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Physical Review Letters

Quantum State Reduction and Conditional Time Evolution of Wave-Particle Correlations in Cavity QED

G. T. Foster, Luis A. Orozco, H. M. Castro-Beltrán, H. J. Carmichael

Published Oct 9, 2000• 141 citations
Publisher

Abstract

We report measurements in cavity QED of a wave-particle correlation function which records the conditional time evolution of the field of a fraction of a photon. Detection of a photon prepares a state of well-defined phase that evolves back to equilibrium via a damped vacuum Rabi oscillation. We record the regression of the field amplitude. The recorded correlation function is nonclassical and provides an efficiency independent path to the spectrum of squeezing. Nonclassicality is observed even when the intensity fluctuations are classical.

DOI
10.1103/physrevlett.85.3149

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We present the conditional time evolution of the electromagnetic field produced by a cavity QED system in the strongly coupled regime. We obtain the conditional evolution through an intensity–amplitude correlation function that measures the time evolution of the field after the detection of a photon. A connection exists between this correlation function and the spectrum of squeezing, which permits the study of squeezed states in the time domain. We calculate the spectrum of squeezing from the master equation for the reduced density matrix using both the quantum-regression theorem and quantum trajectories. Our calculations not only show that spontaneous emission degrades the squeezing signal, they also point to the dynamical processes that cause this degradation.
The correlation of a photon detection with the record of a homodyne photocurrent has allowed the observation of time-resolved quantum fluctuations of the amplitude of a squeezed electromagnetic wave in a cavity quantum electrodynamic system. The photon detection, through the wavefunction collapse, prepares a conditional state whose evolution back to equilibrium is observed. The perturbation of the wave amplitude is measured at the sub-photon level. The third-order correlation is manifestly non-classical, giving a minimum amplitude at zero delay where there would classically be a maximum; its Fourier transform gives the squeezing spectrum.
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Summary form only given. We have developed a correlation technique that permits us to follow the evolution of the mean amplitude of the conditional electromagnetic field emitted from an optical system. The detection of a photon is correlated with the photocurrent record from a balanced homodyne detector. The measured signal in the homodyne detector is proportional to a correlation between particle wave aspects of light. One application of this technique is the measurement of quadrature amplitude fluctuations which violate classical bounds by orders of magnitude.We use this wave-particle correlator to study the time evolution of the electromagnetic field emitted from a cavity QED system. We operate in the optical regime of cavity QED where a beam of optically pumped two-level Rb atoms traverse the TEM/sub 00/ mode of a high finesse Fabry-Perot cavity.