James Raftery

Observation of a Dissipation-Induced Classical to Quantum Transition

We report here the experimental observation of a dynamical quantum phase transition in a strongly interacting open photonic system. The system studied, comprising a Jaynes-Cummings dimer realized on a superconducting circuit platform, exhibits a dissipation driven localization transition. Signatures of the transition in the homodyne signal and photon number reveal this transition to be from a regime of classical oscillations into a macroscopically self-trapped state manifesting revivals, a fundamentally quantum phenomenon. This experiment also demonstrates a small-scale realization of a new class of quantum simulator, whose well controlled coherent and dissipative dynamics is suited to the study of quantum many-body phenomena out of equilibrium.

Short Injector Quantum Cascade Lasers

We report our study on the effects of shortened quantum cascade (QC) laser injector regions. While conventional short-wavelength QC lasers typically have around seven or more injector region quantum wells, we investigate QC structures with three and two injector wells. Improvements in threshold currents, output powers, and wall-plug efficiencies are expected for fundamental reasons. At heat sink temperatures near 80 K, we observe threshold current densities less than 0.5 kA/cm2, nearly 4 W peak output power, and wall-plug efficiencies in excess of 20%. At room temperature, we see threshold current densities around 2.3 kA/cm2, output powers in excess of 1 W, and wall-plug efficiencies around 7.6%. We also observe new effects in midinfrared QC lasers, such as a pronounced negative differential resistance, pulse instabilities, and multiple and varied turn-off mechanisms. These effects result from the greatly abbreviated injector regions with highly discrete states.

Digital quantum simulators in a scalable architecture of hybrid spin-photon qubits

Resolving quantum many-body problems represents one of the greatest challenges in physics and physical chemistry, due to the prohibitively large computational resources that would be required by using classical computers. A solution has been foreseen by directly simulating the time evolution through sequences of quantum gates applied to arrays of qubits, i.e. by implementing a digital quantum simulator. Superconducting circuits and resonators are emerging as an extremely promising platform for quantum computation architectures, but a digital quantum simulator proposal that is straightforwardly scalable, universal, and realizable with state-of-the-art technology is presently lacking. Here we propose a viable scheme to implement a universal quantum simulator with hybrid spin-photon qubits in an array of superconducting resonators, which is intrinsically scalable and allows for local control. As representative examples we consider the transverse-field Ising model, a spin-1 Hamiltonian, and the two-dimensional Hubbard model and we numerically simulate the scheme by including the main sources of decoherence.