UC Berkeley

The laws of quantum mechanics imply the existence of an intrinsic uncertainty, or noise, in the electromagnetic field. These noise fluctuations are central to many processes in atomic physics and quantum optics, including spontaneous emission of radiation by an atomic system, backaction on an atomic system during non-demolition measurement by probe light, and intrinsic bounds on the noise performance of an ideal amplifier. While quantum noise cannot be eliminated, the noise of one observable quantity may be reduced provided that of the conjugate observable is increased in accord with the relevant Heisenberg uncertainty relation; this process is known as squeezing.

Recently, superconducting circuits have emerged as a powerful platform for studying the interaction of squeezed light and matter, leveraging the low-dimensionality of the circuit environment to efficiently couple atomic systems to squeezed radiation. Beyond enabling the verification of canonical predictions of quantum optics, these experiments explore the potential utility of squeezing for the state readout of quantum bits, or qubits, used for quantum information processing.

In this thesis, we present three experiments probing the interaction of a superconducting qubit with squeezed radiation. First, we observe how the fluorescence spectra emitted by a two-level atomic system are modified by squeezing of a resonant drive. The subnatural linewidths of the resulting spectra provide the first successful verification in any system of predictions from nearly three decades prior, and provide a tool for characterization of microwave squeezed states. Second, we combine injected squeezed noise with a stroboscopic measurement scheme to demonstrate the first improvement of the signal-to-noise ratio of qubit state readout due to input squeezing. This study includes a characterization of the effect of squeezing on measurement backaction, exhibiting the first use of squeezing to slow measurement-induced dephasing. Finally, we develop a circuit incorporating the qubit inside of a squeezed-microwave source and extensively study the measurement physics of this hybrid system. This device enables the transfer of quantum information from the qubit at $\sim$30 milliKelvin to a room temperature detector with a marked increase in steady-state efficiency.