Postselected quantum circuits

TL;DR

This paper explores the unusual behaviors of simple post-selected quantum circuits, revealing dualities with acausal physics and demonstrating how noise and interference affect outcomes, with implications for understanding quantum time machines.

Contribution

It introduces a duality between post-selected ensembles and acausal physics, and analyzes how noise influences the behavior of simple quantum circuits with post-selection.

Findings

Post-selected circuits exhibit paradoxical but analyzable behaviors.

Noise can be modeled as destructive interference or decoherence.

The duality links post-selection to acausal physics and time machine models.

Abstract

The purpose of this paper is to show the unusual behavior of a number of simple circuits under the effects of post-selection. A useful duality exists between post-selected ensembles and a consistent picture of acausal physics embodying the application of Novikov's consistency postulate to the wave-function of a time machine. A competing view applies this postulate to density matrices instead, but that lies outside the scope of this paper. To find out the result of a measurement on a particular circuit, we consider a weighted distribution of histories between some initial and final time, such that each individually obeys ordinary quantum mechanics. The weight of a particular history is given by a joint distribution over the values of a part of the system dubbed the time machine, at two chosen times. In the dual post-selection picture the weight is simply the probability for the 'periodic bit' to satisfy the constraint, plus a small noise part, often assumed constant, representing the error rate of the time machine's channel. The emerging bit is one of a Bell pair, its partner kept in reserve to test that the bit entering the time machine later matches the one which emerged. This is done by interfering the incoming bit with the reserve member of the pair and keeping the experimental runs that interfere constructively. Noise can be effectively simulated by keeping a small proportion of runs that interfere destructively, or by perturbing the reserve bit to decohere it by a similar amount. Paradoxes remain but are tractable.