Quantum Beat Laser Research Articles

Decoherence-induced formation of sub-poissonian entangled and steerable states of collective fields

The decoherence process has a tendency to yield the evolution of a pure state into a mixed one and to cause the quantum-to-classical transition by the coupling of a system of interest to the reservoir with infinitely many degrees of freedom. This is the major obstacle to the implementation of quantum computation and hence the realization of quantum computers. We propose a scheme to create unconditionally sub-Poissonian entangled and steerable states of the collective cavity field modes by use of the dissipation process. Based on the suitable choice of combination modes, the scheme uses the inherent, efficient and controllable two-mode squeezed vacuum reservoir coupled to the combination modes of concern rather than the original cavity modes in the two-level quantum beat laser. The decoherence is shown to pull the collective modes into the sub-Poissonian entangled and steerable states in the stationary regime, while the job of the dissipation of the individual cavity fields is to give rise to the degradation of the bipartite entanglement of the two individual modes and to inhibit the occurrence of the quantum steering from one cavity mode to the other. In particular for the case that the external driving field is close to the exact resonance with the atom, the collective fields are eventually prepared asymptotically in the stationary Einstein–Podolsky–Rosen state, while the two individual cavity modes are pulled into the vacuum states and reach steady state. The disappearance of the decoherence disables the nonclassical states of the collective modes, while the ignorance of the dissipation process of the cavity field modes guarantees the generation of the entanglement between the pair of individual modes. The decoherence-induced formation of a nonclassical source is ascribed to the four-wave mixing process together with the intrinsic amplitude and phase locking.

Quantum beat laser as a source of nonclassical wave–particle complementarity

Quantum beat laser as a source of nonclassical wave–particle complementarity

Effect of Classical and Quantum Superposition of Atomic States on Quantum Correlations

In this paper, we report the effect of classical and quantum superposition of atomic states on quantum correlations. Coupled photon pairs generated in a ladder quantum beat laser using coherent-induced classical field and atomic state coherent superposition are considered. Once the quantum coherence get sufficient time, it can generate quantum correlations that include quantum discord, quantum entanglement, and quantum steering, which quickly increase with time until they get their maximum strength. Their strength can be improved further by increasing the number of superposed atoms per unit time, selecting an appropriate amplitude of the classical fields, and managing the amount of temperatures and phase fluctuations. In particular, two-way quantum steering, which is a guarantee for the existence of quantum discord and quantum entanglement, can be achieved by increasing the rate of atomic injection from 2 kHz to 25 kHz even if the mean temperature of the heat bath is considered. The maximum achievable strength of quantum correlations is enhanced by increasing the rate of atomic injection and choosing an appropriate parameters of the coherent-induced classical field in the open quantum system which is treated by using the density operator approach.

Coherence controlled generation of Gaussian quantum discord in a quantum beat laser

Quantum discord, an appropriate measure of quantum correlation, is analyzed in a two-mode Gaussian state of the cavity field evolved by a system of quantum beat laser. In the laser system, two arbitrary single-mode Gaussian states of the cavity fields and an external classical field couple to the atomic lasing medium. We calculate the time-dependent expression for the quantum discord both analytically and numerically, by following the reduced density matrix equation of the resulting cavity field. In this framework, we investigate the generation and control of Gaussian quantum discord in the two-mode Gaussian state of the cavity field at the output in terms of the purity and non-classicality of the two input cavity modes, the Rabi frequency of the classical driving field, the relative phase of the coupling parameters, and the damping rates of the cavity modes. The behaviour of quantum discord appears oscillatory due to the quantum beats in the output cavity field developed by the classical driving field in the medium as time passes. Moreover, we find that quantum discord switches on and off while adjusting the relative phase of the coupling parameters. Analysis of the analytical results agrees well with our numerical simulations.

Quantum steering of a two-mode Gaussian state using a quantum beat laser.

We study the quantum steering of a two-mode Gaussian state evolved by a simple quantum beat laser with the cavity field being initially in arbitrary two single-mode Gaussian states and the lasing medium being driven by a strong classical field. In presence of the cavity damping, the quantum steering time of the two-mode Gaussian state increases with the relative intensity of the coupling fields, whereas the relative phase switches the steerable states into the non-steerable and vice versa. We further show that the higher the non-classicality of the initial cavity modes, the longer the time of steerability in the two-mode Gaussian state. Nonetheless, the quantum steering is irrespective of the influence from the purity of the initial cavity modes of the laser system.

Coherence control of entanglement dynamics of two-mode Gaussian state via Raman driven quantum beat laser using Simon's criterion.

We study the entanglement dynamics of two-mode Gaussian state (TMGS) in a quantum beat laser driven by two classical fields in Raman configuration using Simon's criterion of quantum state separability. The two modes of the cavity field are considered initially in general single-mode Gaussian states. The effect of the non-classicality, purity, and relative phase of the cavity modes on the entanglement phenomenon is studied thoroughly. We show that in the presence of cavity decay rates, the higher the non-classicality of the initial states, the higher is the inseparability of the evolved TMGS of the cavity field. The inseparability, on the other side, is independent of the purity of the initial states. Moreover, the time scale of the entangled state increases with the relative intensity of the driving fields, whereas the relative phase switches the entangled state into the disentangled state of the cavity field and vice versa. The quantum statistics of the mean photons number of the cavity field is analyzed.

Preservation of Gaussian state entanglement in a quantum beat laser by reservoir engineering

Quantum beat lasers have been considered as sources of entangled radiation in continuous variables such as Gaussian states. In order to preserve entanglement and to minimize entanglement degradation due to the system’s interaction with the surrounding environment, we propose to engineer environment modes through insertion of another system in between the laser resonator and the environment. This makes the environment surrounding the two-mode laser a structured reservoir. It not only enhances the entanglement among two modes of the laser but also preserves the entanglement for sufficiently longer times, a stringent requirement for quantum information processing tasks.

Effect of dephasing on transient and steady-state entanglement in a quantum-beat laser

We study the role of dephasing on entanglement evolution in a two-photon quantum-beat laser. We rigorously derived the pertinent master equation by taking into account dephasing, spontaneous emission, and cavity losses. The dephasing rate, in general, is lager than the rates of other dissipation processes and might influence the realizable quantum properties of the cavity field. We investigate the extent to which the dephasing rate affects the transient as well as steady-state entanglement between the cavity modes. We show that the coherence induced by initial coherent superposition of atomic levels leads to transient as well as steady-state entanglement, which turns out to be susceptible to decoherence. However, the entanglement created via coherence induced by a strong laser field exists for only short times and is relatively robust against decoherence.

Phase-controlled entanglement in a quantum-beat laser: application to quantum lithography

We study entanglement generation and control in a quantum-beat laser coupled to a two-mode squeezed vacuum reservoir. We show that the generated entanglement is robust against cavity losses and environmental decoherence and can be controlled by tuning the phases of the microwave and the squeezed input fields. Moreover, we discuss two-photon correlations, absorption and implementations in quantum optical lithography.

Gaussian-state entanglement in a quantum beat laser

Recently quantum beat lasers have been considered as a source of entangled radiation [S. Qamar, F. Ghafoor, M. Hillery, and M. S. Zubairy, Phys. Rev. A 77, 062308 (2008)]. We investigate and quantify the entanglement of this system when the initial cavity modes are prepared in a Gaussian two-mode state, one being a nonclassical state and the other a thermal state. It is investigated how the output entanglement varies with the nonclassicality of the input Gaussian state, thermal noise, and the strength of the driving field.

Generation of two-mode Gaussian-type entangled states of light via a quantum beat laser

The generation of two-mode Gaussian-type entangled states of the cavity field is studied in a quantum beat laser, where three-level atoms of the $V$-type level configuration are coupled to two quantized modes of the cavity field and two upper levels of the atom are driven by a strongly classical field. According to Simon's criterion that is a necessary and sufficient condition for the inseparability of two-mode Gaussian states with a general form of covariance matrices, we analytically and numerically investigate the influence of phase and Rabi frequency of the classical driving field, cavity loss, and the purity and nonclassicality of the initial state of the cavity field on the entanglement property of the resulting two-mode Gaussian state. In the limit of weak cavity loss and strong driving, we show that the cavity field can be periodically into an ideal two-mode entangled Gaussian-type state.

Entanglement in correlated spontaneous emission lasers

Recent studies show that macroscopic entangled states of the radiation field can be generated using correlated spontaneous emission lasers (CEL) even in the presence of cavity losses. Some of the basic schemes based on two-photon CEL and quantum beat laser (QBL) are reviewed in this article.

Entangled radiation via a Raman-driven quantum-beat laser

We propose a scheme for the entanglement generation of two cavity modes using a four-level Raman-driven quantum-beat laser (QBL). The atomic coherence is generated by a Raman-type two-photon process. Two different sufficient conditions for entanglement generation have been investigated. The effect of cavity losses and strength of the driving fields on entanglement generation is studied for different initial states of the resonant cavity modes. The results are in agreement with the earlier scheme for the entanglement generation using a directly driven quantum-beat laser. We observe entanglement generation even for high cavity losses and with low strength of the driving fields. Further, there is no dipole-forbidden transition in the Raman-driven QBL system. Thus the proposed scheme for the entanglement generation is experimentally more viable as compared to the previous directly driven QBL system.

Tripartite entanglement in quantum-beat lasers

We examine the quantum correlations at steady state in a three-mode quantum-beat laser that operates well above threshold. The analysis is presented by employing the dressed-atom combination-mode picture, in which only one combination mode of three cavity fields is coupled to the active medium, while the other two combination modes are decoupled. For suitable operating conditions, the coupled mode has the intensity fluctuations below the shot noise, while the two decoupled modes retain in their vacuum states. This determines that all of the original modes exhibit sub-Poissonian statistics. When the system operates well above threshold, the van Loock-Furusawa inequality is satisfied, concluding that the genuine tripartite entanglement among three sub-Poissonian fields is established. Physically, the responsible mechanism is the combined effect of atomic coherence controlled correlated-spontaneous emission and intrinsic feedback.

Quantum beat laser as a source of entangled radiation

We consider a quantum beat laser [Scully and Zubairy, Phys. Rev. A 35 752 (1987)] as a source of entangled radiation. The system essentially consists of three-level atoms inside a doubly resonant cavity such that coherence is introduced by driving the upper two levels with a strong classical field of Rabi frequency $\ensuremath{\Omega}$. We study the dynamics of this system for different values of Rabi frequencies in the presence of cavity losses. It is shown that entanglement can be generated in this system for different initial states of the field in the two modes.

Entanglement generation in a two-mode quantum beat laser

We analyze the quantum correlations between side modes of a quantum beat laser when a two-level atomic medium is driven strongly by a classical field. The squeezing and the entanglement generation of the cavity radiation are investigated. It turns out that there is neither squeezing nor entanglement when the strong driving field is resonant with the atomic transition but the generated light exhibits both two-mode squeezing and entanglement when the driving field is tuned away from the atomic transition.

Lambda quantum-beat laser with another laser cascaded

In this paper we investigate the quantum statistical properties in a cascade laser system with the lower transition being in a quantum-beat configuration. It is shown that a type of three-mode squeezing may be generated. The fluctuations of the intensity difference between the sum-mode from the quantum-beat and the cascade single mode may be reduced below the shot-noise limit. The best achievable noise reduction is up to 50%. Good squeezing occurs when the sum mode and the cascade single mode have normalized intensities much larger than unity and the intensity difference is much smaller than the respective intensity.

Squeezed two-mode lasers without and with inversion

On the basis of the nondegenerate quantum-beat laser model, we introduce a coherent field which drives the transition between the upper lasing level and an auxiliary level. We demonstrate that such a four-level system can produce squeezed two-mode laser without and with inversion. When the laser is operated well above threshold, the intensity fluctuation in the average mode is reduced below the shot noise with an optimum Mandel parameter Q=- 1/2. At the same time, the noises in the relative amplitude and the relative phase drop to their vaccum noise levels. Furthermore, regardless of inversion, noninversion, and transition between inversion and noninversion, the optimum Mandel Q parameter of Q=- 1/2 is retained when the system operates well above threshold. A simple physical explanation of the squeezing mechanism for two-mode squeezing is given.

Sub-Poissonian quantum-beat laser without inversion

Using the `dressed-atom-dressed-mode' approach and separating the decoupled atomic state, we investigate the statistical properties of a quantum-beat laser without inversion. While the fluctuations of the relative amplitude and phase are retained at their vacuum noise levels, the intensity fluctuations in the sum mode are reduced below the shot noise limit with an optimum Mandel parameter when the laser is operated well above threshold. In particular, we show that the decoupled atomic state significantly relaxes the parameter requirement for squeezing. The physical cause is also explained.

Squeezed Quantum-Beat Laser Without Inversion

A quantum-beat laser without inversion is proposed for producing two-mode suqeezing. Using the dressed-atom-dressed-mode approach, the master equation is derived and the variances in the sum mode intensity and in the relative amplitude and phase are calculated. When the laser is operated well above threshold, the intensity fluctuations in the sum mode are reduced below the shot noise limit with an optimum Mandel parameter of -0.5. At the same time, the fluctuations in the relative amplitude and phase are retained at their vaccum noise levels.