Microwave devices based on inelastic Cooper pair tunneling

随着量子微波（即在电子电路中使用微波光子的量子光学）的出现，光量子光学中几个极其困难的问题得到了解决，新的物理学得到了探索。然而，到目前为止，只有一种量子微波的状态得到了充分的探索：腔（或电路）量子电动力学状态，光子驻留在电谐振器中。***我的研究计划的目标是开发电路量子光学的另一种制度，宽带制度，光子是流动的，即紧凑的波包沿着传输线传播。为了做到这一点，我将重点放在一类新的约瑟夫森结器件上，这种器件基于非弹性库珀对穿过约瑟夫森结的隧道，这是我首创的。这些装置的关键思想是，一个隧道库珀对的2eV势能被转移到一个或多个光子上。***正如我所发现的，这种光-电荷相互作用过程是非常通用的，可以通过设计约瑟夫森结嵌入的线性电路来定制各种各样的设备，例如单光子或纠缠光子的源，量子力学施加的最小噪声放大器，或单光子探测器，这是一种在微波环境中仍然缺失的设备。***与其他约瑟夫森量子器件使用约瑟夫森结产生非线性自由度不同，非弹性库珀对隧道中的约瑟夫森结被用作强非线性驱动器，从简单的直流电压产生宽带纠缠微波辐射。这允许更宽的带宽和更高的工作频率，仅受超导体间隙的限制。我的目标是揭示非弹性库珀对隧道装置的全部潜力，并使它们实际有用。更准确地说，我想：***-开发一个宽带量子微波设备工具箱，产生和检测流动量子态；***-将这些设备的频率范围从GHz扩展到低太赫兹范围，以便与广泛的量子系统兼容；***-使这些设备易于使用，以便其中许多设备可以轻松地部署在量子系统中，从而简化放大。***一旦优化，这些设备将成为在单光子水平上加速涉及1ghz和1thz辐射的活动进展的宝贵工具。它们将成为量子信息处理的关键组成部分，作为经典控制电子学和高度相干量子比特之间的接口层。

With quantum microwaves (i.e. quantum optics with microwave photons in electronic circuits) several extremely hard problems in quantum optics with light have been tackled and new physics been explored. However, so far only one regime of quantum microwaves has been fully explored: The cavity (or circuit) quantum electrodynamics regime, where photons reside in electrical resonators.***The goal of my research program is to develop the other regime of circuit quantum optics, the wideband regime, where photons are itinerant, i.e. compact wave packets propagating along transmission lines. To do so I will focus on a new class of Josephson junction devices based on inelastic Cooper pair tunneling through Josephson junctions, which I have pioneered. The key idea of these devices is that the potential energy 2eV of a tunneling Cooper pair is transferred to one or more photons.***As I have discovered, this light-charge interaction process is extremely versatile and can be tailored into a large variety of devices by engineering the linear circuit into which the Josephson junction is embedded, e.g. sources of single photons or entangled photons, amplifiers with minimal noise as imposed by quantum mechanics, or a single photon detector, a device still dearly missing in the microwave regime.***Contrarily to other Josephson quantum devices, where the Josephson junction is used to produce nonlinear degrees of freedom, the Josephson junction in inelastic Cooper pair tunneling is used as a strongly nonlinear drive generating broadband entangled microwave radiation from a simple DC voltage. This allows for much wider bandwidths and higher operation frequencies only limited by the gap of the superconductor.***My goal is to reveal the full potential of inelastic Cooper pair tunneling devices and making them practically useful. More precisely, I want to:***- develop a toolbox of wideband quantum microwave devices generating and detecting itinerant quantum states,***- extend the frequency range of these devices from the GHz to low THz range, in order to be compatible with a broad range of quantum systems,***- make these devices simple to use, so that many of them can easily be deployed in quantum systems, thereby simplifying scale-up.***Once optimized, these devices will be a valuable tool for accelerating progress in activities involving radiation between 1 GHz and 1 THz, at the single photon level. They will be an key ingredient for quantum information processing as an interface layer between classical control electronics and highly coherent qubits.