arXiv subjects

I. Bloch

Publications and source records attributed to I. Bloch.

At least 55 records · Page 3Linked to original sources

arXiv2014

Further studies of the photoproduction of isolated photons with a jet at HERA

In this extended analysis using the ZEUS detector at HERA, the photoproduction of isolated photons together with a jet is measured for different ranges of the fractional photon energy, $x_γ^{\mathrm{meas}}$, contributing to the photon-jet final state. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges $6 < E_T^γ < 15$ GeV and $-0.7 < η^γ < 0.9$, and for jet transverse-energy and pseudorapidity ranges $4 < E_T^{\rm jet} < 35$ GeV and $-1.5 < η^{\rm jet} < 1.8$, for an integrated luminosity of 374 $\mathrm{pb}^{-1}$. The kinematic observables studied comprise the transverse energy and pseudorapidity of the photon and the jet, the azimuthal difference between them, the fraction of proton energy taking part in the interaction, and the difference between the pseudorapidities of the photon and the jet. Higher-order theoretical calculations are compared to the results.

hep-ex↗

arXiv2014

Measurement of D* photoproduction at three different centre-of-mass energies at HERA

The photoproduction of $D^{*\pm}$ mesons has been measured with the ZEUS detector at HERA at three different ep centre-of-mass energies, $\sqrt{s}$, of 318, 251 and 225 GeV. For each data set, $D^*$ mesons were required to have transverse momentum, $p_T^{D^*}$, and pseudorapidity, $η^{D^*}$, in the ranges $1.9 < p_T^{D^*} < 20$ GeV and $|η^{D^*}|<1.6$. The events were required to have a virtuality of the incoming photon, $Q^2$, of less than 1 GeV$^2$. The dependence on $\sqrt{s}$ was studied by normalising to the high-statistics measurement at $\sqrt{s} =318$ GeV. This led to the cancellation of a number of systematic effects both in data and theory. Predictions from next-to-leading-order QCD describe the $\sqrt{s}$ dependence of the data well.

hep-ex↗

arXiv2014

Deep inelastic cross-section measurements at large y with the ZEUS detector at HERA

The reduced cross sections for $e^{+}p$ deep inelastic scattering have been measured with the ZEUS detector at HERA at three different centre-of-mass energies, $318$, $251$ and $225$ GeV. The cross sections, measured double differentially in Bjorken $x$ and the virtuality, $Q^2$, were obtained in the region $0.13\ \leq\ y\ \leq\ 0.75$, where $y$ denotes the inelasticity and $5\ \leq\ Q^2\ \leq\ 110$ GeV$^2$. The proton structure functions $F_2$ and $F_L$ were extracted from the measured cross sections.

hep-ex↗

arXiv2014

Measurement of neutral current e+/-p cross sections at high Bjorken x with the ZEUS detector

The neutral current e+/-p cross section has been measured up to values of Bjorken x of approximately 1 with the ZEUS detector at HERA using an integrated luminosity of 187 inv. pb of e-p and 142 inv. pb of e+p collisions at sqrt(s) = 318GeV. Differential cross sections in x and Q2, the exchanged boson virtuality, are presented for Q2 geq 725GeV2. An improved reconstruction method and greatly increased amount of data allows a finer binning in the high-x region of the neutral current cross section and leads to a measurement with much improved precision compared to a similar earlier analysis. The measurements are compared to Standard Model expectations based on a variety of recent parton distribution functions.

hep-ex↗

arXiv2014

Emergence of coherence and the dynamics of quantum phase transitions

The dynamics of quantum phase transitions poses one of the most challenging problems in modern many-body physics. Here, we study a prototypical example in a clean and well-controlled ultracold atom setup by observing the emergence of coherence when crossing the Mott insulator to superfluid quantum phase transition. In the one-dimensional Bose-Hubbard model, we find perfect agreement between experimental observations and numerical simulations for the resulting coherence length. We thereby perform a largely certified analogue quantum simulation of this strongly correlated system reaching beyond the regime of free quasiparticles. Experimentally, we additionally explore the emergence of coherence in higher dimensions where no classical simulations are available, as well as for negative temperatures. For intermediate quench velocities, we observe a power-law behaviour of the coherence length, reminiscent of the Kibble-Zurek mechanism. However, we find exponents that strongly depend on the final interaction strength and thus lie outside the scope of this mechanism.

cond-mat.quant-gas↗

arXiv2014

Observation of the Meissner effect with ultracold atoms in bosonic ladders

We report on the observation of the Meissner effect in bosonic flux ladders of ultracold atoms. Using artificial gauge fields induced by laser-assisted tunneling, we realize arrays of decoupled ladder systems that are exposed to a uniform magnetic field. By suddenly decoupling the ladders and projecting into isolated double wells, we are able to measure the currents on each side of the ladder. For large coupling strengths along the rungs of the ladder, we find a saturated maximum chiral current corresponding to a full screening of the artificial magnetic field. For lower coupling strengths, the chiral current decreases in good agreement with expectations of a vortex lattice phase. Our work marks the first realization of a low-dimensional Meissner effect and, furthermore, it opens the path to exploring interacting particles in low dimensions exposed to a uniform magnetic field.

cond-mat.quant-gas↗

arXiv2013

Realization of the Hofstadter Hamiltonian with ultracold atoms in optical lattices

We demonstrate the experimental implementation of an optical lattice that allows for the generation of large homogeneous and tunable artificial magnetic fields with ultracold atoms. Using laser-assisted tunneling in a tilted optical potential we engineer spatially dependent complex tunneling amplitudes. Thereby atoms hopping in the lattice accumulate a phase shift equivalent to the Aharonov-Bohm phase of charged particles in a magnetic field. We determine the local distribution of fluxes through the observation of cyclotron orbits of the atoms on lattice plaquettes, showing that the system is described by the Hofstadter model. Furthermore, we show that for two atomic spin states with opposite magnetic moments, our system naturally realizes the time-reversal symmetric Hamiltonian underlying the quantum spin Hall effect, i.e., two different spin components experience opposite directions of the magnetic field.

cond-mat.quant-gas↗

arXiv2011

Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators

Quantum phases of matter are characterized by the underlying correlations of the many-body system. Although this is typically captured by a local order parameter, it has been shown that a broad class of many-body systems possesses a hidden non-local order. In the case of bosonic Mott insulators, the ground state properties are governed by quantum fluctuations in the form of correlated particle-hole pairs that lead to the emergence of a non-local string order in one dimension. Using high-resolution imaging of low-dimensional quantum gases in an optical lattice, we directly detect these pairs with single-site and single-particle sensitivity and observe string order in the one-dimensional case.

cond-mat.quant-gas↗

arXiv2009

Suppression of the critical temperature for superfluidity near the Mott transition: validating a quantum simulator

Ultracold atomic gases in optical lattices have proven to be a controllable, tunable and clean implementation of strongly interacting quantum many-body systems. An essential prospect for such quantum simulators is their ability to map out the phase diagram of fundamental many-body model Hamiltonians. However, the results need to be validated first for representative benchmark problems via state-of-the-art numerical methods of quantum many-body theory. Here we present the first ab-initio comparison between experiments and quantum Monte Carlo simulations for strongly interacting Bose gases on a lattice for large systems (up to N = 3e5 particles). The comparison has enabled us to perform thermometry for the interacting quantum gas and to experimentally determine the finite temperature phase diagram for bosonic superfluids in an optical lattice. Our results reveal a downshift of the critical temperature as the transition to the Mott insulator is approached.

cond-mat.quant-gas↗

arXiv2009

Electromagnetically Induced Transparency and Light Storage in an Atomic Mott Insulator

We experimentally demonstrate electromagnetically induced transparency and light storage with ultracold 87Rb atoms in a Mott insulating state in a three dimensional optical lattice. We have observed light storage times of about 240 ms, to our knowledge the longest ever achieved in ultracold atomic samples. Using the differential light shift caused by a spatially inhomogeneous far detuned light field we imprint a "phase gradient" across the atomic sample, resulting in controlled angular redirection of the retrieved light pulse.

quant-ph↗

arXiv2008

Metallic and Insulating Phases of Repulsively Interacting Fermions in a 3D Optical Lattice

The fermionic Hubbard model plays a fundamental role in the description of strongly correlated materials. Here we report on the realization of this Hamiltonian using a repulsively interacting spin mixture of ultracold $^{40}$K atoms in a 3D optical lattice. We have implemented a new method to directly measure the compressibility of the quantum gas in the trap using in-situ imaging and independent control of external confinement and lattice depth. Together with a comparison to ab-initio Dynamical Mean Field Theory calculations, we show how the system evolves for increasing confinement from a compressible dilute metal over a strongly-interacting Fermi liquid into a band insulating state. For strong interactions, we find evidence for an emergent incompressible Mott insulating phase.

cond-mat.str-el↗

arXiv2008

Expansion of a quantum gas released from an optical lattice

We analyze the interference pattern produced by ultracold atoms released from an optical lattice. Such interference patterns are commonly interpreted as the momentum distributions of the trapped quantum gas. We show that for finite time-of-flights the resulting density distribution can, however, be significantly altered, similar to a near-field diffraction regime in optics. We illustrate our findings with a simple model and realistic quantum Monte Carlo simulations for bosonic atoms, and compare the latter to experiments.

cond-mat.other↗

arXiv2008

Role of interactions in 87Rb-40K Bose-Fermi mixtures in a 3d optical lattice

We investigate the effect of interspecies interaction on a degenerate mixture of bosonic 87Rb and fermionic 40K atoms in a three-dimensional optical lattice potential. Using a Feshbach resonance, the 87Rb-40K interaction is tuned over a wide range. Through an analysis of the 87Rb momentum distribution, we find a pronounced asymmetry between strong repulsion and strong attraction. In the latter case, the Bose-Hubbard parameters are renormalized due to self-trapping, leading to a marked shift in the superfluid to Mott insulator transition with increasing Bose-Fermi interaction.

cond-mat.other↗

arXiv2008

Counting atoms using interaction blockade in an optical superlattice

We report on the observation of an interaction blockade effect for ultracold atoms in optical lattices, analogous to Coulomb blockade observed in mesoscopic solid state systems. When the lattice sites are converted into biased double wells, we detect a discrete set of steps in the well population for increasing bias potentials. These correspond to tunneling resonances where the atom number on each side of the barrier changes one by one. This allows us to count and control the number of atoms within a given well. By evaluating the amplitude of the different plateaus, we can fully determine the number distribution of the atoms in the lattice, which we demonstrate for the case of a superfluid and Mott insulating regime of 87Rb.

cond-mat.other↗

arXiv2007

Time-resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices

Quantum mechanical superexchange interactions form the basis of quantum magnetism in strongly correlated electronic media. We report on the direct measurement of superexchange interactions with ultracold atoms in optical lattices. After preparing a spin-mixture of ultracold atoms in an antiferromagnetically ordered state, we measure a coherent superexchange-mediated spin dynamics with coupling energies from 5 Hz up to 1 kHz. By dynamically modifying the potential bias between neighboring lattice sites, the magnitude and sign of the superexchange interaction can be controlled, thus allowing the system to be switched between antiferromagnetic or ferromagnetic spin interactions. We compare our findings to predictions of a two-site Bose-Hubbard model and find very good agreement, but are also able to identify corrections which can be explained by the inclusion of direct nearest-neighbor interactions.

cond-mat.other↗

arXiv2007

Preparation and detection of magnetic quantum phases in optical superlattices

We describe a novel approach to prepare, detect and characterize magnetic quantum phases in ultra-cold spinor atoms loaded in optical superlattices. Our technique makes use of singlet-triplet spin manipulations in an array of isolated double well potentials in analogy to recently demonstrated quantum control in semiconductor quantum dots. We also discuss the many-body singlet-triplet spin dynamics arising from coherent coupling between nearest neighbor double wells and derive an effective description for such system. We use it to study the generation of complex magnetic states by adiabatic and non-equilibrium dynamics.

cond-mat.other↗

arXiv2007

The design and performance of the ZEUS Micro Vertex detector

In order to extend the tracking acceptance, to improve the primary and secondary vertex reconstruction and thus enhancing the tagging capabilities for short lived particles, the ZEUS experiment at the HERA Collider at DESY installed a silicon strip vertex detector. The barrel part of the detector is a 63 cm long cylinder with silicon sensors arranged around an elliptical beampipe. The forward part consists of four circular shaped disks. In total just over 200k channels are read out using $2.9 {\rm m^2}$ of silicon. In this report a detailed overview of the design and construction of the detector is given and the performance of the completed system is reviewed.

physics.ins-det↗

arXiv2007

Direct Observation of Second Order Atom Tunnelling

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing novel dynamical behaviour can arise where particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report on the first direct and time-resolved observation of correlated tunnelling of two interacting atoms through a barrier in a double well potential. We show that for weak interactions between the atoms and dominating tunnel coupling, individual atoms can tunnel independently, similar to the case in a normal Josephson junction. With strong repulsive interactions present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime, where a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Our work constitutes the first direct observation of second order tunnelling events with ultracold atoms, which are the dominating dynamical effect in the strongly interacting regime. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of quantum magnetism.

cond-mat.other↗