Possibilities of ultra‐low noise acoustic parametric amplification in liquid helium Free

Liquid helium at temperatures less than 0.1 K is a nearly ideal medium for acoustic parametric amplification at microwave frequencies. The desirable qualities of liquid helium include low attenuation, large upward dispersion, and strong nonlinear acoustic interaction. Calculations indicate that an acoustic parametric amplifier operating at gigahertz frequencies can be constructed with signal gain in excess of 50 dB per millimeter of propagation path and would require less than 1 mW/cm² of pump intensity. When T⩽hf/k, where T is the helium temperature, h is Planck's constant,f is the signal frequency, and k is Boltzmann's constant, then the noise performance of the amplifier is quantum limited and the noise temperature is given by T_N = hf/k ln 2. At 2 GHz this corresponds to T_N = 0.14 K. Even when transduction losses are included, such an amplifier would have electrical noise performance superior to today's lowest noise maser amplifiers. Another interesting possibility is to use a degenerate parametric amplifier to produce phonon squeezed states. Squeezed states, which have attracted considerable interest in quantum optics, exhibit reduced quantum noise in one quadrature of phase.