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arXiv:1211.6364 · Physical Review A

Spontaneous creation and persistence of ground-state coherence in a resonantly driven intracavity atomic ensemble

D. G. Norris, Andres D. Cimmarusti, Luis A. Orozco, Pablo Barberis-Blostein, H. J. Carmichael

Published Nov 15, 2012• 8 citations
arXivHTMLPDF

Abstract

The spontaneous creation and persistence of ground-state coherence in an ensemble of intracavity Rb atoms has been observed as a quantum beat. Our system realizes a quantum eraser, where the detection of a first photon prepares a superposition of ground-state Zeeman sublevels, while detection of a second erases the stored information. Beats appear in the time-delayed photon-photon coincidence rate (intensity correlation function). We study the beats theoretically and experimentally as a function of system parameters, and find them remarkably robust against perturbations such as spontaneous emission. Although beats arise most simply through single-atom-mediated quantum interference, scattering pathways involving pairs of atoms interfere also in our intracavity experiment. We present a detailed model which identifies all sources of interference and accounts for experimental realities such as imperfect prepumping of the atomic beam, cavity birefringence, and the transit of atoms across the cavity mode.

DOI
10.1103/physreva.86.053816

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•Jan 1, 2016•0 citations
The spontaneous creation and persistence of ground-state coherence in an ensemble of intracavity Rb atoms has been observed as a quantum beat. Our system realizes a quantum eraser, where the detection of a first photon prepares a superposition of ground-state Zeeman sublevels, while detection of a second erases the stored information. Beats appear in the time-delayed photon-photon coincidence rate (intensity correlation function). We study the beats theoretically and experimentally as a function of system parameters, and find them remarkably robust against perturbations such as spontaneous emission. Although beats arise most simply through single-atom-mediated quantum interference, scattering pathways involving pairs of atoms interfere also in our intracavity experiment. We present a detailed model which identifies all sources of interference and accounts for experimental realities such as imperfect prepumping of the atomic beam, cavity birefringence, and the transit of atoms across the cavity mode.
Abstract. We probe a ground-state superposition that produces a quantum beat in the intensity correlation of a two-mode cavity QED system. We mix drive with scattered light from an atomic beam traversing the cavity, and effectively measure the interference between the drive and the light from the atom. When a photon escapes the cavity, and upon detection, it triggers our feedback which modulates the drive at the same beat frequency but opposite phase for a given time window. This results in a partial interruption of the beat oscillation in the correlation function, that then returns to oscillate. 1.
Cavity quantum electrodynamics (QED) allows the study of light-matter in-teractions at the most basic level, through precise identification of the coherent and incoherent (dissipative) parts of the system evolution. We present measurements of light from a cavity QED system consisting of a high-finesse optical resonator coupled to a beam of cold Rb atoms. The novelty of the design lies in the interplay of two degenerate and orthogonal polarization modes. One mode (driven) behaves as the canonical cavity mode of the Jaynes-Cummings Hamiltonian, coherently exciting the atoms with a modest coupling strength; the other mode (undriven) collects a small fraction of spontaneously emitted light and provides a probe of the dissipative processes. We first demonstrate the ability to detect individual atoms passing through the cavity modes in real time by coincidence detection of photons from the un-driven mode. Calculation of statistics and correlation functions from the complete photon detection record allows the determination of detection probabilities and the reconstruction of atomic trajectories. We next present evidence of quantum coher-ence that is created, modified, and measured in the excitation-spontaneous emission cycle. The coherence appears as a long-lived quantum beat at the ground-state Larmor frequency, visible in the intensity autocorrelation function of the undriven mode. Quantum jumps of the atomic state, occurring in between the detections of photons from the cavity, result in substantial changes in the frequency and spectral width of the beats. We present the results of a full quantum Monte Carlo calculation in order to quantitatively explain the measurements.
We probe a ground-state superposition that produces a quantum beat in the intensity correlation of a two-mode cavity QED system. We mix drive with scattered light from an atomic beam traversing the cavity, and effectively measure the interference between the drive and the light from the atom. When a photon escapes the cavity, and upon detection, it triggers our feedback which modulates the drive at the same beat frequency but opposite phase for a given time window. This results in a partial interruption of the beat oscillation in the correlation function, that then returns to oscillate.
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.
The realization of nonlinear systems able to mediate strong interactions between light fields at the few photon level in an environment with minimal absorption represents a cornerstone for future developments of quantum information science and photonic quantum technologies.One approach to reach this goal is via strong coupling of single emitters to optical cavities as provided by cavity quantum electrodynamics (QED).The other one is via electromagnetically induced transparency (EIT) with single atoms or with atomic ensembles.This thesis reports on a system which merges both single-atom cavity QED in the strong coupling regime and EIT.First, we characterize this system theoretically and show that its energy eigenstates are very similar to the well known Jaynes-Cummings ladder of eigenstates.However, here, the eigenstates are optically controllable via the control field Rabi frequency provided by EIT.We show that increasing the control field Rabi frequency enhances the normalmode splitting, and therefore the effective light-matter coupling.Moreover, with this system we are able to observe photon blockade and a two-photon gateway for the same input probe field and to tune the behavior of the secondorder correlation function optically.Thanks to the long coherence times of EIT, photon blockade can be prolonged in time for longer times than the decay rates of the system.We refer to this effect as photon blockade with memory.Next, we verify our theoretical findings experimentally.By performing spectroscopy measurements on the single-atom cavity EIT system, we show an unprecedented EIT on/off contrast of about 80 % thanks to our strong atomcavity coupling rate.Later, we show experimentally, the ability to generate both, photon blockade and a two-photon gateway, for the same input field by just varying the control field Rabi frequency.Finally, by performing timedependent second-order correlations we observe strong oscillations in the correlations which are a result of the creation of a new light field in the cavity which beats with the impinging probe laser.We show experimentally that this new field has a coherence time longer than the decay rates of the system, confirming its EIT origin.In a final experiment, we take advantage of the strong atom-cavity coupling achieved to induce a group delay on probing pulses.Delays up to approximately 200 ns are measured.This is the first time a group delay is observed with a single atom as the delaying medium.iii