Superconducting Devices in Quantum Optics

Hironaru Murakami, Ryohei Kitamura, Iwao Kawayama +1 more

The properties of an YBa2Cu3O7−d superconducting quantum interference device (SQUID) have been evaluated magneto-optically and electrically. It was found that the operating characteristics of the SQUID observed by means of the magneto-optical (MO) method show a good consistency with those measured by the electrical method. Furthermore, we have succeeded in the MO detection of local magnetic signals corresponding to the single flux quantum pulses generated in the SQUID loop applied with an external pulsed magnetic field. These results indicate that the MO probe has the potential to be applied to an optical output interface for superconducting magnetic flux devices.

Andrew L. DiRienzo, Daniel N. Rogovin, Marlan O. Scully +3 more

Frans Godschalk

In this dissertation, we study a novel class of laser devices, referred to as half-Josephson lasers (HJL). Here, lasing at half the Josephson frequency is induced by a parametric instability, resulting from an oscillating supercurrent flowing through a Josephson junction that contains quantum emitters. The device combines the electronic coherence of superconductivity with the optical coherence of lasers, resulting in a phase lock between the optical phase and the superconducting phase difference across the junction. Apart from the introductory first chapter, this dissertation consists of two parts. In the first part, the essentials of HJL devices are investigated. First, we study a model containing a Josephson junction with a single quantum emitter. Spontaneous switchings in the quantum emitter lead to decoherence of the laser. After this, a general HJL model is derived for the case of many quantum emitters driving the laser mode. Here, small fluctuations in the laser lead to a broad background in the laser spectrum. Large fluctuations, occurring on exponentially long timescales, cause decoherence. In the second part of the dissertation, two possible applications of the HJL are investigated, both exploiting the phase lock. First, a well-known optical feedback technique is found to lead to significant stabilization of fluctuations in the bias voltage of the HJL, which cause drift of the optical and superconducting phases. After this, a HJL device is proposed based on the Superconducting Quantum Interference Device. In this `light-superconducting interference device’ (LSID), magnetic fields can be used to manipulate the laser light. The LSID is also shown to facilitate multimode lasing.

Barry C. Sanders, Dmitry N. Chigrin

Simon Hönl

Quantum computing is one of the great scientific challenges of the 21st century. Small-scale systems today promise to surpass classical computers in the coming years and to enable the solution of classically intractable computational tasks in the fields of quantum chemistry, optimization, cryptography and more. In contrast to classical computers, quantum computers based on superconducting quantum bits (qubits) can to date not be linked over long distance in a network to improve their computing capacity, since devices, which preserve the quantumstate when it is transferred from one machine to another, are not available. Several approaches are being pursued to realize such a component, one of themost promising to date makes use of an intermediary, micromechanical element that enables quantum coherent conversion between the information present in the quantum computer and an optical fiber, without compromising the quantum nature of the information, via optomechanical interaction. This approach could allow fiber-optic quantum networks between separate quantum computers based on superconducting qubits in the future. In this work a platformfor such a microwave-to-optic link was developed based on the piezoelectric material gallium phosphide. This III-V semiconductor offers not only a piezoelectric coupling between the electric field of a microwave circuit and a mechanicalmode, but also a wide optical bandgap E_g = 2.26eV which reduces nonlinear optical absorption in the device and a large refractive index n(1550nm) = 3.01 which allows strong optical confinement at near-infrared wavelengths. Importantly and in contrast to other approaches with gallium phosphide, an epitaxially grown, single crystal thin film of the material is integrated directly on a silicon wafer with pre-structured niobium electrodes by direct wafer-bonding. This opens up the possibility of integrating the device design presented here directly with superconducting qubits fabricated with this material system. A microwave-to-optical transducer design was simulated and fabricated in the galliumphosphideon- silicon platform. The device was found to exhibit large vacuum optomechanical coupling rates g0/2 pi ~ 290kHz and a high intrinsic optical quality factor Q >10^5 while at the same time permitting electromechanical coupling to a microwave electrode. Coherent microwave-tooptical transductionwas shown at room temperature for this device and the electromechanical coupling rate could be extracted from a model derived by input-output theory. The electromechanical coupling between the electro-optomechanical device and a superconducting qubit was estimated to be g/2 pi = O(200kHz) which indicates that strong coupling between the here presented device and a superconducting transmon qubit is achievable. In addition, superconducting microwave cavities with high quality factor at single photon energy Q ~ 5x10^5 were fabricated and measured to verify that fabrication process of the microwave-to-optical transducer is compatible with high-quality superconducting microwave circuits.