Adrian Lupascu

Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime

A superconducting artificial atom coupled to a 1D waveguide tests the limits of light–matter interaction in an unexplored coupling regime, which may enable new perspectives for quantum technologies.

Flux qubits in a planar circuit quantum electrodynamics architecture: Quantum control and decoherence

We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of 72%. We measure energy relaxation times at the symmetry point of 5 and

Probing the strongly driven spin-boson model in a superconducting quantum circuit

10\ensuremath{\mu}\mathrm{s}

Ultrasensitive magnetic field detection using a single artificial atom

, corresponding to 7 and

Dynamics of parametric fluctuations induced by quasiparticle tunneling in superconducting flux qubits

20\ensuremath{\mu}\mathrm{s}

An analysis method for transmission measurements of superconducting resonators with applications to quantum-regime dielectric-loss measurements

when relaxation through the resonator due to Purcell effect is subtracted out, and levels of flux noise of 2.6 and

Suspended graphene devices with local gate control on an insulating substrate.

2.7\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}{\mathrm{\ensuremath{\Phi}}}_{0}/\sqrt{\text{Hz}}

Insertable system for fast turnaround time microwave experiments in a dilution refrigerator

at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. The strong coupling between the qubits and the cavity leads to a large, cavity-mediated, qubit-qubit coupling. This coupling, which is characterized spectroscopically, reaches 38 MHz. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.

Dynamics of a two-level system under strong driving: Quantum-gate optimization based on Floquet theory

Quantum two-level systems interacting with the surroundings are ubiquitous in nature. The interaction suppresses quantum coherence and forces the system towards a steady state. Such dissipative processes are captured by the paradigmatic spin-boson model, describing a two-state particle, the “spin”, interacting with an environment formed by harmonic oscillators. A fundamental question to date is to what extent intense coherent driving impacts a strongly dissipative system. Here we investigate experimentally and theoretically a superconducting qubit strongly coupled to an electromagnetic environment and subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson model. We show that the drive reinforces environmental suppression of quantum coherence, and that a coherent-to-incoherent transition can be achieved by tuning the drive amplitude. An out-of-equilibrium detailed balance relation is demonstrated. These results advance fundamental understanding of open quantum systems and bear potential for the design of entangled light-matter states.Two-level systems interacting with a bosonic environment appear everywhere in physics. Here, the authors use a superconducting device to study this spin-boson model in the presence of coherent driving, showing that the drive enhances dissipation into the environment and can localize or delocalize the system.

Characterization of low-temperature microwave loss of thin aluminum oxide formed by plasma oxidation

Efficient detection of magnetic fields is central to many areas of research and technology. High-sensitivity detectors are commonly built using direct-current superconducting quantum interference devices or atomic systems. Here we use a single artificial atom to implement an ultrasensitive magnetometer with micron range size. The artificial atom, a superconducting two-level system, is operated similarly to atom and diamond nitrogen-vacancy centre-based magnetometers. The high sensitivity results from quantum coherence combined with strong coupling to magnetic field. We obtain a sensitivity of 3.3 pT Hz(-1/2) for a frequency at 10 MHz. We discuss feasible improvements to increase sensitivity by one order of magnitude. The intrinsic sensitivity of this detector at frequencies in the 100 kHz-10 MHz range compares favourably with direct-current superconducting quantum interference devices and atomic magnetometers of equivalent spatial resolution. This result illustrates the potential of artificial quantum systems for sensitive detection and related applications.