[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 so-called 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 zero-field-cooled and field-cooled 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 low-dimensional 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 dot--cavity system in a
high-mobility two-dimensional electron gas, and obtain an extended spin-singlet
state in the regime of strong dot--cavity 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
non-equilibrium photonic lattice system exhibiting a polariton flat band as
described by a variant of the Jaynes-Cummings-Hubbard 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 short-ranged crystalline order
with period doubling. The latter manifests itself in strong spatial
correlations, i.e., on-site and nearest-neighbor anti-bunching combined with
extended density-wave oscillations at larger distances. We propose a
state-of-the-art 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 two-dimensional trap and
subject to an optical square lattice. The repulsive long-range 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 minimal-energy
states at the commensurate density $n = 1/b^2$ and establish the complete
square-to-triangular transformation pathway of the lattice with decreasing $V$
involving period-doubled, solitonic, and distorted-triangular configurations.
[Show abstract][Hide abstract] ABSTRACT: Repeated measurements as typically occurring in two-time correlators rely on
von Neumann's projection postulate, telling how to restart the system after a
measurement. We describe an alternative procedure where co-evolving 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 system-memory 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 two-photon scattering properties of a Jaynes-Cummings (JC)
nonlinearity consisting of a two-level 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 Hong-Ou-Mandel effect arising due to quantum interference
and effective photon-photon interactions induced by the presence of the qubit.
We calculate the two-photon scattering matrix of this system analytically and
identify signatures of interference and interaction in the second order auto-
and cross-correlation 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, in-situ tunable beam splitter
giving rise to ideal Hong-Ou-Mandel interference, generating a highly
path-entangled two-photon NOON state of the scattered photons. The latter
manifests itself in strongly suppressed waveguide cross-correlations and
Poissonian photon number statistics in each waveguide. If the two-level 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
sub-poissonian photon number statistics. In the latter regime, we find that
photon blockade is associated with bunched auto-correlations in both
waveguides, while a two-polariton resonance can lead to bunched as well as
anti-bunched correlations.
Physical Review A 12/2014; 91(3). DOI:10.1103/PhysRevA.91.033816 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study a system of dipolar molecules confined in a two-dimensional trap and subject to an optical square lattice. The repulsive long-range 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 minimal-energy states at the commensurate density n = 1/b(2) and establish the complete square-to-triangular transformation pathway of the lattice with decreasing V involving period-doubled, solitonic, and distorted-triangular configurations.
Physical Review B 08/2014; 90(6). DOI:10.1103/PhysRevB.90.060101 · 3.74 Impact Factor
[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 single-particle wave packets. We
contrast the two limiting cases of weak and strong system--detector coupling
exerting vanishing and strong back-action on the system and analyze the
resulting differences in the charge-charge 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.
Physical Review B 06/2014; 90(7). DOI:10.1103/PhysRevB.90.075312 · 3.74 Impact Factor
[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 dc-SQUID 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.
Physical Review B 02/2014; 89(10). DOI:10.1103/PhysRevB.89.104514 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the single-photon transport properties of a one-dimensional
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 light-matter
coupling regime, the transmission through a long array exhibits two
ultra-narrow resonances corresponding to long-lived self-protected 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.
Physical Review A 02/2014; 89(2). DOI:10.1103/PhysRevA.89.025801 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the random directed polymer problem—the short-scale
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 short-scale 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 ill-defined and the problem turns out to be
nonperturbative in the lower dimensions and for an infinite-length
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 Larkin-type
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 lowest-order
results for disorder-induced 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.
Journal of Experimental and Theoretical Physics 09/2013; 117(3):570-578. DOI:10.1134/S1063776113110022 · 0.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We discuss the Jaynes-Cummings-Hubbard model (JCHM) describing the
superfluid-Mott insulator transition of polaritons (i.e., dressed photon-qubit
states) in coupled qubit-cavity 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 weak-coupling mean-field
theory (MFT) for a generalised multi-mode 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 qubit-like. 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 slave-boson 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.
Journal of Physics B Atomic Molecular and Optical Physics 07/2013; 46(22). DOI:10.1088/0953-4075/46/22/224020 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a quantum-enhanced iterative (with $K$ steps) measurement scheme
based on an ensemble of $N$ two-level 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 shot-noise 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 two-component atomic cloud of Bosons in the precision
measurement of a magnetic field.
Physical Review A 04/2013; 89(1). DOI:10.1103/PhysRevA.89.012118 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose and analyze a mesoscopic device producing on-demand entangled
pairs of electrons. The system consists of two capacitively coupled
Mach-Zehnder interferometers implemented in a quantum Hall structure. A pair of
electron wave-packets is injected into the chiral edge states of two (of the
four) incoming arms; scattering on the incoming interferometers splits the
wave-packets 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 two-particle case and determine the conditions to
reach and observe full entanglement. We extend our two-particle analysis to
include the underlying Fermi seas in the quantum Hall device; the change in
shape of the wave-function, the generation of electron-hole 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.
[Show abstract][Hide abstract] ABSTRACT: Iron pnictides are layered high T(c) superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosov-like to Josephson-like in the pnictide superconductor SmFeAs(O,F) with T(c)~48-50 K on cooling below a temperature T(*)~41-42 K, despite its moderate electronic anisotropy γ~4-6. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the flux-flow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T(*) coincides well with the temperature where the coherence length ξ(c) perpendicular to the layers matches half of the FeAs-layer spacing. For fields slightly out-of-plane (> 0.1°- 0.15°) the vortices are completely immobilized as well-pinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from well-pinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.
Nature Material 11/2012; 12(2). DOI:10.1038/nmat3489 · 36.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the modification in mesoscopic electronic transport due to electron-electron interactions making use of scattering states. We demonstrate that for a specific (finite range) interaction kernel, the knowledge of the scattering matrix is sufficient to take interaction effects into account. We calculate perturbatively the corrections to the current and current-current correlator; in agreement with previous work, we find that, in linear response, interaction effects can be accounted for by an effective (renormalized) transmission probability. Beyond linear response, simple renormalization of scattering coefficients is not sufficient to describe the current-current correlator, as additional corrections arise due to irreducible two-particle processes. Furthermore, we find that the correlations between opposite-spin currents induced by interaction are enhanced for an asymmetric scatterer, generating a nonzero result already to lowest order in the interaction.
[Show abstract][Hide abstract] ABSTRACT: We study the coherence and fluorescence properties of the coherently pumped and dissipative Jaynes-Cummings-Hubbard model describing polaritons in a coupled-cavity array. At weak hopping we find strong signatures of photon blockade similar to single-cavity systems. At strong hopping the state of the photons in the array depends on its size. While the photon blockade persists in a dimer consisting of two coupled cavities, a coherent state forms on an extended lattice, which can be described in terms of a semi-classical model.
[Show abstract][Hide abstract] ABSTRACT: We determine the current-voltage characteristic of type-II superconductors in the presence of strong pinning centers. Focusing on a small density of defects, we derive a generic form for the characteristic with a linear flux-flow branch shifted by the critical current (excess-current characteristic). The details near onset, a hysteretic jump (for κ≫1) or a smooth velocity turn-on (κ→1), depend on the Labusch parameter κ characterizing the pinning centers. Pushing the single-pin analysis into the weak pinning domain, we reproduce the collective pinning results for the critical current.