While relatively easy to engineer, static transverse coupling between a qubit and a cavity mode satisfies the criteria for a quantum non-demolition (QND) measurement only if the couplingbetween the qubit and cavity is much less than their mutual detuning. This can put significant limits on the speed of the measurement, requiring trade-offs in the circuit design between coupling, detuning, and decoherence introduced by the cavity mode. Here, we study a circuit in which the qubit-cavity and the cavity-feedline coupling can be turned on and off, which helps to isolate the qubit. We do not rely on the rotating-wave or dispersive approximations, but solve the full transverse interaction between the qubit and the cavity mode. We show that by carefully choosing the detuning and interaction time, we can exploit a recurrence in the qubit-cavity dynamics in a way that makes it possible to perform very fast, high fidelity, QND measurements. Here, the qubit measurement is performed more like a gate operation between the qubit and the cavity, where the cavity state can be amplified, squeezed, and released in a time-sequenced fashion. In addition, we also show that the non-demolition property of the off-resonant approximation breaks down much faster than its dispersive property, suggesting that many of the dispersive measurements to date have been implemented outside the QND regime.
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 demonstrate a fully cryogenic microwave feedback network composed of
modular superconducting devices interconnected by transmission lines and
designed to control a mechanical oscillatorcoupled to one of the devices. The
network is partitioned into an electromechanical device and a dynamically
tunable controller that coherently receives, processes and feeds back
continuous microwave signals that modify the dynamics and readout of the
mechanical state. While previous electromechanical systems represent some
compromise between efficient control and efficient readout of the mechanical
state, as set by the electromagnetic decay rate, this flexible controller
yields a closed-loop network that can be dynamically and continuously tuned
between both extremes much faster than the mechanical response time. We
demonstrate that the microwave decay rate may be modulated by at least a factor
of 10 at a rate greater than $10^4$ times the mechanical response rate.
Routers, switches, and repeaters are essential components of modern
information-processing systems. Similar devices will be needed in future
superconducting quantum computers. In thiswork we investigate experimentally
the time evolution of Autler-Townes splitting in a superconducting phase qubit
under the application of a control tone resonantly coupled to the second
transition. A three-level model that includes independently determined
parameters for relaxation and dephasing gives excellent agreement with the
experiment. The results demonstrate that the qubit can be used as a ON/OFF
switch with 100 ns operating time-scale for the reflection/transmission of
photons coming from an applied probe microwave tone. The ON state is realized
when the control tone is sufficiently strong to generate an Autler-Townes
doublet, suppressing the absorption of the probe tone photons and resulting in
a maximum of transmission.