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arXiv:quant-ph/0010081 · Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences

Theory of the quantum speed-up

Giuseppe Castagnoli, David Ritz Finkelstein

Published Jul 8, 2001• 16 citations

Abstract

We analyse the quantum speed-up: the fact that some quantum algorithms are faster than their classical counterparts. The classical formulation of problem solving must be modified. In general a problem statement defines the solution only implicitly, giving a property that a solution must possess, and does not represent a process that constructs the solution explicitly. A solution algorithm must then specify explicitly a finite step-by-step procedure that constructs the solution through a dynamical development. In the quantum speed-up, however, an implicit definition directly determines the solution through an extra-dynamical irreversible non-algorithmic quantum measurement. While in an algorithmic solution, the initial conditions determine the solution in a one-way manner, in a solution by measurement, the initial conditions and a final condition influence the solution jointly. The quantum computation is as implicit as the problem statement. Implicit determination with joint influence is richer than explicit one-way dynamical determination, as it yields the speed-up. Quantum problem solving with speed-up is essentially extra-dynamical and irreversible. Other forms of implicit quantum problem solving may exploit extra-dynamical projections for speed-up.

DOI: 10.1098/rspa.2001.0797

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We analyse the quantum speed-up: the fact that some quantum algorithms are faster than their classical counterparts. The classical formulation of problem solving must be modified. In general a problem statement defines the solution only implicitly, giving a property that a solution must possess, and does not represent a process that constructs the solution explicitly. A solution algorithm must then specify explicitly a finite step-by-step procedure that constructs the solution through a dynamical development. In the quantum speed-up, however, an implicit definition directly determines the solution through an extra-dynamical irreversible non-algorithmic quantum measurement. While in an algorithmic solution, the initial conditions determine the solution in a one-way manner, in a solution by measurement, the initial conditions and a final condition influence the solution jointly. The quantum computation is as implicit as the problem statement. Implicit determination with joint influence is richer than explicit one-way dynamical determination, as it yields the speed-up. Quantum problem solving with speed-up is essentially extra-dynamical and irreversible. Other forms of implicit quantum problem solving may exploit extra-dynamical projections for speed-up.

We investigate the reason for the quantum speedup (quantum algorithms require fewer computation steps than their classical counterparts). We extend the representation of the quantum algorithm to the process of setting the problem, namely choosing the function computed by the black box. The initial measurement selects a setting at random, Bob (the problem setter) unitarily changes it into the desired one. With reference to the observer dependent quantum states of relational quantum mechanics, this representation is with respect to Bob and any external observer, it cannot be with respect to Alice (the problem solver). It would tell her the function computed by the black box, which to her should be hidden. To Alice, the projection of the quantum state due to the initial measurement is retarded at the end of her problem solving action, so that the algorithm input state remains one of complete ignorance of the setting. By black box computations, she unitarily sends it into the output state that, for each possible setting, encodes the corresponding solution, acquired by the final measurement. Mathematically, we can ascribe to the final measurement the selection of any fraction R of the random outcome of the initial measurement. This projects the input state to Alice on one of lower entropy where she knows the corresponding fraction of the problem setting. Given the appropriate value of R, the quantum algorithm is a sum over classical histories in each of which Alice, knowing in advance one of the R-th parts of the setting, performs the black box computations still required to identify the solution. Given a quantum algorithm, this retrocausality model provides the value of R that explains its speed up; in the major quantum algorithms, R is 1/2 or slightly above it. Conversely, given the problem, R=1/2 always yields the order of magnitude of the number of black box computations required to solve it in an optimal quantum way. Quanta 2016; 5: 34–52.

The usual representation of quantum algorithms, limited to the problem solving process, is incomplete as it lacks the initial measurement. Completing it involves three steps: (i) representing the problem setting process, (ii) relativizing this representation to Alice, the problem solver, to whom the problem setting must be concealed, (iii) representing the reversibility of the computation process by symmetrizing it for time reversal. The latter step shows that all is as if Alice, before beginning her problem solving action, knew half of the information that specifies the solution of the problem she will read in the future. An optimal quantum algorithm is a sum over classical histories in each of which Alice knows in advance one of the possible halves of the solution and performs the computation steps (oracle queries) necessary to identify the missing half. This explains the quantum computational speedup and allows to predict the speedup offered by the optimal quantum solution of any oracle problem.

We explain the mechanism of the quantum speed-up -quantum algorithms requiring fewer computation steps than their classical equivalentfor a family of algorithms.Bob chooses a function and gives to Alice the black box that computes it.Alice, without knowing Bob's choice, should find a character of the function (e. g. its period) by computing its value for different arguments.There is naturally correlation between Bob's choice and the solution found by Alice.We show that, in quantum algorithms, this correlation becomes quantum.This highlights an overlooked measurement problem: sharing between two measurements the determination of correlated (thus redundant) measurement outcomes.All is like Alice, by reading the solution at the end of the algorithm, contributed to the initial choice of Bob, for half of it in quantum superposition for all the possible ways of taking this half.This contribution, back evolved to before running the algorithm, where Bob's choice is located, becomes Alice knowing in advance half of this choice.The quantum algorithm is the quantum superposition of all the possible ways of taking half of Bob's choice and, given the advanced knowledge of it, classically computing the missing half.This yields a speed-up with respect to the classical case where, initially, Bob's choice is completely unknown to Alice.

The reason for the higher efficiency exhibited by some quantum algorithms over their classical counterparts is examined by considering the interplay between the reversible actions required to prepare the computer registers in an entangled state before measurement (the "initial actions"), and the final measurement action -- whereas measurement is interpreted in a new way, particularly suited to a problem solving context. This unification shows that the computation process, comprising the measurement outcome, is significantly influenced by both the initial actions and the need to satisfy the constraints set by the final measurement action. Reviewing the existing quantum algorithms in the light of this dual influence, yields new valuable insight in the nature of the quantum computation speed up.