Quantum Degeneracy Research Articles

Collisional ionization and recombination in degenerate plasmas beyond the free-electron-gas approximation.

One of the most successful ways to model the multitude of electron and photon processes in plasmas is the approach used in collisional radiative (CR) codes. The accuracy of CR codes depends largely on the accuracy of the rates of each process. These rates are generally well approximated in hot, classical plasmas. However, in degenerate plasmas quantum effects can influence these rates and must be accounted for. Previous approaches have developed corrections to the classical rates using the free-electron-gas (FEG) approximation. Here, we use electronic structures beyond the FEG approximation and show how the collisional rates are affected by degeneracy in aluminum and iron plasmas. We find that the FEG is a good approximation for aluminum, whereas more complex electronic structures that include d orbitals, such as iron, deviate from the FEG approximation. This results in different degeneracy corrections to the collisional rates relative to those for the FEG. Although the general trend of the corrections to degenerate plasmas is captured by assuming an FEG, we show that more complex electronic structures can result in deviations, even outside the degenerate regime. This study further advances the treatment of free-electron quantum effects in collisional radiative models.

Electron quasi-itinerancy intertwined with quantum order by disorder in pyrochlore iridate magnetism

We point out the emergence of magnetism from the interplay of electron quasi-itinerancy and quantum order by disorder in pyrochlore iridates. Like other Mott insulating iridates, the Ir$^{4+}$ ion in pyrochlore iridates develops an effective ${J=1/2}$ moment from the on-site spin-orbit coupling. We consider the generic symmetry-allowed exchange between these local moments on a pyrochlore lattice and obtain the mean-field phase diagram. Assuming the superexchange is mediated by direct and/or indirect electron hopping via intermediate oxygens, we derive the exchange interactions in the strong coupling regime from the Hubbard model. This exchange has a degenerate classical ground state manifold and quantum fluctuation selects a non-coplanar ground state, known as quantum order by disorder. Extending to the intermediate coupling regime, the same non-coplanar order is selected from the degenerate manifold by the kinetic energy, which is dubbed "electron quasi-itinerancy". We discuss the experimental relevance of our results and electron quasi-itinerancy among other iridates and $4d/5d$ magnets.

Single-electron emission from degenerate quantum levels

Single-electron sources on-demand are requisite for a promising fully electronic platform for solid-state quantum information processing. Most of the experimentally realized sources use the fact that only one electron can be taken from a singly occupied quantum level. Here I take the next step and discuss emission from orbitally degenerate quantum levels that arise, for example, in quantum dots with a nontrivial ring topology. I show that degeneracy provides additional flexibility for single- and two-electron emission. Indeed, the small Aharonov-Bohm flux, which slightly lifts the degeneracy, is a powerful tool for changing the relative width of the emitted wave packets over a wide range. In a ring with one lead, even electrons emitted from completely degenerate levels can be separated in time if the driving potential changes at the appropriate rate. In a ring with two leads, electrons can be emitted to different leads if one of the leads has a bound state with Fermi energy at its end.

Stability in turbulence: The interplay between shocks and vorticity in a superfluid with higher-order dispersion

Higher-order dispersion can lead to intriguing dynamics that are becoming a focus of modern hydrodynamics research. Such systems occur naturally, for example in shallow water waves and nonlinear optics, for which several types of novel dispersive shocks structures have been identified. Here we introduce ultracold atoms as a tunable quantum simulations platform for higher-order systems. Degenerate quantum gases are well controlled model systems for the experimental study of dispersive hydrodynamics in superfluids and have been used to investigate phenomena such as vortices, solitons, dispersive shock waves and quantum turbulence. With the advent of Raman-induced spin-orbit coupling, the dispersion of a dilute gas Bose-Einstein condensate can be modified in a flexible way, allowing for detailed investigations of higher-order dispersion dynamics. Here we present a combined experimental and theoretical study of shock structures generated in such a system. The breaking of Galilean invariance by the spin-orbit coupling allows two different types of shock structures to emerge simultaneously in a single system. Numerical simulations suggest that the behavior of these shock structures is affected by interactions with vortices in a manner reminiscent of emerging viscous hydrodynamics due to an underlying quantum turbulence in the system. This result suggests that spin-orbit coupling can be used as a powerful means to tun the effective viscosity in cold-atom experiments serving as quantum simulators of turbulent hydrodynamics, with applications from condensed matter and optics to quantum simulations of neutron stars.

3-D Cylindrical Waves in a Self-Gravitating Degenerate Quantum Plasma

A theoretical investigation is carried out for understanding the properties of three-dimensional degenerate pressure-driven (DPD) gravitoelectrostatic solitary waves (SWs) in a general (but realistic) self-gravitating degenerate quantum plasma (SGDQP) medium whose constituents are noninertial degenerate electron and inertial degenerate heavy and light ions/nuclei/elements. The formation of solitary electrostatic and self-gravitational potential structures in such an SGDQP system is examined. The standard reductive perturbation method is employed to derive the (3+1)-D cylindrical Korteweg-de Vries equation (also known as cylindrical Kadomtsev-Petviashvili equation). The parametric regimes for the existence of solitary electrostatic and self-gravitational potential structures are found. It is shown that the present plasma model supports the DPD SWs with positive and negative electrostatic potentials as well as self-gravitational potentials. The effects of the degenerate plasma particles are found to significantly modify the basic properties (viz., the amplitude, width, and speed) of the electrostatic and self-gravitational SWs. The present investigation can be of relevance to the electrostatic and self-gravitational solitary pulses observed in various plasma space environments (viz., white dwarfs, neutron stars, and so on).

Negative contributions to entropy production induced by quantum coherences

The entropy production in dissipative processes is the essence of the arrow of time and the second law of thermodynamics. For dissipation of quantum systems, it was recently shown that the entropy production contains indeed two contributions: a classical one and a quantum one. Here we show that for degenerate (or near-degenerate) quantum systems there are additional quantum contributions which, remarkably, can become negative. Furthermore, such negative contributions are related to significant changes in the ongoing thermodynamics. This includes phenomena such as generation of coherences between degenerate energy levels (called horizontal coherences), alteration of energy exchanges and, last but not least, reversal of the natural convergence of the populations toward the thermal equilibrium state. Going further, we establish a complementarity relation between horizontal coherences and population convergence, particularly enlightening for understanding heat flow reversals. Conservation laws of the different types of coherences are derived. Some consequences for thermal machines and resource theory of coherence are suggested.

Short-wavelength soliton in a fully degenerate quantum plasma

We present a novel one-dimensional nonlinear evolution equation governing the dynamics short-wavelength longitudinal waves in a nonrelativistic fully degenerate quantum plasma using kinetic equation for the Wigner function. The linear dispersion of the equation has a form of “zero sound” ∼k exp (−k2), where k is the wave number, and it strongly differs from previously known nonlinear evolution equations. We numerically find the corresponding soliton solutions and demonstrate that the collisions between three solitons turn out to be elastic, resulting only in phase shifts.

Non-Abelian quantum adiabatic dynamics and phase simulation with classical resonant oscillators

It is known that the dynamics and geometric phase of a quantum system could be mapped mathematically into a classical system of coupled oscillators without loss of physics. In this work, we show that this quantum-classical mapping can also be used to simulate non-Abelian dynamics and phase of the quantum system with energy degeneracy by interpreting the quantum degeneracy as the resonance between the eigenfrequencies of the classical oscillators. The quantum evolution and Wilczek-Zee phase in the degenerate subspace can also be described by the classical non-Abelian evolution and a non-Abelian geometric angle in the resonant subspace. By studying the adiabatic evolution in this classical non-Abelian case, our results can also be used to generalize the concept of averaging principle and Hannay's angle into the classical system with resonant eigenfrequencies. The quantum-classical mapping of a tripod-scheme Hamiltonian with two degenerate dark states is employed to illustrate the classical non-Abelian dynamics and geometric angles.

Ultracold collisions of the lithium monoxide radical

Ultracold collisions of LiO molecules in the $^{2}\Pi_{3/2}$ ground state are considered, under the influence of either an external magnetic or electric field. Inelastic collisions are shown to be suppressed in the presence of modest laboratory strength magnetic and electric fields. The rate of elastic collisions that rethermalize the thermal distribution, and the corresponding low rate of heating state-changing collisions, suggest that quantum degeneracy or even molecular Bose-Einstein condensation of LiO gas may be attainable, provided that the initial temperatures in the milliKelvin range are achievable.

Biexcitons in highly excited CdSe nanoplatelets

We present the phase diagram of free charges (electrons and holes), excitons, and biexcitons in highly excited CdSe nanoplatelets that predicts a crossover to a biexciton-dominated region at easily attainable low temperatures or high photoexcitation densities. Our findings extend previous work describing only free charges and excitons by introducing biexcitons into the equation of state, while keeping the exciton and biexciton binding energies constant in view of the relatively low density of free charges in this material. Our predictions are experimentally testable in the near future and offer the prospect of creating a quantum degenerate, and possibly even superfluid, biexciton gas. Furthermore, we also provide simple expressions giving analytical insight into the regimes of photoexcitation densities and temperatures in which excitons and biexcitons dominate the response of the nanoplatelets.

Suppression of Penning ionization by orbital angular momentum conservation

The efficient suppression of autoionizing collisions is a stringent requirement to achieve quantum degeneracy in metastable rare gases. Here, we report on the suppression of Penning ionization in collisions between metastable He and laser-excited Li atoms. Our results show that the suppression is achieved by the conservation of both the total electron spin and $\mathrm{\ensuremath{\Lambda}}$, i.e., the projection of the total molecular orbital angular momentum along the internuclear axis. Our findings suggest that $\mathrm{\ensuremath{\Lambda}}$ conservation can be used as a more general means of reaction control, in particular, to improve schemes for the simultaneous laser cooling and trapping of metastable He and alkali-metal atoms.

Emergent Fermi surface in a many-body non-Hermitian fermionic chain

Quantum degeneracy pressure (QDP) underscores the stability of matter and is arguably the most ubiquitous many-body effect. The associated Fermi surface (FS) has broad implications for physical phenomena, ranging from electromagnetic responses to entanglement entropy (EE) area law violations. Given recent fruitful studies in condensed-matter physics under effectively non-Hermitian descriptions, it becomes urgent to study how QDP and many-body interactions interplay with non-Hermitian effects. Through a prototypical critical 1D fermionic lattice with asymmetric gain/loss, a real space FS is shown to naturally emerge, in addition to any existing momentum space FS. We also carefully characterize such real space FS with the EE, via a renormalized temperature that encapsulates the interplay of thermal excitations and non-Hermiticity. Nearest neighbor repulsion is also found to induce competing charge density wave (CDW) that may erode the real space FS. The underlying physics surrounding criticality and localization is further analyzed with complex flux spectral flows. Our findings can be experimentally demonstrated with ultracold fermions in a suitably designed optical lattice.

Rapid production of a Rb85 Bose-Einstein condensate in a double compressible optical dipole trap

We develop an all-optical method to rapidly produce a large $^{85}\mathrm{Rb}$ Bose-Einstein condensate. $^{85}\mathrm{Rb}$ has a reputation as a difficult species to cool to quantum degeneracy due to its collisional properties. We solve these intrinsic difficulties by a combination of polarization gradient cooling in a 3D optical lattice and a compressible double-crossed dipole trap (DCDT). This lattice cooling circumvents the low rethermalization rate and loads more than ${10}^{7}$ atoms with a phase-space density larger than ${10}^{\ensuremath{-}3}$ into the enlarged DCDT. The subsequent compressions and evaporations in the size-adjustable DCDT can maximize the cooling efficiency while reducing unnecessary inelastic losses. We fix the bias magnetic field to 161 G where the $s$-wave scattering length is positive and large. The forced evaporations function well against inelastic collisions at an average density of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. A nearly pure condensate of $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms is produced in 3.2 s of evaporation time.

Robustness of chiral symmetry in atomic nuclei with reflection-asymmetric shapes

Abstract Nearly degenerate quantum states arise from fundamental symmetries, which are often broken in finite many-body systems like atomic nuclei. Chirality can be static, like in molecules composed of more than four atoms, or dynamic, like in particle physics distinguishing between parallel and antiparallel orientation of the spin with respect to the momentum of massless fermions. Frauendorf and Meng.

Heavy nucleus acoustic periodic waves in a degenerate relativistic magneto-rotating quantum plasma

The study of heavy nucleus acoustic (HNA) periodic and solitary waves in degenerate relativistic magneto-rotating quantum plasma (DRMQP) composed of relativistic degenerate electrons, light nuclei and non-degenerate mobile heavy nuclei is presented. Reductive perturbation technique is adopted to derive the Korteweg–de Vries (KdV) type equation for studying the characteristics of HNA periodic waves. Further energy balance equation is derived by using Sagdeev potential approach and HNA cnoidal wave solution is determined. Only positive potential nonlinear periodic and solitary structures are observed. The impact of number density ratio, charge density, magnetic field strength and rotational effect is analyzed on the characteristics of HNA cnoidal and solitary waves. The results of this investigation may shed light on the salient features of different nonlinear HNA waves in different astrophysical environments, especially in white dwarfs.

Machine learning for achieving Bose-Einstein condensation of thulium atoms

Bose-Einstein condensation (BEC) is a powerful tool for a wide range of research activities, a large fraction of which are related to quantum simulations. Various problems may benefit from different atomic species, but cooling down novel species interesting for quantum simulations to BEC temperatures requires a substantial amount of optimization and is usually considered as a hard experimental task. In this work, we implemented the Bayesian machine learning technique to optimize the evaporative cooling of thulium atoms and achieved BEC in an optical dipole trap operating near 532 nm. The developed approach could be used to cool down other novel atomic species to quantum degeneracy without additional studies of their properties.

Experimental setup for the production of ultracold strongly correlated fermionic superfluids of 6Li

We present our experimental setup to produce ultracold strongly correlated fermionic superfluids made of a two-component spin-mixture of 6Li atoms. Employing standard cooling techniques, we achieve quantum degeneracy in a single-beam optical dipole trap. Our setup is capable of generating spin-balanced samples at temperatures as low as T/TF = 0.1 containing up to 5 × 10^4 atomic pairs. We can access different superfluid regimes by tuning the interparticle interactions close to a broad magnetic Feshbach resonance. In particular, we are able to explore the crossover from the molecular Bose-Einstein condensate (BEC) to the Bardeen-Cooper-Schrieffer (BCS) superfluid regimes. In the near future, we plan to study different collective excitations in these superfluid samples.

Self-Gravitational Shock Structures in Self-Gravitating, Super-Dense, Degenerate Quantum Plasmas

The nonlinear propagation of self-gravitational shock structures (SGSSs) in a self-gravitating, super-dense, degenerate quantum plasma system (containing non-degenerate extremely heavy nuclei and ultra-relativistic degenerate electrons) has been investigated. The well-known reductive perturbation technique, which is valid in the small but finite amplitude limit, has been used to examine the nonlinear propagation of these SGSSs in such degenerate quantum plasma systems. The nonlinear dynamics of these SGSSs has been found to be governed by the Burgers equation, which is derived analytically and solved numerically in planar coordinates. These SGSSs in such plasma systems are shown to be formed due to the presence of a viscous force (which is the source of dissipation) acting on inertial extremely heavy nuclei species of the plasma system. The fundamental properties (viz., amplitude, width, speed, etc.) of these SGSSs are influenced in the ultra-relativistic limit. Our considered plasma model and the numerical analysis of the Burgers equation can be applied to astrophysical compact objects like neutron stars.

Cascade of electronic transitions in magic-angle twisted bilayer graphene.

Magic-angle twisted bilayer graphene exhibits a variety of electronic states, including correlated insulators1-3, superconductors2-4 and topological phases3,5,6. Understanding the microscopic mechanisms responsible for these phases requires determination of the interplay between electron-electron interactions and quantum degeneracy (the latter is due to spin and valley degrees of freedom). Signatures of strong electron-electron correlations have been observed at partial fillings of the flat electronic bands in recent spectroscopic measurements7-10, and transport experiments have shown changes in the Landau level degeneracy at fillings corresponding to an integer number of electrons per moiré unit cell2-4. However, the interplay between interaction effects and the degeneracy of the system is currently unclear. Here we report a cascade of transitions in the spectroscopic properties of magic-angle twisted bilayer graphene as a function of electron filling, determined using high-resolution scanning tunnelling microscopy. We find distinct changes in the chemical potential and a rearrangement of the low-energy excitations at each integer filling of the moiré flat bands. These spectroscopic features are a direct consequence of Coulomb interactions, which split the degenerate flat bands into Hubbard sub-bands. We find these interactions, the strength of which we can extract experimentally, to be surprisingly sensitive to the presence of a perpendicular magnetic field, which strongly modifies the spectroscopic transitions. The cascade of transitions thatwe report here characterizes the correlated high-temperature parent phase11,12 from which various insulating and superconducting ground-state phases emerge at low temperatures in magic-angle twisted bilayer graphene.

Bipartite entanglement in the spin-1/2 Ising-Heisenberg planar lattice constituted of identical trigonal bipyramidal plaquettes

The bipartite entanglement is rigorously examined in the spin-1/2 Ising-Heisenberg planar lattice composed of identical inter-connected bipyramidal plaquettes at zero and finite temperatures using the quantity called concurrence. It is shown that the Heisenberg spins of the same plaquette are twice stronger entangled in the two-fold degenerate quantum ground state than in the macroscopically degenerate quantum chiral one. The bipartite entanglement with chiral features completely disappears below or exactly at the critical temperature of the model, while that with no chirality may survive even above the critical temperature of the model. Non-monotonous temperature variations of the concurrence clearly evidence the activation of the entangled Heisenberg states also above classical ground state as well as their re-appearance above the critical temperature of the model.