Joshua Strong

Measurement crosstalk between two phase qubits coupled by a coplanar waveguide

We investigate measurement crosstalk in a system with two flux-biased phase qubits coupled by a resonant coplanar waveguide cavity. After qubit measurement, the superconducting phase undergoes damped oscillations in a deep anharmonic potential producing a frequency chirped voltage or crosstalk signal. We show experimentally that a coplanar waveguide cavity acts as a bandpass filter that can significantly reduce the propagation of this crosstalk signal when the qubits are far off resonance from the cavity. The transmission of the crosstalk signal

Low-loss superconducting resonant circuits using vacuum-gap-based microwave components

\ensuremath{\propto}{({\ensuremath{\omega}}_{q}{C}_{x})}^{2}

rf-SQUID-Mediated Coherent Tunable Coupling between a Superconducting Phase Qubit and a Lumped-Element Resonator

can be further minimized by reducing the qubit frequencies and the coupling capacitance to the cavity. We model the large amplitude crosstalk signal and qubit response classically with results that agree well with the experimental data. We find that the maximum energy transferred by the crosstalk generating qubit roughly saturates for long energy relaxation times

Tripartite interactions between two phase qubits and a resonant cavity

({T}_{1}g100\text{ }\text{ns})

Frequency-Tunable Josephson Junction Resonator for Quantum Computing

while the delay time necessary for the crosstalk signal to propagate to the cavity scales linearly with

Coherent interactions between phase qubits, cavities, and TLS defects

{T}_{1}

Vacuum-Gap Capacitors for Low-Loss Superconducting Resonant Circuits

. Ultimately, the use of resonant cavities as coupling elements and crosstalk filters is extremely beneficial for future architectures incorporating many coupled qubits.

Josephson Junction Microwave Modulators

We have produced high-quality complex microwave circuits, such as multiplexed resonators and superconducting phase qubits, using a “vacuum-gap” technology that eliminates lossy dielectric materials. We have improved our design and fabrication strategy beyond our earlier work, leading to increased yield, enabling the realization of these complex circuits. We incorporate both novel vacuum-gap wiring crossovers for gradiometric inductors and vacuum-gap capacitors (VGC) on chip to produce resonant circuits that have large internal quality factors (30 000<QI<165 000) at 50 mK, outperforming most dielectric-filled devices. Resonators with VGCs as large as 180 pF confirm single mode behavior of our lumped-element components.

Josephson Junction Double-Balanced Modulator for Qubit Control

We demonstrate coherent tunable coupling between a superconducting phase qubit and a lumped-element resonator. The coupling strength is mediated by a flux-biased rf SQUID operated in the nonhysteretic regime. By tuning the applied flux bias to the rf SQUID we change the effective mutual inductance, and thus the coupling energy, between the phase qubit and resonator. We verify the modulation of coupling strength from 0 to 100 MHz by observing modulation in the size of the splitting in the phase qubits spectroscopy, as well as coherently by observing modulation in the vacuum Rabi oscillation frequency when on resonance. The measured spectroscopic splittings and vacuum Rabi oscillations agree well with theoretical predictions.

Time-Domain Detection of Weakly Coupled TLS Flucuators in Phase Qubits

Micrometre-scale superconducting circuits are at present explored as the building blocks for scalable quantum information processors. In a system where two such qubits are coupled to a resonant cavity, tripartite interactions and controlled coherent dynamics have now been demonstrated. This platform should allow for a fuller exploration of multipartite quantum states and their deterministic preparation.