10 research outputs found
Tunable coupling to a mechanical oscillator circuit using a coherent feedback network
We demonstrate a fully cryogenic microwave feedback network composed of
modular superconducting devices connected by transmission lines and designed to
control a mechanical oscillator coupled to one of the devices. The network
features an electromechanical device and a 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, the
tunable controller produces 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 times the mechanical
response rate. The system is easy to build and suggests that some useful
functions may arise most naturally at the network-level of modular, quantum
electromagnetic devices.Comment: 11 pages, 6 figures, final published versio
Vacuum-Gap Capacitors for Low-Loss Superconducting Resonant Circuits
Low-loss microwave components are used in many superconducting resonant
circuits from multiplexed readouts of low-temperature detector arrays to
quantum bits. Two-level system defects in amorphous dielectric materials cause
excess energy loss. In an effort to improve capacitor components, we have used
optical lithography and micromachining techniques to develop superconducting
parallel-plate capacitors in which lossy dielectrics are replaced by vacuum
gaps. Resonance measurements at 50 mK on lumped LC circuits that incorporate
these vacuum-gap capacitors (VGCs) reveal loss tangents at low powers as low as
4x10^{-5}, significantly lower than with capacitors using amorphous
dielectrics. VGCs are structurally robust, small, and easily scaled to
capacitance values above 100 pF.Comment: 5 pages, 6 figures, .docx forma
Measurement crosstalk between two phase qubits coupled by a coplanar waveguide
We analyze the measurement crosstalk between two flux-biased phase qubits
coupled by a resonant coplanar waveguide cavity. After the first qubit is
measured, the superconducting phase can undergo damped oscillations resulting
in an a.c. voltage that produces a frequency chirped noise signal whose
frequency crosses that of the cavity. We show experimentally that the coplanar
waveguide cavity acts as a bandpass filter that can significantly reduce the
crosstalk signal seen by the second qubit when its frequency is far from the
cavity's resonant frequency. We present a simple classical description of the
qubit behavior that agrees well with the experimental data. These results
suggest that measurement crosstalk between superconducting phase qubits can be
reduced by use of linear or possibly nonlinear resonant cavities as coupling
elements.Comment: 4 pages, 3 figure
Trade off-Free Entanglement Stabilization in a Superconducting Qutrit-Qubit System
Quantum reservoir engineering is a powerful framework for autonomous quantum
state preparation and error correction. However, traditional approaches to
reservoir engineering are hindered by unavoidable coherent leakage out of the
target state, which imposes an inherent trade off between achievable
steady-state state fidelity and stabilization rate. In this work we demonstrate
a protocol that achieves trade off-free Bell state stabilization in a
qutrit-qubit system realized on a circuit-QED platform. We accomplish this by
creating a purely dissipative channel for population transfer into the target
state, mediated by strong parametric interactions coupling the second-excited
state of a superconducting transmon and the engineered bath resonator. Our
scheme achieves a state preparation fidelity of 84% with a stabilization time
constant of 339 ns, leading to the lowest error-time product reported in
solid-state quantum information platforms to date.Comment: 19 pages, 14 figure
Autler-Townes effect in a superconducting three-level system
When a three-level quantum system is irradiated by an intense coupling field
resonant with one of the three possible transitions, the absorption peak of an
additional probe field involving the remaining level is split. This process is
known in quantum optics as the Autler-Townes effect. We observe these phenomena
in a superconducting Josephson phase qubit, which can be considered an
"artificial atom" with a multilevel quantum structure. The spectroscopy peaks
can be explained reasonably well by a simple three-level Hamiltonian model.
Simulation of a more complete model (including dissipation, higher levels, and
cross-coupling) provides excellent agreement with all the experimental data
Decoherence, Autler-Townes effect, and dark states in two-tone driving of a three-level superconducting system
We present a detailed theoretical analysis of a multi-level quantum system
coupled to two radiation fields and subject to decoherence. We concentrate on
an effect known from quantum optics as the Autler-Townes splitting, which has
been recently demonstrated experimentally [M. A. Sillanpaa et al., Phys. Rev.
Lett. 103, 193601 (2009)] in a superconducting phase qubit. In the three-level
approximation, we derive analytical solutions and describe how they can be used
to extract the decoherence rates and to account for the measurement data.
Better agreement with the experiment can be obtained by extending this model to
five levels. Finally, we investigate the stationary states created in the
experiment and show that their structure is close to that of dark states.Comment: 16 pages, 8 figure