Johnson noise thermometry enables direct measurement of the electron temperature, a valuable probe of many-body systems. Practical use of this technique calls for nonequilibrium generalizations of the Johnson-Nyquist theorem. For a hydrodynamic Corbino device, however, a naive use of the Shockley-Ramo theorem alongside the “Corbino paradox” leads to yet another paradox: current noise through the contacts would seem to be completely insensitive to bulk fluctuations. In this work, we resolve the unphysical “Corbino Shockley-Ramo paradox” by correctly formulating the hydrodynamic Shockley-Ramo problem. This allows us to properly formulate the problem of current noise in a hydrodynamic multiterminal device of arbitrary geometry, as well as validate a previously unjustified assumption for rectangular geometry results. As an example, we compute the Johnson noise in a hydrodynamic Corbino device, where we find a suppression of Johnson noise with magnetic field. This unusual characteristic serves as a qualitative signature of viscous hydrodynamic behavior.

Dressed states are eigenstates of the full Hamiltonian which also includes the interaction of the atoms with the driving field. From this point of view, photons of the field interacting with the atom externally induce quantum coherence, and thus modify the inherent properties of atomic system, such as Mollow triplets in the resonance fluorescence spectrum. Because of the rigorous requirement on the ratio of driving strength to the intrinsic linewidth, the associated demonstrations in neutral atoms are restricted to optical dressing in a cavity quantum electrodynamics system and microwave dressing in the Rydberg excitation. Here, we report the realization of coherent dressing of a ground state driven by a radio-frequency magnetic field in an antirelaxation coated warm vapor cell. We reveal the properties of the radio-frequency dressed states, a linear scaling in the resonance splitting, and the improved resonance linewidth. Such a system is robust to the variation of detection frequency and atom number and is able to determine the driving frequency. It thus may emerge as a unique and versatile tool in quantum metrology, for instance, rf atomic magnetometry in a large detection frequency range.

We study measurement-induced phases of free fermion systems with $U\left(1\right)$ symmetry. Following a recent approach developed for Majorana chains, we derive a field theory description for the purity and bipartite entanglement at large space and timescales. We focus on a multiflavor one-dimensional chain with random complex hoppings and continuous monitoring of the local fermion density. By means of the replica trick, and using the number of flavors as a large parameter controlling our approximations, we derive an effective field theory made up of a SU(N) nonlinear sigma model ($\mathrm{NL}\sigma \mathrm{M}$) coupled to fluctuating hydrodynamics. Contrary to the case of noninteracting Majorana fermions, displaying no $U\left(1\right)$ symmetry, we find that the bipartite entanglement entropy satisfies an area law for all monitoring rates but with a nontrivial scaling of entanglement when the correlation length is large. We provide numerical evidence supporting our claims. We briefly show how imposing a reality condition on the hoppings can change the $\mathrm{NL}\sigma \mathrm{M}$ and also discuss higher-dimensional generalizations.

The discovery of van der Waals superconductors in recent years has generated a lot of excitement for their potentially novel pairing mechanisms. However, their typical atomic-scale thickness and micrometer-scale lateral dimensions impose severe challenges to investigations of pairing symmetry by conventional methods. We demonstrate an improved technique that employs high-quality-factor superconducting resonators to measure the kinetic inductance—up to one part per million—and loss of a van der Waals superconductor. We analyze the equivalent circuit model to extract the kinetic inductance, superfluid stiffness, penetration depth, and ratio of imaginary and real parts of the complex conductivity. We validate the technique by measuring aluminum and finding excellent agreement in both the zero-temperature superconducting gap as well as the complex conductivity data when compared with BCS theory. We then demonstrate the utility of the technique by measuring the kinetic inductance of multilayered niobium diselenide and discuss the limits to the accuracy of our technique when the transition temperature of the sample, ${\mathrm{NbSe}}_2$ at 7.06 K, approaches our Nb probe resonator at 8.59 K. Our method will be useful for practitioners in the growing fields of superconducting physics, materials science, and quantum sensing, as a means of characterizing superconducting circuit components and studying pairing mechanisms of the novel superconducting states which arise in layered two-dimensional materials and heterostructures.

Orbital angular momentum (OAM) light beams for high-order harmonic generation (HHG) provide an additional degree of freedom to study the light-matter interaction at ultrafast timescales. A more sophisticated configuration is a perfect optical vortex (POV) beam, a light beam with a helical wave front characterized by a phase singularity at its center and an azimuthal phase variation. POV beams are characterized by a radial profile which is independent of the OAM. Here, we study the nonperturbative process of gas-phase HHG using a linearly polarized POV beam. We observe that the harmonics are emitted with similar divergence due to the perfectness of the POV-driven harmonic generation. Furthermore, the topological charge upscaling is rigorously followed. We show that a POV beam is more advantageous than that of the Laguerre-Gaussian beam for cases where a large topological charge with a small core size is required. Our research establishes a pathway for producing bright structured extreme ultraviolet coherent radiation sources—a pivotal tool with multifaceted applications across various technological domains.

Attosecond x-ray pulses play a crucial role in the study of ultrafast phenomena involving inner and valence electrons. Especially isolated attosecond pulses with high photon energy and high peak power are of great significance in single-shot imaging in the soft x-ray region, life sciences, and attosecond pump-probe experiments. In modern accelerators, laser manipulation of electrons can be used to tailor the ultrafast properties of free-electron laser pulses. In this paper, we propose a novel laser manipulation technique that makes use of two many-cycle, obliquely incident laser beams with mutual delays to synthesize microbunching rotation on the scale of infrared laser wavelengths within the electron bunch. This synthesis microbunching rotation ultimately leads to an enhanced current contrast ratio between the main peak and the surrounding satellite peaks within the bunch. By properly accounting for the longitudinal space-charge fields within the undulator, a tapered undulator can further suppress the side peaks in the radiation pulse and enable the selection of an isolated, hundred-attosecond, GW-level soft x-ray pulse.

Constrained combinatorial optimization problems, which are ubiquitous in industry, can be solved by quantum algorithms such as quantum annealing (QA) and the quantum approximate optimization algorithm (QAOA). In these quantum algorithms, constraints are typically implemented with quadratic penalty functions. This penalty method can introduce large energy scales and make interaction graphs much more dense. These effects can result in worse performance of quantum optimization, particularly on near-term devices that have sparse hardware graphs and other physical limitations. In this work, we consider linear Ising penalty functions, which are applied with local fields in the Ising model, as an alternative method for implementing constraints that makes more efficient use of physical resources. We study the behavior of the penalty method in the context of quantum optimization for customer data science problems. Our theoretical analysis and numerical simulations of QA and the QAOA indicate that this penalty method can lead to better performance in quantum optimization than the quadratic method. However, the linear Ising penalty method is not suitable for all problems as it cannot always exactly implement the desired constraint. In cases where the linear method is not successful in implementing all constraints, we propose that schemes involving both quadratic and linear Ising penalties can be effective.

Motivated by recent experiments on correlated van der Waals materials, including twisted and rhombohedral graphene and twisted ${\mathrm{WSe}}_2$, we perform an analytical and numerical study of the effects of strong on-site and short-range interactions in fractionally filled ideal Chern bands. We uncover an extensive nontrivial ground state manifold within the band filling range $0<\nu <1$ and introduce a general principle, the “three-rule,” for combining flat band wave functions, which governs their zero-energy property on the torus geometry. Based on the structure of these wave functions, we develop a variational approach that reveals distinct phases under different perturbations: metallic behavior emerges from a finite dispersion, and superconductivity is induced by attractive Cooper channel interactions. Our approach, not reliant on the commonly applied mean-field approximations, provides an analytical expression for the macroscopic wave function of the off-diagonal long-range order correlator, attributing pairing susceptibility to the set of nontrivial zero-energy ground state wave functions. Extending to finite screening lengths and beyond the ideal limit using exact diagonalization simulations, we demonstrate the peculiar structure in the many-body wave function's coefficients to be imprinted in the low-energy spectrum of the topologically ordered Halperin spin-singlet state. Our findings also make connections to frustration-free models of noncommuting projector Hamiltonians, potentially aiding the future construction of exact ground states for various fractional fillings.

In this study, we rigorously derived the survival probability in a time-dependent Ising chain coupled with a bosonic system at zero temperature. We found that nonadiabatic transitions can occur in finite systems due to effects of the external environment, even in parameter regions that are adiabatic in isolated systems. This phenomenon is attributed to cooperative effects similar to those in the Dicke model, becoming particularly pronounced under strong-coupling conditions. Furthermore, considering a continuous-mode boson bath, this cooperative effect can potentially enhance the accuracy of estimating the coupling strength with the environment. These findings may provide new insights and serve as a criterion for evaluating existing and future simulation methods of open many-body quantum systems.

We report measurements of counting statistics, average and variance, of microwave photons of ill-defined frequency: bichromatic photons, i.e., photons involving two well separated frequencies, and “white” broadband photons. Our setup allows for the analysis of single photonic modes of arbitrary waveform over the 1–10 GHz frequency range. The photon statistics are obtained by on-the-fly numerical calculation from the sampled time-dependent voltage. Using an $\mathrm{ac}+\mathrm{dc}$ biased tunnel junction as a source of quantum microwave, we report an ultrawide squeezing spectrum representing a competitive source for entanglement generation (up to 0.9 billion measured entangled bits per second) easily achievable experimentally. We also report the observation of quantum steering by the tunnel junction, and show how the presence of squeezing of a broadband mode implies the existence of entanglement between two modes it encompasses.

We experimentally study the statistics of the transition path time taken by a submicron bead to successfully traverse an energy barrier created by two optical tweezers in two prototypical viscoelastic fluids, namely, aqueous polymer and micellar solutions. We find a very good agreement between our experimental distributions and a theoretical expression derived from the generalized Langevin equation for the particle motion. Our results reveal that the mean transition path times measured in such viscoelastic fluids have a nontrivial dependence on the barrier curvature and they can be significantly reduced when compared with those determined in Newtonian fluids of the same zero-shear viscosity. We verify that the decrease of the mean transition path time can be described in terms of an effective viscosity that quantitatively coincides with that measured by linear microrheology at a frequency determined by the reactive mode that gives rise to the unstable motion over the barrier. Therefore our results uncover the linear response of the particle during its thermally activated escape from a metastable state even when taking place in a non-Markovian bath.

We present a theoretical model and numerical optimization of double Bragg diffraction, a widely used technique in atom interferometry. We derive an effective two-level-system Hamiltonian based on the Magnus expansion in the so-called quasi-Bragg regime, where most Bragg-pulse atom interferometers operate. Furthermore, we extend the theory to a five-level description to account for Doppler detuning. Using these derived effective Hamiltonians, we investigate the impacts of AC-Stark shift and polarization errors on the double Bragg beam splitter, along with their mitigations through detuning control. Notably, we design a linear detuning sweep that demonstrates robust efficiency exceeding 99.5% against polarization errors up to 8.5%. Moreover, we develop an artificial-intelligence-aided optimal detuning control protocol, showcasing enhanced robustness against both polarization errors and Doppler effects. This protocol achieves an average efficiency of 99.92% for samples with a finite momentum width of $0.05\hslash k_L$ within an extended polarization error range of up to 10%.

We experimentally investigate peculiar light propagation inside a three-dimensional (3D) superlattice of resonant cavities that are confined within a 3D photonic band gap. To this end, we fabricated 3D diamondlike photonic crystals from silicon with a broad 3D band gap in the near-infrared and doped them with a periodic array of point defects. In position-resolved reflectivity and scattering microscopy, we observe narrow spectral features that match well with superlattice bands in band structures computed with the plane-wave expansion. The cavities are coupled in all three dimensions when they are closely spaced $(a_{\mathrm{SL}}\le 3a)$, and uncoupled when they are further apart. The superlattice bands correspond to light that hops in high-symmetry directions in 3D (“Cartesian light”) that opens applications in 3D photonic networks, 3D Anderson localization of light, and future 3D quantum photonic networks.

Data visualization is important in understanding the characteristics of data that are difficult to see directly. It is used to visualize loss landscapes and optimization trajectories to analyze optimization performance. Popular optimization analysis is performed by visualizing a loss landscape around the reached local or global minimum using principal component analysis. However, this visualization depends on the variational parameters of a quantum circuit rather than quantum states, which makes it difficult to understand the mechanism of the optimization process through the property of quantum states. Here, we propose a quantum data visualization method using quantum kernels, which enables us to offer fast and highly accurate visualization of quantum states. In our numerical experiments, we visualize a hand-written digits data set and apply a $k$-nearest neighbor algorithm to the low-dimensional data to quantitatively evaluate our proposed method compared with a classical kernel method. As a result, our proposed method achieves comparable accuracy to the state-of-the-art classical kernel method, meaning that the proposed visualization method based on quantum machine learning does not degrade the separability of the input higher-dimensional data. Furthermore, we visualize the optimization trajectories of finding the ground states of the transverse field Ising model, and we successfully find the trajectory characteristics. Since quantum states are higher-dimensional objects that can only be seen via observables, our visualization method, which inherits the similarity of quantum data, would be useful in understanding the behavior of quantum circuits and algorithms.

Multiband superconductivity plays an important role in many emergent novel superconductors and has attracted great interest over the years. Various related experimental aspects have been intensely researched, but a quantitative understanding on the Cooper-pair transport remains still elusive, despite its fundamental and technological importance. We study a Josephson junction with a scanning tunneling microscope (STM), where both tip and sample are Pb, a prototypical type I two-band superconductor. We map the properties of the junction across a wide range of normal state conductances revealing in-gap features originating from multiple Andreev reflections (MARs) and the Josephson effect. We present the theoretical framework to extract the transmission through the transport channels and describe the Cooper-pair tunneling with quantitative precision through two superconducting bands. This paves the way for the understanding of increasingly complicated superconductors.

We study the properties of a cryogenic buffer gas source that uses a low-temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about $82\mathrm{m}{\mathrm{s}}^{-1}$ and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of $56\mathrm{m}{\mathrm{s}}^{-1}$ and a flux of $9\times {10}^9$ ground-state molecules per steradian per pulse, with a substantial fraction at speeds below $40\mathrm{m}{\mathrm{s}}^{-1}$. These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap.

The simultaneous occurrence of electric-field controlled superconductivity and spin-orbit interaction makes two-dimensional electron systems (2DES) constructed from perovskite transition metal oxides promising candidates for the next generation of spintronics and quantum computing. It is, however, essential to understand the electronic bands thoroughly and verify the predicted electronic states experimentally in these 2DES to advance technological applications. Here, we present insights into the electronic states of the 2DES at oxide interfaces through comprehensive investigations of Shubnikov–de Haas oscillations in three different systems: EuO/${\mathrm{KTaO}}_3$, ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$, and amorphous-${\mathrm{LaAlO}}_3$/${\mathrm{KTaO}}_3$. To accurately resolve these oscillations, we conducted transport measurements in high magnetic fields up to 60 T and low temperatures down to 100 mK. For 2D confined electrons at these interfaces, we observed a progressive increase of oscillations frequency and cyclotron mass with the magnetic field. We interpret these universal and intriguing findings by considering the existence of nontrivial electronic bands, for which the $E-k$ dispersion incorporates both linear and parabolic relations. In addition to providing experimental evidence for nonparabolic electronic states in ${\mathrm{KTaO}}_3$ and ${\mathrm{SrTiO}}_3$ 2DES, the unconventional oscillations presented in this study establish a paradigm for quantum oscillations in 2DES based on perovskite transition metal oxides, where the oscillation frequencies in $1/B$ exhibit quadratic dependence on the magnetic field.