Switchable Ultrastrong Coupling in Circuit QED

Abstract

We propose different designs of switchable coupling between a superconducting flux qubit and a microwave transmission line. They are based on two or more loops of Josephson junctions which are directly connected to a closed (cavity) or open transmission line. In both cases the circuit induces a coupling that can be modulated in strength, reaching the so-called ultrastrong coupling regime in which the coupling is comparable to the qubit and photon frequencies. Furthermore, we suggest a wide set of applications for the introduced architectures. DOI: 10.1103/PhysRevLett.105.023601 PACS numbers: 42.50.Àp, 03.67.Lx, 85.25.Àj Superconducting quantum circuits [1] possess ingredients for quantum information processing and for developing on-chip microwave quantum optics [2]. After the first manipulations of few-level superconducting systems (qubits) [3][4][5], the real boost came with the achievement of the strong coupling regime between qubits and confined microwave photons [6][7][8]. The initial qubit-cavity couplings of 10-100 MHz exceeded by orders of magnitude the rate at which photons leak out of the resonator, but the use of the transmon qubit [9] improved those numbers by a factor of 2-3 reaching a strength that is comparable only to the state of the art in microwave quantum optics [10,11]. More recently, proof-of-principle theoretical and experimental studies have paved the way to the ultrastrong coupling regime [12][13][14], where the coupling approaches the qubit transition frequency and the Jaynes-Cummings model of cavity QED [10,14] breaks down [15,16], and a door opens to the rather unexplored physics beyond the rotating-wave approximation [17,18].The strong coupling regime in circuit QED has made possible an incredible variety of experiments, such as dispersive readouts of qubits [19], resolving the photon numbers in cavity [20], multiphoton excitations of the Jaynes-Cummings model [21], preparing nonclassical states of a resonator [22], full quantum tomography of the microwave radiation field [23], or the TavisCummings model [24], etc. However, all those experiments have something in common: The microwave field is confined inside a resonator. In other words, the transmission line spectrum is discrete and the coupling between qubits and photons could be switched on and off by tuning the qubit [25] or cavity frequency [26]. While the switchability of the coupling has been proposed for open lines [27,28], this has not been achieved in the ultrastrong coupling regimes.In this work, we will introduce a novel circuit QED design where the qubit is ultrastrongly coupled to a transmission line, open or not, with a coupling that can be tuned in strength and kind by applying an external flux bias. Our proposal uses the type of designs shown in Fig. 1, where the qubit is built in direct contact with the transmission line. It has been shown theoretically [14], and demonstrated experimentally [13], that the system admits an effective description based on a two-level system-the current in the loop-ultrastrongly coup...

Zeno physics in ultrastrong-coupling circuit QED

“…Arguably, the most promising setup would consist on a superconducting qubit interacting via a tunable coupling with the microwave resonator [29]. By controlling the interaction we should be able not only to choose between strong and ultrastrong couplings, but also to completely deactivate the interaction for brief periods of time in order to measure the system.…”

Light-shift-induced quantum phase transitions of a Bose-Einstein condensate in an optical cavity

“…More importantly, recent experiment has reported the existence of the ultrastrong coupling with the ratio 0.12 between the coupling strength and the microwave photon frequency [13]. Moreover, this ratio maybe approach unit due to the current efforts [14,15].…”

“…More importantly, recent experiment has reported the existence of the ultrastrong coupling with the ratio 0.12 between the coupling strength and the microwave photon frequency [13]. Moreover, this ratio maybe approach unit due to the current efforts [14,15].However, in this ultrastrong coupling regime the wellknown RWA breaks down and the whole Hamiltonian is written aswhere a † and a are creation and annihilation operators for photon with frequency ω, σ ± are the raising and lowering operators of the two-level atom in the basis of σ z , Ω is the atomic resonant frequency and g is the atomphoton coupling strength. Due to the existence of the counter-rotating terms (σ + a + and σ − a), Hamiltonian (1) is very difficult to be solved analytically except for Ω = 0.…”

Circuit quantum electrodynamics in the ultrastrong-coupling regime

“…Our system can act as an on-chip prototype for unveiling the physics of ultrastrong light-matter interaction. Future explorations may include squeezing, switchable ultrastrong coupling 29 , causality effects in quantum field theory 30 , and the generation of bound states of qubits and photons.…”