We measure the dispersive energy-level shift of an LC resonator magnetically coupled to a superconducting qubit, which clearly shows that our system operates in the ultrastrong coupling regime. The large mutual kinetic inductance provides a coupling energy of ≈ 0.82 GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model. We find a 50 MHz Bloch-Siegert shift when the qubit is in its symmetry point, fully consistent with our analytical model.
superconducting circuits, semiconductor quantum wells, and other hybrid quantum systems. Finally, anticipated applications are highlighted utilizing USC and DSC regimes, including novel quantum optical phenomena, quantum simulation, and quantum computation. CONTENTSCaltech Caltech Caltech LKB Paris LKB Paris LKB Paris LKB Paris LKB Paris Harvard Harvard Würzburg U Tokyo ETH U Tokyo Stanford Princeton (MW) ETH (MW) Yale Delft, NTT IMS WMI, Delft WMI NICT ETH ETH UPD ISSP UPD UPD U Reg IMS U Tokyo CNRS CNRS NCU CNR ICL Caltech Caltech Caltech LKB Paris LKB Paris LKB Paris Harvard Harvard Würzburg LKB Paris LKB Paris NICT Yale Delft, NTT WMI, Delft WMI UPD IMS UPD UPD IMS ETH ETH
The study of light-matter interaction has led to many fundamental discoveries as well as numerous important technologies. Over the last decades, great strides have been made in increasing the strength of this interaction at the single-photon level, leading to a continual exploration of new physics and applications. Recently, a major achievement has been the demonstration of the so-called strong coupling regime [1, 2], a key advancement enabling great progress in quantum information science. Here, we demonstrate light-matter interaction over an order of magnitude stronger than previously reported, reaching the nonperturbative regime of ultrastrong coupling (USC). We achieve this using a superconducting artificial atom tunably coupled to the electromagnetic continuum of a one-dimensional waveguide. For the largest coupling, the spontaneous emission rate of the atom exceeds its transition frequency. In this USC regime, the description of atom and light as distinct entities breaks down, and a new description in terms of hybrid states is required [4, 8]. Our results open the door to a wealth of new physics and applications. Beyond light-matter interaction itself, the tunability of our system makes it a promising tool to study a number of important physical systems such as the well-known spin-boson [9] and Kondo models [12]. * These authors contributed equally to this work.
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...
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