Simultaneous weak measurement of non-commuting observables

TL;DR

This paper develops a systematic framework for performing simultaneous weak measurements of non-commuting quantum observables, extending the Arthurs-Kelly protocol to weak measurement regimes with explicit measurement operators.

Contribution

It introduces a method to construct weak measurement operators for non-commuting observables, satisfying POVM normalization and generalizing the AK protocol with broadened detector wavefunctions.

Findings

Derived a Gaussian-like weak measurement operator for non-commuting observables.

Showed the measurement operators form a valid POVM in position-momentum space.

Extended the AK protocol to include weak measurement with broadened initial detector states.

Abstract

In contrast to a projective quantum measurement in which the system is projected onto an eigenstate of the measured operator, in a weak measurement the system is only weakly perturbed while only partial information on the measured observable is obtained. A full description of such measurement should describe the measurement protocol and provide an explicit form of the measurement operator that transform the quantum state to its post measurement form. A simultaneous measurement of non-commuting observables cannot be projective, however the strongest possible such measurement can be defined as providing their values at the smallest uncertainty limit. Starting with the Arthurs and Kelly (AK) protocol for such measurement of position and momentum, we derive a systematic extension to a corresponding weak measurement along three steps: First, a plausible form of the weak measurement operator analogous to the Gaussian Kraus operator often used to model a weak measurement of a single observable is obtained by projecting a na\"ive extension (valid for commuting observable) onto the corresponding Gabor space. Second, we show that the so obtained set of measurement operators satisfies the normalization condition for the probability to obtain given values of the position and momentum in the weak measurement operation, namely that this set constitutes a positive operator valued measure (POVM) in the position-momentum space. Finally, we show that the so-obtained measurement operator corresponds to a generalization of the AK measurement protocol in which the initial detector wavefunctions is suitable broadened.