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.
In quantum mechanics, the process of measurement is a subtle interplay between extraction of information and disturbance of the state of the quantum system. A
We present a new readout method for a superconducting flux qubit, based on the measurement of the Josephson inductance of a superconducting quantum interference device that is inductively coupled to the qubit. The intrinsic flux detection efficiency and backaction are suitable for a fast and nondestructive determination of the quantum state of the qubit, as needed for readout of multiple qubits in a quantum computer. We performed spectroscopy of a flux qubit and we measured relaxation times of the order of 80 s. DOI: 10.1103/PhysRevLett.93.177006 PACS numbers: 85.25.Cp, 03.67.Lx, 74.50.+r, 85.25.Dq Suitably designed superconducting circuits, based on Josephson junctions, behave as quantum two level systems. Because of scalability and flexibility in their design parameters, they are promising candidates for quantum bits (or qubits), which are the basic units in a quantum information processor [1]. In such circuits, coherent evolution for single qubits was observed [2,3], and a conditional gate for two qubits was demonstrated [4].Flux qubits consist of a superconducting loop interrupted by one or more Josephson junctions. The basis states have opposite persistent current. The quantum state can be read out by measuring the generated magnetic flux, using a dc superconducting quantum interference device (dc-SQUID). The critical current of the SQUID depends on this flux and is usually measured by determining the maximum supercurrent, where the device switches to a finite voltage. When the SQUID is in this voltage state it generates quasiparticles that later recombine with a burst of energy. It also radiates strong high frequency signals into the whole circuit with, in future, multiple qubits and readout devices. Inevitably, significant quantum information is destroyed apart from the consequence of reading out one qubit. In contrast, we now measured the SQUID critical current by determining the Josephson inductance, without dissipation in the SQUID system. In this Letter, we discuss the intrinsic properties of this inductive readout and we present the first results of measurements on a flux qubit.In the experiments, we use a persistent current qubit (PCQ) [5]. The PCQ is a flux qubit consisting of a small inductance superconducting loop interrupted by three Josephson junctions. Two of the three junctions are equal, characterized by the Josephson coupling energy E J and the charging energy E C , and the third one is smaller by a factor . At low temperatures and appropriate values of E J > E C and , and with an external magnetic flux qb close to 2n 1 0 =2 (n integer) in the loop, the circuit behaves as a two level system. In the basis of two flux states, the Hamiltonian isin which i i x; y; z are the Pauli matrices and 2I p qb ÿ 2n 1 0 =2. The minimum energy level splitting and the maximum qubit persistent current I p are determined by E J , E C , and . The two energy eigenstates (which are linear superpositions of the current states) each have an expectation value of the circulating current given by the derivative o...
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