arXiv subjects

Ralf Schützhold

Publications and source records attributed to Ralf Schützhold.

At least 109 records · Page 6Linked to original sources

arXiv2010

Dynamical quantum phase transitions

A sweep through a quantum phase transition by means of a time-dependent external parameter (e.g., pressure) entails non-equilibrium phenomena associated with a break-down of adiabaticity: At the critical point, the energy gap vanishes and the response time diverges (in the thermodynamic limit). Consequently, the external time-dependence inevitably drives the system out of equilibrium, i.e., away from the ground state, if we assume zero temperature initially. In this way, the initial quantum fluctuations can be drastically amplified and may become observable -- especially for symmetry-breaking (restoring) transitions. By means of several examples, possible effects of these amplified quantum fluctuations are studied and universal features (such as freezing) are discussed.

quant-ph↗

arXiv2010

Quantum radiation by electrons in lasers and the Unruh effect

In addition to the Larmor radiation known from classical electrodynamics, electrons in a laser field may emit pairs of entangled photons -- which is a pure quantum effect. We investigate this quantum effect and discuss why it is suppressed in comparison with the classical Larmor radiation (which is just Thomson backscattering of the laser photons). Further, we provide an intuitive explanation of this process (in a simplified setting) in terms of the Unruh effect.

hep-th↗

arXiv2010

On Quantum Correlations across the Black Hole Horizon

Inspired by the condensed-matter analogues of black holes, we study the quantum correlations across the event horizon reflecting the entanglement between the outgoing particles of the Hawking radiation and their in-falling partners. For a perfectly covariant theory, the total correlation is conserved in time and piles up arbitrary close to the horizon in the past, where it merges into the singularity of the vacuum two-point function at the light cone. After modifying the dispersion relation (i.e., breaking Lorentz invariance) for large $k$, on the other hand, the light cone is smeared out and the entanglement is not conserved but actually created in a given rate per unit time.

gr-qc↗

arXiv2010

O(N) symmetry-breaking quantum quench: Topological defects versus quasiparticles

We present an analytical derivation of the winding number counting topological defects created by an O(N) symmetry-breaking quantum quench in N spatial dimensions. Our approach is universal in the sense that we do not employ any approximations apart from the large-$N$ limit. The final result is nonperturbative in N, i.e., it cannot be obtained by %the usual an expansion in 1/N, and we obtain far less topological defects than quasiparticle excitations, in sharp distinction to previous, low-dimensional investigations.

cond-mat.stat-mech↗

arXiv2009

Catalysis of Schwinger Vacuum Pair Production

We propose a new catalysis mechanism for non-perturbative vacuum electron-positron pair production, by superimposing a plane-wave X-ray probe beam with a strongly focused optical laser pulse, such as is planned at the Extreme Light Infrastructure (ELI) facility. We compute the absorption coefficient arising from vacuum polarization effects for photons below threshold in a strong electric field. This set-up should facilitate the (first) observation of this non-perturbative QED effect with planned light sources such as ELI yielding an envisioned intensity of order 10^{26}W/cm^2.

hep-ph↗

arXiv2009

The role of symmetries in adiabatic quantum algorithms

Exploiting the similarity between adiabatic quantum algorithms and quantum phase transitions, we argue that second-order transitions -- typically associated with broken or restored symmetries -- should be advantageous in comparison to first-order transitions. Guided by simple examples we construct an alternative adiabatic algorithm for the NP-complete problem {\em Exact Cover 3}. We show numerically that its average performance (for the considered cases up to ${\cal O}\{20\}$ qubits) is better than that of the conventional scheme. The run-time of adiabatic algorithms is not just determined by the minimum value of the fundamental energy gap (between the ground state and the exited states), but also by its curvature at the critical point. The proposed symmetry-restoring adiabatic quantum algorithm only contains contributions linear and quadratic in the Pauli matrices and can be generalized to other problem Hamiltonians which are decomposed of terms involving one and two qubits. We show how the factoring problem can be cast into such a quadratic form. These findings suggest that adiabatic quantum algorithms can solve a large class of NP problems much faster than the Grover search routine (which corresponds to a first-order transition and yields a quadratic enhancement only).

quant-ph↗

arXiv2008

Tunneling-induced damping of phase coherence revivals in deep optical lattices

We consider phase coherence collapse and revival in deep optical lattices, and calculate within the Bose-Hubbard model the revival amplitude damping incurred by a finite tunneling coupling of the lattice wells (after sweeping from the superfluid to the Mott phase). Deriving scaling laws for the corresponding decay of first-order coherence revival in terms of filling factor, final lattice depth, and number of tunneling coupling partners, we estimate whether revival-damping related to tunneling between sites can be or even has already been observed in experiment.

cond-mat.other↗

arXiv2008

Effect of fluctuations on the superfluid-supersolid phase transition on the lattice

We derive a controlled expansion into mean field plus fluctuations for the extended Bose-Hubbard model, involving interactions with many neighbors on an arbitrary periodic lattice, and study the superfluid-supersolid phase transition. Near the critical point, the impact of (thermal and quantum) fluctuations on top of the mean field grows, which entails striking effects, such as negative superfluid densities and thermodynamical instability of the superfluid phase -- earlier as expected from mean-field dynamics. We also predict the existence of long-lived "supercooled" states with anomalously large quantum fluctuations.

cond-mat.other↗

arXiv2008

Dynamically assisted Schwinger mechanism

We study electron-positron pair creation {from} the Dirac vacuum induced by a strong and slowly varying electric field (Schwinger effect) which is superimposed by a weak and rapidly changing electromagnetic field (dynamical pair creation). In the sub-critical regime where both mechanisms separately are strongly suppressed, their combined impact yields a pair creation rate which is {dramatically} enhanced. Intuitively speaking, the strong electric field lowers the threshold for dynamical particle creation -- or, alternatively, the fast electromagnetic field generates additional seeds for the Schwinger mechanism. These findings could be relevant for planned ultra-high intensity lasers.

hep-th↗

arXiv2008

Quantum simulator for the Ising model with electrons floating on a helium film

We propose a physical setup that can be used to simulate the quantum dynamics of the Ising model with present-day technology. Our scheme consists of electrons floating on superfluid helium which interact via Coulomb forces. In the limit of low temperatures, the system will stay near the ground state where its Hamiltonian is equivalent to the Ising model and thus shows phenomena such as quantum criticality. Furthermore, the proposed design could be generalized in order to study interacting field theories (e.g., $λϕ^4$) and adiabatic quantum computers.

quant-ph↗

arXiv2008

Bogoliubov theory of quantum correlations in the time-dependent Bose-Hubbard model

By means of an adapted mean-field expansion for large fillings $n\gg1$, we study the evolution of quantum fluctuations in the time-dependent Bose-Hubbard model, starting in the superfluid state and approaching the Mott phase by decreasing the tunneling rate or increasing the interaction strength in time. For experimentally relevant cases, we derive analytical results for the temporal behavior of the number and phase fluctuations, respectively. This allows us to calculate the growth of the quantum depletion and the decay of off-diagonal long-range order. We estimate the conditions for the observability of the time dependence in the correlation functions in the experimental setups with external trapping present. Finally, we discuss the analogy to quantum effects in the early universe during the inflationary epoch.

cond-mat.other↗

arXiv2008

On the origin of the particles in black hole evaporation

We present an analytic derivation of Hawking radiation for an arbitrary (spatial) dispersion relation $ω(k)$ as a model for ultra-high energy deviations from general covariance. It turns out that the Hawking temperature is proportional to the product of the group $dω/dk$ and phase $ω/k$ velocities evaluated at the frequency $ω$ of the outgoing radiation far away, which suggests that Hawking radiation is basically a low-energy phenomenon. Nevertheless, a group velocity growing too fast at ultra-short distances would generate Hawking radiation at ultra-high energies (``ultra-violet catastrophe'') and hence should not be a realistic model for the microscopic structure of quantum gravity.

gr-qc↗

arXiv2007

Quantum back-reaction problems

The macroscopic behavior of many physical systems can be approximately described by classical quantities. However, quantum theory demands the existence of omnipresent quantum fluctuations on top of this classical background -- which, albeit small, should have some impact onto its dynamics. The correct treatment of this quantum back-reaction is one of the main problems in quantum gravity and related to fundamental questions such as the initial (big bang) singularity or the cosmological constant. By means of the qualitative analogy between gravity and fluid dynamics, we try to shed some light onto these problems and show some of the difficulties associated with the calculation of the quantum back-reaction starting from the classical (macroscopic) equation of motion.

gr-qc↗

arXiv2007

Decoherence in the dynamical quantum phase transition of the transverse Ising chain

For the prototypical example of the Ising chain in a transverse field, we study the impact of decoherence on the sweep through a second-order quantum phase transition. Apart from the advance in the general understanding of the dynamics of quantum phase transitions, these findings are relevant for adiabatic quantum algorithms due to the similarities between them. It turns out that (in contrast to first-order transitions studied previously) the impact of decoherence caused by a weak coupling to a rather general environment increases with system size (i.e., number of spins/qubits), which might limit the scalability of the system.

quant-ph↗

arXiv2007

Vortex quantum creation and winding number scaling in a quenched spinor Bose gas

Motivated by a recent experiment, we study non-equilibrium quantum phenomena taking place in the quench of a spinor Bose-Einstein condensate through the zero-temperature phase transition separating the polar paramagnetic and planar ferromagnetic phases. We derive the typical spin domain structure (correlations of the effective magnetization) created by the quench arising due to spin-mode quantum fluctuations, and establish a sample-size scaling law for the creation of spin vortices, which are topological defects in the transverse magnetization.

cond-mat.other↗

arXiv2007

Quantum toy model for black-hole back-reaction

We propose a simple quantum field theoretical toy model for black hole evaporation and study the back-reaction of Hawking radiation onto the classical background. It turns out that the horizon is also ``pushed back'' in this situation (i.e., the interior region shrinks) but this back-reaction is not caused by energy conservation but by momentum balance. The effective heat capacity and the induced entropy variation can have both signs -- depending on the parameters of the model. PACS: 04.62.+v, 04.70.Dy.

gr-qc↗

arXiv2007

Table-top creation of entangled multi-keV photon pairs via the Unruh effect

Electrons moving in a strong periodic electromagnetic field (e.g., laser or undulator) may convert quantum vacuum fluctuations into pairs of entangled photons, which can be understood as a signature of the Unruh effect. Apart from verifying this striking phenomenon, the considered effect may allow the construction of a table-top source for entangled photons (``photon pair laser'') and the associated quantum-optics applications in the multi-keV regime with near-future facilities. 04.62.+v, 12.20.Fv, 41.60.-m, 42.50.Dv.

quant-ph↗

arXiv2007

Analogue of cosmological particle creation in an ion trap

We study phonons in a dynamical chain of ions confined by a trap with a time-dependent (axial) potential strength and demonstrate that they behave in the same way as quantum fields in an expanding/contracting universe. Based on this analogy, we present a scheme for the detection of the analogue of cosmological particle creation which should be feasible with present-day technology. In order to test the quantum nature of the particle creation mechanism and to distinguish it from classical effects such as heating, we propose to measure the two-phonon amplitude via the $2^{\rm nd}$ red side-band and to compare it with the one-phonon amplitude ($1^{\rm st}$ red side-band). PACS: 04.62.+v, 98.80.-k, 42.50.Vk, 32.80.Pj.

quant-ph↗