We measured angle-resolved high-energy electron spectra emitted from ${\mathrm{C}}_2{\mathrm{H}}_4$ in an intense laser field, extracted field-free electron-ion elastic scattering differential cross sections (DCSs) according to quantitative rescattering theory, and obtained molecular contrast factors (MCFs) subtracting the incoherent sum of DCSs of all the atoms in the molecule. Comparing the results with ab initio scattering calculations and employing least-squares fitting, we have extracted the C-C and C-H bond lengths of the molecule with $\sim 5\%$ uncertainty. This approach opens the way to retrieve the structure of hydrocarbon molecules, potentially at high temporal resolution, employing low collision energies where electron scattering is sensitive to the hydrogen atoms; and where the independent atom model calculations may fail to reproduce the experimentally extracted MCF.

We report on the realization of a transversely loaded two-dimensional magneto-optical trap serving as a source for cold strontium atoms. We analyze the dependence of the source's properties on various parameters, in particular the intensity of a pushing beam accelerating the atoms out of the source. An atomic flux exceeding ${10}^9\mathrm{atoms}/\mathrm{s}$ at a rather moderate oven temperature of $500{}^{\circ }\mathrm{C}$ is achieved. The longitudinal velocity of the atomic beam can be tuned over several tens of m/s by adjusting the power of the pushing laser beam. The beam divergence is around 60 mrad, determined by the transverse velocity distribution of the cold atoms. The slow atom source is used to load a three-dimensional magneto-optical trap realizing loading rates up to ${10}^9\mathrm{atoms}/\mathrm{s}$ without indication of saturation of the loading rate for increasing oven temperature. The compact setup avoids undesired effects found in alternative sources like, e.g., Zeeman slowers, such as vacuum contamination and black-body radiation due to the hot strontium oven.

We present a universal theory for the critical behavior of an impurity at the two-dimensional superfluid–Mott-insulator transition. Our analysis is motivated by a numerical study of the Bose-Hubbard model with an impurity site by Huang et al. [Phys. Rev. B 94, 220502 (2016)]; they found an impurity phase transition as a function of the trapping potential, while the bulk was critical. The bulk theory is described by the $\text{O}\left(2\right)$ symmetric Wilson-Fisher conformal field theory, and we model the impurity by a localized spin-1/2 degree of freedom. We also consider a generalized model by considering an $\text{O}\left(N\right)$ symmetric bulk theory coupled to a spin-$S$ degree of freedom. We study this field theory using the $\epsilon =3-d$ expansion, where the impurity-bulk interaction flows to an infrared stable fixed point at the critical trapping potential. We determine the scaling dimensions of the impurity degree of freedom and the associated critical exponents near the critical point. We also determine the universal contribution of the impurity to the finite-temperature compressibility of the system at criticality. Our results are compared with numerical simulations.

One of the challenges for fermionic cold-atom experiments in optical lattices is to cool the systems to low enough temperature so that they can form quantum degenerate ordered phases. In particular, there has been significant work in trying to find the antiferromagnetic phase transition of the Hubbard model in three dimensions, without success. Here, we attack this problem from a different angle by enhancing the ordering temperature via an increase in the degeneracy of the atomic species trapped in the optical lattice. In addition to developing the general theory, we also discuss some potential systems where one might be able to achieve these results experimentally.

We study theoretically stimulated Brillouin scattering (SBS) in silicon nitride (SiN) waveguides created as a phononic line defect inside a pillar-based phononic-crystal membrane of the same material for efficient confinement of the generated acoustic phonons to the optical waveguides. The phononic defect is carefully designed to confine the transversally resonating breathing acoustic mode inside the phononic band gap of the host phononic crystal. These breathing acoustic modes are well excited by the fundamental optical modes of the waveguides. By optimizing this structure, we show the possibility of achieving high SBS gain in an integrated platform with full complementary metal-oxide-semiconductor (CMOS) compatibility with other photonic and electronic functionalities. The combination of low-loss traveling photons and long-lasting resonating phonons in the proposed SiN waveguide paves the way for the demonstration of efficient on-chip SBS devices.

We propose an extension of quantum mechanics based on the idea that the underlying “quantum noise” has a nonzero, albeit very small, correlation time ${\tau }_c$. The standard (nonrelativistic) Schrödinger equation is recovered to zeroth order in ${\tau }_c$, and the first correction to energy levels is computed. Some consequences are discussed, in particular, the violation of Heisenberg's uncertainty principle and the restoration of locality at short times.

Quantum coherence has recently emerged as a key candidate for use as a resource in various quantum information processing tasks. Therefore, it is of utmost importance to explore the possibility of creating a greater number of coherent states from an existing coherent pair. In other words, we start with an initial incoherent pair and induce coherence via quantum cloning. More specifically, we start with a genuinely incoherent state that remains incoherent with the change of basis and make it a coherent state at the end. This process is known as broadcasting of coherence via cloning, which can either be optimal or nonoptimal. Interestingly, in this work, we are able to give a method by which we can introduce coherence in the genuinely incoherent state. We use the computational basis representation of the most general two-qubit mixed state, shared between Alice and Bob, as the input state for the universal symmetric optimal Buzek-Hillery cloner. First of all we show that it is impossible to ensure optimal broadcast of coherence. Second, in the case of nonoptimal broadcasting, we show that the coherence introduced in the output states of the cloner will always be lesser than the initial coherence of the input state. Finally, we take the examples of statistical mixture of the most coherent state and most incoherent state, and then the Bell-diagonal states, to obtain their respective ranges of nonoptimal broadcasting in terms of their input state parameters.

We present a blueprint for building a fault-tolerant universal quantum computer with Rydberg atoms. Our scheme, which is based on the surface code, uses individually addressable, optically trapped atoms as qubits and exploits electromagnetically induced transparency to perform the multiqubit gates required for error correction and computation. We discuss the advantages and challenges of using Rydberg atoms to build such a quantum computer, and we perform error correction simulations to obtain an error threshold for our scheme. Our findings suggest that Rydberg atoms are a promising candidate for quantum computation, but gate fidelities need to improve before fault-tolerant universal quantum computation can be achieved.

We use a magnetometer probe based on the Zeeman shift of the rubidium resonant optical transition to explore the atomic magnetic response for a wide range of field values. We record optical spectra for fields from a few tesla up to 60 T, the limit of the coil producing the magnetic field. The atomic absorption is detected by the fluorescence emissions from a very small region with a submillimeter size. We investigate a wide range of magnetic interactions from the hyperfine Paschen-Back regime to the fine one and the transitions between them. The magnetic field measurement is based on the rubidium absorption itself. The rubidium spectroscopic constants were previously measured with high precision, except the excited state Landé $g$-factor that we derive from the position of the absorption lines in the transition to the fine Paschen-Back regime. Our spectroscopic investigation, even if limited by the Doppler broadening of the absorption lines, measures the field with a 20 ppm uncertainty at the explored high magnetic fields. Its accuracy is limited to 75 ppm by the excited state Landé $g$-factor determination.

We report a detailed molecular spectroscopy study of the lowest excited electronic states of ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ for producing ultracold ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ molecules in the electronic, rovibrational, and hyperfine ground state. Starting from weakly bound Feshbach molecules, a series of vibrational levels of the $A{}^1{\mathrm{\Sigma }}^{+}\text{−}b{}^3\mathrm{\Pi }$ coupled excited states is investigated. After resolving, modeling, and interpreting the hyperfine structure of several lines, we successfully identify a long-lived level resulting from the accidental hyperfine coupling between the $0^{+}$ and $0^{-}$ components of the $b{}^3\mathrm{\Pi }$ state, satisfying all the requirements for the population transfer toward the lowest rovibrational level of the $X{}^1{\mathrm{\Sigma }}^{+}$ state. Using two-photon spectroscopy, its binding energy is measured to be 4977.308(3) ${\mathrm{cm}}^{-1}$. We calibrate all the transition strengths carefully and also demonstrate Raman transfer of Feshbach molecules to the absolute ground state.

The quantum theory of coherent Ising machines, based on degenerate optical parametric oscillators and measurement-feedback circuits, is developed using the positive $P(\alpha ,\beta )$ representation of the density operator and the master equation. The theory is composed of the c-number stochastic differential equations for describing open dissipative quantum dynamics and the replicator dynamics equations for handling measurement-induced collapse of the density operator. We apply the present theory to simulate two simple Ising spin models and elucidate the unique features of this computing machine.

Recently, the coherent Ising machine (CIM) as a degenerate optical parametric oscillator (DOPO) network has been researched to solve Ising combinatorial optimization problems. We formulate a theoretical model for the CIM with discrete-time measurement feedback processes, and perform numerical simulations for the simplest network, composed of two degenerate optical parametric oscillator pulses with the antiferromagnetic mutual coupling. We evaluate the extent to which quantum coherence exists during the optimization process.

The addition or subtraction of a photon from a Gaussian state of light is a versatile and experimentally feasible procedure to create non-Gaussian states. In multimode setups, these states manifest a wide range of phenomena when the photon is added or subtracted in a mode-tunable way. In this paper we derive the truncated correlations, which are multimode generalizations of cumulants, between quadratures in different modes as statistical signatures of these states. These correlations are then used to obtain the full multimode Wigner function, the properties of which are subsequently studied. In particular we investigate the effect of impurity in the subtraction or addition process and evaluate its impact on the negativity of the Wigner function. Finally, we elaborate on the generation of inherent entanglement through subtraction or addition of a photon from a pure squeezed vacuum.

van der Waals interactions govern the physics of a plethora of molecular structures. It is well known that the leading term in the distance-based London expansion of the van der Waals energy for atomic and molecular dimers decays as $1/R^6$, where $R$ is the dimer distance. Using perturbation theory, we find the leading term in the distance-based expansion of the intracule pair density at the interatomic distance. Our results unveil a universal $1/R^3$ decay, which is less prone to numerical errors than the $1/R^6$ dependency, and it is confirmed numerically in ${\mathrm{H}}_2$ and ${\mathrm{He}}_2$ molecules. This signature of van der Waals interactions can be directly used in the construction of approximate pair density and energy functionals including vdW corrections.

The super Efimov effect is a recently proposed three-body effect characterized by a double-exponential scaling, which has not yet been observed experimentally. Here, we present the general dynamic equations determining the cloud size of a scale-invariant quantum gas in a time-dependent harmonic trap. We show that a double-logarithmic periodicity as the hallmark of the super Efimov effect emerges when the trap frequency is decreased with a specially designed time dependence. We also demonstrate that this dynamic super Efimov effect can be realized with realistic choices of parameters in current experiments.

We develop a class of supercell photonic crystals supporting complete photonic band gaps based on breaking spatial symmetries of the underlying primitive photonic crystal. One member of this class based on a two-dimensional honeycomb structure supports a complete band gap for an index-contrast ratio as low as $n_{\mathrm{high}}/n_{\mathrm{low}}=2.1$, the lowest index contrast known to support a complete band gap for a two-dimensional (2D) photonic crystal. This same design principle is used to develop the first photonic crystal slab that supports a band gap for dual-polarization visible light. The complete band gaps found in such supercell photonic crystals do not necessarily monotonically increase as the index contrast in the system is increased.

Several recent results on thermodynamics have been obtained using the tools of quantum information theory and resource theories. So far, the resource theories utilized to describe thermodynamics have assumed the existence of an infinite thermal reservoir, by declaring that thermal states at some background temperature come for free. Here, we propose a resource theory of quantum thermodynamics without a background temperature, so that no states at all come for free. We apply this resource theory to the case of many noninteracting systems and show that all quantum states are classified by their entropy and average energy, even arbitrarily far away from equilibrium. This implies that thermodynamics takes place in a two-dimensional convex set that we call the energy-entropy diagram. The answers to many resource-theoretic questions about thermodynamics can be read off from this diagram, such as the efficiency of a heat engine consisting of finite reservoirs, or the rate of conversion between two states. This allows us to consider a resource theory which puts work and heat on an equal footing, and serves as a model for other resource theories.

It is by now well established that noise itself can be useful for performing quantum information processing tasks. We present results which show how one can effectively reduce the error rate associated with a noisy quantum channel by counteracting its detrimental effects with another form of noise. In particular, we consider the effect of adding on top of a purely Markovian (Lindblad) dynamics, a more general form of dissipation, which we refer to as generalized-Markovian noise. This noise has an associated memory kernel and the resulting dynamics are described by an integrodifferential equation. The overall dynamics are characterized by decay rates which depend not only on the original dissipative time scales but also on the new integral kernel. We find that one can engineer this kernel such that the overall rate of decay is lowered by the addition of this noise term. We illustrate this technique for the case where the bare noise is described by a dephasing Pauli channel. We analytically solve this model and show that one can effectively double (or even triple) the length of the channel, while achieving the same fidelity, entanglement, and error threshold. We numerically verify this scheme can also be used to protect against thermal Markovian noise (at nonzero temperature), which models spontaneous emission and excitation processes. A physical interpretation of this scheme is discussed, whereby the added generalized-Markovian noise causes the system to become periodically decoupled from the background Markovian noise.

There has been an increasing interest in the quantification of nearly deterministic work extraction from a finite number of copies of microscopic particles in finite time. This paradigm, the so-called single-shot $\epsilon$-deterministic work extraction, considers processes with small failure probabilities. However, the resulting fluctuations in the extracted work entailed by this failure probability have yet to be studied. In the standard thermodynamics, paradigm fluctuation theorems are powerful tools to study fluctuating quantities. Given that standard fluctuation theorems are inadequate for a single-shot scenario, here we formulate and prove a fluctuation relation specific to the single-shot $\epsilon$-deterministic work extraction to bridge this gap. Our results are general in the sense that we allow the system to be in contact with the heat bath at all times. As a corollary of our theorem, we derive the known bounds on the $\epsilon$-deterministic work.

For a quantum system undergoing a unitary, process work is commonly defined based on the two projective measurement protocols, which measures the energies of the system before and after the process. However, it is well known that projective measurements disregard quantum coherences of the system with respect to the energy basis, thus removing potential quantum signatures in the work distribution. Here we consider weak measurements of the system's energy difference and establish corrections to work averages arising from initial system coherences. We discuss two weak measurement protocols that couple the system to a detector, prepared and measured either in the momentum or the position eigenstates. Work averages are derived for when the system starts in the proper thermal state versus when the initial system state is a pure state with thermal diagonal elements and coherences characterized by a set of phases. We show that by controlling only the phase differences between the energy eigenstate contributions in the system's initial pure state, the average work done during the same unitary process can be controlled. By changing the phases alone, one can toggle from regimes where the system absorbs energy, i.e., a work cost, to the ones where it emits energy, i.e., work can be drawn. This suggests that the coherences are additional resources that can be used to manipulate or store energy in a quantum system.