Entangling superconducting quantum processors via light would enable new means of secure communication and distributed quantum computing. However, transducing quantum signals betweenthese disparate regimes of the electromagnetic spectrum remains an outstanding goal, and interfacing superconducting qubits with electro-optic transducers presents significant challenges due to the deleterious effects of optical photons on superconductors. Moreover, many remote entanglement protocols require multiple qubit gates both preceding and following the upconversion of the quantum state, and thus an ideal transducer should leave the state of the qubit unchanged: more precisely, the backaction from the transducer on the qubit should be minimal. Here we demonstrate non-destructive optical readout of a superconducting transmon qubit via a continuously operated electro-optic transducer. The modular nature of the transducer and circuit QED system used in this work enable complete isolation of the qubit from optical photons, and the backaction on the qubit from the transducer is less than that imparted by thermal radiation from the environment. Moderate improvements in transducer bandwidth and added noise will enable us to leverage the full suite of tools available in circuit QED to demonstrate transduction of non-classical signals from a superconducting qubit to the optical domain.
An electro-optomechanical device capable of microwave-to-optics conversion has recently been demonstrated, with the vision of enabling optical networks of superconducting qubits. Herewe present an improved converter design that uses a three-dimensional (3D) microwave cavity for coupling between the microwave transmission line and an integrated LC resonator on the converter chip. The new design simplifies the optical assembly and decouples it from the microwave part of the setup. Experimental demonstrations show that the modular device assembly allows us to flexibly tune the microwave coupling to the converter chip while maintaining small loss. We also find that electromechanical experiments are not impacted by the additional microwave cavity. Our design is compatible with a high-finesse optical cavity and will improve optical performance.
We propose to use a cryogenic nonlinear resonator for the projective readout, classical memory, and feedback for a superconducting qubit. This approach sidesteps many of the inefficienciesinherent in two-way communication between temperature stages in typical systems with room temperature controllers, and avoids increasing the cryogenic heat load. This controller may find a broad range of uses in multi-qubit systems, but here we analyze two specific demonstrative cases in single qubit-control. In the first case, the nonlinear controller is used to initialize the qubit in a definite eigenstate. And in the second case, the qubit’s state is read into the controller’s classical memory, where it is stored for an indefinite period of time, and then used to reinstate the measured state after the qubit has decayed. We analyze the properties of this system and we show simulations of the time evolution for the full system dynamics.