Emergence of nonlinear friction from quantum fluctuations
ArXiv 2104.06464 (2021)
Abstract:Nonlinear damping, a force of friction that depends on the amplitude of motion, plays an important role in many electrical, mechanical and even biological oscillators. In novel technologies such as carbon nanotubes, graphene membranes or superconducting resonators, the origin of nonlinear damping is sometimes unclear. This presents a problem, as the damping rate is a key figure of merit in the application of these systems to extremely precise sensors or quantum computers. Through measurements of a superconducting circuit, we show that nonlinear damping can emerge as a direct consequence of quantum fluctuations and the conservative nonlinearity of a Josephson junction. The phenomenon can be understood and visualized through the flow of quasi-probability in phase space, and accurately describes our experimental observations. Crucially, the effect is not restricted to superconducting circuits: we expect that quantum fluctuations or other sources of noise give rise to nonlinear damping in other systems with a similar conservative nonlinearity, such as nano-mechanical oscillators or even macroscopic systems.
Superconducting electro-mechanics to explore the effect of general relativity in massive superpositions
ArXiv 2103.12729 (2021)
Abstract:Attempting to reconcile general relativity with quantum mechanics is one of the great undertakings of contemporary physics. Here we present how the incompatibility between the two theories arises in the simple thought experiment of preparing a heavy object in a quantum superposition. Following Penrose's analysis of the problem, we determine the requirements on physical parameters to perform experiments where both theories potentially interplay. We use these requirements to compare different systems, focusing on mechanical oscillators which can be coupled to superconducting circuits.
Phonon-number resolution of voltage-biased mechanical oscillators with weakly-anharmonic superconducting circuits
ArXiv 2103.04829 (2021)
Abstract:Observing quantum phenomena in macroscopic objects, and the potential discovery of a fundamental limit in the applicability of quantum mechanics, has been a central topic of modern experimental physics. Highly coherent and heavy micro-mechanical oscillators controlled by superconducting circuits are a promising system for this task. Here, we focus in particular on the electrostatic coupling of motion to a weakly anharmonic circuit, namely the transmon qubit. In the case of a megahertz mechanical oscillator coupled to a gigahertz transmon, we explain the difficulties in bridging the large electro-mechanical frequency gap. To remedy this issue, we explore the requirements to reach phonon-number resolution in the resonant coupling of a megahertz transmon and a mechanical oscillator.
Measuring and controlling radio-frequency quanta with superconducting circuits
ArXiv 2004.09153 (2020)
Abstract:In this PhD thesis, we will present the theoretical and experimental work that led to the realization of a radio-frequency circuit quantum electrodynamics system (RFcQED). In chapter 2, we provide a detailed derivation of the Hamiltonian of circuit QED formulated in the context of the Rabi model, and extract expressions for the cross-Kerr interaction. The resulting requirements for the coupling rate in RFcQED are discussed, one of them being the need to dramatically increase the coupling rate compared to typical circuit QED device. In chapter 3 we cover two experimental approaches to increasing the coupling in a circuit QED system, one making use of a high impedance resonator, the second utilizing a large coupling capacitor. In chapter 4, we combine these two approaches to implement RFcQED. Through strong dispersive coupling, we could measure individual photons in a megahertz resonator, demonstrate quantum control by cooling the resonator to the ground state or preparing Fock states, and finally observe with nanosecond resolution the re-thermalization of these states. In chapter 5 we present QuCAT or Quantum Circuit Analyzer Tool in Python, a software package that can be used for the design of circuit QED systems such as the one presented in this thesis. In chapter 6 we discuss how certain interplays between general relativity and quantum mechanics cannot be described using our current laws of physics. In particular, we show how radio-frequency mechanical oscillators are perfect candidates to perform experiments in this regime. In chapter 7 we present the prospects for coupling such mechanical oscillator to weakly anharmonic superconducting circuits such as the transmon qubit or RFcQED systems.
QuCAT: quantum circuit analyzer tool in Python
NEW JOURNAL OF PHYSICS 22:1 (2020) ARTN 013025