Loops and Strings in a Superconducting Lattice Gauge Simulator.

We propose an architecture for an analog quantum simulator of electromagnetism in 2+1 dimensions, based on an array of superconducting fluxonium devices. The encoding is in the integer (spin-1) representation of the quantum link model formulation of compact U(1) lattice gauge theory. We show how to engineer Gauss' law via an ancilla mediated gadget construction, and how to tune between the strongly coupled and intermediately coupled regimes. The witnesses to the existence of the predicted confining phase of the model are provided by nonlocal order parameters from Wilson loops and disorder parameters from 't Hooft strings. We show how to construct such operators in this model and how to measure them nondestructively via dispersive coupling of the fluxonium islands to a microwave cavity mode. Numerical evidence is found for the existence of the confined phase in the ground state of the simulation Hamiltonian on a ladder geometry.

Foliated Quantum Error-Correcting Codes.

We show how to construct a large class of quantum error-correcting codes, known as Calderbank-Steane-Shor codes, from highly entangled cluster states. This becomes a primitive in a protocol that foliates a series of such cluster states into a much larger cluster state, implementing foliated quantum error correction. We exemplify this construction with several familiar quantum error-correction codes and propose a generic method for decoding foliated codes. We numerically evaluate the error-correction performance of a family of finite-rate Calderbank-Steane-Shor codes known as turbo codes, finding that they perform well over moderate depth foliations. Foliated codes have applications for quantum repeaters and fault-tolerant measurement-based quantum computation.

Accurate lineshape spectroscopy and the Boltzmann constant.

Spectroscopy has an illustrious history delivering serendipitous discoveries and providing a stringent testbed for new physical predictions, including applications from trace materials detection, to understanding the atmospheres of stars and planets, and even constraining cosmological models. Reaching fundamental-noise limits permits optimal extraction of spectroscopic information from an absorption measurement. Here, we demonstrate a quantum-limited spectrometer that delivers high-precision measurements of the absorption lineshape. These measurements yield a very accurate measurement of the excited-state (6P1/2) hyperfine splitting in Cs, and reveals a breakdown in the well-known Voigt spectral profile. We develop a theoretical model that accounts for this breakdown, explaining the observations to within the shot-noise limit. Our model enables us to infer the thermal velocity dispersion of the Cs vapour with an uncertainty of 35 p.p.m. within an hour. This allows us to determine a value for Boltzmann's constant with a precision of 6 p.p.m., and an uncertainty of 71 p.p.m.

Superabsorption of light via quantum engineering.

Almost 60 years ago Dicke introduced the term superradiance to describe a signature quantum effect: N atoms can collectively emit light at a rate proportional to N(2). Structures that superradiate must also have enhanced absorption, but the former always dominates in natural systems. Here we show that this restriction can be overcome by combining several well-established quantum control techniques. Our analytical and numerical calculations show that superabsorption can then be achieved and sustained in certain simple nanostructures, by trapping the system in a highly excited state through transition rate engineering. This opens the prospect of a new class of quantum nanotechnology with potential applications including photon detection and light-based power transmission. An array of quantum dots or a molecular ring structure could provide a suitable platform for an experimental demonstration.

Raman phonon emission in a driven double quantum dot.

The compound semiconductor gallium-arsenide (GaAs) provides an ultra-clean platform for storing and manipulating quantum information, encoded in the charge or spin states of electrons confined in nanostructures. The absence of inversion symmetry in the zinc-blende crystal structure of GaAs however, results in a strong piezoelectric interaction between lattice acoustic phonons and qubit states with an electric dipole, a potential source of decoherence during charge-sensitive operations. Here we report phonon generation in a GaAs double quantum dot, configured as a single- or two-electron charge qubit, and driven by the application of microwaves via surface gates. In a process that is a microwave analogue of the Raman effect, phonon emission produces population inversion of the two-level system and leads to rapid decoherence of the qubit when the microwave energy exceeds the level splitting. Comparing data with a theoretical model suggests that phonon emission is a sensitive function of the device geometry.

Dynamical steady States in driven quantum systems.

We derive dynamical equations for a driven, dissipative quantum system in which the environment-induced relaxation rate is comparable to the Rabi frequency, avoiding assumptions on the frequency dependence of the environmental coupling. When the environmental coupling varies significantly on the scale of the Rabi frequency, secular or rotating wave approximations break down. We avoid these approximations, yielding dynamical steady states which account for the interaction between driven quantum dots and their phonon environment. The theory, which is motivated by recent experimental observations, qualitatively and quantitatively describes the transition from asymmetric unsaturated resonances at weak driving to population inversion at strong driving.

Continuous measurement of a microwave-driven solid state qubit.

We analyze the dynamics of a continuously observed, damped, microwave-driven solid state charge qubit, consisting of a single electron in a double well potential. The microwave field induces transitions between the qubit eigenstates, which have a profound effect on the detector output current. Useful information about the qubit dynamics, such as dephasing and relaxation rates, and the Rabi frequency, can be extracted from the detector conductance and output noise power spectrum. We also propose a technique for single-shot electron spin readout, for spin based quantum information processing, which has a number of practical advantages over existing schemes.

Population inversion of a driven two-level system in a structureless bath.

We derive a master equation for a driven double quantum dot damped by an unstructured phonon bath, and calculate the spectral density. We find that bath-mediated photon absorption is important at relatively strong driving, and may even dominate the dynamics, inducing population inversion of the double-dot system. This phenomenon is consistent with recent experimental observations.

Mesoscopic one-way channels for quantum state transfer via the quantum Hall effect.

We show that the one-way channel formalism of quantum optics has a physical realization in electronic systems. In particular, we show that magnetic edge states form unidirectional quantum channels capable of coherently transporting electronic quantum information. Using the equivalence between one-way photonic channels and magnetic edge states, we adapt a proposal for quantum state transfer to mesoscopic systems using edge states as a quantum channel, and show that it is feasible with reasonable experimental parameters. We discuss how this protocol may be used to transfer information encoded in number, charge, or spin states of quantum dots, so it may prove useful for transferring quantum information between parts of a solid-state quantum computer.

Continuous quantum measurement: inelastic tunneling and lack of current oscillations.

We study the dynamics of a charge qubit, consisting of a single electron in a double well potential coupled to a point-contact (PC) electrometer, using the quantum trajectories formalism. Contrary to previous predictions, we show formally that, in the sub-Zeno limit, coherent oscillations in the detector output are suppressed, and the dynamics is dominated by inelastic processes in the PC. Furthermore, these reduce the detector efficiency and induce relaxation even when the source-drain bias is zero. This is of practical significance since it means the detector will act as a source of decoherence. Finally, we show that the sub-Zeno dynamics is divided into two regimes: low and high bias in which the PC current power spectra show markedly different behavior.

An electrochemical model of the transport of charged molecules through the capillary glycocalyx.

An electrochemical theory of the glycocalyx surface layer on capillary endothelial cells is developed as a model to study the electrochemical dynamics of anionic molecular transport within capillaries. Combining a constitutive relationship for electrochemical transport, derived from Fick's and Ohm's laws, with the conservation of mass and Gauss's law from electrostatics, a system of three nonlinear, coupled, second-order, partial, integro-differential equations is obtained for the concentrations of the diffusing anionic molecules and the cations and anions in the blood. With the exception of small departures from electroneutrality that arise locally near the apical region of the glycocalyx, the model assumes that cations in the blood counterbalance the fixed negative charges bound to the macromolecular matrix of the glycocalyx in equilibrium. In the presence of anionic molecular tracers injected into the capillary lumen, the model predicts the size- and charge-dependent electrophoretic mobility of ions and tracers within the layer. In particular, the model predicts that anionic molecules are excluded from the glycocalyx at equilibrium and that the extent of this exclusion, which increases with increasing tracer and/or glycocalyx electronegativity, is a fundamental determinant of anionic molecular transport through the layer. The model equations were integrated numerically using a Crank-Nicolson finite-difference scheme and Newton-Raphson iteration. When the concentration of the anionic molecular tracer is small compared with the concentration of ions in the blood, a linearized version of the model can be obtained and solved as an eigenvalue problem. The results of the linear and nonlinear models were found to be in good agreement for this physiologically important case. Furthermore, if the fixed-charge density of the glycocalyx is of the order of the concentration of ions in the blood, or larger, or if the magnitude of the anionic molecular valence is large, a closed-form asymptotic solution for the diffusion time can be obtained from the eigenvalue problem that compares favorably with the numerical solution. In either case, if leakage of anionic molecules out of the capillary occurs, diffusion time is seen to vary exponentially with anionic valence and in inverse proportion to the steady-state anionic tracer concentration in the layer relative to the lumen. These findings suggest several methods for obtaining an estimate of the glycocalyx fixed-charge density in vivo.