Publications (250)977.75 Total impact
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ABSTRACT: Repeated measurements that typically occur in twotime or multitime correlators rely on von Neumann's projection postulate, telling how to restart the system after an intermediate measurement. We invoke the principle of deferred measurement to describe an alternative procedure in which coevolving quantum memories extract system information through entanglement, combined with a final readout of the memories described by Born's rule. Our approach to repeated quantum measurements respects the unitary evolution of quantum mechanics during intermediate times, unifies the treatment of strong and weak measurements, and reproduces the projected and (anti)symmetrized correlators in the two limits. As an illustration, we apply our formalism to the calculation of the electron charge correlator in a mesoscopic physics setting, where single electron pulses assume the role of flying memory qubits. We propose an experimental setup that reduces the measurement of the time correlator to the measurement of currents and noise, exploiting the (pulsed) injection of electrons to cope with the challenge of performing shorttime measurements.  [Show abstract] [Hide abstract]
ABSTRACT: TypeII superconductors owe their magnetic and transport properties to vortex pinning, the immobilization of flux quanta through material inhomogeneities or defects. Characterizing the potential energy landscape for vortices, the pinning landscape (or short, pinscape), is of great technological importance. Besides measurement of the critical current density $j_c$ and of creep rates $S$, the $ac$ magnetic response provides valuable information on the pinscape which is different from that obtained through $j_c$ or $S$, with the Campbell penetration depth $\lambda_{\rm \scriptscriptstyle C}$ defining a characteristic quantity well accessible in an experiment. Here, we derive a microscopic expression for the Campbell penetration depth $\lambda_{\rm \scriptscriptstyle C}$ using strong pinning theory. Our results explain the dependence of $\lambda_{\rm \scriptscriptstyle C}$ on the state preparation of the vortex system and the appearance of hysteretic response. Analyzing different pinning models, metallic or insulating inclusions as well as $\delta T_c$ and $\delta \ell$pinning, we discuss the behavior of the Campbell length for different vortex state preparations within the phenomenological $H$$T$ phase diagram and compare our results with recent experiments.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum engineering requires controllable artificial systems with quantum coherence exceeding the device size and operation time. This can be achieved with geometrically confined lowdimensional electronic structures embedded within ultraclean materials, with prominent examples being artificial atoms (quantum dots) and quantum corrals (electronic cavities). Combining the two structures, we implement a mesoscopic coupled dotcavity system in a highmobility twodimensional electron gas, and obtain an extended spinsinglet state in the regime of strong dotcavity coupling. Engineering such extended quantum states presents a viable route for nonlocal spin coupling that is applicable for quantum information processing.  [Show abstract] [Hide abstract]
ABSTRACT: The penetration of an $ac$ magnetic signal into a type II superconductor residing in the Shubnikov phase depends on the pinning properties of Abrikosov vortices. Within a phenomenological theory, the socalled Campbell penetration depth $\lambda_{\rm \scriptscriptstyle C}$ is determined by the curvature $\alpha$ at the bottom of the effective pinning potential. Preparing the sample into a Bean critical state, this curvature vanishes and the Campbell length formally diverges. We make use of the microscopic expression for the pinning force density derived within strong pinning theory and show how flux penetration on top of a critical state proceeds in a regular way.  [Show abstract] [Hide abstract]
ABSTRACT: The penetration of an $ac$ magnetic signal into a type II superconductor residing in the Shubnikov phase depends on the pinning properties of Abrikosov vortices. Within a phenomenological theory, the socalled Campbell penetration depth $\lambda_{\rm \scriptscriptstyle C}$ is determined by the curvature $\alpha$ at the bottom of the effective pinning potential. Preparing the sample into a Bean critical state, this curvature vanishes and the Campbell length formally diverges. We make use of the microscopic expression for the pinning force density derived within strong pinning theory and show how flux penetration on top of a critical state proceeds in a regular way.  [Show abstract] [Hide abstract]
ABSTRACT: Measuring the $ac$ magnetic response of a type II superconductor provides valuable information on the pinning landscape (pinscape) of the material. We use strong pinning theory to derive a microscopic expression for the Campbell length $\lambda_{\rm \scriptscriptstyle C}$, the penetration depth of the $ac$ signal. We show that $\lambda_{\rm \scriptscriptstyle C}$ is determined by the jump in the pinning force, in contrast to the critical current $j_c$ which involves the jump in pinning energy. We demonstrate that the Campbell lengths generically differ for zerofieldcooled and fieldcooled samples and predict that hysteretic behavior can appear in the latter situation. We compare our findings with new experimental data and show the potential of this technique in providing information on the material's pinscape.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum engineering requires controllable artificial systems with quantum coherence exceeding the device size and operation time. This can be achieved with geometrically confined lowdimensional electronic structures embedded within ultraclean materials, with prominent examples being artificial atoms (quantum dots) and quantum corrals (electronic cavities). Combining the two structures, we implement a mesoscopic coupled dotcavity system in a highmobility twodimensional electron gas, and obtain an extended spinsinglet state in the regime of strong dotcavity coupling. Engineering such extended quantum states presents a viable route for nonlocal spin coupling that is applicable for quantum information processing.  [Show abstract] [Hide abstract]
ABSTRACT: We study the interplay of geometric frustration and interactions in a nonequilibrium photonic lattice system exhibiting a polariton flat band as described by a variant of the JaynesCummingsHubbard model. We show how to engineer strong photonic correlations in such a driven, dissipative system by quenching the kinetic energy through frustration. This produces an incompressible state of photons characterized by shortranged crystalline order with period doubling. The latter manifests itself in strong spatial correlations, i.e., onsite and nearestneighbor antibunching combined with extended densitywave oscillations at larger distances. We propose a stateoftheart circuit QED realization of our system, which is tunable in situ.  [Show abstract] [Hide abstract]
ABSTRACT: We study a system of dipolar molecules confined in a twodimensional trap and subject to an optical square lattice. The repulsive longrange dipolar interaction $D/r^3$ favors an equilateral triangular arrangement of the molecules, which competes against the square symmetry of the underlying optical lattice with lattice constant $b$ and amplitude $V$. We find the minimalenergy states at the commensurate density $n = 1/b^2$ and establish the complete squaretotriangular transformation pathway of the lattice with decreasing $V$ involving perioddoubled, solitonic, and distortedtriangular configurations.  [Show abstract] [Hide abstract]
ABSTRACT: Repeated measurements as typically occurring in twotime correlators rely on von Neumann's projection postulate, telling how to restart the system after a measurement. We describe an alternative procedure where coevolving quantum memories extract system information through entanglement, combined with a final readout of the memories according to Born's rule. We apply this procedure to the calculation of the electron charge correlator in mesoscopic physics and the photon intensity correlator in quantum optics. While our approach to repeated quantum measurements deals with any systemmemory coupling, we show that the limits of strong (weak) measurements are correctly reproduced at strong (weak) coupling.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the twophoton scattering properties of a JaynesCummings (JC) nonlinearity consisting of a twolevel system (qubit) interacting with a single mode cavity, which is coupled to two waveguides, each containing a single incident photon wave packet initially. In this scattering setup, we study the interplay between the HongOuMandel effect arising due to quantum interference and effective photonphoton interactions induced by the presence of the qubit. We calculate the twophoton scattering matrix of this system analytically and identify signatures of interference and interaction in the second order auto and crosscorrelation functions of the scattered photons. In the dispersive regime, when qubit and cavity are far detuned from each other, we find that the JC nonlinearity can be used as an almost linear, insitu tunable beam splitter giving rise to ideal HongOuMandel interference, generating a highly pathentangled twophoton NOON state of the scattered photons. The latter manifests itself in strongly suppressed waveguide crosscorrelations and Poissonian photon number statistics in each waveguide. If the twolevel system and the cavity are on resonance, the JC nonlinearity strongly modifies the ideal HOM conditions leading to a smaller degree of path entanglement and subpoissonian photon number statistics. In the latter regime, we find that photon blockade is associated with bunched autocorrelations in both waveguides, while a twopolariton resonance can lead to bunched as well as antibunched correlations.  [Show abstract] [Hide abstract]
ABSTRACT: We study a system of dipolar molecules confined in a twodimensional trap and subject to an optical square lattice. The repulsive longrange dipolar interaction D/r(3) favors an equilateral triangular arrangement of the molecules, which competes against the square symmetry of the underlying optical lattice with lattice constant b and amplitude V. We find the minimalenergy states at the commensurate density n = 1/b(2) and establish the complete squaretotriangular transformation pathway of the lattice with decreasing V involving perioddoubled, solitonic, and distortedtriangular configurations.  [Show abstract] [Hide abstract]
ABSTRACT: We study the charge dynamics of a quantum dot as measured by a nearby quantum point contact probing the dot via individual singleparticle wave packets. We contrast the two limiting cases of weak and strong systemdetector coupling exerting vanishing and strong backaction on the system and analyze the resulting differences in the chargecharge correlator. Extending the study to multiple projective measurements modelling a continuous strong measurement, we identify a transition from a charge dynamics dominated by the system's properties to a universal dynamics governed by the measurement.  [Show abstract] [Hide abstract]
ABSTRACT: In a coupled system of one classical and one quantum mechanical degree of freedom, the quantum degree of freedom can facilitate the escape of the whole system. Such unusual escape characteristics have been theoretically predicted as "M\"unchhausen effect". We implement such a system by shunting one of the two junctions of a dcSQUID with an additional capacitance. In our experiments, we detect a crossover between quantum and classical escape processes related to the direction of escape. We find that, under varying external magnetic flux, macroscopic quantum tunneling periodically alternates with thermally activated escape, a hallmark of the "M\"unchhausen effect".  [Show abstract] [Hide abstract]
ABSTRACT: We study the magnetic response of a superconducting double strip, i.e., two parallel coplanar thin strips of width $2w$, thickness $d \ll w$ and of infinite length, separated by a gap of width $2s$ and subject to a perpendicular magnetic field $H$. The magnetic properties of this system are governed by the presence of a geometric energy barrier for vortex penetration which we investigate as a function of applied field $H$ and gap parameter $s$. The new results deal with the case of a narrow gap $s \ll w$, where the field penetration from the inner edges is facilitated by large flux focusing. Upon reducing the gap width $2s$, we observe a considerable rearrangement of the screening currents, leading to a strong reduction of the penetration field and the overall magnetization loop, with a suppression factor reaching $\sim (d/w)^{1/2}$ as the gap drops below the sample thickness, $2s < d$. We compare our results with similar systems of different shapes (elliptic, rectangular platelet) and include effects of surface barriers as well. Furthermore, we verify that corrections arising from the magnetic response of the Shubnikov phase in the penetrated state are small and can be omitted. Extending the analysis to multiple strips, we determine the specific sequence of flux penetrations into the different strips. Our studies are relevant for the understanding of platelet shaped samples with cracks or the penetration into layered superconductors at oblique magnetic fields.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the singlephoton transport properties of a onedimensional coupled cavity array (CCA) containing a single qubit in its central site by coupling the CCA to two transmission lines supporting propagating bosonic modes with linear dispersion. We find that even in the nominally weak lightmatter coupling regime, the transmission through a long array exhibits two ultranarrow resonances corresponding to longlived selfprotected polaritonic states localized around the site containing the qubit. The lifetime of these states is found to increase exponentially with the number of array sites in sharp distinction to the polaritonic Bloch modes of the cavity array.  [Show abstract] [Hide abstract]
ABSTRACT: We study the random directed polymer problem—the shortscale behavior of an elastic string (or polymer) in one transverse dimension subject to a disorder potential and finite temperature fluctuations. We are interested in the polymer shortscale wandering expressed through the displacement correlator <[δ u( X)]2>, with δ u( X) being the difference in the displacements at two points separated by a distance X. While this object can be calculated at short scales using the perturbation theory in higher dimensions d > 2, this approach becomes illdefined and the problem turns out to be nonperturbative in the lower dimensions and for an infinitelength polymer. In order to make progress, we redefine the task and analyze the wandering of a string of a finite length L. At zero temperature, we find that the displacement fluctuations <[δ u( X)]2> ∝ LX 2 depend on L and scale with the square of the segment length X, which differs from a straightforward Larkintype scaling. The result is best understood in terms of a typical squared angle <α2> ∝ L, where α = ∂ x u, from which the displacement scaling for the segment X follows naturally, <[δ u( X)]2> ∝ <α2> X 2. At high temperatures, thermal fluctuations smear the disorder potential and the lowestorder results for disorderinduced fluctuations in both the displacement field and the angle vanish in the thermodynamic limit L → ∞. The calculation up to the second order allows us to identify the regime of validity of the perturbative approach and provides a finite expression for the displacement correlator, albeit depending on the boundary conditions and the location relative to the boundaries.  [Show abstract] [Hide abstract]
ABSTRACT: We discuss the JaynesCummingsHubbard model (JCHM) describing the superfluidMott insulator transition of polaritons (i.e., dressed photonqubit states) in coupled qubitcavity arrays in the crossover from strong to weak correlations. In the strongly correlated regime the phase diagram and the elementary excitations of lattice polaritons near the Mott lobes are calculated analytically using a slave boson theory (SBT). The opposite regime of weakly interacting polariton superfluids is described by a weakcoupling meanfield theory (MFT) for a generalised multimode Dicke model. We show that a remarkable relation between the two theories exists in the limit of large photon bandwidth and large negative detuning, i.e., when the nature of polariton quasiparticles becomes qubitlike. In this regime, the weak coupling theory predicts the existence of a single Mott lobe with a change of the universality class of the phase transition at the tip of the lobe, in perfect agreement with the slaveboson theory. Moreover, the spectra of low energy excitations, i.e., the sound velocity of the Goldstone mode and the gap of the amplitude mode match exactly as calculated from both theories.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a quantumenhanced iterative (with $K$ steps) measurement scheme based on an ensemble of $N$ twolevel probes which asymptotically approaches the Heisenberg limit $\delta_K \propto R^{K/(K+1)}$, $R$ the number of quantum resources. The protocol is inspired by Kitaev's phase estimation algorithm and involves only collective manipulation and measurement of the ensemble. The iterative procedure takes the shotnoise limited primary measurement with precision $\delta_1\propto N^{1/2}$ to increasingly precise results $\delta_K\propto N^{K/2}$. A straightforward implementation of the algorithm makes use of a twocomponent atomic cloud of Bosons in the precision measurement of a magnetic field.  [Show abstract] [Hide abstract]
ABSTRACT: We propose and analyze a mesoscopic device producing ondemand entangled pairs of electrons. The system consists of two capacitively coupled MachZehnder interferometers implemented in a quantum Hall structure. A pair of electron wavepackets is injected into the chiral edge states of two (of the four) incoming arms; scattering on the incoming interferometers splits the wavepackets into four components of which two interact. The resulting interaction phase associated with this component leads to the entanglement of the state; the latter is scattered at the outgoing beam splitter and analyzed in a Bell violation test measuring the presence of particles in the four outgoing leads. We study the twoparticle case and determine the conditions to reach and observe full entanglement. We extend our twoparticle analysis to include the underlying Fermi seas in the quantum Hall device; the change in shape of the wavefunction, the generation of electronhole pairs in the interaction regime, and a time delay between the pulses all reduce the degree of visible entanglement and the violation of the Bell inequality, effects which we analyze quantitatively. We determine the device settings optimizing the entanglement and the Bell test and find that violation is still possible in the presence of the Fermi seas, with a maximal Bell parameter reaching ${\cal B} = 2.18 > 2$ in our setup.
Publication Stats
6k  Citations  
977.75  Total Impact Points  
Top Journals
Institutions

19702015

ETH Zurich
 Institute for Theoretical Physics
Zürich, Zurich, Switzerland


19912011

Landau Institute for Theoretical Physics
Moskva, Moscow, Russia


19872007

Cornell University
 Laboratory of Atomic and Solid State Physics
Ithaca, NY, United States


1999

Hochschule für Technik Zürich
Zürich, Zurich, Switzerland
