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数理解析研究所講究録

Quantum Oracles and Computational Complexity (Algebraic Systems, Formal Languages and Computations)

Harumichi Nishimura, Masanao Ozawa

Published Aug 1, 2000• 0 citations
OpenAlex

Abstract

In this paper, we consider the following problem (the subroutine problem).If the operation performed by an oracle is efficiently computable, can a non-oracle quantum computer simulate a quantum computer with this oracle efficiently ?Bennett et al. and Aharonov et al. respectively solved the subroutine problem when their oracles evaluate deterministic and probabilistic functions.However, both of them considered only the case where machines enter the query mode deterministically.We extend the notion of an oracle of a quantum computer to a device for performing a unitary transformation, and solve the subroutine problem in the general setting where a query state and a nonquery state may superpose.Using this result, we show that EQP and BQP are robust in the general setting, which extends the robustness of BQP shown by Bennett et al. in the case where machines enter the query mode deterministically.Moreover, we prove the robustness of ZQP by improving.themethod of Bennett et al. employed for the above BQP result.

Related papers

Quantum subroutine problem and the robustness of quantum complexity classes

Harumichi Nishimura, Masanao Ozawa

This paper positively solves the quantum subroutine problem for fully quantum oracles. The quantum subroutine problem asks whether a quantum computer with an efficiently computable oracle can be efficiently simulated by a non-oracle quantum computer. We extends the earlier results obtained by Bennett, Bernstein, Brassard, and Vazirani, and by Aharonov, Kitaev, and Nisan to the case where the oracle evaluates a unitary operator and the computer is allowed to be in the superposition of a query state and a non-query state during computation. We also prove the robustness of {\bf EQP}, {\bf BQP}, and {\bf ZQP} under the above general formulation, extending the earlier results on the robustness of {\bf BQP} shown by Bennett et al.

arXiv:quant-ph/0107089•Jul 17, 2001•0 citations

Quantum complexity theory

Ethan Bernstein, Umesh V. Vazirani

Abstract. In this paper we study quantum computation from a complexity theoretic viewpoint. Our rst result is the existence of an ecient universal quantum Turing machine in Deutsch’s model of a quantum Turing machine (QTM) [Proc. Roy. Soc. London Ser. A, 400 (1985), pp. 97{117]. This construction is substantially more complicated than the corresponding construction for classical Turing machines (TMs); in fact, even simple primitives such as looping, branching, and composition are not straightforward in the context of quantum Turing machines. We establish how these familiar primitives can be implemented and introduce some new, purely quantum mechanical primitives, such as changing the computational basis and carrying out an arbitrary unitary transformation of polynomially bounded dimension. We also consider the precision to which the transition amplitudes of a quantum Turing machine need to be specied. We prove that O(log T) bits of precision suce to support a T step computation. This justies the claim that the quantum Turing machine model should be regarded as a discrete model of computation and not an analog one. We give the rst formal evidence that quantum Turing machines violate the modern (complexity theoretic) formulation of the Church{Turing thesis. We show the existence of a problem, relative to an oracle, that can be solved in polynomial time on a quantum Turing machine, but requires superpolynomial time on a bounded-error probabilistic Turing machine, and thus not in the class BPP. The class BQP of languages that are eciently decidable (with small error-probability) on a quantum Turing machine satises BPP ⊆ BQP ⊆ P]P. Therefore, there is no possibility of giving a mathematical proof that quantum Turing machines are more powerful than classical probabilistic Turing machines (in the unrelativized setting) unless there is a major breakthrough in complexity theory.

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Quantum bounded query complexity

Harry Buhrman, Wim van Dam

We combine the classical notions and techniques for bounded query classes with those developed in quantum computing. We give strong evidence that quantum queries to an oracle in the class NP does indeed reduce the query, complexity of decision problems. Under traditional complexity assumptions, we obtain an exponential speedup between the quantum and the classical query complexity of function classes. For decision problems and function classes we obtain the following results: P/sub /spl par///sup NP[2k]//spl sube/EQP/sub /spl par///sup NP[k]/; P/sub /spl par///sup NP[2k+1-2]//spl sube/EQP/sup NP[k]/; FP/sub /spl par///sup NP[2k=1-2]//spl sube/FEQP/sup NP[2k]/; FP/sub /spl par///sup NP/spl sube/FEQP(NP[Olog n)]/. For sets A that are many-one complete for PSPACE or EXP we show that Fp/sup A//spl sube/FEQP/sup A[1]/. Sets A that are many-one complete for PP have the property that FP/sub /spl par///sup A//spl sube/FEQP/sup A[1]/. In general we prove that for any set A there is a set X such that FP/sup A//spl sube/FEQP/sup X[1]/, establishing that no set is superterse in the quantum setting.

arXiv:quant-ph/9903035•IEEE Conference on Computational Complexity (CCC)•Jan 20, 2003•8 citations

Complexity analysis and semantics for quantum computation

Elham Kashefi

This thesis focuses on two different aspects of quantum computation: quantum complexity and quantum semantics. In the first part, I study the quantum complexity mainly within the quantum query model. Together with my colleagues, I introduce a new framework for quantum query complexity, phrased in terms of the minimal oracle and analyse limits and strengths of this new model in comparison to their classical and quantum counterparts. Working within the query model I study quantum one-way functions. I show that in the quantum setting, the problem of the existence of a quantum one-way permutation can be reduced to the problem of constructing polynomial size networks for performing the specific task of the reflection about a sequence of states. Furthermore, I extend these results to the domain of the state and operator complexity. I show that if a quantum one-way function exists, then we can construct a sequence of so called “hard” states with the property that the reflection operators about those states are efficiently implementable. In the second part, I study the extension of domain theory to the quantum setting and develop the semantics of quantum computation. By defining a quantum domain I introduce a rigourous definition of quantum computability for quantum states and operators. Furthermore I show that the denotational semantics of quantum computation has the same semantical structure as the denotational semantics of classical probabilistic computation introduced by Kozen. This could be considered as a foundation for designing functional programming languages for quantum computation. Finally, I continue with an abstract mathematical approach to study a general formalism for describing entanglement manipulation and introduce a new approach to derive a unique measure of entanglement for bipartite quantum pure states.

•Jan 1, 2003•1 citation

Oracle separations of hybrid quantum-classical circuits

Atul Singh Arora, Alexandru Gheorghiu, Singh, Uttam

An important theoretical problem in the study of quantum computation, that is also practically relevant in the context of near-term quantum devices, is to understand the computational power of hybrid models, that combine poly-time classical computation with short-depth quantum computation. Here, we consider two such models: CQ_d which captures the scenario of a polynomial-time classical algorithm that queries a d-depth quantum computer many times; and QC_d which is more analogous to measurement-based quantum computation and captures the scenario of a d-depth quantum computer with the ability to change the sequence of gates being applied depending on measurement outcomes processed by a classical computation. Chia, Chung & Lai (STOC 2020) and Coudron & Menda (STOC 2020) showed that these models (with d=log^O(1) (n)) are strictly weaker than BQP (the class of problems solvable by poly-time quantum computation), relative to an oracle, disproving a conjecture of Jozsa in the relativised world. We show that, despite the similarities between CQ_d and QC_d, the two models are incomparable, i.e. CQ_d $\nsubseteq$ QC_d and QC_d $\nsubseteq$ CQ_d relative to an oracle. In other words, there exist problems that one model can solve but not the other and vice versa. We do this by considering new oracle problems that capture the distinctions between the two models and by introducing the notion of an intrinsically stochastic oracle, an oracle whose responses are inherently randomised, which is used for our second result. While we leave showing the second separation relative to a standard oracle as an open problem, we believe the notion of stochastic oracles could be of independent interest for studying complexity classes which have resisted separation in the standard oracle model. Our constructions also yield simpler oracle separations between the hybrid models and BQP, compared to earlier works.

arXiv:2201.01904•Jan 6, 2022•3 citations

Quantum Complexity Theory

Ethan Bernstein, Umesh V. Vazirani

In this paper we study quantum computation from a complexity theoretic viewpoint. Our first result is the existence of an efficient universal quantum Turing machine in Deutsch's model of a quantum Turing machine (QTM) [Proc. Roy. Soc. London Ser. A, 400 (1985), pp. 97--117]. This construction is substantially more complicated than the corresponding construction for classical Turing machines (TMs); in fact, even simple primitives such as looping, branching, and composition are not straightforward in the context of quantum Turing machines. We establish how these familiar primitives can be implemented and introduce some new, purely quantum mechanical primitives, such as changing the computational basis and carrying out an arbitrary unitary transformation of polynomially bounded dimension. We also consider the precision to which the transition amplitudes of a quantum Turing machine need to be specified. We prove that $O(\log T)$ bits of precision suffice to support a T step computation. This justifies the claim that the quantum Turing machine model should be regarded as a discrete model of computation and not an analog one. We give the first formal evidence that quantum Turing machines violate the modern (complexity theoretic) formulation of the Church--Turing thesis. We show the existence of a problem, relative to an oracle, that can be solved in polynomial time on a quantum Turing machine, but requires superpolynomial time on a bounded-error probabilistic Turing machine, and thus not in the class $\BPP$. The class $\BQP$ of languages that are efficiently decidable (with small error-probability) on a quantum Turing machine satisfies $\BPP \subseteq \BQP \subseteq \Ptime^{\SP}$. Therefore, there is no possibility of giving a mathematical proof that quantum Turing machines are more powerful than classical probabilistic Turing machines (in the unrelativized setting) unless there is a major breakthrough in complexity theory.

SIAM Journal on Computing•Oct 1, 1997•1,500 citations
In this paper, we consider the following problem (the subroutine problem).If the operation performed by an oracle is efficiently computable, can a non-oracle quantum computer simulate a quantum computer with this oracle efficiently ?Bennett et al. and Aharonov et al. respectively solved the subroutine problem when their oracles evaluate deterministic and probabilistic functions.However, both of them considered only the case where machines enter the query mode deterministically.We extend the notion of an oracle of a quantum computer to a device for performing a unitary transformation, and solve the subroutine problem in the general setting where a query state and a nonquery state may superpose.Using this result, we show that EQP and BQP are robust in the general setting, which extends the robustness of BQP shown by Bennett et al. in the case where machines enter the query mode deterministically.Moreover, we prove the robustness of ZQP by improving.themethod of Bennett et al. employed for the above BQP result.
This paper positively solves the quantum subroutine problem for fully quantum oracles. The quantum subroutine problem asks whether a quantum computer with an efficiently computable oracle can be efficiently simulated by a non-oracle quantum computer. We extends the earlier results obtained by Bennett, Bernstein, Brassard, and Vazirani, and by Aharonov, Kitaev, and Nisan to the case where the oracle evaluates a unitary operator and the computer is allowed to be in the superposition of a query state and a non-query state during computation. We also prove the robustness of {\bf EQP}, {\bf BQP}, and {\bf ZQP} under the above general formulation, extending the earlier results on the robustness of {\bf BQP} shown by Bennett et al.
Abstract. In this paper we study quantum computation from a complexity theoretic viewpoint. Our rst result is the existence of an ecient universal quantum Turing machine in Deutsch’s model of a quantum Turing machine (QTM) [Proc. Roy. Soc. London Ser. A, 400 (1985), pp. 97{117]. This construction is substantially more complicated than the corresponding construction for classical Turing machines (TMs); in fact, even simple primitives such as looping, branching, and composition are not straightforward in the context of quantum Turing machines. We establish how these familiar primitives can be implemented and introduce some new, purely quantum mechanical primitives, such as changing the computational basis and carrying out an arbitrary unitary transformation of polynomially bounded dimension. We also consider the precision to which the transition amplitudes of a quantum Turing machine need to be specied. We prove that O(log T) bits of precision suce to support a T step computation. This justies the claim that the quantum Turing machine model should be regarded as a discrete model of computation and not an analog one. We give the rst formal evidence that quantum Turing machines violate the modern (complexity theoretic) formulation of the Church{Turing thesis. We show the existence of a problem, relative to an oracle, that can be solved in polynomial time on a quantum Turing machine, but requires superpolynomial time on a bounded-error probabilistic Turing machine, and thus not in the class BPP. The class BQP of languages that are eciently decidable (with small error-probability) on a quantum Turing machine satises BPP ⊆ BQP ⊆ P]P. Therefore, there is no possibility of giving a mathematical proof that quantum Turing machines are more powerful than classical probabilistic Turing machines (in the unrelativized setting) unless there is a major breakthrough in complexity theory.
We combine the classical notions and techniques for bounded query classes with those developed in quantum computing. We give strong evidence that quantum queries to an oracle in the class NP does indeed reduce the query, complexity of decision problems. Under traditional complexity assumptions, we obtain an exponential speedup between the quantum and the classical query complexity of function classes. For decision problems and function classes we obtain the following results: P/sub /spl par///sup NP[2k]//spl sube/EQP/sub /spl par///sup NP[k]/; P/sub /spl par///sup NP[2k+1-2]//spl sube/EQP/sup NP[k]/; FP/sub /spl par///sup NP[2k=1-2]//spl sube/FEQP/sup NP[2k]/; FP/sub /spl par///sup NP/spl sube/FEQP(NP[Olog n)]/. For sets A that are many-one complete for PSPACE or EXP we show that Fp/sup A//spl sube/FEQP/sup A[1]/. Sets A that are many-one complete for PP have the property that FP/sub /spl par///sup A//spl sube/FEQP/sup A[1]/. In general we prove that for any set A there is a set X such that FP/sup A//spl sube/FEQP/sup X[1]/, establishing that no set is superterse in the quantum setting.
This thesis focuses on two different aspects of quantum computation: quantum complexity and quantum semantics. In the first part, I study the quantum complexity mainly within the quantum query model. Together with my colleagues, I introduce a new framework for quantum query complexity, phrased in terms of the minimal oracle and analyse limits and strengths of this new model in comparison to their classical and quantum counterparts. Working within the query model I study quantum one-way functions. I show that in the quantum setting, the problem of the existence of a quantum one-way permutation can be reduced to the problem of constructing polynomial size networks for performing the specific task of the reflection about a sequence of states. Furthermore, I extend these results to the domain of the state and operator complexity. I show that if a quantum one-way function exists, then we can construct a sequence of so called “hard” states with the property that the reflection operators about those states are efficiently implementable. In the second part, I study the extension of domain theory to the quantum setting and develop the semantics of quantum computation. By defining a quantum domain I introduce a rigourous definition of quantum computability for quantum states and operators. Furthermore I show that the denotational semantics of quantum computation has the same semantical structure as the denotational semantics of classical probabilistic computation introduced by Kozen. This could be considered as a foundation for designing functional programming languages for quantum computation. Finally, I continue with an abstract mathematical approach to study a general formalism for describing entanglement manipulation and introduce a new approach to derive a unique measure of entanglement for bipartite quantum pure states.
An important theoretical problem in the study of quantum computation, that is also practically relevant in the context of near-term quantum devices, is to understand the computational power of hybrid models, that combine poly-time classical computation with short-depth quantum computation. Here, we consider two such models: CQ_d which captures the scenario of a polynomial-time classical algorithm that queries a d-depth quantum computer many times; and QC_d which is more analogous to measurement-based quantum computation and captures the scenario of a d-depth quantum computer with the ability to change the sequence of gates being applied depending on measurement outcomes processed by a classical computation. Chia, Chung & Lai (STOC 2020) and Coudron & Menda (STOC 2020) showed that these models (with d=log^O(1) (n)) are strictly weaker than BQP (the class of problems solvable by poly-time quantum computation), relative to an oracle, disproving a conjecture of Jozsa in the relativised world. We show that, despite the similarities between CQ_d and QC_d, the two models are incomparable, i.e. CQ_d ⊈\nsubseteq⊈ QC_d and QC_d ⊈\nsubseteq⊈ CQ_d relative to an oracle. In other words, there exist problems that one model can solve but not the other and vice versa. We do this by considering new oracle problems that capture the distinctions between the two models and by introducing the notion of an intrinsically stochastic oracle, an oracle whose responses are inherently randomised, which is used for our second result. While we leave showing the second separation relative to a standard oracle as an open problem, we believe the notion of stochastic oracles could be of independent interest for studying complexity classes which have resisted separation in the standard oracle model. Our constructions also yield simpler oracle separations between the hybrid models and BQP, compared to earlier works.
In this paper we study quantum computation from a complexity theoretic viewpoint. Our first result is the existence of an efficient universal quantum Turing machine in Deutsch's model of a quantum Turing machine (QTM) [Proc. Roy. Soc. London Ser. A, 400 (1985), pp. 97--117]. This construction is substantially more complicated than the corresponding construction for classical Turing machines (TMs); in fact, even simple primitives such as looping, branching, and composition are not straightforward in the context of quantum Turing machines. We establish how these familiar primitives can be implemented and introduce some new, purely quantum mechanical primitives, such as changing the computational basis and carrying out an arbitrary unitary transformation of polynomially bounded dimension. We also consider the precision to which the transition amplitudes of a quantum Turing machine need to be specified. We prove that O(log⁡T)O(\log T)O(logT) bits of precision suffice to support a T step computation. This justifies the claim that the quantum Turing machine model should be regarded as a discrete model of computation and not an analog one. We give the first formal evidence that quantum Turing machines violate the modern (complexity theoretic) formulation of the Church--Turing thesis. We show the existence of a problem, relative to an oracle, that can be solved in polynomial time on a quantum Turing machine, but requires superpolynomial time on a bounded-error probabilistic Turing machine, and thus not in the class $\BPP$. The class $\BQP$ of languages that are efficiently decidable (with small error-probability) on a quantum Turing machine satisfies $\BPP \subseteq \BQP \subseteq \Ptime^{\SP}$. Therefore, there is no possibility of giving a mathematical proof that quantum Turing machines are more powerful than classical probabilistic Turing machines (in the unrelativized setting) unless there is a major breakthrough in complexity theory.