1. Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
2. These authors contributed equally to this work.
3. Present addresses: Department of Physics, ETH Zurich, CH-8093 Zürich, Switzerland (A.W.); Département de Physique et Regroupement Québécois sur les Matériaux de Pointe, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 (A.B.).

Correspondence to: R. J. Schoelkopf¹ Correspondence and requests for materials should be addressed to R.J.S. (Email: Robert.Schoelkopf@yale.edu).

Abstract

Electromagnetic signals are always composed of photons, although in the circuit domain those signals are carried as voltages and currents on wires, and the discreteness of the photon's energy is usually not evident. However, by coupling a superconducting quantum bit (qubit) to signals on a microwave transmission line, it is possible to construct an integrated circuit in which the presence or absence of even a single photon can have a dramatic effect. Such a system¹ can be described by circuit quantum electrodynamics (QED)—the circuit equivalent of cavity QED, where photons interact with atoms or quantum dots. Previously, circuit QED devices were shown to reach the resonant strong coupling regime, where a single qubit could absorb and re-emit a single photon many times². Here we report a circuit QED experiment in the strong dispersive limit, a new regime where a single photon has a large effect on the qubit without ever being absorbed. The hallmark of this strong dispersive regime is that the qubit transition energy can be resolved into a separate spectral line for each photon number state of the microwave field. The strength of each line is a measure of the probability of finding the corresponding photon number in the cavity. This effect is used to distinguish between coherent and thermal fields, and could be used to create a photon statistics analyser. As no photons are absorbed by this process, it should be possible to generate non-classical states of light by measurement and perform qubit–photon conditional logic, the basis of a logic bus for a quantum computer.

1. Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520, USA
2. These authors contributed equally to this work.
3. Present addresses: Department of Physics, ETH Zurich, CH-8093 Zürich, Switzerland (A.W.); Département de Physique et Regroupement Québécois sur les Matériaux de Pointe, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 (A.B.).

Correspondence to: R. J. Schoelkopf¹ Correspondence and requests for materials should be addressed to R.J.S. (Email: Robert.Schoelkopf@yale.edu).

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