Raymond W. Simmonds

We describe a tunable-cavity quantum electrodynamics (QED) architecture with an rf SQUID phase qubit inductively coupled to a single-mode, resonant cavity with a tunable frequency that allows for both microwave readout of tunneling and dispersive measurements of the qubit. Dispersive measurement is well characterized by a three-level model, strongly dependent on qubit anharmonicity, qubit-cavity coupling, and detuning. A tunable-cavity frequency provides a way to strongly vary both the qubit-cavity detuning and coupling strength, which can reduce Purcell losses, cavity-induced dephasing of the qubit, and residual bus coupling for a system with multiple qubits. With our qubit-cavity system, we show that dynamic control over the cavity frequency enables one to avoid Purcell losses during coherent qubit evolutions and optimize state readout during qubit measurements. The maximum qubit decay time ${T}_{1}=1.5\phantom{\rule{0.222222em}{0ex}}$\mu${}$s is found to be limited by surface dielectric losses from a design geometry similar to planar transmon qubits.

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 $\propto${}($\omega${}qCx)2 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 (T1>100$\hskip0.3em${}ns) while the delay time necessary for the crosstalk signal to propagate to the cavity scales linearly with T1. Ultimately, the use of resonant cavities as coupling elements and crosstalk filters is extremely beneficial for future architectures incorporating many coupled qubits.

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 qubit's 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.

We report on a method for detecting weakly coupled spurious two-level system fluctuators (TLSs) in superconducting qubits. This method is more sensitive that standard spectroscopic techniques for locating TLSs with a reduced data acquisition time.

The creation and manipulation of multipartite entangled states is important for advancements in quantum computation and communication, and for testing our fundamental understanding of quantum mechanics and precision measurements. Multipartite entanglement has been achieved by use of various forms of quantum bits (qubits), such as trapped ions, photons, and atoms passing through microwave cavities. Quantum systems based on superconducting circuits have been used to control pair-wise interactions of qubits, either directly, through a quantum bus, or via controllable coupling. Here, we describe the first demonstration of coherent interactions of three directly coupled superconducting quantum systems, two phase qubits and a resonant cavity. We introduce a simple Bloch-sphere-like representation to help one visualize the unitary evolution of this tripartite system as it shares a single microwave photon. With careful control and timing of the initial conditions, this leads to a protocol for creating a rich variety of entangled states. Experimentally, we provide evidence for the deterministic evolution from a simple product state, through a tripartite W-state, into a bipartite Bell-state. These experiments are another step towards deterministically generating multipartite entanglement in superconducting systems with more than two qubits.