Energetics of a Single Qubit Gate.

Qubits are physical, a quantum gate thus not only acts on the information carried by the qubit but also on its energy. What is then the corresponding flow of energy between the qubit and the controller that implements the gate? Here we exploit a superconducting platform to answer this question in the case of a quantum gate realized by a resonant drive field. During the gate, the superconducting qubit becomes entangled with the microwave drive pulse so that there is a quantum superposition between energy flows.

Quantum Rifling: Protecting a Qubit from Measurement Back Action.

Quantum mechanics postulates that measuring the qubit's wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate that the qubit collapsed into. We show that this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the time-averaged expectation value of the measurement operator, erasing information about the initial state of the qubit while completely suppressing the measurement backaction.

Supercurrent Interference in Few-Mode Nanowire Josephson Junctions.

Junctions created by coupling two superconductors via a semiconductor nanowire in the presence of high magnetic fields are the basis for the potential detection, fusion, and braiding of Majorana bound states. We study NbTiN/InSb nanowire/NbTiN Josephson junctions and find that the dependence of the critical current on the magnetic field exhibits gate-tunable nodes. This is in contrast with a well-known Fraunhofer effect, under which critical current nodes form a regular pattern with a period fixed by the junction area.

Formation and electronic properties of InSb nanocrosses.

Signatures of Majorana fermions have recently been reported from measurements on hybrid superconductor-semiconductor nanowire devices. Majorana fermions are predicted to obey special quantum statistics, known as non-Abelian statistics. To probe this requires an exchange operation, in which two Majorana fermions are moved around one another, which requires at least a simple network of nanowires.

Probing dynamics of an electron-spin ensemble via a superconducting resonator.

We study spin relaxation and diffusion in an electron-spin ensemble of nitrogen impurities in diamond at low temperature (0.25-1.2 K) and polarizing magnetic field (80-300 mT).