Motivated by recent advancements highlighting Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have exploredthe effect of cryogenic temperatures on the growth of Ta and its integration in superconducting circuits. Cryogenic growth of Ta using a low temperature molecular beam epitaxy (MBE) system is found to stabilize single phase α-Ta on several different substrates, which include Al2O3(0001), Si(001), Si(111), SiNx, and GaAs(001). The substrates are actively cooled down to cryogenic temperatures and remain < 20 K during the Ta deposition. X-ray θ-2θ diffraction after warming to room temperature indicates the formation of polycrystalline α-Ta. The 50 nm α-Ta films grown on Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (ρ300K) of 13.4 μΩcm, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature (TC) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an RMS roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher ρ300K, lower RRR and Tc, and increased β-Ta content. Coplanar waveguide resonators with a gap width of 3 μm fabricated from cryogenically grown Ta on Si(111) and Al2O3(0001) show low power Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline α-Ta films may be promising for superconducting qubit applications even though they are not fully epitaxial.[/expand]
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.