|Title||Development of a Josephson Parametric Ampliﬁer for the Preparation and Detection of Nonclassical States of Microwave Fields|
|Year of Publication||2010|
Recent innovations in the technology of superconducting circuits have made it possible to create nonclassical states of microwave light ﬁelds. These states are usually associated with the subject of quantum optics. The ability to manipulate quantum states of microwave light ﬁelds holds out the promise of building dense integrable cir- cuits to process quantum information electrically. However, a major diﬃculty lies in the fact that there is no general purpose method of eﬃciently measuring the quantum state of a microwave ﬁeld. In this thesis, I describe the development of an ampliﬁer, based on a Josephson parametric ampliﬁer. This new parametric ampliﬁer enables this kind of eﬃcient measurement. Although parametric ampliﬁers are narrowband (only ampli- fying signals in a narrow frequency range around a central frequency), I have developed an ampliﬁer whose central frequency is widely tunable (a full octave, 4–8 GHz), greatly improving its usefulness. I have studied the gain, bandwidth, dynamic range, and added noise of the ampliﬁer. I have shown that when operated in a particular manner, known as degenerate mode, the parametric ampliﬁer adds less noise than the noise associated with the quantum ﬂuctuations of the electromagnetic vacuum. In addition, its gain is large enough to ensure that the ampliﬁed vacuum noise overwhelms the noise added by conventional ampliﬁers that follow the parametric ampliﬁer. Together, these two fea- tures make it possible to measure either the phase or amplitude of a microwave signal where the dominant uncertainty comes from the quantum noise of the signal itself. In addition, a degenerate parametric ampliﬁer also prepares particular nonclassical states called squeezed states. Our parametric ampliﬁer prepares squeezed states, which have (in one of its quadratures) noise with a variance less than one tenth that of the vacuum noise. I have employed the parametric ampliﬁer in two experiments that require its abil- ity to make eﬃcient measurements. First, I used the parametric ampliﬁer to determine the full density matrix of a squeezed state generated by a second parametric ampliﬁer. Second, I used the ampliﬁer to enable a measurement of position with precision bet- ter than the value at the standard quantum limit. These demonstrations are powerful evidence that we have indeed realized a general-purpose quantum-eﬃcient method of measuring microwave ﬁelds.