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Squeezed States Engineering by Coherent Pulses Train Acting on Single Atom Laser

V. Stefanov, S. Ya. Kilin

Published Jan 1, 2019• 3 citations
OpenAlex

Abstract

We propose a method of squeezed light generation by a single-atom laser excited by \naperiodic train of coherent resonant pulses of various duration. The action of three pulses \ntrain has been analyzed and it is shown that the maximal squeezing is achieved not in the case \nwhen one creates maximal squeezing during action of each pulse but rather by a nontrivial \nsequence of pulses. This coherent pulses train is determined by solution of optimization \nproblem.

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arXiv:quant-ph/0507091•Physical Review Letters•Jan 18, 2006•50 citations
We propose a method of squeezed light generation by a single-atom laser excited by \naperiodic train of coherent resonant pulses of various duration. The action of three pulses \ntrain has been analyzed and it is shown that the maximal squeezing is achieved not in the case \nwhen one creates maximal squeezing during action of each pulse but rather by a nontrivial \nsequence of pulses. This coherent pulses train is determined by solution of optimization \nproblem.
A proof is given using, a quantized single mode field in interaction with an atom in non-relativistic approximation, that the scattered light in strong laser–atom interaction can be made coherent to squeezed by properly choosing the wavelength of the laser, the density of the sample of atoms at the laser beam and the pulse duration. In the experiments with gases for harmonic generation, at the current laser wavelengths this effect is too small to be observed and the term of the square of the field in the Hamiltonian can be safely neglected. Then, the observed light is purely coherent.
A proof is given using, a quantized single mode field in interaction with an atom in non-relativistic approximation, that the scattered light in strong laser–atom interaction can be made coherent to squeezed by properly choosing the wavelength of the laser, the density of the sample of atoms at the laser beam and the pulse duration. In the experiments with gases for harmonic generation, at the current laser wavelengths this effect is too small to be observed and the term of the square of the field in the Hamiltonian can be safely neglected. Then, the observed light is purely coherent.
Light emitted from laser-driven atoms often has squeezed quadrature fluctuations reflecting the phase dependence of the atomic source excited into a coherent superposition by the driving field. In this paper, we study the maximum squeezing in the emitted light fields from three-level atoms, paying particular attention to the role of atomic coherences in governing the optimum squeezing which is possible. Some phase dependent nonlinear optical processes have the potential for substantial quantum noise reduction at useful intensities: for example, four-wave mixing involving atomic systems has already been used to generate quadrature-squeezed light. The threoretical description of such a source is generally complicated with damping processes, coherent and incoherent pumping, and so on, all needing to be taken into account. Fully quantum treatments are available, but the basic physics of the process is often difficult to grasp in the necessarily complicated analysis. Although less important from the point of view of efficient practical sources of squeezed light, studying the generation of squeezed light in simpler systems such as two- or three-level atoms interacting with one or two modes (as in Jaynes-Cummings models) or in spontaneous emission or resonance fluorescence gives greater insight into the fundamentals of the squeezing process. In particular, the relationship between the atomic coherences and the degree of squeezing in the generated fields can be explored. We study the maximum (optimized) squeezing that can be obtained in the emitted fields from the irreversible decay from three-level atoms for any choice of initial conditions, excitations and decay process, for all quadrature components and for all possible configurations (V, \ensuremath{\Lambda}, and ``ladder'' systems).We show that for V and \ensuremath{\Lambda} systems, optimum squeezing is actually associated with a two-level state (in a suitable basis) involving a single one-photon coherence, but for ladder systems the state for maximum noise reduction is not equivalent to a two-level system and involves a single two-photon coherence and no intermediate state population. We pay particular attention to whether the total field is in a minimum uncertainty state, and the nature of the atomic state associated with maximum squeezing, especially whether it is a mixed or pure state. In the case where the initial free field has a zero amplitude at the detector and the source atoms are confined to a region small compared to the wavelength (Dicke source) we show that the squeezing in the total field is given in terms of the squeezing in the source field, and hence related to atomic populations and atomic one-photon and two-photon coherences for the case of three level atom. Previously unconsidered source-free field interferences terms are shown to be zero. The choice of quadrature phase and frequency is optimized to minimize the source field normally ordered variance. We then minimize further with respect to the choice of atomic density matrix elements subject to the constraints that the density matrix is Hermitean, positive, and has a trace equal to unity. In all cases we find that the optimum squeezing is produced when the source field is in a minimum uncertainty state.
A scheme is proposed, in which the pulse train of squeezed light is generated via forward four-wave mixing through a nonlinear medium with third-order polarizability, coherently pumped by a train of pulses of modelocked laser. One of the quadrature noise compoments of the squeezed light is measured with a balanced homodyne detector using a pulse train as the local oscillation. The theoretical results show that the fairly high-intensity output of squeezed light can be achieved, and the noise-power spectrum of the light is almost completely squeezed and exhibits the form of a train of pulses, but the shape of squeezed spectrum is reversed with respect to that of the pulse train of the input light.
The coherent interaction between a laser-driven single trapped atom and an optical high-finesse resonator allows one to produce entangled multiphoton light pulses on demand. The mechanism is based on the mechanical effect of light. The degree of entanglement can be controlled through the parameters of the laser excitation. Experimental realization of the scheme is within reach of current technology. A variation of the technique allows for controlled generation of entangled subsequent pulses, with the atomic motion serving as intermediate memory of the quantum state.