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Quantum Electronics

Quantum computing based on one-photon polarisation states of light pulses propagating in a doped resonance medium

Alexei V. Prokhorov, A. P. Alodjants, Sergei M Arakelyan

Published Dec 31, 2007• 2 citations
Publisher

Abstract

The propagation of one-photon light pulses in optical media doped with rare-earth atoms is considered under conditions close to the atomic optical resonance. The four-beam polarisation interaction M-scheme with two probe pulses and two pump pulses is analysed. The regimes of induced cross-interaction between probe optical pulses are studied and the conditions are found for controlling efficiently their group velocities in the medium, which is determined by the tunings of the atomic optical resonance. An algorithm is proposed for quantum information processing by coding and controlling the time delays of one-photon light pulses in this system.

DOI
10.1070/qe2007v037n12abeh013678

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The propagation of one-photon light pulses in optical media doped with rare-earth atoms is considered under conditions close to the atomic optical resonance. The four-beam polarisation interaction M-scheme with two probe pulses and two pump pulses is analysed. The regimes of induced cross-interaction between probe optical pulses are studied and the conditions are found for controlling efficiently their group velocities in the medium, which is determined by the tunings of the atomic optical resonance. An algorithm is proposed for quantum information processing by coding and controlling the time delays of one-photon light pulses in this system.
Engineered strong photon-photon interactions would enable a broad range of applications in quantum information processing. We demonstrate, using an ensemble of atoms strongly coupled to an optical cavity, that light transmission through an opaque medium may be controlled with a few photons and even with the electromagnetic vacuum field. The vacuum field induces 40% transmission, accompanied by a group delay of 25 ns, which corresponds to the light pulse propagating at 1600 m/s through the medium. The transmission increases to 80% when ~10 photons are injected into the cavity. The strong optical nonlinearity at a few-photon level observed in this system opens prospects for advanced quantum devices such as photon-number-state filters.
Results of study of nonlinear processes occurring during the propagation of light pulses in optical fibers doped with atoms of rare-earth elements under conditions of atomic coherence and interference are presented. For a three-level Λ scheme of interaction, the linear (χ (1) ) and nonlinear (χ (3) ) susceptibilities of such a medium are calculated. It is shown that the coefficients of Kerr nonlinearity and nonlinear absorption can reach extremely large values and can be negative. The competition between linear and nonlinear processes in the Λ scheme allows one to obtain compensation regimes when the coefficients of dispersion or absorption of the optical fiber material vanish. The efficient control of the optical properties of such a system over wide limits proves to be possible owing to variations in the parameters of light pulses at the input of the medium. The necessary conditions for realizing regimes with “slow” light, as well as for self-compression of a probing pulse on ultimately small spatial scales in a doped optical fiber, were obtained.
Well-controlled nonlinear interactions between light-field pulses and single atoms could be used to implement optical quantum information technologies based on qubits encoded in superpositions of coherent states of light. Here, we investigate the transformation of a coherent light field input at a single atom sufficiently far from resonance to limit the decoherence effects associated with random excitations of the atom. The conditions for suppressing multiphoton scattering to implement arbitrarily large shifts of the coherent light amplitude without decoherence are studied. It is shown that quantum-controlled coherent shifts can be achieved by sufficiently long coherent pulses, indicating the possibility of generating superpositions of coherent states with large amplitude differences. The dominant multiphoton scattering effect associated with four-wave mixing is also identified, and the spectral and temporal characteristics of the entangled photon pairs generated by this process are discussed.
We consider the propagation of a light pulse in a medium with a single resonance. If the frequency of the pulse is tuned far from resonance and the pulse duration is much shorter than the lifetime of the excited state of the resonant particles in the medium (atoms in a gas, impurity ions in a solid, etc.), the group velocity of the pulse is appreciably reduced. It is shown that the slowing down of the group velocity of the pulse is accompanied with a pulse chirp, which produces a pulse broadening in time. It is proposed to use two samples in sequence with opposite chirps (up chirp and down chirp or vice versa) compensating the pulse broadening. Then the pulse can be delayed with almost no losses, distortion, and broadening. However, there is a maximum distance, beyond which the pulse experiences corruption. Pumping with an auxiliary laser beam can control the delay time of the light pulse in the medium. Conditions to eliminate the contribution of the dephasing processes in the pulse propagation are considered.
Electromagnetically-induced transparency (EIT) of atomic medium interacting with two laser beams can exhibit effects of reciprocal influence of both beams on optical properties of the medium. If the probe beam reaches instantaneous intensity comparable to that of the control beam, considerable energy is transferred to the latter. Resulting increase in the control-beam intensity temporarily modifies EIT conditions and consequently affects propagation of probe-beam pulses. The effect is particularly strong in atomic media exhibiting significant dissipation of radiation energy due to relaxation processes efficiently counteracting the optical pumping. Coherent interaction of atoms with both beams causes reduction of dissipation of the control beam (reciprocal EIT conditions). Our experiments with hot rubidium vapor demonstrate the possibility of dual control over slow propagation of light pulses in such media. Besides the well-known dependence on input control-beam intensity, the delays of output probe pulses depended significantly also on the energy of input probe pulses. Moreover, for relatively short input pulses, very specific shape of output pulses was observed, invariant in wide ranges of input probe pulse energies and control-beam intensities. The shapes of the output pulses were also the same for short input pulses of different shapes as well as for pulses temporarily stored in the medium by switching off the control beam during their propagation.