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Physical Review A

Quantum-beat lasers as bright sources of entangled sub-Poissonian light

Xiangming Hu, Jin-hua Zou

Published Oct 8, 2008• 35 citations
Publisher

Abstract

We show that a quantum-beat system with incoherent pump or coherent driving produces an entangled sub-shot-noise laser that operates well above threshold. The numerical results and physical analyses are presented using the combination modes of the lasing fields. The relative mode is decoupled from the active medium and thus remains in its vacuum state, while the sum mode operates well above threshold and has sub-shot-noise. The quantum beat and the sum mode intensity noise reduction combine to yield entanglement between two bright beams and sub-Poissonian photon statistics of the respective beams.

DOI
10.1103/physreva.78.045801

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We show that a quantum-beat system with incoherent pump or coherent driving produces an entangled sub-shot-noise laser that operates well above threshold. The numerical results and physical analyses are presented using the combination modes of the lasing fields. The relative mode is decoupled from the active medium and thus remains in its vacuum state, while the sum mode operates well above threshold and has sub-shot-noise. The quantum beat and the sum mode intensity noise reduction combine to yield entanglement between two bright beams and sub-Poissonian photon statistics of the respective beams.
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.
We present a collection of our recent schemes that have been proposed for continuous variable entanglement. Included in this collection are two types of optical systems. One is the four-wave mixing in resonantly or near-resonantly driven atomic systems. The numerical results and physical analyses are presented by using dressed atomic states and squeeze transformed cavity modes. Two dissipation channels are identified, through which the dressed atoms simultaneously absorb in the excitations from the pair of squeeze transformed modes. It is in the presence of such two channels that the entanglement is greatly enhanced and the best achievable state is the original Einstein-Podolsky-Rosen (EPR) entangled state. This scheme is applicable in the optical regime where atomic spontaneous emission has to be taken into account, unlike the two-step atomic reservoir engineering scheme, which is limited to the microwave regime. The other kind of systems is the quantum-beat laser with incoherent pump or coherent driving. Such a laser, when it operates well above threshold, produces entangled light. The numerical results and physical analyses are presented by using the collective modes of the lasing fields. The relative mode is decoupled from the active medium and thus remains in its vacuum state, while the sum mode operates well above threshold and displays sub-shot noise. The quantum-beat and the sum mode intensity noise reduction combine to yield entanglement between two bright beams and sub-Poissonian photon statistics of respective beams.
It is shown that the Λ quantum beat system, into which a new atomic coherence is introduced, can produce bright two-mode sub-Poissonian light. The Mandel Q parameter is calculated using Scully-Lamb theory. On one hand, when quantum noises of the relative amplitude and relative phase drop to vaccum noise levels, the average mode exhibits sub-Poissonian photon distribution; when quantum noises of the average amplitude and average phase drop to vaccum noise levels, the relative mode exhibits sub-Poissonian photon statistics; the optimum Mandel Q parameter is -0.5. On the other hand, in the above two cases, each of the correlated spontaneous emission laser modes exhibits sub-Poissonian photon statistics with optimum Mandel Q parameter of -0.25.
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.
We theoretically consider a three-level laser with an incoherent one-way pump from a ground state into an intermediate excited state and a coherent drive into the upper laser state. The possibilities of inversionless lasing are studied. It is demonstrated that, in the case of a strong coupling with the drive laser (when its generation depends essentially on the three-level medium), the quantum properties of laser radiation can be improved by using m-quantum coherent excitation or by applying a sub-Poissonian coherent pump.
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.