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arXiv:quant-ph/0604174 · Quantum Information and Computation

Quantum measurements for hidden subgroup problems with optimal sample complexity

Masahito Hayashi, A. Kawachi, H. Kobayashi

Published Mar 1, 2008• 35 citations
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Abstract

One of the central issues in the hidden subgroup problem is to bound the sample complexity, i.e., the number of identical samples of coset states sufficient and necessary to solve the problem. In this paper, we present general bounds for the sample complexity of the identification and decision versions of the hidden subgroup problem. As a consequence of the bounds, we show that the sample complexity for both of the decision and identification versions is $\Theta(\log|\HH|/\log p)$ for a candidate set $\HH$ of hidden subgroups in the case \REVISE{where the candidate nontrivial subgroups} have the same prime order $p$, which implies that the decision version is at least as hard as the identification version in this case. In particular, it does so for the important \REVISE{cases} such as the dihedral and the symmetric hidden subgroup problems. Moreover, the upper bound of the identification is attained \REVISE{by a variant of the pretty good measurement}. \REVISE{This implies that the concept of the pretty good measurement is quite useful for identification of hidden subgroups over an arbitrary group with optimal sample complexity}.

DOI
10.26421/qic8.3-4-8

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•Jan 1, 2011•0 citations
One of the central issues in the hidden subgroup problem is to bound the sample complexity, i.e., the number of identical samples of coset states sufficient and necessary to solve the problem. In this paper, we present general bounds for the sample complexity of the identification and decision versions of the hidden subgroup problem. As a consequence of the bounds, we show that the sample complexity for both of the decision and identification versions is Θ(log⁡∣\HH∣/log⁡p)\Theta(\log|\HH|/\log p)Θ(log∣\HH∣/logp) for a candidate set \HH\HH\HH of hidden subgroups in the case \REVISE{where the candidate nontrivial subgroups} have the same prime order ppp, which implies that the decision version is at least as hard as the identification version in this case. In particular, it does so for the important \REVISE{cases} such as the dihedral and the symmetric hidden subgroup problems. Moreover, the upper bound of the identification is attained \REVISE{by a variant of the pretty good measurement}. \REVISE{This implies that the concept of the pretty good measurement is quite useful for identification of hidden subgroups over an arbitrary group with optimal sample complexity}.
We consider the dihedral hidden subgroup problem as the problem of distinguishing hidden subgroup states.We show that the optimal measurement for solving this problem is the so-called pretty good measurement.We then prove that the success probability of this measurement exhibits a sharp threshold as a function of the density ν = k/ log 2 N , where k is the number of copies of the hidden subgroup state and 2N is the order of the dihedral group.In particular, for ν < 1 the optimal measurement (and hence any measurement) identifies the hidden subgroup with a probability that is exponentially small in log N , while for ν > 1 the optimal measurement identifies the hidden subgroup with a probability of order unity.Thus the dihedral group provides an example of a group G for which Ω(log |G|) hidden subgroup states are necessary to solve the hidden subgroup problem.We also consider the optimal measurement for determining a single bit of the answer, and show that it exhibits the same threshold.Finally, we consider implementing the optimal measurement by a quantum circuit, and thereby establish further connections between the dihedral hidden subgroup problem and average case subset sum problems.In particular, we show that an efficient quantum algorithm for a restricted version of the optimal measurement would imply an efficient quantum algorithm for the subset sum problem, and conversely, that the ability to quantum sample from subset sum solutions allows one to implement the optimal measurement.
We approach the hidden subgroup problem by performing the so-called pretty good measurement on hidden subgroup states. For various groups that can be expressed as the semidirect product of an abelian group and a cyclic group, we show that the pretty good measurement is optimal and that its probability of success and unitary implementation are closely related to an average-case algebraic problem. By solving this problem, we find efficient quantum algorithms for a number of nonabelian hidden subgroup problems, including some for which no efficient algorithm was previously known: certain metacyclic groups as well as all groups of the form /spl Zopf//sub p/ /sup r/ /spl times/ /spl Zopf//sub p/ fixed r (including the Heisenberg group, r = 2). In particular our results show that entangled measurements across multiple copies of hidden subgroup states can be useful for efficiently solving the nonabelian HSP.
We study the computational complexity of the HIDDEN SUBGROUP problem, a well-studied problem in quantum computing. First we show that several proposed generalizations or variants of this problem, including HIDDEN COSET, HIDDEN SHIFT, and ORBIT COSET, are all equivalent or reducible to HIDDEN SUBGROUP. Then we study the relationship between the decision version and search version of HIDDEN SUBGROUP over various group classes. We show that the two versions are polynomial-time equivalent over permutation groups, and over dihedral groups given the order of the group is smooth. Finally, we give nonadaptive program checkers for HIDDEN SUBGROUP and its decision version.
In the quest to identify further quantum algorithms exhibiting superpolynomial speed-ups, a recurring theme is that the complexity of a problem is largely shaped by the input access model. Here, we study this phenomenon for the state hidden subgroup problem (StateHSP), a quantum generalization of the hidden subgroup problem in which the goal is to identify the symmetries of an unknown quantum state. For finite abelian groups, existing Fourier-sampling algorithms use O(log⁡(∣G∣)/ε)O(\log(|G|)/ε)O(log(∣G∣)/ε) copies of the state, but whether this scaling is optimal has remained open. We settle the complexity of the abelian StateHSP in both the previously studied sample model and a new query model, which is a stronger and operationally natural generalization that provides access to the state-preparation unitary and its inverse. In the query model, we give a time-efficient quantum algorithm using O(log⁡(∣G/H∣)/ε)O(\log(|G/H|)/\sqrtε)O(log(∣G/H∣)/ε​) forward and inverse queries, and prove a matching Ω(log⁡(∣G/H∣)/ε)Ω(\log(|G/H|)/\sqrtε)Ω(log(∣G/H∣)/ε​) lower bound which holds even in the stronger conjugate-query and controlled-query settings. By contrast, we show that in the sample model, Θ(log⁡(∣G/H∣)/ε)Θ(\log(|G/H|)/ε)Θ(log(∣G/H∣)/ε) copies are both sufficient and information-theoretically necessary, even if one allows for arbitrary collective measurements. Thus, the quadratic improvement in εεε genuinely arises from coherent access to the preparation circuit. As applications, we obtain faster algorithms for learning stabilizer groups, locating unentanglement, and identifying hidden translation symmetries.
This dissertation concerns the Hidden Subgroup Problem (HSP) in quantum computation. We explore certain facets of the problem which, we expect, will lead towards a general framework for understanding such problems. We first present an implementation of an optimal measurement, the Pretty Good Measurement (PGM), for the problem of the hidden subgroup problem over the dihedral group. This is a problem for which no efficient algorithm is known. This problem is relevant to the shortest vector in a lattice problem (SVP) in cryptography. We give a representation theoretic implementation of the PGM, achievable with only abelian transforms. The subset sum problem, an NP-complete problem, is needed in identification of certain subspaces. There are some novel ideas and techniques used in the implementation that make it realizable. This brings into focus how a measurement specified through a POVM (positive operator valued measure) is achievable through a basis change in a particular representation. This has connections to the HSP for other groups. We also study the question of optimal query selection to maximize the probability of subgroup identification. This part of the research complements a well-researched topic of finding optimal measurements in the spirit of state discrimination as applied to the hidden subgroup problem. We find the optimal query, the "character query", over a class of queries that are an equal superposition over the group, for oracles that take values in a finite abelian group. This query outperforms the query used in the "standard method" of the HSP. This approach has connections to other domains of quantum computation. We then extend the query selection idea to the case of multiple queries in the HSP and find examples in which this gives significant gains over the standard method. We consider the problems that the analysis of this more general problem entails, and outline a possible program to be followed. This is a part of continuing research