Showing papers by "Howard J. Carmichael published in 2013"

Phonon-mediated population inversion in a semiconductor quantum-dot cavity system

Abstract: We investigate pump-induced exciton inversion in a quantum-dot cavity system with continuous wave drive. Using a polaron-based master equation, we demonstrate excited-state populations above 0.9 for an InAs quantum dot at a phonon bath temperature of 4 K. In an exciton-driven system, the dominant mechanism is incoherent excitation from the phonon bath. For cavity driving, the mechanism is phonon-mediated switching between ground- and excited-state branches of the ladder of photon states, as quantum trajectory simulations clearly show. The exciton inversion as a function of detuning is found to be qualitatively different for exciton and cavity driving, primarily due to cavity filtering. The master equation approach allows us to include important radiative and non-radiative decay processes on the zero phonon line, provides a clear underlying dynamic in terms of photon and phonon scattering, and admits simple analytical approximations that help to explain the physics.

Breakdown of Photon Blockade: A Dissipative Quantum Phase Transition in Zero Dimensions

Abstract: A two-state system coupled to a driven mode of the electromagnetic field undergoes a dissipative quantum phase transition at a critical drive strength. The phase transition marks the breakdown of photon blockade in the Jaynes-Cummings model.

Control of conditional quantum beats in cavity QED: amplitude decoherence and phase shifts

Abstract: We implement a simple feedback mechanism on a two-mode cavity QED system to preserve the Zeeman coherence of a ground state superposition that generates quantum beats on the second-order correlation function. Our investigation includes theoretical and experimental studies that show how to prevent a shift away from the Larmor frequency and associated decoherence caused by Rayleigh scattering. The protocol consists of turning off the drive of the system after the detection of a first photon and letting it evolve in the dark. Turning the drive back on after a pre-set time reveals a phase accumulated only from Larmor precession, with the amplitude of the quantum beat more than a factor of two larger than with continuous drive.

Micromaser Revisited: Sub-Poissonian Fields from Poissonian Pumping

Abstract: We perform quantum trajectory simulations of the micromaser/laser with Poissonian pump, lifting all restriction on the number of simultaneously interacting atoms. For mean numbers of intracavity atoms between zero and two, highly sub-Poissonian fields are obtained from the Poissonian pumping, with a Mandel \(Q\) parameter inside the cavity below \(-0.6\) under suitable conditions. Simulation of a Hanbury Brown and Twiss-type measurement performed on the cavity emissions recovers the intracavity value of the Mandel \(Q\) parameter from the intensity correlation function of the output photon stream.

Feedback in a cavity QED system for control of quantum beats

Abstract: Conditional measurements on the undriven mode of a two-mode cavity QED system prepare a coherent superposition of ground states which generate quantum beats. The continuous system drive induces decoherence through the phase interruptions from Rayleigh scattering, which manifests as a decrease of the beat amplitude and an increase of the frequency of oscillation. We report recent experiments that implement a simple feedback mechanism to protect the quantum beat. We continuously drive the system until a photon is detected, heralding the presence of a coherent superposition. We then turn off the drive and let the superposition evolve in the dark, protecting it against decoherence. At a later time we reinstate the drive to measure the amplitude, phase, and frequency of the beats. The amplitude can increase by more than fifty percent, while the frequency is unchanged by the feedback.

Feedback in a cavity QED system for control of quantum beats

Abstract: Conditional measurements on the undriven mode of a two-mode cavity QED system prepare a coherent superposition of ground states which generate quantum beats. The continuous system drive induces decoherence through the phase interruptions from Rayleigh scattering, which manifests as a decrease of the beat amplitude and an increase of the frequency of oscillation. We report recent experiments that implement a simple feedback mechanism to protect the quantum beat. We continuously drive the system until a photon is detected, heralding the presence of a coherent superposition. We then turn off the drive and let the superposition evolve in the dark, protecting it against decoherence. At a later time we reinstate the drive to measure the amplitude, phase, and frequency of the beats. The amplitude can increase by more than fifty percent, while the frequency is unchanged by the feedback.

Control of conditional quantum beats in cavity QED: amplitude decoherence and phase shifts

Abstract: We implement a simple feedback mechanism on a two-mode cavity QED system to preserve the Zeeman coherence of a ground state superposition that generates quantum beats on the second-order correlation function. Our investigation includes theoretical and experimental studies that show how to prevent a shift away from the Larmor frequency and associated decoherence caused by Rayleigh scattering. The protocol consists of turning off the drive of the system after the detection of a first photon and letting it evolve in the dark. Turning the drive back on after a pre-set time reveals a phase accumulated only from Larmor precession, with the amplitude of the quantum beat more than a factor of two larger than with continuous drive.

Ground state quantum coherences, from quantum beats to strong control

Abstract: Second order correlations reveal quantum beats from a coherent ground-state superposition on the undriven mode of a two-mode cavity QED system. Continuous drive induces decoherence due to Rayleigh scattering. We control this with feedback.