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Conference on Lasers and Electro-Optics

Electromagnetically induced quantum memory

A. Nazarkin, Ralf Netz, Roland A. Sauerbrey

Published Jan 1, 2004• 1 citation
Publisher

Abstract

The problem of creating coherence in an optically driven quantum system in conditions where decoherence is caused by the laser field itself is discussed. It is shown that by applying an additional laser field to the system the relaxation channel can be suppressed, giving raise to long living Rabi oscillations.
DOI
10.1364/iqec.2004.imp4

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We discuss the problem of creating coherence in an optically driven quantum system in conditions where decoherence is caused by the laser field itself, due to coupling of the system to a rapidly decaying state or continuum. It is shown that by applying an additional laser field between this state and a bound state the relaxation channel can be suppressed as a result of a "dark state" formation, giving rise to long living Rabi oscillations in the system. It is found that the same mechanism of preserving coherence exists in systems with level splitting or degeneracy, where the driving field interacts with multiple resonant sublevels simultaneously. We also show that specific coherent propagation phenomena assisted by the interference suppression of decoherence can be observed under these conditions.
Summary form only given. High-intensity pulse propagation of femtosecond laser pulses in optically thick semiconductor crystals can lead to self-induced transmission when the laser is tuned to a free exciton resonance. This effect means that at intensities on the order of MW/cm/sup 2/, pulses in the subpicosecond range lead to Rabi flopping of the carrier density. This causes the pulses to break up temporally after long-distance coherent propagation. The effect depends strongly on the coherence of the incident light field with the polarization of the exciton transition created in the semiconductor. Despite the expected excitation-induced dephasing, a large amount of coherent nonlinear transmission and a high contrast ratio of the Rabi flops are found at sample temperatures /spl les/10 K. In order to study the dynamics of the coherently driven polarization and the dependence on external dephasing, we introduced phonon-scattering by raising the temperature of our samples. The experiments were conducted with 800 fs pulses of an optical parametric amplifier in epitaxially grown CdSe. Our experiments clearly demonstrate that the coherent coupling of the free carrier polarization to the driving field of a high-intensity subpicosecond pulse with an area larger than /spl pi/ effectively prevents the expected dephasing by carrier-carrier scattering. Opening an additional dephasing channel by introducing phonon-scattering gradually weakens the coupling and suppresses characteristic features of self-induced transmission.
Interaction between quantum two-level systems (qubits) and electromagnetic fields can provide additional coupling channels to qubit states. In particular, the interwell relaxation or Rabi oscillations, resulting, respectively, from the multi- or single-mode interaction, can produce effective crossovers, leading to electromagnetically induced interference in microwave driven qubits. The environment is modeled by a multimode thermal bath, generating the interwell relaxation. Relaxation induced interference, independent of the tunnel coupling, provides deeper understanding to the interaction between the qubits and their environment. It also supplies a useful tool to characterize the relaxation strength as well as the characteristic frequency of the bath. In addition, we demonstrate the relaxation can generate population inversion in a strongly driving two-level system. On the other hand, different from Rabi oscillations, Rabi-oscillation-induced interference involves more complicated and modulated photon exchange thus offers an alternative means to manipulate the qubit, with more controllable parameters including the strength and position of the tunnel coupling. It also provides a testing ground for exploring nonlinear quantum phenomena and quantum state manipulation in qubits either with or without crossover structure.
A coherently driven quantum system suffers a loss of phase coherence through the excitation of unstable levels. This effect is relevant for three-level systems which might serve as very accurate frequency standards. The effect explains why excitation of the doubly driven three-level system is suppressed, a result which has been recently derived in the context of the optical Bloch equations. Dynamical response times of the driven system play an important role in our discussion, and a formalism which highlights these dynamical effects is proposed.