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Physical Review A

Generation of a sub-Poissonian state with quantum high- and low-pass filters

Haibo Wang, Yongmin Li, Satoru Odate, Takayoshi Kobayashi

Published Jul 27, 2005• 1 citation
Publisher

Abstract

We introduce a quantum component, the quantum high- and low-pass filter, and analyze its output characteristic. It is shown that a quantum bandpass filter can be achieved by using passive linear optics and projection measurements. Furthermore, we show that the sub-Poissonian state can be generated by using a cascaded quantum low-pass filter and high-pass filter.

DOI
10.1103/physreva.72.013822

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We introduce a quantum component, the quantum high- and low-pass filter, and analyze its output characteristic. It is shown that a quantum bandpass filter can be achieved by using passive linear optics and projection measurements. Furthermore, we show that the sub-Poissonian state can be generated by using a cascaded quantum low-pass filter and high-pass filter.
The author reviews methods for generating sub-Poissonian light and related concepts. This light has energy fluctuations reduced below the level which corresponds to a classical Poissonian process (shot-noise level). After an introduction to the concept of nonclassical light, an overview is given of the main methods of quantum-noise reduction. Sub-Poissonian processes are exemplified in different areas of optics, ranging from single-atom resonance fluorescence to nonlinear optics, laser physics, and cavity quantum electrodynamics. Emphasis is placed on the conceptual foundations, and on developments in laser theory that lead to the possibility, already demonstrated experimentally, of linewidth narrowing and sub-Poissonian light generation in lasers and masers. The sources of quantum noise in these devices are analyzed, and four noise-suppression methods are discussed in detail: regularization of the pumping, suppression of spontaneous-emission noise, nonadiabatic evolution of the atomic variables, and twin-beam generation.
In this thesis, we study a system consisting of qubits—two-level quantum systems— interacting in the strong-coupling regime for a finite amount of time with a mode of the electromagnetic field inside a resonator. In particular, we are interested in the dynamics of the photon number and the possibility of controlling its statistics to produce sub-Poissonian light. To describe the system we use quantum trajectory theory where the time evolution of the state vector consists of both a continuous part and jumps, which account for the resonator losses. We also incorporate a variable number of qubits (changing in time) interacting with the mode. A computer simulation was developed and optimised to calculate the quantum trajectories. When one sets the qubits initially to the excited state and fixes the interaction time accordingly, so-called “trapping states” (photon numbers which block the addition of a photon by a single qubit) appear. These states are instrumental in the generation of sub-Poisson fields, for some cases even when the number of interacting qubits follow a Poisson process. We take advantage of the existence of trapping states to implement control protocols based on the measurement of the output field of the resonator, with the objective of lowering the uncertainty in the number of photons. Furthermore, we explore the possibility of creating a source of sub-Poissonian pulses by first using a random interaction time (which bypasses the trapping states) and then utilising the control protocols developed. We are able to achieve fields with a Mandel Q parameter of QM = −0.98 and a photon number of n 68.
A near-number state features strongly sub-Poissonian photon-number fluctuation and, unlike a quadrature squeezed state, the photonnumber noise can approach zero, with a finite average photon number. It is now well established that a beam with a sub-Poissonian photon-number fluctuation can be generated directly from a semiconductor laser,l,2 The basic physical processes are a) quantum mechanical interference between the transmitted internal field and the reflected vacuum field at the output coupler and b) high impedance suppression of the pump noise.
The paper considers the problem of equalization of passive linear quantum systems. While our previous work was concerned with the analysis and synthesis of passive equalizers, in this paper we analyze coherent quantum equalizers whose annihilation (respectively, creation) operator dynamics in the Heisenberg picture are driven by both quadratures of the channel output field. We show that the characteristics of the input field must be taken into consideration when choosing the type of the equalizing filter. In particular, we show that for thermal fields allowing the filter to process both quadratures of the channel output may not improve mean square accuracy of the input field estimate, in comparison with passive filters. This situation changes when the input field is `squeezed'.