Partial Measurements of Quantum Systems

TL;DR

This paper explores the use of partial quantum measurements to study uncertainty relations and irreversible dynamics, demonstrating their effects on entropy and measurement outcomes in superconducting qubits.

Contribution

It introduces novel experiments employing partial measurements to analyze quantum uncertainty and thermodynamics, along with a new fabrication technique for superconducting circuits.

Findings

Partial measurements can reduce uncertainty between incompatible observables.

Statistics of partial measurements align with generalized second laws of thermodynamics.

A new fabrication method enables fast, high-coherence superconducting qubits.

Abstract

Projective measurement is a commonly used assumption in quantum mechanics. However, advances in quantum measurement techniques allow for partial measurements, which accurately estimate state information while keeping the wavefunction intact. In this dissertation, we employ partial measurements to study two phenomena. First, we investigate an uncertainty relation -- in the style of Heisenberg's 1929 thought experiment -- which includes partial measurements in addition to projective measurements. We find that a weak partial measurement can decrease the uncertainty between two incompatible (non-commuting) observables. In the second study, we investigate the foundation of irreversible dynamics resulting from partial measurements. We do so by comparing the forward and time-reversed probabilities of measurement outcomes resulting from post-selected feedback protocols with both causal and reversed-causal order. We find that the statistics of partial measurements produce entropy in accordance with generalized second laws of thermodynamics. We perform these experiments using superconducting qubits. This dissertation also describes the fabrication process for these devices and details a novel fabrication technique that allows fast, single-step lithography of Josephson-junction superconducting circuits. The technique simplifies processing by utilizing a direct-write photolithography system, in contrast to traditional electron-beam lithography. Despite their large lithographic area, Josephson junctions made with this method have low critical currents and high coherence times.