Resolving photon number states in a superconducting circuit
Feb, 2007Citations per year
Abstract: (Springer)
In cavity quantum electrodynamics (QED), atoms or quantum dots are made to strongly interact with single photons. Recent work showed that a regime of 'strong coupling' can be obtained, where a single photon is absorbed and re-emitted many times. Schuster et al. have built a special type of cavity QED system that is embedded within an electronic circuit; in it a superconducting quantum bit (qubit) interacts with photons from a microwave transmission line. A novel regime can be produced in this system, namely the strong dispersive limit, where a single photon has a large effect on the qubit without being absorbed. This opens the possibility of nondestructive counting of photons that are present in the cavity. This effect could be used as a basis for qubit–photon conditional logic, a requirement for quantum computing. A special version of a cavity quantum electrodynamics system, namely one that is embedded within an electronic circuit has been constructed. A superconducting quantum bit interacts with photons from a microwave transmission line. A novel regime can be obtained with this system, namely the strong dispersive limit, where a single photon has a large effect on the qubit without ever being absorbed. 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.References(30)
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