We present transport measurements on a dipolar superfluid using a Bose-Einstein condensate of ${}^{162}\mathrm{Dy}$ with strong magnetic dipole-dipole interactions. By moving an attractive laser beam through the condensate we observe an anisotropy in superfluid flow. This observation is compatible with an anisotropic critical velocity for the breakdown of dissipationless flow, which, in the spirit of the Landau criterion, can directly be connected to the anisotropy of the underlying dipolar excitation spectrum. In addition, the heating rate above this critical velocity reflects the same anisotropy. Our observations are in excellent agreement with simulations based on the Gross-Pitaevskii equation and highlight the effect of dipolar interactions on macroscopic transport properties, rendering dissipation anisotropic.

We use continuous weak measurements of a driven superconducting qubit to experimentally study the information dynamics of a quantum Maxwell’s demon. We show how information gained by a demon who can track single quantum trajectories of the qubit can be converted into work using quantum coherent feedback. We verify the validity of a quantum fluctuation theorem with feedback by utilizing information obtained along single trajectories. We demonstrate, in particular, that quantum backaction can lead to a loss of information in imperfect measurements. We furthermore probe the transition between information gain and loss by varying the initial purity of the qubit.

We propose a mechanism generating primordial magnetic fields after the $e^{+}{e}^{-}$ annihilations. Our mechanism involves an ultralight axionlike particle (ALP) which constitutes the dark matter and a dark $U(1{)}_X$ gauge boson introduced to bypass the obstacle placed by the conductivity of cosmic plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon field, and part of the amplified dark photon field is concurrently converted to the ordinary magnetic field through the ALP-induced magnetic mixing. For the relevant ALP mass range ${10}^{-21}\mathrm{eV}\lesssim m_{\varphi }\lesssim {10}^{-17}\mathrm{eV}$, our mechanism can generate $B\sim {10}^{-24}\mathrm{G}(m_{\varphi }/{10}^{-17}\mathrm{eV}{)}^{5/4}$ with a coherent length $\lambda \sim (m_{\varphi }/{10}^{-17}\mathrm{eV}{)}^{-1/2}\mathrm{kpc}$, which is large enough to provide a seed of the galactic magnetic fields. The mechanism also predicts a dark $U(1{)}_X$ electromagnetic field $E_X\sim B_X\sim 80\mathrm{nG}(m_{\varphi }/{10}^{-17}\mathrm{eV}{)}^{-1/4}$, which can result in interesting astrophysical or cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.

Conversions between magnetic and kinetic energy occur over a range of plasma scales in astrophysical and solar dynamos and reconnection in the solar corona and the laboratory. Canonical flux tubes reconcile all plasma regimes with concepts of twists, writhes, and linkages. We present measurements of canonical flux tubes, their helicity, and their helicity transport in a gyrating plasma kink. The helicity gauge is removed with general techniques valid even if only a limited section of the plasma is measured. Temporal asymmetries in the helicities confirm irreducible 3D fields in the kink.

We measured the electromagnetic stress-induced local strain distribution on a centimeter-sized parallel-plate metallic resonant unit illuminated with microwave radiation. Using a fiber interferometer, we found that the strain changes sign across the resonant unit, in agreement with theoretical predictions that the attractive electric and repulsive magnetic forces act at different locations. The enhancement of the corresponding maximum local electromagnetic stress is stronger than the enhancement of the net force, reaching a factor of $>600$ compared to the ordinary radiation pressure.

Coupled 2D sheets of electrons and holes are predicted to support novel quantum phases. Two experiments of Coulomb drag in electron-hole ($e\text{−}h$) double bilayer graphene (DBLG) have reported an unexplained and puzzling sign reversal of the drag signal. However, we show that this effect is due to the multiband character of DBLG. Our multiband Fermi liquid theory produces excellent agreement and captures the key features of the experimental drag resistance for all temperatures. This demonstrates the importance of multiband effects in DBLG: they have a strong effect not only on superfluidity, but also on the drag.

Two-dimensional topological superconductivity has attracted great interest due to the emergence of Majorana modes bound to vortices and propagating along edges. However, due to its rare appearance in natural compounds, experimental realizations rely on a delicate artificial engineering involving materials with helical states, magnetic fields, and conventional superconductors. Here we introduce an alternative path using a class of three-dimensional antiferromagnets to engineer a two-dimensional topological superconductor. Our proposal exploits the appearance of solitonic states at the interface between a topologically trivial antiferromagnet and a conventional superconductor, which realize a topological superconducting phase when their spectrum is gapped by intrinsic spin-orbit coupling. We show that these interfacial states do not require fine-tuning, but are protected by asymptotic boundary conditions.

In minimally twisted bilayer graphene, a moiré pattern consisting of $AB$ and $BA$ stacking regions separated by domain walls forms. These domain walls are predicted to support counterpropogating topologically protected helical (TPH) edge states when the $AB$ and $BA$ regions are gapped. We fabricate designer moiré crystals with wavelengths longer than 50 nm and demonstrate the emergence of TPH states on the domain wall network by scanning tunneling spectroscopy measurements. We observe a double-line profile of the TPH states on the domain walls, only occurring when the $AB$ and $BA$ regions are gapped. Our results demonstrate a practical and flexible method for TPH state network construction.

Measurement is one of the key concepts which discriminates classical and quantum physics. Unlike classical systems, a measurement on a quantum system typically alters it drastically as a result of wave function collapse. Here we suggest that this feature can be exploited for inducing quench dynamics in a many-body system while leaving its Hamiltonian unchanged. Importantly, by doing away with dedicated macroscopic devices for inducing a quench—using instead the indispensable measurement apparatus only—the protocol is expected to be easier to implement and more resilient against decoherence. By way of various case studies, we show that our scheme also has decisive advantages beyond reducing decoherence—for spectroscopy purposes and probing nonequilibrium scaling of critical and quantum impurity many-body systems.

We show that apparently thermalized states still store relevant amounts of information about their past, information that can be tracked by experiments involving nonequilibrium processes. We provide a condition for the microcanonical quantum Crook’s theorem, and we test it by means of numerical experiments. In the Lipkin-Meshkov-Glick model, two different procedures leading to the same equilibrium states give rise to different statistics of work in nonequilibrium processes. In the Dicke model, two different trajectories for the same nonequilibrium protocol produce different statistics of work. Microcanonical averages provide the correct results for the expectation values of physical observables in all the cases; the microcanonical quantum Crook’s theorem fails in some of them. We conclude that testing quantum fluctuation theorems is mandatory to verify if a system is properly thermalized.

We compute the joint statistics of the momenta $p_i$ of $N$ noninteracting fermions in a trap, near the Fermi edge, with a particular focus on the largest one $p_{\max }$. For a 1D harmonic trap, momenta and positions play a symmetric role, and hence the joint statistics of momenta are identical to that of the positions. In particular, $p_{\max }$, as $x_{\max }$, is distributed according to the Tracy-Widom distribution. Here we show that novel “momentum edge statistics” emerge when the curvature of the potential vanishes, i.e., for “flat traps" near their minimum, with $V(x)\sim x^{2n}$ and $n>1$. These are based on generalizations of the Airy kernel that we obtain explicitly. The fluctuations of $p_{\max }$ are governed by new universal distributions determined from the $n$th member of the second Painlevé hierarchy of nonlinear differential equations, with connections to multicritical random matrix models. Finite temperature extensions and possible experimental signatures in cold atoms are discussed.

Quantum backaction (QBA) of a measurement limits the precision of observation of the motion of a free mass. This profound effect, dubbed the “Heisenberg microscope” in the early days of quantum mechanics, leads to the standard quantum limit (SQL) stemming from the balance between the measurement sensitivity and the QBA. We consider the measurement of motion of a free mass performed in a quantum reference frame with an effective negative mass which is not limited by QBA. As a result, the disturbance on the motion of a free mass can be measured beyond the SQL. QBA-limited detection of motion for a free mass is extremely challenging, but there are devices where this effect is expected to play an essential role, namely, gravitational wave detectors (GWDs) such as LIGO and Virgo. Recent reports on the observations of gravitational waves have opened new horizons in cosmology and astrophysics. We present a general idea and a detailed numerical analysis for QBA-evading measurement of the gravitational wave effect on the GWD mirrors, which can be considered free masses under relevant conditions. The measurement is performed by two entangled beams of light, probing the GWD and an auxiliary atomic spin ensemble, respectively. The latter plays the role of a free negative mass. We show that under realistic conditions the sensitivity of the GWD in $m/\sqrt{\mathrm{Hz}}$ can be increased by 6 dB over the entire frequency band of interest.

In this work we introduce boundary time crystals. Here continuous time-translation symmetry breaking occurs only in a macroscopic fraction of a many-body quantum system. After introducing their definition and properties, we analyze in detail a solvable model where an accurate scaling analysis can be performed. The existence of the boundary time crystals is intimately connected to the emergence of a time-periodic steady state in the thermodynamic limit of a many-body open quantum system. We also discuss connections to quantum synchronization.

The slow Debye-like relaxation in the dielectric spectra of monohydroxy alcohols is a matter of long-standing debate. In the present Letter, we probe reorientational dynamics of 5-methyl-2-hexanol with dielectric spectroscopy and depolarized dynamic light scattering (DDLS) in the supercooled regime. While in a previous study of a primary alcohol no indication of the Debye peak in the DDLS spectra was found, we now for the first time report clear evidence of a Debye contribution in a monoalcohol in DDLS. A quantitative comparison between the dielectric and DDLS manifestation of the Debye peak reveals that while the dielectric Debye process represents fluctuations in the end-to-end vector dipole moment of the transient chains, its occurrence in DDLS shows a more local signature and is related to residual correlations that occur due to a slight anisotropy of the $\alpha$ relaxation caused by the chain formation.

We study transient dynamics in a two-dimensional system of interacting Dirac fermions subject to a quenched drive with circularly polarized light. In the absence of interactions, the drive opens a gap at the Dirac point in the quasienergy spectrum, inducing nontrivial band topology. We investigate the dynamics of this gap opening process, taking into account the essential role of electron-electron interactions. Crucially, scattering due to interactions (1) induces dephasing, which erases memory of the system’s prequench state and yields the intrinsic timescale for gap emergence, and (2) provides a mechanism for the system to absorb energy of the drive, leading to heating which must be mitigated to ensure the success of Floquet band engineering. We characterize the gap opening process via the system’s generalized spectral function and correlators probed by photoemission experiments, and we identify a parameter regime at moderate driving frequencies where a hierarchy of timescales allows a well-defined Floquet gap to be produced and studied before the deleterious effects of heating set in.

Inducing superconductivity in a topological insulator can lead to novel quantum effects. However, experimental approaches to turn a topological insulator into a superconductor are limited. Here, we report on superconductivity in topological insulator ${\mathrm{Bi}}_{0.91}{\mathrm{Sb}}_{0.09}$ induced via focused ion-beam deposition of a Pt thin film. The superconducting phase exhibits a Berezinski-Kosterlitz-Thouless transition, demonstrative of its two-dimensional character. From the in-plane upper critical field measurements, we estimate the superconducting thickness to be $\sim 17\mathrm{nm}$ for a $5.5\text{−}\mu \mathrm{m}$-thick sample. Our results provide evidence that the interface superconductivity could originate from the surface states of ${\mathrm{Bi}}_{0.91}{\mathrm{Sb}}_{0.09}$.

Thermally injected pure spin current phenomena have been investigated in $\mathrm{Bi}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ and $\mathrm{Bi}/\mathrm{Ag}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ structures at room temperature. We show that although pure spin current has been injected into the Bi layer and the $\mathrm{Bi}/\mathrm{Ag}$ bilayer, there is little detectable signal of spin-to-charge conversion, except the distinctive Nernst signal from the Bi layer, in sharp contrast to the inverse Rashba-Edelstein effect claimed in these systems.