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Physical Review A

Reversing the weak measurement of an arbitrary field with finite photon number

Qingqing Sun, M. Al-Amri, Muhammad Suhail Zubairy

Published Sep 25, 2009• 95 citations
Publisher

Abstract

For an arbitrary field with finite photon number inside a leaky cavity, we show that the null-result measurement can be conditionally reversed, using either multiple atoms or a multilevel atom. Even with photons detected, a certain class of fields can still be probabilistically recovered. The state protection probability in such a system can be significantly high.

DOI
10.1103/physreva.80.033838

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For an arbitrary field with finite photon number inside a leaky cavity, we show that the null-result measurement can be conditionally reversed, using either multiple atoms or a multilevel atom. Even with photons detected, a certain class of fields can still be probabilistically recovered. The state protection probability in such a system can be significantly high.
We present a proposal for reversing the weak (partial-collapse) quantum measurement on a cavity field with arbitrary maximum photon number. We start by putting forth a protocol to realize quantum phase gates between the cavity field and an ancilla qubit. Afterward, adopting these phase gates and some other quantum gates, we can determine the reversal of the cavity state just by observing the ancilla qubit. Compared to previous proposals, our proposal does not need any other weak measurement and can save reversal time, which is significantly important in quantum informatics and quantum computation.
We propose a class of probabilistic reversing operations on the state of a system that was disturbed by a weak measurement. It can approximately recover the original state from the disturbed state especially with an additional information gain using the Hermitian conjugate of the measurement operator. We illustrate the general scheme by considering a quantum measurement consisting of spin systems with an experimentally feasible interaction and show that the reversing operation simultaneously increases both the fidelity to the original state and the information gain with such a high probability of success that their average values increase simultaneously.
A scheme for the preparation of Fock states and general superposition states of the electromagnetic field in a cavity is studied in detail. The scheme uses adiabatic passage in a strongly coupled atom-cavity system to ``map'' atomic ground-state Zeeman coherence onto the cavity-mode field. We model photon-counting and homodyne measurements of the field exiting the cavity and demonstrate the possibility of generating and detecting highly nonclassical states of the field parameter values close to currently realizable experimental values. The adiabatic passage process is also reversible, enabling cavity-mode fields to be mapped onto atomic ground-state Zeeman coherence. Application of this property to the measurement of cavity fields is discussed, with particular consideration given to a possible scheme for quantum measurements of the intracavity photon number.
We investigate the information distribution among different entities in the weak measurements protocol. Focusing on multilevel, decaying systems under continuous (no-click) monitoring, we derive exact, conservation-type information relations that hold for each outcome in the record. Analogous relations hold when an explicit reversal is applied, with the reversal success probability entering the relation. We extend the framework to finite-count outcomes (arbitrary photon numbers) obtaining quantitative trade-offs that link information change in the weak measurement process to the entities to which the information is distributed. These results provide a unified, outcome-resolved account of information flow in monitored open quantum dynamics and provide insight into a deeper understanding of open-system dynamics and its control.