Quantum Transport with ultracold atoms

The fundamental challenge in the growing field of metrology is to measure a physical quantity with high precision and how the precision improves with increasing number of resources. We report a novel interferometer using the technique of storage and retrieve of light in spin-1 atomic medium where two dark state polaritons interfere and as a result both optical and atomic phase is measured with high precision. More over we report that the sensitivity of measured magnetic field increases inversely with number of atoms reaching the Heisenberg limit of sensitivity. Another use of this interferometer is to quantify the amount of synchronization in a spin-1 system with the phase of external drive. synchronization between different physical systems has gained lots of attention in research. While in classical system, numerous research articles report the signature of synchronization, synchronization in the quantum regime has not been observed yet. In particular, when we store and destructively interfere two dark state polaritons, we observe an otherwise delocalized limit-cycle spin-1 state gets localized and entrained to classically controlled phases, only in presence of artificially engineered, anisotropic decay channels.

The transport of particles through disordered potential landscapes has been actively studied for the last decades. The majority of these studies, e.g. of Anderson localization, addressed the regime in which the disordered potential is static. However, it seems natural to investigate the influence of time-dependent disorder on transport properties. More specifically, a crossover from localization to diffusive, even hyper-ballistic, transport is expected to occur when the disorder varies in time. We experimentally investigate the dynamics of ultracold, spin-polarized fermionic lithium atoms when exposed to such a time-dependent optical speckle potential. Here, we observe a strong dependence of the system’s diffusion exponent on the so-called correlation time, a measure for the speckle’s variation rate.

Recent experimental observations of spin–orbit coupling (SOC) in Bose–Einstein condensates (BECs) open the way for investigating novel physics of nonlinear waves with promising applications in atomic physics and condensed matter physics. The interplay between atomic interactions and SOC are crucial for the understanding of the dynamics of nonlinear waves in BECs with SOC. Here, in the small linear coupling regime, an approach is presented which allows us to derive an infinite number of novel approximate solutions of the Gross–Pitaevskii equations (GPEs) in one and two dimensions including SOCs, time-dependent external potentials, and nonlinearities leading to breathers and periodic as well as quasiperiodic nonlinear waves. To verify the theoretical predictions we perform numerical simulations which show for several cases a very good agreement with the analytics. For the case of one spatial dimension, it is shown that functions describing the external potential and nonlinearities cannot be chosen independently. The management of the solutions is clarified along with some important physical properties such as Josephson oscillations and Rosen–Zener oscillations.

We report on the experimental realization of a Kapitza pendulum for ultracold $^{87}Rb$ atoms. We show the dynamical stabilization of the atomic motion by a time periodic modulated potential. The time average of the modulated potential vanishes and the corresponding Floquet-Hamiltonian results in an effective time independent potential, which acts as a trap for the atoms. The Kapitza pendulum is generated by two time modulated Gaussian shaped laser beams, which create an attractive and repulsive potential. We analyse the stability of the trap depending on the driving frequency, as well as the, role of experimental imperfections. To continue the studies on Floquet driven systems and extend them to transport processes in time modulated environments, we are currently upgrading our ultracold quantum gas experiment to combine a scanning electron microscope and a high resolution optical objective. This will allow us to image and manipulate a cloud of ultracold $^{87}Rb$ atoms with high spatial resolution employing both techniques. Our system will be able to study resonance phenomena of a quasi 1D BEC tunneling through time modulated optical potentials in the presence of dissipation.

We address the depletion of cold Bose atoms via an open channel as a tool for probing chaos in many-body quantum systems. We theoretically analyze the depletion dynamics in a quasi-one-dimensional optical lattice, where atoms in one of the lattice sites are subject to decay due to ionization by an electron beam. The laboratory experiment has been realized in H. Ott's group, see for instance [1]. Unlike the previous studies of this problem we consider a completely different initial quantum state of the system, which is chaotic in a sense of both classical and quantum chaos. To analyze the problem we use the pseudoclassical approach [2], which allows us to treat the experimentally relevant situation of $~10^3$ atoms per lattice site. We show that by measuring occupations of the affected and two neighboring sites one can extract important information about chaotic dynamics of the Bose-Hubbard model. [1] R. Labouvie, B. Santra, S. Heun, and H. Ott, Bistability in a Driven Dissipative Superfluid, Phys. Rev. Lett. 116, 235302 (2016). [2] A. A. Bychek, P. S. Muraev, D. N. Maksimov, and A. R. Kolovsky, Open Bose-Hubbard chain: Pseudoclassical approach, Phys. Rev. E, 101(1), 012208 (2020). [3] A. A. Bychek, P. S. Muraev, and A. R. Kolovsky, Probing quantum chaos in many-body quantum systems by the induced dissipation. Phys. Rev. A, 100(1), 013610 (2020).

Authors: Pavel S. Muraev and Anna A. Bychek Abstract: We analyse stationary current of non-interacting and interacting Bose particles across the tight-binding chain connecting two particles reservoirs. Unlike the standard approach based on Markovian master equation for bosonic carriers in the chain, we introduce non-Makovian master equation which is capable to capture the phenomenon of resonant transport. The effect of inter-particle interactions on the resonant transport is quantified.

We analyze the two main physical observables related to the momenta of strongly correlated SU($N$) fermions in ring-shaped lattices pierced by an effective magnetic flux: homodyne (momentum distribution) and self-heterodyne interference patterns. We demonstrate how their analysis allows us to monitor the persistent current pattern. We find that both homodyne and self-heterodyne interference display a specific dependence on the structure of the Fermi distribution and particles' correlations. For homodyne protocols, the momentum distribution is affected by the particle statistics in two distinctive ways. The first effect is a purely statistical one: at zero interactions, the characteristic hole in the momentum distribution around the momentum $\mathbf{k}=0$ opens up once half of the SU($N$) Fermi sphere is displaced. The second effect originates from interaction: the fractionalization in the interacting system manifests itself by an additional `delay' in the flux for the occurrence of the hole, that now becomes a depression at $\mathbf{k}=0$. In the case of self-heterodyne interference patterns, we are not only able to monitor, but also observe the fractionalization. Indeed, the fractionalized angular momenta, due to level crossings in the system, are reflected in dislocations present in interferograms. Our analysis demonstrate how the study of the interference fringes grants us access to both number of particles and number of components of SU($N$) fermions.

Quantum computers are currently affected by many sources of decoherence. These prevent us from performing high-fidelity quantum operations, but various techniques can be adopted to increase the performance of a quantum system. In particular, in our study, we faced the problem with two approaches: in the first one, we analyzed different counter-diabatic state-transfer protocols affected by various decoherence channels. The study was performed on a single-qubit system and two-qubit entangling gate. The results show that, according to the decoherence channel affecting the system, one can mitigate the degradation of the fidelity by properly optimize the driving [1]; in the second, instead, we exploit the interatomic interactions for compensating static errors in the control parameters. We show that the interaction can be tuned in order to recover essentially the error-free dynamics of the atoms. Our calculations show that there exists a specific condition for which the compensation is indeed optimal [2]. A natural experimental realization are ultracold Rydberg atoms with imperfect excitation pulses. Their nonlocal interaction allows for many possible scenarios for the realization of qubit gates and excitation transport. [1] M. Delvecchio, F. Petiziol, and S. Wimberger, The Renewed role of Sweep Functions in Noisy Shortcuts to Adiabaticity, Entropy, 23(7), 897 (2021) [2] M. Delvecchio, F. Petiziol, E. Arimondo, and S. Wimberger, Atomic interactions for qubit-error compensations, Phys. Rev. A 105, 042431 (2022)

We theoretically investigate a bosonic Josephson junction by using the path-integral formalism with relative phase and population imbalance as dynamical variables. We derive an only-phase action by performing functional integral over the population imbalance. We then analyze the quantum phase action, which formally contains all the quantum corrections. To the second order in the derivative expansion and to the lowest order in $\hbar$-expansion, we obtain the leading quantum correction to the Josephson frequency of oscillation. Finally, we can find the same quantum correction by adopting an alternative approach through the equation of motion. Our predictions would be a useful theoretical tool for experiments with atomic or superconducting Josephson junctions.

Persistent oscillatory dynamics in non-equilibrium many-body systems is a tantalizing manifestation of ergodicity breakdown that continues to attract much attention. Recent works have focused on two classes of such systems: discrete time crystals and quantum many-body scars (QMBS). While both systems host oscillatory dynamics, its origin is expected to be fundamentally different: discrete time crystal is a phase of matter which spontaneously breaks the $\mathbb{Z}_2$ symmetry of the external periodic drive, while QMBS span a subspace of non-thermalizing eigenstates forming an su(2) algebra representation. Here we ask a basic question: is there a physical system that allows to tune between these two dynamical phenomena? In contrast to much previous work, we investigate the possibility of a continuous time crystal (CTC) in undriven, energy-conserving systems exhibiting prethermalization. We introduce a long-range XYZ spin model and show that it encompasses both a CTC phase as well as QMBS. We map out the dynamical phase diagram using numerical simulations based on exact diagonalization and time-dependent variational principle in the thermodynamic limit. We identify a regime where QMBS and CTC order co-exist, and we discuss experimental protocols that reveal their similarities as well as key differences.

Diffusion is a transport phenomenon that appears as a fundamental process in almost all physical systems. Therefore, diffusion can occur in very different regimes, ranging from subdiffusion to hyperballistic diffusion, depending on the external parameters. In addition to the properties of the bath or the diffusing particle, the diffusion in systems subjected to external forces is critical for understanding transport phenomena in complex systems. Here we present a system where we can observe the diffusion dynamics of single atoms in tilted optical lattices in the underdamped regime. A one-dimensional optical lattice allows transporting individual cesium atoms with variable lattice depth, constant velocity or acceleration, and thus force. For example, the force exerted on individual atoms can be huge, exceeding standard gravitation by orders of magnitude. Thereby, very different regimes of diffusion can be experimentally accessed. We find that we can tune the system’s macroscopic diffusion coefficient by varying the lattice depth and acceleration while applying optical molasses onto the atoms as a “bath of light” for the diffusion. Additionally, the atoms can be transported through a bath of ultracold rubidium atoms. We observe the interplay of the large Rb-bath and the single Cs-atoms trapped in the accelerated lattice and report its effective friction.

Time crystals are novel phases of matter with broken time-translational symmetry. Based on the type of symmetry breaking involved, these non-equilibrium phases of matter come in two varieties, namely discrete time crystals and continuous-time crystals (CTC). We show how CTC can be induced in the Zeno limit of strong measurement in a spin star system (arXiv:2206.14438). Strong measurement usually restricts the dynamics of a finite system to the Zeno subspace, where subsequent evolution is unitary. We demonstrate in a spin star system how competition between strong measurement and thermodynamic limit could result in qualitative changes in steady-state properties. We see a dissipative phase transition in the ancilla spins between a phase with stationary magnetization and the time crystal phase with persistent oscillations of magnetization.

We propose to investigate transition metal ions in crystals, for new solid state systems - which can lead electrons and photons at low and high temperature. This coupling can be increased using photonic structures embedded within the host crystal. This work is based on the combination of guided propagation of light through crystals containing transition metal. We set out to demonstrate that these materials can sustain a coherent spin-photon link at liquid-nitrogen temperatures, that various photonic structures can be directly fabricated in them by laser writing, and that these two features can be combined in the demonstration of a high-temperature, fully integrated quantum spin-photon interface.

Thouless pumping is a fundamental instance of quantized transport, which is topologically protected. Although its theoretical importance, the adiabaticity condition is an obstacle for further practical applications. Here, focusing on the Rice-Mele model, we provide a family of finite-frequency examples that ensure both the absence of excitations and the perfect quantization of the pumped charge at the end of each cycle. This family, which contains an adiabatic protocol as a limiting case, is obtained through a mapping onto the zero curvature representation of the Euclidean sinh-Gordon equation.

I discuss how quantum correlations can be converted into energy in out-of-equilibrium processes. This general mechanism is illustrated analytically for free fermion systems and numerically for strongly interacting systems. We consider a quench protocol for two initially independent systems described by thermal states. The von Neumann entropy of each system increases once the systems are coupled. At the moment of decoupling, there is an energy transfer to the systems set by the correlations accumulated during joint evolution. This energy transfer appears as work produced by the quench to decouple the systems and results into measurable temperature change of the system. We discuss plausible setups to verify both qualitative and quantitative predictions in ultracold atom experiments.

We show that nonequilibrium quantum states can be prepared and stabilized by combining time-periodic driving (Floquet engineering) with synthetic quantum baths, as they can be realized in quantum simulators based on superconducting circuits. Considering lattices of periodically driven artificial atoms coupled to pumped-damped cavities, we characterize regimes, where, on the one hand, the periodic driving can produce effective Hamiltonians with desired properties while, on the other, the cavities induce controlled dissipation cooling the systems to the effective ground state. We will illustrate this mechanism in the robust preparation of non-trivial states such as chiral currents for interacting photons and Aharonov-Bohm cages, where quantum interference constrains the systems to localized states.

Ionization of targets such as atoms, ions, and molecules by charged projectiles such as electrons / positrons has been studied from a long time and has various applications; few may be listed as diagnostics of fusion plasmas, modeling of physics and chemistry related to atmosphere, understanding the effect of ionizing radiation on biological tissues etc. The ionization cross sections are essential in the modeling of plasma in fusion research. Beryllium (Be) is one of the materials which is directly exposed to the plasma components in the International Thermonuclear Experimental Reactor (ITER) [1]. Formation of gas-phase Be in various charge states and of hydrides of Be, takes place when the erosion of Be walls occurs in contact with the hot plasma containing hydrogen and its isotopes. Electron collision processes on the beryllium and its charged states play an important role in the fusion edge and diverter plasmas. The tungsten (W) and tungsten based materials have also been recommended as one of the materials to be used as plasma facing components for the International Thermonuclear Experimental Reactor (ITER) [1], and it is also been used in the number of current tokamaks such as JET, ASDEX-Upgrade and DIII-D. Electron induced processes are prevalent in such magnetic fusion devices in a wide range of energies. We report the results of our recent work on calculation of electron impact ionization cross sections for Be, W atoms, charged states of Be and W [2-4]. The status of charged particle ionization processes from targets with introductory idea about the theoretical formalism involved will be reviewed and results for the electron impact ionization of atomic / ionic / molecular targets will be discussed. References: [1]G. Federici, Phys. Scr. T 124, 1 (2006). [2] G. Purohit, D. Kato, I. Murakami, Shivani Gupta and P. Sinha, Eur. Phys. J. D 75, 9 (2021). [3] G. Purohit, D. Kato and I. Murakami, Plasma and Fusion Research 13, 3401026 (2018). [4] G. Purohit and D. Kato, J. Phys. B: At. Mol. Opt. Phys. 51, 135201 (2018).

We experimentally investigate the behavior of a driven-dissipative Bose-Einstein condensate of weakly interacting $^{87}$Rb atoms in a 1-D optical lattice. The dissipation is induced by a scanning electron microscope setup, which allows us to observe a single site time resolved. Tunneling from the neighboring sites makes up the driving force. By changing the tunnel coupling $J$ of the lattice, a dissipative phase transition from a coherent super fluid phase to an incoherent phase can be seen. In the vicinity of the phase transition, both branches coexist in a meta stable region depending on the initial state. Measuring the relaxation rates between the two states allows us to approximate the effective Liouvillian gap and find the critical point. In every individual realization of the experiment, the filling of the site shows a digital behavior, which is visible as pronounced jumps in the site occupation. We find that the switching between both states takes only a few tunneling times despite hundreds of atoms tunneling. Furthermore, starting from an initially filled site, the losses induce a super fluid current which keeps the site filled. This complete extinction of a matter wave within a medium indicates the onset of coherent perfect absorption.

A major signature of Quantum Chaos is the fast scrambling of quantum correlations, quantified by the exponential initial (pre-Ehrenfest time) growth of out-of-time-order correlators (OTOCs) and by their later saturation. As previously shown by [1] and [2], there is a significant difference in the short time dynamics of the OTOCs in integrable systems around hyperbolic fixed points depending on the initial state being localized or uniform (high-temperature). In these cases, the exponential regime is given respectively by twice the instability-exponent $2\lambda$ or only once the stability-exponent $\lambda$ of the hyperbolic fixed point. We show that a local wave-packet can have a clear dynamical transition between these two re- ported exponential-regions within the pre-Ehrenfest-time regime. Thus, the question arises on how to decide, based on the properties of the hy- perbolic fixed point which of the two scenarios applies in each particular situation. 1. Hummel, Q., Geiger, B., Urbina, J. D. & Richter, K. Reversible Quantum Information Spreading in Many-Body Systems near Criticality. Phys. Rev. Lett. 123, 160401 (2019). 2. Xu, T., Scaffidi, T. & Cao, X. Does Scrambling Equal Chaos? Phys. Rev. Lett. 124, 140602 (2020).

The simulation of non-equilibrium many-body systems with many baths and finite temperatures is challenging. The requirement to represent infinite environments and long-time evolution to reach a steady-state makes it exceptionally difficult to study with numerical techniques. An increasingly utilized and natural approach to control the complexity of the task is to cast the baths within an open quantum system. There are many ways to do so, ranging from continuous Lindblad relaxation to discrete refresh events. We introduce an accumulative reservoir construction that employs a series of partial renewals of the reservoir modes. Through this series, the representation accumulates the character of an infinite reservoir. We study the range of behavior within this framework and how it impacts both accuracy and computational cost. These results provide the basis for comparing different open system methods for obtaining steady state dynamics. [1] Purkayastha, Guarnieri, Campbell, Prior, Goold, Phys. Rev. B (2021) [2] Wójtowicz, Elenewski, Rams, Zwolak, J. Chem. Phys. (2021) [3] Wójtowicz, Elenewski, Rams, Zwolak, Phys. Rev. A (2020)

Ultracold atoms offer a unique opportunity to study many-body physics in a clean, well-controlled environment. However, the isolated nature of quantum gases makes it difficult to study transport properties of the system, which are among the key observables in condensed matter physics. In this work, we employ Markovian feedback control to synthesize two effective thermal baths that couple to the boundaries of a one-dimensional Bose-Hubbard chain. This allows the realization of a heat-current carrying state. We investigate the steady state heat current, including its system-size scaling behaviour and its response to disorder. Our findings can be tested by available experimental techniques.