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arXiv:quant-ph/9711045

A reductionistic approach to quantum computation

Giuseppe Castagnoli, Dalida Monti

Published Nov 20, 1997• 0 citations
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Abstract

In the reductionistic approach, mechanisms are divided into simpler parts interconnected in some standard way (e.g. by a mechanical transmission). We explore the possibility of porting reductionism in quantum operations. Conceptually, first parts are made independent of each other by assuming that all ``transmissions'' are removed. The overall state would thus become a superposition of tensor products of the eigenstates of the independent parts. Transmissions are restored by projecting off all the tensor products which violate them. This would be performed by particle statistics; the plausibility of this scheme is based on the interpretation of particle statistics as projection. The problem of the satisfiability of a Boolean network is approached in this way. This form of quantum reductionism appears to be able of taming the quantum whole without clipping its richness.

DOI
10.48550/arxiv.quant-ph/9711045

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In the reductionistic approach, mechanisms are divided into simpler parts interconnected in some standard way (e.g. by a mechanical transmission). We explore the possibility of porting reductionism in quantum operations. Conceptually, first parts are made independent of each other by assuming that all ``transmissions'' are removed. The overall state would thus become a superposition of tensor products of the eigenstates of the independent parts. Transmissions are restored by projecting off all the tensor products which violate them. This would be performed by particle statistics; the plausibility of this scheme is based on the interpretation of particle statistics as projection. The problem of the satisfiability of a Boolean network is approached in this way. This form of quantum reductionism appears to be able of taming the quantum whole without clipping its richness.
In the diakoptic approach, mechanisms are divided into simpler parts interconnected in some standard way (say by a "mechanical connection''). We explore the possibility of applying this approach to quantum mechanisms: the specialties of the quantum domain seem to yield a richer result. First parts are made independent of each other by assuming that connections are removed. The overall state would thus become a superposition of tensor products of the eigenstates of the independent parts. Connections are restored by projecting off all the tensor products which violate them. This would be performed by particle statistics, under a special interpretation thereof. The NP-complete problem of testing the satisfiability of a Boolean network is approached in this way. The diakoptic approach appears to be able of taming the quantum whole without clipping its richness.
In this thesis I confront the problem of indeterminism in quantum physics. The first pioneers accepted indeterminism as part of their Copenhagen interpretation and it was incorporated in Von Neumann's formulation (as the "projection postulate"). But how deep does this indeterminism go? With the arrival of quantum physics, the idea that the world is fundamentally deterministic has been shattered. Redhead's work showed that the experimental confirmation of the violation of Bell's inequality means that determinism has broken down; he even formulated a version of the Bell inequality that is not dependent on local hidden variables and showed that the violation thereof negates even what might be called "stochastic" determinism - at least in the framework of the Lorentzian space-time manifold (Redhead 1987:83, 103). With the acceptance of non-determinism as part of our world, we are confronted with the question: How can the possibility of non-determinism be explained and how can it be reconciled with determinism in one coherent conceptualization of the world? I use the philosophy of science that Immanuel Kant developed in the Critique of the Power of Judgment to develop an answer to these questions. In this part of his philosophy of science Kant is concerned with the possibility and conceivability of a spontaneous (albeit effective) causality in nature's producing its products. Kant developed the idea of this causality in analogy to the achievement of human ends, conceptualizing it as a capacity that non-extended wholes and parts have to be realized as parts forming an aggregated whole in nature. An important feature of my approach is that I develop a new interpretation of Kant's philosophy as presented in his well-known Critique of Pure Reason. In contrast with the two-object and twoaspect views in Kantian interpretation, I argue that the noumenal realm refers to an ontologically distinct realm outside nature, problematically assumed. This agrees with Kant's view in the Critique of the Power of Judgment where it is conceptualized as the substratum of nature and forms a central part of his philosophy of science. It is only when the noumenal (supersensible) realm is conceptualized in this manner that spontaneous causality becomes not only logically possible, but also conceivable within the framework of his philosophy. In my Kantian interpretation of quantum physics his concept of nature is taken as referring to the "classical realm" (where the theories of relativity would apply). His non-extended wholes and parts, which belong to the supersensible realm, find application in superpositions of states, which I argue belong to an ontologically distinct realm - the pre-measurement "quantum realm". Since the classical and quantum realms are taken as ontologically distinct realms, there is no contradiction in ascribing two heterogeneous laws, namely of deterministic and spontaneous causality (which is manifest in the reduction of the wave packet), to these realms which are combined in one description in quantum mechanics. In my view spontaneous causality grounds a non-spatiotemporal potentiality which explains why superpositions of states have the ability to collapse to reduced states. I develop these ideas in the framework of the various interpretations of quantum mechanics.
In this thesis, I describe research results on three topics : (i) a phase transition in the area-law regime of quantum circuits driven by projection measurements; (ii) ultra slow dynamics in two dimensional spin circuits; and (iii) tensor network methods applied to boolean satisfiability problems. (i) Nonuniversal entanglement level statistics in projection-driven quantum circuits; Non-thermalized closed quantum many-body systems have drawn considerable attention, due to their relevance to experimentally controllable quantum systems. In the first part of the thesis, we study the level-spacing statistics in the entanglement spectrum of output states of random universal quantum circuits where, at each time step, qubits are subject to a finite probability of projection onto states of the computational basis. We encounter two phase transitions with increasing projection rate: The first is the volume-to-area law transition observed in quantum circuits with projective measurements; The second separates the pure Poisson level statistics phase at large projective measurement rates from a regime of residual level repulsion in the entanglement spectrum within the area-law phase, characterized by non-universal level spacing statistics that interpolates between the Wigner-Dyson and Poisson distributions. The same behavior is observed in both circuits of random two-qubit unitaries and circuits of universal gates, including the set implemented by Google in its Sycamore circuits. (ii) Ultra-slow dynamics in a translationally invariant spin model for multiplication and factorization; Slow relaxation of glassy systems in the absence of disorder remains one of the most intriguing problems in condensed matter physics. In the second part of the thesis we investigate slow relaxation in a classical model of short-range interacting Ising spins on a translationally invariant two-dimensional lattice that mimics a reversible circuit that, depending on the choice of boundary conditions, either multiplies or factorizes integers. We prove that, for open boundary conditions, the model exhibits no finite-temperature phase transition. Yet we find that it displays glassy dynamics with astronomically slow relaxation times, numerically consistent with a double exponential dependence on the inverse temperature. The slowness of the dynamics arises due to errors that occur during thermal annealing that cost little energy but flip an extensive number of spins. We argue that the energy barrier that needs to be overcome in order to heal such defects scales linearly with the correlation length, which diverges exponentially with inverse temperature, thus yielding the double exponential behavior of the relaxation time. (iii) Reversible circuit embedding on tensor networks for Boolean satisfiability; Finally, in the third part of the thesis we present an embedding of Boolean satisfiability (SAT) problems on a two-dimensional tensor network. The embedding uses reversible circuits encoded into the tensor network whose trace counts the number of solutions of the satisfiability problem. We specifically present the formulation of #2SAT, #3SAT, and #3XORSAT formulas into planar tensor networks. We use a compression-decimation algorithm introduced by us to propagate constraints in the network before coarse-graining the boundary tensors. Iterations of these two steps gradually collapse the network while slowing down the growth of bond dimensions. For the case of #3XORSAT, we show numerically that this procedure recognizes, at least partially, the simplicity of XOR constraints for which it achieves subexponential time to solution. For a #P-complete subset of #2SAT we find that our algorithm scales with size in the same way as state-of-the-art #SAT counters, albeit with a larger prefactor. We find that the compression step performs less efficiently for #3SAT than for #2SAT.