Coherence of superconducting qubits can be improved by implementing designs that protect the parity of Cooper pairs on superconducting islands. Here, we introduce a parity-protected qubit based on voltage-controlled semiconductor nanowire Josephson j
unctions, taking advantage of the higher harmonic content in the energy-phase relation of few-channel junctions. A symmetric interferometer formed by two such junctions, gate-tuned into balance and frustrated by a half-quantum of applied flux, yields a cos(2{phi}) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed tenfold by tuning into the protected regime.
We propose a novel platform for the study of quantum phase transitions in one dimension (1D QPT). The system consists of a specially designed chain of asymmetric SQUIDs; each SQUID contains several Josephson junctions with one junction shared between
the nearest-neighbor SQUIDs.We develop the theoretical description of the low energy part of the spectrum. In particular, we show that the system exhibits the quantum phase transition of Ising type. In the vicinity of the transition the low energy excitations of the system can be described by Majorana fermions. This allow us to compute the matrix elements of the physical perturbations in the low energy sector. In the microwave experiments with this system, we explored the phase boundaries between the ordered and disordered phases and the critical behavior of the systems low-energy modes close to the transition. Due to the flexible chain design and control of the parameters of individual Josephson junctions, future experiments will be able to address the effects of non-integrability and disorder on the 1D QPT.
We studied the Shubnikov-de Haas (SdH) oscillations in high-mobility Si-MOS samples over a wide range of carrier densities $nsimeq (1-50) times 10^{11}$cm$^{-2}$, which includes the vicinity of the apparent metal-insulator transition in two dimension
s (2D MIT). Using a novel technique of measuring the SdH oscillations in superimposed and independently controlled parallel and perpendicular magnetic fields, we determined the spin susceptibility $chi^*$, the effective mass $m^*$, and the $g^*$-factor for mobile electrons. These quantities increase gradually with decreasing density; near the 2D MIT, we observed enhancement of $chi^*$ by a factor of $sim 4.7$.
We have measured directly the thermal conductance between electrons and phonons in ultra-thin Hf and Ti films at millikelvin temperatures. The experimental data indicate that electron-phonon coupling in these films is significantly suppressed by diso
rder. The electron cooling time $tau_epsilon$ follows the $T^{-4}$-dependence with a record-long value $tau_epsilon=25ms$ at $T=0.04K$. The hot-electron detectors of far-infrared radiation, fabricated from such films, are expected to have a very high sensitivity. The noise equivalent power of a detector with the area $1mum^2$ would be $(2-3)10^{-20}W/Hz^{1/2}$, which is two orders of magnitude smaller than that of the state-of-the-art bolometers.
