesis test.

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arXiv2010

On the monotone metric of classical channel and distribution spaces: asymptotic theory

The aim of the manuscript is to characterize monotone metric in the space of Markov map. Here, metric may not be Riemanian, or equivalently, may not be induced from an inner product. So far, there have been plenty of literatures on the metric in the space of probability distributions and quantum states. Among them, Cencov and Petz characterized all the monotone metrics in the classical and quantum state space. As for channels, however, only a little is known about its geometrical structures. In that author's previous manuscript, the upper and the lower bound of monotone channel metric was derived using resource conversion theory, and it is proved that any monotone metric cannot be Riemanian. . Due to the latter result, we cannot rely on Cencov's theory, to build a geometric theory consistent across probability distributions and channels. To dispense with the assumption that a metric is Riemanian, we introduce some assumptions on asymptotic behavior;weak asymptotic additivity and lower asymptotic continuity. The proof utilizes resource conversion technique. In the end of the paper, an implication on quantum state metrics is discussed.

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arXiv2010

On metric of quantum channel spaces

So far, there have been plenty of literatures on the metric in the space of probability distributions and quantum states. As for channels, however, only a little had been known. In this paper, we impose monotonicity by concatenation of channels before and after the given channel families, and invariance by tensoring identity channels. Under these axioms, we identify the largest and the smallest metrics. Also, we studied asymptotic theory of metric in parallel and adaptive repetition settings, and applied them to the study of channel estimation. First we express the achievable lower bound of the mean square error (MSE) of an estimate by a monotone channel metric, and show this equals O(1/n) for noisy channels, where n is the number of times of channel use. This result shows Heisenberg rate, or O(1/n^2)-rate of the MSE observed in case of estimation of unitary, collapses with very small arbitrary noise.

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arXiv2010

On the First Order Asymptotic Theory of Quantum Estimation

We give a rigorous treatment on the foundation of the first order asymptotic theory of quantum estimation, with tractable and reasonable regularity conditions. Different from past works, we do not use Fisher information nor MLE, and an optimal estimator is constructed based on locally unbiased estimators. Also, we treat state estimation by local operations and classical communications (LOCC), and estimation of quantum operations.

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arXiv2010

Thomae type formula for K3 surfaces given by double covers of the projective plane branching along six lines

In this paper, we give Thomae type formula for \KK surfaces $\cS$ given by double covers of the projective plane branching along six lines. This formula gives relations between theta constants on the bounded symmetric domain of type $I_{22}$ and period integrals of $X$. Moreover, we express the period integrals by using the hypergeometric function $F_S$ of four variables. As an application of our main theorem, we define $\R^4$-valued sequences by mean iterations of four terms, and express their common limits by the hypergeometric function $F_S$.

math.AG↗

arXiv2010

Degenerations of triple coverings and Thomae's formula

In this paper, we prove Thomae's formula for a triple covering of $\bold P^1$ with arbitrary index. This formula gives a relation between theta constants, determinants of period integrals and the difference products of branch points. To specify a symplectic basis of the curve, we use the combinatorics of binary trees on $\bold P^1$. This symplectic basis behaves so well for degenerations that we obtain the absolute constant in this formula and reduce it to a special case treated in [Bershadsky-Radul], [Nakayashiki].

math.AG↗

arXiv2010

On monotone `metrics' of the classical channel space:non-asymptotic theory

The aim of the manuscript is to characterize monotone `metric' in the space of Markov map. Here, `metric' means the square of the norm defined on the tangent space, and not necessarily induced from an inner product (this property hereafter will be called inner-product-assumption), different from usual metric used in differential geometry. As for metrics in So far, there have been plenty of literatures on the metric in the space of probability distributions and quantum states. Among them, Cencov proved the monotone metric in probability distribution space is unique up to constant multiple, and identical to Fisher information metric. Petz characterized all the monotone metrics in the quantum state space using operator mean. As for channels, however, only a little had been known. In this paper, we impose monotonicity by concatenation of channels before and after the given channel families, and invariance by tensoring identity channels. (Notably, we do not use the inner-product-assumption.) To obtain this result, `resource conversion' technique, which is widely used in quantum information, is used. We consider distillation from and formation to a family of channels. Under these axioms, we identify the largest and the smallest `metrics'. Interestingly, they are not induced from any inner product, i.e., not a metric. Indeed, one can prove that any `metric' satisfying our axioms can not be a metric. This result has some impact on the axiomatic study of the monotone metric in the space of classical and quantum states, since both conventional theory relies on the inner-product-assumption. Also, we compute the lower and the upper bound for some concrete examples.

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arXiv2010

Computing on Anonymous Quantum Network

This paper considers distributed computing on an anonymous quantum network, a network in which no party has a unique identifier and quantum communication and computation are available. It is proved that the leader election problem can exactly (i.e., without error in bounded time) be solved with at most the same complexity up to a constant factor as that of exactly computing symmetric functions (without intermediate measurements for a distributed and superposed input), if the number of parties is given to every party. A corollary of this result is a more efficient quantum leader election algorithm than existing ones: the new quantum algorithm runs in O(n) rounds with bit complexity O(mn^2), on an anonymous quantum network with n parties and m communication links. Another corollary is the first quantum algorithm that exactly computes any computable Boolean function with round complexity O(n) and with smaller bit complexity than that of existing classical algorithms in the worst case over all (computable) Boolean functions and network topologies. More generally, any n-qubit state can be shared with that complexity on an anonymous quantum network with n parties.

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arXiv2010

Arithmetic-geometric means for hyperelliptic curves and Calabi-Yau varieties

In this paper, we define a generalized arithmetic-geometric mean $μ_g$ among $2^g$ terms motivated by $2τ$-formulas of theta constants. By using Thomae's formula, we give two expressions of $μ_g$ when initial terms satisfy some conditions. One is given in terms of period integrals of a hyperelliptic curve $C$ of genus $g$. The other is by a period integral of a certain Calabi-Yau $g$-fold given as a double cover of the $g$-dimensional projective space $\mathbf{P}^g$.

math.AG↗

arXiv2008

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

A central problem in quantum computational complexity is how to prevent entanglement-assisted cheating in multi-prover interactive proof systems. It is well-known that the standard oracularization technique completely fails in some proof systems under the existence of prior entanglement. This paper studies two constructions of two-prover one-round interactive proof systems based on oracularization. First, it is proved that the two-prover one-round interactive proof system for PSPACE by Cai, Condon, and Lipton 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. Second, it is proved that the two-prover one-round interactive proof system obtained by oracularizing a three-query probabilistically checkable proof system becomes sound in a weak sense even against dishonest entangled provers with the help of a dummy question. As a consequence, every language in NEXP has a two-prover one-round interactive proof system of perfect completeness, albeit with exponentially small gap between completeness and soundness, in which each prover responds with only two bits. In other words, it is NP-hard to approximate within an inverse-polynomial the value of a classical two-prover one-round game, even when provers are entangled and each sends a two-bit answer to a verifier.

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arXiv2008

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

This paper introduces quantum ``multiple-Merlin''-Arthur proof systems in which Arthur receives multiple quantum proofs that are unentangled with each other. Although classical multi-proof systems are obviously equivalent to classical single-proof systems (i.e., usual Merlin-Arthur proof systems), it is unclear whether or not quantum multi-proof systems collapse to quantum single-proof systems (i.e., usual quantum Merlin-Arthur 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, in the case of perfect soundness, using multiple quantum proofs does not increase the power of quantum Merlin-Arthur proof systems.

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arXiv2007

Exact Quantum Algorithms for the Leader Election Problem

This paper gives the first separation of quantum and classical pure (i.e., non-cryptographic) computing abilities with no restriction on the amount of available computing resources, by considering the exact solvability of a celebrated unsolvable problem in classical distributed computing, the ``leader election problem'' on anonymous networks. The goal of the leader election problem is to elect a unique leader from among distributed parties. The paper considers this problem for anonymous networks, in which each party has the same identifier. It is well-known that no classical algorithm can solve exactly (i.e., in bounded time without error) the leader election problem in anonymous networks, even if it is given the number of parties. This paper gives two quantum algorithms that, given the number of parties, can exactly solve the problem for any network topology in polynomial rounds and polynomial communication/time complexity with respect to the number of parties, when the parties are connected by quantum communication links.

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arXiv2007

Using Entanglement in Quantum Multi-Prover Interactive Proofs

The central question in quantum multi-prover interactive proof systems is whether or not entanglement shared between provers affects the verification power of the proof system. We study for the first time positive aspects of prior entanglement and show that entanglement is useful even for honest provers. We show how to use shared entanglement to parallelize any multi-prover quantum interactive proof system to a one-round system with perfect completeness, with one extra prover. 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.

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arXiv2007

Entangled games are hard to approximate

We establish the first hardness results for the problem of computing the value of one-round games played by a verifier and a team of provers who can share quantum entanglement. In particular, we show that it is NP-hard to approximate within an inverse polynomial the value of a one-round game with (i) quantum verifier and two entangled provers or (ii) classical verifier and three entangled provers. Previously it was not even known if computing the value exactly is NP-hard. We also describe a mathematical conjecture, which, if true, would imply hardness of approximation to within a constant. We start our proof by describing two ways to modify classical multi-prover games to make them resistant to entangled provers. We then show that a strategy for the modified game that uses entanglement can be ``rounded'' to one that does not. The results then follow from classical inapproximability bounds. Our work implies that, unless P=NP, the values of entangled-prover games cannot be computed by semidefinite programs that are polynomial in the size of the verifier's system, a method that has been successful for more restricted quantum games.

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