We demonstrate that atomic superposition states are sensitive to phase-dependent properties of radiation fields and that atoms prepared in such states can be employed as detectors. We concentrate on three-level detectors in two configurations, \ensuremath{\lambda} and ladder systems. \ensuremath{\lambda} systems prepared in a superposition state are shown to be sensitive to the relative phase between field modes and are relevant to measurements of fields produced by correlated-emission lasers. Ladder systems prepared in a superposition state are sensitive to the overall field phase and are relevant to measurements of fields generated in squeezing experiments. Such atomic-superposition-state detectors could be of value in detecting phase-dependent properties of microwave radiation in Rydberg atom masers and for investigating the properties of optical radiation generated in correlated-emission lasers.
We demonstrate that atomic superposition states are sensitive to phase-dependent properties of radiation fields and that atoms prepared in such states can be employed as detectors. We concentrate on three-level detectors in two configurations, \ensuremath{\lambda} and ladder systems. \ensuremath{\lambda} systems prepared in a superposition state are shown to be sensitive to the relative phase between field modes and are relevant to measurements of fields produced by correlated-emission lasers. Ladder systems prepared in a superposition state are sensitive to the overall field phase and are relevant to measurements of fields generated in squeezing experiments. Such atomic-superposition-state detectors could be of value in detecting phase-dependent properties of microwave radiation in Rydberg atom masers and for investigating the properties of optical radiation generated in correlated-emission lasers.
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.
This paper contains a theoretical treatment of a scheme for determining phase-dependent features of a single mode of quantized light fields in general states. The proposal involves using a three-level atom, in a cascade configuration, as a detector. The cascade atom, which is on two-photon resonance with the field, is prepared in a coherent superposition of the upper and lower levels, and detection involves measuring the probability of the cascade atom being in the upper level. The results depend on the phase difference between the amplitudes for the field being in N and N + 2 photon states in the number state basis, and are important in describing nonclassical states, such as squeezed and Schrödinger cat states. The analysis is based on a standard dressed atom model approach and a Fourier treatment of the measured signal. The experimental possibilities of realizing this scheme are also discussed.
A three-level atom with two competing transitions can sense correlations between modes of an electromagnetic field with which it interacts. We examine the sensitivity of three-level atoms in ladder, \ensuremath{\Lambda}, and V configurations to correlations between modes in a broadband correlated reservoir of field modes. Our model of a correlated reservoir includes pairwise mode correlations around a center frequency. When such correlations are set to zero, the reservoir becomes a simple thermal heat bath, but in the opposite extreme when the modes are perfectly correlated, we recover a minimum-uncertainty broadband squeezed vacuum. We show that atomic populations are extremely sensitive to these correlations, and can, in the ladder system, become inverted as a result of the phase-sensitive noise. We also show how atomic population trapping is affected by mode correlations.
We consider a phase-sensitive amplifier that consists of a system of three-level atoms in a cascade configuration that is prepared in a coherent superposition of upper and lower states by a strong classical field. We have studied the role of pump phase fluctuations associated with the finite linewidth on the quantum-statistical properties of the radiation field. A study of the output field characteristics reveals that the presence of a very weak pump field gives rise to equal variances in both field quadratures which are independent of linewidth. On the other hand, the presence of a strong pump field alters the output field characteristics with the generation of squeezing which is later on destroyed due to the presence of phase fluctuations.
We investigate the behavior of a three-level atomic system with \ensuremath{\Lambda} structure of levels in the field of a sequence of short frequency-chirped bichromatic laser pulses (BLP's) in the adiabatic passage regime of interaction. We show that the efficiency of the population transfer in this atomic system is sensitive to the relative phase of the pulses forming the BLP when the atom is prepared in a coherent superposition of its two ground states initially. We propose to use this sensitivity for coherent fast and robust writing and reading of optical phase information. The initial preparation of the atom in the ``dark'' superposition state is suggested through the action of a sequence of a few frequency-chirped BLP's with duration longer than the atomic relaxation time.
A linear theory of two-photon amplification by three-level atoms in the cascade configuration is developed, where a coherence is induced between the top and bottom levels, by an external classical driving field. It is shown that this system becomes an ideal parametric amplifier for sufficiently strong driving field, whereas for a weak driving field it is a phase-insensitive amplifier. In between these two extremes, one finds phase-sensitive amplification as well as squeezing for a certain range. The system does not, however, reduce to a model studied previously where the atomic coherence was treated as an initial condition. The system is also studied in a cavity configuration: It is predicted that the oscillator may behave as a two-photon correlated emission laser, i.e., its phase diffusion coefficient vanishes.