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Engineered dissipative reservoirs have the potential to steer many-body quantum systems toward correlated steady states useful for quantum simulation of high-temperature superconductivity or quantum magnetism. Using up to 49 superconducting qubits, we prepared low-energy states of the transverse-field Ising model through coupling to dissipative auxiliary qubits. In one dimension, we observed long-range quantum correlations and a ground-state fidelity of 0.86 for 18 qubits at the critical point. In two dimensions, we found mutual information that extends beyond nearest neighbors. Lastly, by coupling the system to auxiliaries emulating reservoirs with different chemical potentials, we explored transport in the quantum Heisenberg model. Our results establish engineered dissipation as a scalable alternative to unitary evolution for preparing entangled many-body states on noisy quantum processors.

Undesired coupling to the surrounding environment destroys long-range correlations on quantum processors and hinders the coherent evolution in the nominally available computational space. This incoherent noise is an outstanding challenge to fully leverage the computation power of near-term quantum processors. It has been shown that benchmarking Random Circuit Sampling (RCS) with Cross-Entropy Benchmarking (XEB) can provide a reliable estimate of the effective size of the Hilbert space coherently available. The extent to which the presence of noise can trivialize the outputs of a given quantum algorithm, i.e. making it spoofable by a classical computation, is an unanswered question. Here, by implementing an RCS algorithm we demonstrate experimentally that there are two phase transitions observable with XEB, which we explain theoretically with a statistical model. The first is a dynamical transition as a function of the number of cycles and is the continuation of the anti-concentration point in the noiseless case. The second is a quantum phase transition controlled by the error per cycle; to identify it analytically and experimentally, we create a weak link model which allows varying the strength of noise versus coherent evolution. Furthermore, by presenting an RCS experiment with 67 qubits at 32 cycles, we demonstrate that the computational cost of our experiment is beyond the capabilities of existing classical supercomputers, even when accounting for the inevitable presence of noise. Our experimental and theoretical work establishes the existence of transitions to a stable computationally complex phase that is reachable with current quantum processors.

An important measure of the development of quantum computing platforms has been the simulation of increasingly complex physical systems. Prior to fault-tolerant quantum computing, robust error mitigation strategies are necessary to continue this growth. Here, we study physical simulation within the seniority-zero electron pairing subspace, which affords both a computational stepping stone to a fully correlated model, and an opportunity to validate recently introduced ``purification-based'' error-mitigation strategies. We compare the performance of error mitigation based on doubling quantum resources in time (echo verification) or in space (virtual distillation), on up to $20$ qubits of a superconducting qubit quantum processor. We observe a reduction of error by one to two orders of magnitude below less sophisticated techniques (e.g. post-selection); the gain from error mitigation is seen to increase with the system size. Employing these error mitigation strategies enables the implementation of the largest variational algorithm for a correlated chemistry system to-date. Extrapolating performance from these results allows us to estimate minimum requirements for a beyond-classical simulation of electronic structure. We find that, despite the impressive gains from purification-based error mitigation, significant hardware improvements will be required for classically intractable variational chemistry simulations., Comment: 10 pages, 13 page supplementary material, 12 figures. Experimental data available at https://doi.org/10.5281/zenodo.7225821

The detection of individual quanta of light is important for quantum computation, fluorescence lifetime imaging, single-molecule detection, remote sensing, correlation spectroscopy, and more. Thanks to their broadband operation, high detection efficiency, exceptional signal-to-noise ratio, and fast recovery times, superconducting nanowire single-photon detectors (SNSPDs) have become a critical component in these applications. The operation of SNSPDs based on conventional superconductors, which have a low critical temperature ($T_c$), requires costly and bulky cryocoolers. This motivated exploration of other superconducting materials with higher $T_c$ that would enable single-photon detection at elevated temperatures, yet this task has proven exceedingly difficult. Here we show that with proper processing, high-$T_c$ cuprate superconductors can meet this challenge. We fabricated superconducting nanowires (SNWs) out of thin flakes of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ (LSCO-LCO) bilayer films and demonstrated their single-photon response up to $25$ and $8$ K, respectively. The single-photon operation is revealed through the linear scaling of the photon count rate (PCR) on the radiation power. Both of our cuprate-based SNSPDs exhibited single-photon sensitivity at the technologically-important $1.5$ ${\mu}$m telecommunications wavelength. Our work expands the family of superconducting materials for SNSPD technology, opens the prospects of raising the temperature ceiling, and raises important questions about the underlying mechanisms of single-photon detection by unconventional superconductors., Comment: 8 pages, 3 figures

In high-temperature cuprate superconductors, the anti-ferromagnetic spin fluctuations are thought to have a very important role in naturally producing an attractive interaction between the electrons in the $d$-wave channel. The connection between superconductivity and spin fluctuations is expected to be especially consequential at the overdoped end point of the superconducting dome. In some materials, that point seems to coincide with a Lifshitz transition, where the Fermi surface changes from the hole-like centered at ($\pi, \pi$) to the electron-like, centered at the $\Gamma$ point causing a loss of large momentum anti-ferromagnetic fluctuations. Here, we study the doping dependence of the electronic structure of Bi$_{1.8}$Pb$_{0.4}$Sr$_2$CuO$_{6+\delta}$ in angle-resolved photoemission and find that the superconductivity vanishes at lower doping than at which the Lifshitz transition occurs. This requires a more detailed re-examination of a spin-fluctuation scenario., Comment: 6 pages, 3 Figures, 1 Table

Mixing of topological states with superconductivity could result in topological superconductivity with the elusive Majorana fermions potentially applicable in fault-tolerant quantum computing. One possible candidate considered for realization of topological superconductivity is thin bismuth films on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212). Here, we present angle-resolved and core-level photoemission spectroscopy studies of thin Bi films grown {\it in-situ} on as-grown Bi2212 that show the absence of proximity effect. We find that the electron transfer from the film to the substrate and the resulting severe underdoping of Bi2212 at the interface is a likely origin for the absence of proximity effect. We also propose a possible way of preventing a total loss of proximity effect in this system. Our results offer a better and more universal understanding of the film/cuprate interface and resolve many issues related to the proximity effect., Comment: 6 pages, 4 figures

The effects of structural supermodulation with the period $\lambda \approx26$ \AA \ along the $b$-axis of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ have been observed in photoemission studies from the early days as the presence of diffraction replicas of the intrinsic electronic structure. Although predicted to affect the electronic structure of the Cu-O plane, the influence of supermodulation potential on Cu-O electrons has never been observed in photoemission. In the present study, we clearly see, for the first time, the effects on the Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ electronic structure - we observe a hybridization of the intrinsic bands with the supermodulation replica bands in the form of avoided crossings and a corresponding reconstruction of the Fermi surface. We estimate the hybridization gap, $2\Delta_h\sim25$ meV in the slightly underdoped samples. The hybridization weakens with doping and the anti-crossing can no longer be resolved in strongly overdoped samples. In contrast, the shadow replica, shifted by $(\pi, \pi)$, is found not to hybridize with the original bands within our detection limits., Comment: 6 pages, 4 figures

In high-temperature cuprate superconductors, superconductivity is accompanied by a "plethora of orders", and phenomena that may compete, or cooperate with superconductivity, but which certainly complicate our understanding of origins of superconductivity in these materials. While prominent in the underdoped regime, these orders are known to significantly weaken or completely vanish with overdoping. Here, we approach the superconducting phase from the more conventional highly overdoped side. We present angle-resolved photoemission spectroscopy (ARPES) studies of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals cleaved and annealed in ozone to increase the doping all the way to the metallic, non-superconducting phase. We show that the mass renormalization in the antinodal region of the Fermi surface, associated with the structure in the quasiparticle self-energy, that possibly reflects the pairing interaction, monotonically weakens with increasing doping and completely disappears precisely where superconductivity disappears. This is the direct evidence that in the overdoped regime, superconductivity is determined by the coupling strength. A strong doping dependence and an abrupt disappearance above the transition temperature ($T_{\mathrm c}$) eliminate the conventional phononic mechanism of the observed mass renormalization and identify the onset of spin-fluctuations as its likely origin., Comment: 8 pages, 4 figures

In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level $p$. In most materials, $p$ cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, $T_\mathrm{c}$, using the assumption that $T_\mathrm{c}$ dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to $T_\mathrm{c}=91$ K. We show that $p$ can be determined from the underlying Fermi surfaces and that $in-situ$ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of $2\Delta_0=(4\pm1)k_\mathrm{B}T_\mathrm{c}$, Comment: 8 pages, 4 figures

The formalism of local maximization for entropy gradient producing the evolution and dynamical equations for closed systems. It eliminates the inconsistency between the reversibilty of time in dynamical equations and the strict direction of irreversible evolution for complex systems, causality contradictions and ambiguity of time flow in different systems. Independently it leads to basic principles of special relativity.

The desorption kinetics was modelled with the both interface- and surface reactions as rate-controlling steps. It has been shown analytically, that in the model of 'shrinking core' desorption, the finite hydride-decomposition-rate causes a modified slope of kinetics. The dependence of desorption time on the powder particle size has the same power of order as for the surface controlled desorption.

The observable results for desorption kinetics by powder of metal hydride on the example of mangesium hydride are reproduced with the model formulated in terms of specific surface of powder. A volumetric measurement of hydrogen desorption process is evaluated on an example of wet ball milled magnesium hydride, and can be applied generally for any metal hydride. The exact solution of the model reproduces the shape of experimental curves for desorption process providing a satisfying agreement with experimental data.

The proposed model concerns the 'confluent shrinking core' scenario and reproduces the desorption kinetic after the complete decay of the stoichiometric hydride ($\beta$-phase). The exact analytical solution is obtained, the numerical values are demonstrated by the example of magnesium hydride.

Accounting of effective volumetric factor for the measurement at inhomogeneous temperature allows to the simplify the measurement process avoiding a pre-calibration with a noble gas. The volumetric correction in form of the volumetric factor is calculated analytically by modeling of the temperature profile in the volumetric apparatus, and provides a quite agreement with the measured results. The calculation can be applied for each volumetric measurement with an inhomogeneous temperature distribution.

Topological crystalline insulators represent a novel topological phase of matter in which the surface states are protected by discrete point group-symmetries of the underlying lattice. Rock-salt lead-tin-selenide alloy is one possible realization of this phase which undergoes a topological phase transition upon changing the lead content. We used scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES) to probe the surface states on (001) Pb$_{1-x}$Sn$_{x}$Se in the topologically non-trivial (x=0.23) and topologically trivial (x=0) phases. We observed quasiparticle interference with STM on the surface of the topological crystalline insulator and demonstrated that the measured interference can be understood from ARPES studies and a simple band structure model. Furthermore, our findings support the fact that Pb$_{0.77}$Sn$_{0.23}$Se and PbSe have different topological nature., Comment: 5 pages, 4 figures

We describe the topological surface states of Bi$_4$Se$_3$, a compound in the infinitely adaptive Bi$_2$-Bi$_2$Se$_3$ natural superlattice phase series, determined by a combination of experimental and theoretical methods. Two observable cleavage surfaces, terminating at Bi or Se, are characterized by angle resolved photoelectron spectroscopy and scanning tunneling microscopy, and modeled by ab-initio density functional theory calculations. Topological surface states are observed on both surfaces, but with markedly different dispersions and Kramers point energies. Bi$_4$Se$_3$ therefore represents the only known compound with different topological states on differently terminated surfaces., Comment: 5 figures references added Published in PRB: http://link.aps.org/doi/10.1103/PhysRevB.88.081108

We propose an easy-to-build easy-to-detect scheme for realizing Majorana fermions at the ends of a chain of magnetic atoms on the surface of a superconductor. Model calculations show that such chains can be easily tuned between trivial and topological ground state. In the latter, spatial resolved spectroscopy can be used to probe the Majorana fermion end states. Decoupled Majorana bound states can form even in short magnetic chains consisting of only tens of atoms. We propose scanning tunneling microscopy as the ideal technique to fabricate such systems and probe their topological properties., Comment: 5 pages, 5 figures + supplementary

We report on the observation of a new spin mode in a quantum Hall system in the vicinity of odd electron filling factors under experimental conditions excluding the possibility of Skyrmion excitations. The new mode having presumably zero energy at odd filling factors emerges at small deviations from odd filling factors and couples to the spin-exciton. The existence of an extra spin mode assumes a nontrivial magnetic order at partial fillings of Landau levels surrounding quantum Hall ferromagnets other then the Skyrmion crystal., Comment: 9 pages, 4 figures

An antiphased magnetoplasma (MP) mode in a two-dimensional electron gas (2DEG) has been studied by means of inelastic light scattering (ILS) spectroscopy. Unlike the cophased MP mode it is purely quantum excitation which has no classic plasma analogue. It is found that zero momentum degeneracy for the antiphased and cophased modes predicted by the first-order perturbation approach in terms of the {\it e-e} interaction is lifted. The zero momentum energy gap is determined by a negative correlation shift of the antiphased mode. This shift, observed experimentally and calculated theoretically within the second-order perturbation approach, is proportional to the effective Rydberg constant in a semiconductor material., Comment: Submitted to Phys. Rev. B

An interpretation of an electromagnetic quantum as a single pulse is suggested. In this context the Planck formula is shown to be equivalent to the Heisenberg time-energy uncertainty relation. This allows to treat the photon frequency as an inverse time of emission. Such an ansatz avoids the inherent problems of the conventional approach., Comment: 6 pages

It is proposed to apply a recently developed concept of local wave velocities to the dynamical field characteristics, especially for the canonical field energy density. It is shown that local energy velocities can be derived from the lagrangian directly. The local velocities of zero- and first- order for energy propagation has been obtained for special cases of scalar and vector fields. Some important special cases of these results are discussed., Comment: 8 Pages

The classical characterization of \wp, as a typical concept for far field phenomena, has been successfully applied to many \w phenomena in past decades. The recent reports of superluminal tunnelling times and negative group velocities challenged this concept. A new local approach for the definition of \wvs avoiding these difficulties while including the classical definitions as particular cases is proposed here. This generalisation of the conventional non-local approach can be applied to arbitrary \w forms and propagation media. Some applications of the formalism are presented and basic properties of the concept are summarized., Comment: 18 pages 5 figures

A general local approach for the definition of velocities and especially phase velocities for waves recently proposed for one-dimensional waves is generalized for 2 and 3 dimensional scalar wave. A geometrically consistent generalization has been found for the local wave velocities of order zero and one., Comment: 5 pages

The classical far field concept of wave velocities has its merits while exibiting intrinsic difficulties. A general local approach for the definition of velocities and especially phase velocities for waves avoiding these difficulties is proposed. It includes the classical definitions as particular cases and can be applied to waves of an arbitrary structure, and to arbitrary propagation media as well. Applications of the formalism are elucidated and some basic properties of the local concept defined here are discussed., Comment: 16 pages, 5 Figures

The ground state energy of a quantum field in the background of classical field configurations is considered. The subject of the ground state energy in framework of the quantum field theory is explained. The short review of calculation methods (generalized zeta function and heat kernel expansion) and their mathematical foundations is given. We use the zeta-functional regularization and express the ground state energy as an integral involving the Jost function of a two dimensional scattering problem.We perform the renormalization by subtracting the contributions from first several heat kernel coefficients. The ground state energy is presented as a convergent expression suited for numerical evaluation. The investigation for three models has been carried out: scalar quantum field on the background of scalar string with rectangular shape, spinor vacuum polarized by magnetic string of the similar shape and spinor vacuum interacting with the Nielsen-Olesen vortex. Using the uniform asymptotic expansion of the special functions entering the Jost function we are also able to calculate higher order heat kernel coefficients. Several features of vacuum energy have been investigated numerically. We discuss corresponding numerical results., Comment: 151 pages, 82 citations, 23 figures, 3 tables, mathematical appendix

We calculate the vacuum energy of a spinor field in the background of a Nielsen-Olesen vortex. We use the method of representing the vacuum energy in terms of the Jost function on the imaginary momentum axis. Renormalization is carried out using the heat kernel expansion and zeta functional regularization. With this method well convergent sums and integrals emerge which allow for an efficient numerical calculation of the vacuum energy in the given case where the background is not known analytically but only numerically. The vacuum energy is calculated for several choices of the parameters and it turns out to give small corrections to the classical energy., Comment: 22 pages, 6 figures

We consider the ground state energy of a spinor field in the background of a square well shaped magnetic flux tube. We use the zeta- function regularization and express the ground state energy as an integral involving the Jost function of a two dimensional scattering problem. We perform the renormalization by subtracting the contributions from first several heat kernel coefficients. The ground state energy is presented as a convergent expression suited for numerical evaluation. We discuss corresponding numerical calculations. Using the uniform asymptotic expansion of the special functions entering the Jost function we are able to calculate higher order heat kernel coefficients., Comment: 28 pages, 6 figures