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Quantum Optics Journal of the European Optical Society Part B

The degenerate four-wave mixer: SU(1,1) symmetry and photon statistics

Faige F. Singer, Richard A. Campos, Malvin Carl Teich, Bahaa E. A. Saleh

Published Aug 1, 1990• 9 citations
Publisher

Abstract

The degenerate four-wave mixer is a nonlinear optical device capable of being configured to provide an extraordinarily broad range of photon statistics. The authors examine these statistics at its output in terms of the matrix representation of the process, which belongs to the SU(1,1) group of second-order unimodular matrices. The connection between this group and that of proper Lorentz transformations in two space dimensions and one time dimension permits the field density operators at the input and output ports of the device to be related by means of unitary transformations. This, in turn, provides the joint output photon number distribution for any joint input state.

DOI
10.1088/0954-8998/2/4/003

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arXiv:1311.2479•Journal of Nonlinear Optical Physics & Materials•Jun 1, 2015•7 citations
The degenerate four-wave mixer is a nonlinear optical device capable of being configured to provide an extraordinarily broad range of photon statistics. The authors examine these statistics at its output in terms of the matrix representation of the process, which belongs to the SU(1,1) group of second-order unimodular matrices. The connection between this group and that of proper Lorentz transformations in two space dimensions and one time dimension permits the field density operators at the input and output ports of the device to be related by means of unitary transformations. This, in turn, provides the joint output photon number distribution for any joint input state.
A comprehensive formalism for a two-mode SU(1,1) interferometer (a nondegenerate parametric amplifier, a degenerate four-wave mixer, or a phase-conjugating mirror) is presented. The device changes the quantum state of the two incident modes by an SU(1,1) transformation. Apart from phase shiftings, the quadrature wave functions are Lorentz transformed. A transformation formula in the Fock space of the two modes is derived. Hybrids of s and -s quasiprobability distributions are changed by the inverse-mode transformation. It is related to quantum noise in phase conjugation. Linear amplification caused by Gaussian reservoirs corresponds exactly to a heuristic nondegenerate-parametric-amplifier model. It allows a simple geometrical explanation for the sensitivity of squeezing or quantum superposition with respect to phase-sensitive amplification.
For optical homodyning, the matrix representation of a lossless beam splitter belongs to the SU(2) group of unimodular second-order unitary matrices. The connection between this group and the rotation group in three dimensions permits the field density operators at the input and output ports of the beam splitter to be related by means of well-known angular-momentum transformations. This, in turn, provides the joint output photon-number distribution, which may be written as a Fourier series in the relative phase shift imparted by the beam splitter, for a general joint state at its inputs. The series collapses to a single term if one of the input fields is diagonal in the number-state representation. If the inputs to both ports are further restricted to be pure number states, the joint, as well as the marginal photon-number distributions, turn out to be directly proportional to the square of Jacobi polynomials in the beam-splitter transmittance. These photon-number probabilities are invariant to a set of physical and time-reversal symmetries. When one of the input photon-number states is the vacuum, the beam splitter simply deletes photons from the other port in Bernoulli fashion, as if they were classical particles. The output photon number is then described by the binomial distribution. If the inputs at the two ports are different number states, neither of which is the vacuum, the photon-number distribution is expressible in terms of summed and weighted products of the results for photomixing with the vacuum. If the inputs at the two ports are identical number states, and a beam splitter of transmittance \ensuremath{\tau}=(1/2 is used, the photon-number distribution assumes a simple but interesting form. It vanishes for odd photon numbers, indicating that the photons assemble in pairs at each output port. Finally, it is shown that homodyning quantum fluctuations can be reduced by using a balanced photomixer for arbitrary input states.
A Lie-group-theoretical approach to the analysis of interferometers is presented. Conventional interferometers such as the Mach-Zehnder and Fabry-Perot can be characterized by SU(2). We introduce a class of interferometers characterized by SU(1,1). These interferometers employ active elements such as four-wave mixers or degenerate-parametric amplifiers in their construction. Both the SU(2) and SU(1,1) interferometers can in principle achieve a phase sensitivity \ensuremath{\Delta}\ensuremath{\varphi} approaching 1/N, where N is the total number of quanta entering the interferometer, provided that the light entering the input ports is prepared in a suitable quantum state. SU(1,1) interferometers can achieve this sensitivity with fewer optical elements.
A model of a four-wave mixer operated in a nonlinear regime, studied by Yurke and Stoler [Phys. Rev. A 35, 4846 (1987)], is reexamined. Yurke and Stoler have shown that this device, under a certain condition, acts as an even-odd filter, switching even-photon-number states from the pump mode to the signal mode. An initial coherent state in the pump is converted into an entanglement of even and odd coherent states with vacuum states in the output signal and pump modes. We point out that under a different condition and with an even-photon-number state initially in the pump and a vacuum in the signal, the device creates a maximally entangled state between the number state and the vacuum. Using the device to replace the first beamsplitter of a Mach-Zehnder interferometer, phase uncertainties at the Heisenberg limit $(\ensuremath{\Delta}\ensuremath{\varphi}=1/n)$ can be obtained. Since number states are difficult to generate, we point out that an even coherent state obtained from the output of one device can be used as input to a second to achieve the phase uncertainties $\ensuremath{\Delta}\ensuremath{\varphi}{=1/n}_{e},$ where ne{n}_{e}ne​ is the average photon number of the even coherent state.
The suitability of degenerate four-wave mixing (DFWM) as a source of squeezed-state (two-photon coherent state) light is investigated, both theoretically and experimentally. In previous semiclassical theory, which neglected pump quantization and loss in the nonlinear medium, Yuen and Shapiro [Opt. Lett. 4, 334 (1979)] showed that such states would be generated by 50%-50% combination of the transmitted probe (TP) and the phase-conjugate (PC) output beams of DFWM. Here the effects of quantum fluctuations in the amplitudes and phases of the pump beams are calculated in a traveling-wave finite-interaction-length DFWM configuration. It is shown that in the limit of strong pump beams and weak nonlinear coupling, the classical-pumps assumption of Yuen and Shapiro is valid. With use of this assumption, loss in the nonlinear medium is incorporated into the model by coupling the interacting modes to a set of reservoir modes. It is shown that loss presents an absolute limit on the degree of squeezing that can be achieved with DFWM. Also, for comparison with experimental work, the photoelectron statistics of the TP, PC, and 50%-50% combination modes are calculated for lossy DFWM; significant deviations are found from the predictions for lossless operation. Quantum-noise measurements on the PC and TP beams generated in sodiumvapor DFWM are described. The mixer is pumped and probed by the nearly transform-limited pulses produced by a continuous-wave-oscillator-pulse-amplifier chain dye laser at 589 nm wave-length. The normalized second-factorial moment g2{g}_{2}g2​ of the PC and TP beams is determined from photoelectron-counting measurements. Inability to simultaneously achieve high DFWM reflectivity and acceptably low scattered-plus-fluorescent background, coupled with low photomultiplier quantum efficiency, restrict the experiment to a DFWM regime wherein theory predicts Poisson statistics. Experimental g2{g}_{2}g2​ values corroborate this prediction, indicating that systematic and excess noises have been eliminated. No attempt was made to measure the quantum statistics of the 50%-50% combination mode.