Article

Wallraff, A. et al. Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics. Nature 431, 162-167

Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA.
Nature (Impact Factor: 42.35). 10/2004; 431(7005):162-7. DOI: 10.1038/nature02851
Source: PubMed

ABSTRACT The interaction of matter and light is one of the fundamental processes occurring in nature, and its most elementary form is realized when a single atom interacts with a single photon. Reaching this regime has been a major focus of research in atomic physics and quantum optics for several decades and has generated the field of cavity quantum electrodynamics. Here we perform an experiment in which a superconducting two-level system, playing the role of an artificial atom, is coupled to an on-chip cavity consisting of a superconducting transmission line resonator. We show that the strong coupling regime can be attained in a solid-state system, and we experimentally observe the coherent interaction of a superconducting two-level system with a single microwave photon. The concept of circuit quantum electrodynamics opens many new possibilities for studying the strong interaction of light and matter. This system can also be exploited for quantum information processing and quantum communication and may lead to new approaches for single photon generation and detection.

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    • "When system B is a photon field andˆH B is proportional to the number of photons in the field, thenˆH AB describes the weak probing of the population difference of an ensemble of two-level atoms with far-detuned light [37, 51– 60], or dispersive coupling between a microwave cavity and a superconducting qubit [61] [62] [63]. We will explore this specific case shortly, however, for now we keepˆH B completely general. "
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    ABSTRACT: Quantum-enhanced metrology can be achieved by entangling a probe with an auxiliary system, passing the probe through an interferometer, and subsequently making measurements on both the probe and auxiliary system. Conceptually, this corresponds to performing metrology with the purification of a (mixed) probe state. We demonstrate via the quantum Fisher information how to design mixed states whose purifications are an excellent metrological resource. In particular, we give examples of mixed states with purifications that allow (near) Heisenberg-limited metrology, and provide example entangling Hamiltonians that can generate these states. Finally, we present the optimal measurement and parameter-estimation procedure required to realize these sensitivities (i.e. that saturate the quantum Cram\'er-Rao bound). Since pure states of comparable metrological usefulness are typically challenging to generate, it may prove easier to use this approach of entanglement and measurement of an auxiliary system. An example where this may be the case is atom interferometry, where entanglement with optical systems is potentially easier to engineer than the atomic interactions required to produce nonclassical atomic states.
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    • "We assume a superconducting qubit is integrated into cavity a. It is initialized in its ground state |g, and exhibits a switchable dispersive interaction [26] [27] [28] "
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    ABSTRACT: Opto- and electromechanical systems offer an effective platform to test quantum theory and its predictions at macroscopic scales. To date, all experiments presuppose the validity of quantum mechanics, and could in principle be described by a hypothetical classical statistical theory. Here we suggest a Bell test using the electromechanical Einstein-Podolski-Rosen entangled state recently generated by Palomaki et al., which would rule out any classical explanation of the measured data without assuming the validity of quantum mechanics at macroscopic scales. The parameter regime required in this scheme has been demonstrated or is within reach of current experiments.
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    • "Subsequently, leveraging ideas from cavity quantum electrodynamics, a superconducting qubit has also been coupled to a microwave cavity (i.e. coplanar waveguide resonator), launching the field of cQED [2], [3]. In such a scheme, coupling a qubit to a resonator with discrete electromagnetic modes modifies the available decay channels (see Fig. 1b) from that of the continuum of free space. "
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    ABSTRACT: Physical implementations of qubits can be extremely sensitive to environmental coupling, which can result in decoherence. While efforts are made for protection, coupling to the environment is necessary to measure and manipulate the state of the qubit. As such, the goal of having long qubit energy relaxation times is in competition with that of achieving high-fidelity qubit control and measurement. Here we propose a method that integrates filtering techniques for preserving superconducting qubit lifetimes together with the dispersive coupling of the qubit to a microwave resonator for control and measurement. The result is a compact circuit that protects qubits from spontaneous loss to the environment, while also retaining the ability to perform fast, high-fidelity readout. Importantly, we show the device operates in a regime that is attainable with current experimental parameters and provide a specific example for superconducting qubits in circuit quantum electrodynamics.
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