Cavity and Circuit Quantum Electrodynamics
Circuit quantum electrodynamics (circuit-QED) and cavity quantum electrodynamics (cavity-QED) are rapidly advancing fields in modern theoretical and experimental physics. They offer a wide range of exciting applications, including the the development of quantum computers and the exploration of fundamental quantum mechanics in open systems.
At their core, these fields studies the behaviour of atoms coupled to discrete photon modes in high-quality resonators. In circuit-QED these resonators are constructed using superconducting transmission lines integrated on a chip. In cavity-QED, they consist of cavities with highly reflective walls capable of trapping individual photons. These simple platforms have been used to great effect in the creation and manipulation of photonic Schrödinger cat states and entangled Bell pairs. In fact, Serge Haroche’s contribution to the 2012 Nobel Prize in Physics was partly due to these achievements.
In the context of quantum computing, circuit-QED employs qubits instead of atoms to store and process information. The resonator, on the other hand, plays a crucial role in tasks such as control and readout. In our research group, we utilize computational and analytical techniques to gain a deeper understanding of the dynamics between the qubit and resonator interactions. Armed with this knowledge, we collaborate with experimental groups working on circuit-QED to assist them in achieving their goals of building the essential components of a quantum computer.
Bi-stability in circuit-QED
Currently, one of our main topics of research is how bi-stability in circuit-QED can be used to improve qubit readout protocols. Bi-stability in physics is most commonly described as a situation when a dynamical system can have two stable (equilibrium) states. Small bi-stabe physical systems can dynamically switch between two very different modes of oscillation as a result of random fluctuations induced by a noisy environment. We can find bi-stability in the classical motion of pendula, in the isotropic turbulence and the response of Bose-Einstein condensates among many different configurations.
In the world of quantum oscillators, it is intriguing that switching is enabled by quantum fluctuations as a consequence of the uncertainty principle and even the process of measurement itself. What is even more exciting is to follow the build-up of simultaneous bi-stability for two coupled quantum degrees of freedom, a qubit and a cavity, in an open (noisy) dissipative setup with varying drive amplitude and frequency. In our case there is increased complexity because the qubit oscillator is a superconducting circuit with an unusual non-linearity.
We show that the system response can be continuously tuned from being essentially borne mostly by the cavity to one where the superconducting qubit fully participates in the dynamics. In the experiments we measure the phase of the oscillator and observe a coherent cancellation of the response, a hallmark of bi-stability in the quantum regime. Theory and experiment capture this signature of nonlinearity with a solid-state device. These devices are a valuable means for fundamental research in quantum electrodynamics as well as a tool in the field of quantum computing.
Simultaneous Bistability of a Qubit and Resonator in Circuit Quantum Electrodynamics Th. K. Mavrogordatos, G. Tancredi, M. Elliott, M. J. Peterer, A. Patterson, J. Rahamim, P. J. Leek, E. Ginossar, and M. H. Szymańska Phys. Rev. Lett. 118, 040402 (2017)
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