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Springer proceedings in physics

Time-Symmetric Electrodynamics and Quantum Measurement

David Thomas Pegg

Published Jan 1, 1986• 0 citations
Publisher
DOI
10.1007/978-3-642-71407-8_25

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arXiv:1906.00765•Physical Review A•May 28, 2019•19 citations
A quantised version of Wheeler-Feynman time symmetric absorber theory sheds new light on an old problem at present experiencing a revival of interest. Some photon correlation experiments, when interpreted in terms of quantum electrodynamics, lead to conceptual difficulties of the Einstein-Podolsky-Rosen type. The results imply that one has to abandon either the concept of strict causality, which would make conventional quantum electrodynamics self-inconsistent, or the concept of an objective reality independent of the observer, which has far-reaching philosophical consequences. In terms of absorber theory, on the other thand, there is no need to abandon objective reality and, in fact, such an experiment is directly interpretable as an example of the manifestation of the future on the past, which is the important distinguishing feature of absorber theory.
For some cavity-quantum-electrodynamics systems, such as a single-electron spin coupled to a passive cavity, it is challenging to reach the strong-coupling regime. In such a weak-coupling regime, the conventional dispersive readout technique cannot be used to resolve the quantum states of the spin. Here we propose an improved dispersive readout method to measure the quantum states of a weakly coupled qubit by harnessing either one or two auxiliary cavities linearly coupled to the passive cavity containing the qubit. With appropriate parameters in both cases, the system excluding the qubit can exhibit a parity-time-symmetric phase transition at the exceptional point (EP). Because the EP can amplify the perturbation induced by the qubit and the parity-time symmetry can narrow the linewidths of the peaks in the transmission spectrum of the passive cavity, we can measure the quantum states of the weakly coupled qubit via this transmission spectrum. Owing to the weak coupling between the qubit and the passive cavity, the backaction due to the measurement of the qubit can also be reduced in comparison with the conventional dispersive readout technique in the strong-coupling regime.