Shay Hacohen-Gourgy

Quantum dynamics of simultaneously measured non-commuting observables

In quantum mechanics, measurements cause wavefunction collapse that yields precise outcomes, whereas for non-commuting observables such as position and momentum Heisenberg’s uncertainty principle limits the intrinsic precision of a state. Although theoretical work has demonstrated that it should be possible to perform simultaneous non-commuting measurements and has revealed the limits on measurement outcomes, only recently has the dynamics of the quantum state been discussed. To realize this unexplored regime, we simultaneously apply two continuous quantum non-demolition probes of non-commuting observables to a superconducting qubit. We implement multiple readout channels by coupling the qubit to multiple modes of a cavity. To control the measurement observables, we implement a ‘single quadrature’ measurement by driving the qubit and applying cavity sidebands with a relative phase that sets the observable. Here, we use this approach to show that the uncertainty principle governs the dynamics of the wavefunction by enforcing a lower bound on the measurement-induced disturbance. Consequently, as we transition from measuring identical to measuring non-commuting observables, the dynamics make a smooth transition from standard wavefunction collapse to localized persistent diffusion and then to isotropic persistent diffusion. Although the evolution of the state differs markedly from that of a conventional measurement, information about both non-commuting observables is extracted by keeping track of the time ordering of the measurement record, enabling quantum state tomography without alternating measurements. Our work creates novel capabilities for quantum control, including rapid state purification, adaptive measurement, measurement-based state steering and continuous quantum error correction. As physical systems often interact continuously with their environment via non-commuting degrees of freedom, our work offers a way to study how notions of contemporary quantum foundations arise in such settings.

Cooling and Autonomous Feedback in a Bose-Hubbard Chain with Attractive Interactions

We engineer a quantum bath that enables entropy and energy exchange with a one-dimensional Bose-Hubbard lattice with attractive on-site interactions. We implement this in an array of three superconducting transmon qubits coupled to a single cavity mode; the transmons represent lattice sites and their excitation quanta embody bosonic particles. Our cooling protocol preserves the particle number-realizing a canonical ensemble-and also affords the efficient preparation of dark states which, due to symmetry, cannot be prepared via coherent drives on the cavity. Furthermore, by applying continuous microwave radiation, we also realize autonomous feedback to indefinitely stabilize particular eigenstates of the array.

Kondo-like behavior near the metal-to-insulator transition of nanoscale granular aluminum

We show that the normal state transport properties of nano-scale granular Aluminum films, near the metal to insulator transition, present striking similarities with those of Kondo systems. Those include a negative magneto-resistance, a minimum of resistance R at a temperature Tm in metallic films, a logarithmic rise at low temperatures and a negative curvature of R(T) at high temperatures. These normal state properties are interpreted in terms of spin-flip scattering of conduction electrons by local magnetic moments, possibly located at the metal/oxide interfaces. Their co-existence with the enhanced superconductivity seen in these films is discussed.

Mott transition in granular aluminum

A Mott transition in granular Al films is observed by probing the increase of the spin-flip scattering rate of conduction electrons as the nanosize metallic grains are being progressively decoupled. The presence of free spins in granular Al films is directly demonstrated by

Observation of Andreev–Saint-James reflections in nano-scale planar superconductor to ferromagnet contacts

\ensuremath{\mu}\mathrm{SR}

Coexistence of a triplet nodal order parameter and a singlet order parameter at the interfaces of ferromagnet/superconductor Co/CoO/In junctions

measurements. Analysis of the magnetoresistance in terms of an effective Fermi energy shows that it becomes of the order of the grains electrostatic charging energy at a room temperature resistivity

Temperature dependance of the tunneling density of states in sub-micron planar metal/oxide/graphene junctions

{\ensuremath{\rho}}_{300\phantom{\rule{4pt}{0ex}}\mathrm{K}}\ensuremath{\approx}50000\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\ensuremath{\Omega}\phantom{\rule{4pt}{0ex}}\mathrm{cm}

Evolution of a bosonic mode across the superconducting dome in the high-Tccuprate Pr2−xCexCuO4−δ

, at which a metal to insulator transition is known to exist. As this transition is approached the magnetoresistance exhibits a heavy-fermion-like behavior, consistent with an increased electron effective mass.

Current-driven transitions in ferromagnetic/insulator/superconductor narrow stripes

We fabricated nanoscale ferromagnetic (F) Nickel/superconducting (S) indium junctions which show that spin polarization effects on the contact conductance can be observed in a planar geometry. The data demonstrate that the Andreev–Saint-James [Sov. Phys. JETP 19, 1228 (1964) and J. Phys. (Paris) 25, 899 (1964)] electron-hole reflections at the F∕S interface, which are sensitive to the polarization of the F side, dominate the conductance of the contact at low bias. The simplicity of fabrication makes these junctions appealing for use in multiterminal SF structures.

Observing Topological Invariants Using Quantum Walks in Superconducting Circuits

Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, 69978 Tel-Aviv, Israel(Dated: July 13, 2011)We present differential conductance measurements of Cobalt / Cobalt-Oxide / Indium planarjunctions, 500nm x 500nm in size. The junctions span a wide range of barriers, from very low to atunnel barrier. The characteristic conductance of all the junctions show a V-shape structure at lowbias instead of the U-shape characteristic of a s-wave order parameter. The bias of the conductancepeaks is, for all junctions, larger than the gap of indium. Both properties exclude pure s-wavepairing. The data is well fitted by a model that assumes the coexistence of s-wave singlet andequal spin p-wave triplet fluids. We find that the values of the s-wave and p-wave gaps follow theBCS temperature dependance and that the amplitude of the s-wave fluid increases with the barrierstrength.I. INTRODUCTION