Resolving photon number states in a superconducting circuit
Resolving photon number states in a superconducting circuit
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DOI：
10.1038/nature05461
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发表时间：
2007-02-01
期刊：
影响因子：
64.8
通讯作者：
Schoelkopf, R. J.
中科院分区：
文献类型：
--
作者：
Schuster, D. I.;Houck, A. A.;Schoelkopf, R. J.
电磁信号总是由光子组成，尽管在电路领域，这些信号以电压和电流的形式在电线上传输，并且光子能量的离散性通常并不明显。然而，通过将超导量子比特（qubit）耦合到微波传输线上的信号，可以构建一个集成电路，其中即使是单个光子的存在或不存在也可以产生巨大的影响。这样的系统（1）可以用电路量子电动力学（QED）来描述-电路等效的腔QED，其中光子与原子或量子点相互作用。以前，电路QED设备被证明可以达到共振强耦合状态，其中单个量子位可以多次吸收和重新发射单个光子（2）。在这里，我们报告了一个电路QED实验在强色散限制，一个新的制度，其中一个单光子有一个很大的影响量子比特没有被吸收。这种强色散机制的特点是量子比特跃迁能量可以被分解为微波场的每个光子数态的单独谱线。每条线的强度是在腔中找到相应光子数的概率的量度。这种效应用于区分相干场和热场，并可用于创建光子统计分析器。由于没有光子被这个过程吸收，因此应该可以通过测量产生光的非经典状态，并执行量子比特-光子条件逻辑，这是量子计算机逻辑总线的基础。
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 system(1) 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 times(2). 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.