A Low Frequency Squeezed State Source
There has been considerable interest in the generation of two-photon coherent states or squeezed states in the optical regime in order to carry out various quantum signal processing schemes. H. Takahasi first noted that degenerate parametric amplifiers (DPAs) could be used to generate two-photon coherent state signals. With the availablility of the Josephson junction inductive nonlinearity there is reason to believe that squeezed state generation with DPAs may be more readily demonstrated in the radio frequency or microwave frequency regime than at optical frequencies. The availability of squeezed state sources at low frequencies is also of interest, particularly in regard to the back-action-evading detection schemes proposed by Thorne, Dreyer, Caves, Zimmerman and Sandberg for reading out the response of a Weber bar to passing gravitational waves. We have shown how two DPAs operating in the negative resistance reflection mode can be coupled together to form a back-action-evading amplifier. This scheme required circulators to properly direct the signal flow through the circuit. Here we present an alternative method of forming a back-action-evading amplifier or squeezed state source. Instead of using circulators, the amplifiers are coupled in a balanced configuration via a 3 dB 90° hybrid coupler. Hybrid couplers can be built out of lumped circuit components and hence are more readily realized at sub-microwave frequencies than a circulator. The configuration of the balanced amplifier is depicted in Fig. 1 below. The two DPAs have equal gains and are pumped in such a manner that they amplify in quadrature. This is most easily accomplished by splitting the pump power coming from the pump oscillator with a 3 dB 180° power splitter.
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