By generalizing our automated algebra approach from homogeneous space to harmonically trapped systems, we have calculated the fourth- and fifth-order virial coefficients of universal spin-$\frac{1}{2}$ fermions in the unitary limit, confined in an isotropic harmonic potential. We present results for said coefficients as a function of trapping frequency (or, equivalently, temperature), which compare favorably with previous Monte Carlo calculations (available only at fourth order) as well as with our previous estimates in the untrapped limit (high temperature, low frequency). In addition to the conventional coefficients, we provide estimates for the contributions from subspaces with varying polarization. We use our results for the virial coefficients, together with resummation techniques, to calculate the compressibility and spin susceptibility.

Achieving near-term quantum advantage will require effective methods for mitigating hardware noise. Data-driven approaches to error mitigation are promising, with popular examples including zero-noise extrapolation (ZNE) and Clifford data regression (CDR). Here, we propose a scalable error mitigation method that conceptually unifies ZNE and CDR. Our approach, called variable-noise Clifford data regression (vnCDR), significantly outperforms these individual methods in numerical benchmarks. vnCDR generates training data first via near-Clifford circuits (which are classically simulable) and second by varying the noise levels in these circuits. We employ a noise model obtained from IBM's Ourense quantum computer to benchmark our method. For the problem of estimating the energy of an 8-qubit Ising model system, vnCDR improves the absolute energy error by a factor of 33 over the unmitigated results and by factors of 20 and 1.8 over ZNE and CDR, respectively. For the problem of correcting observables from random quantum circuits with 64 qubits, vnCDR improves the error by factors of 2.7 and 1.5 over ZNE and CDR, respectively.

The transition metal dichalcogenide $\mathrm{Ti}{\mathrm{Se}}_2$ is an ideal correlated system for studying the interplay between superconductivity (SC) and a charge density wave (CDW) because both symmetry-breaking phases can be easily controlled by either Cu intercalation or physical pressure. SC appears in proximity to a CDW quantum critical point (QCP) induced by both Cu intercalation and applied pressure, raising the possibility of CDW-driven SC. Here, we report tuning the CDW QCP by simultaneously controlling Cu intercalation and external pressure and the appearance of a SC dome centered on the tunable QCP. When subjected to pressure, CDW ordering of Cu-intercalated ${\mathrm{Cu}}_{0.025}\mathrm{Ti}{\mathrm{Se}}_2$ is completely suppressed at 2.3 GPa, where the residual resistivity and the resistivity-temperature exponent decrease sharply, indicating the presence of the CDW QCP. The upper critical field of ${\mathrm{Cu}}_{0.025}\mathrm{Ti}{\mathrm{Se}}_2$ is 3.51 kOe, 16 times larger than that of pristine $\mathrm{Ti}{\mathrm{Se}}_2$, and its temperature dependence is linear, indicating that SC of $\mathrm{Ti}{\mathrm{Se}}_2$ is switched from the two-dimensional- to anisotropic three-dimensional-like by Cu intercalation. These discoveries show that the simultaneous application of Cu intercalation and pressure move the CDW QCP and that the highest SC transition temperature is pinned to the QCP, suggesting that the SC in $\mathrm{Ti}{\mathrm{Se}}_2$ is strongly correlated with CDW quantum criticality.

We probe the collective dynamics of a quantum degenerate Bose-Bose mixture of ${}^{133}\mathrm{Cs}$ and ${}^{174}\mathrm{Yb}$ with attractive interspecies interactions. Specifically, we excite vertical center-of-mass oscillations of the Cs condensate. We observe significant damping for the Cs dipole mode, due to the rapid transfer of energy to the larger Yb component and the ensuing acoustic dissipation. Numerical simulations based on coupled Gross-Pitaevskii equations provide excellent agreement with experiment and additionally reveal the possibility of late-time revivals (beating), which are found to be highly sensitive to the Cs and Yb atom number combinations. By further tuning the interaction strength of Cs using a broad Feshbach resonance, we explore the stability of the degenerate mixture. We observe collapse of the Cs condensate mediated by the attractive Cs-Yb interaction when $a_{\mathrm{Cs}}<50a_0$, well above the single-species collapse threshold, in good agreement with simulations.

Decoherence remains one of the most serious challenges to the implementation of quantum technology. It appears as a result of the transformation over time of a quantum superposition state into a classical mixture due to the quantum system interacting with the environment. Since quantum systems are never completely isolated from their environment, decoherence therefore cannot be avoided in realistic situations. Decoherence has been extensively studied, mostly theoretically, because it has many important implications in quantum technology, such as in the fields of quantum information processing, quantum communication, and quantum computation. Here we report a novel experimental scheme on the study of decoherence of a two-mode squeezed vacuum state via its second harmonic generation signal. Our scheme can directly extract the decoherence of the phase-sensitive quantum correlation $\langle \widehat{a}\widehat{b}\rangle$ between two entangled modes $a$ and $b$. Such a correlation is the most important characteristic of a two-mode squeezed state. More importantly, this is an experimental study on the decoherence effect of a squeezed vacuum state, which has been rarely investigated.

The criticality-enhanced magnetocaloric effect (MCE) near a field-induced quantum critical point (QCP) in the spin systems constitutes a very promising and highly tunable alternative to conventional adiabatic demagnetization refrigeration. Strong fluctuations in the low-$T$ quantum critical regime can give rise to a large thermal entropy change and thus significant cooling effect when approaching the QCP. In this work, through efficient and accurate many-body calculations, we show there exists a significant inverse MCE (iMCE) in the spin-1 quantum chain materials (${\mathrm{CH}}_3{)}_4\mathrm{NNi}{\left({\mathrm{NO}}_2\right)}_3$ (TMNIN) and ${\mathrm{NiCl}}_2\text{−}4\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2$ (DTN), where DTN has substantial low-$T$ refrigeration capacity while requiring only moderate magnetic fields. The iMCE characteristics, including the adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}$, isothermal entropy change $\mathrm{\Delta }S$, differential Grüneisen parameter, and the entropy change rate, are obtained with quantum many-body calculations at finite temperature. The cooling performance, i.e., the efficiency factor and hold time, of the two compounds is also discussed. Based on the many-body calculations on realistic models for the spin-chain materials, we conclude that the compound DTN constitutes a very promising and highly efficient quantum magnetic coolant with pronounced iMCE properties. We advocate that such quantum magnets can be used in cryofree refrigeration for space applications and quantum computing environments.

The idea of self-testing is to render guarantees concerning the inner workings of a device based on the measurement statistics. It is one of the most formidable quantum certification and benchmarking schemes. Recently it was shown [A. Coladangelo, K. T. Goh, and V. Scarani, Nat. Commun. 8, 15485 (2017)] that all pure bipartite entangled states can be self-tested in the device-independent scenario by employing subspace methods introduced by Yang and Navascués [Phys. Rev. A 87, 050102(R) (2013)]. Here, we have adapted their method to show that any bipartite pure entangled state can be certified in the semi-device-independent scenario through quantum steering. Analogous to the tilted Clauser-Horne-Shimony-Holt inequality, we use a steering inequality called the tilted steering inequality for certifying any pure two-qubit entangled state. Furthermore, we use this inequality to certify any bipartite pure entangled state by certifying two-dimensional subspaces of the qudit state by observing the structure of the set of assemblages obtained on the trusted side after measurements are made on the untrusted side. As a feature of quantum state certification via steering, we use the notion of assemblage-based robust state certification to provide robustness bounds for the certification result in the case of pure maximally entangled states of any local dimension.

The noise dynamics of an optical frequency comb based on a mode-locked Ti-sapphire laser is analyzed in terms of noise modes. A spectrally resolved multipixel homodyne detection enables the simultaneous measurement of the amplitude and phase noises of several optical frequency channels, from which the covariance matrices of the amplitude and phase quadratures of the laser field are calculated. The decomposition of these matrices into the four most significant time/frequency modes of the field enables the tracking of the origin of the noises and the correlations between the noise modes. In particular, the correlations between the amplitude and phase noises are measured. These measurements are well reproduced by a model taking into account the correlations between the carrier envelope offset phase noise and the amplitude noise induced by the group velocity dispersion of the laser cavity.

Passive states, i.e., those states from which no work can be extracted via unitary operations, play an important role in the foundations and applications of quantum thermodynamics. They generalize the familiar Gibbs thermal states, which are the sole passive states being stable under tensor product. Here, we introduce a partial order on the set of passive states that captures the idea of a passive state being virtually cooler than another one. This partial order, which we build by defining the notion of relative passivity, offers a fine-grained comparison between passive states based on virtual temperatures (just like thermal states are compared based on their temperatures). We then characterize the quantum operations that are closed on the set of virtually cooler states with respect to some fixed input and output passive states. Viewing the activity, i.e., nonpassivity, of a state as a resource, our main result is then a necessary and sufficient condition on the transformation of a class of pure active states under these relative passivity-preserving operations. This condition gives a quantum thermodynamical meaning to the majorization relation on the set of nonincreasing vectors due to Hoffman. The maximum extractable work under relative passivity-preserving operations is then shown to be equal to the ergotropy of these pure active states. Finally, we are able to fully characterize passivity-preserving operations in the simpler case of qubit systems, and hence to derive a state interconversion condition under passivity-preserving qubit operations. The prospect of this work is a general resource-theoretical framework for the extractable work via quantum operations going beyond thermal operations.

Hybrid quantum-classical variational algorithms are one of the most propitious implementations of quantum computing on near-term devices, offering classical machine-learning support to quantum scale solution spaces. However, numerous studies have demonstrated that the rate at which this space grows in qubit number could preclude learning in deep quantum circuits, a phenomenon known as barren plateaus. In this work, we implicate random entanglement, i.e., entanglement that is formed due to state evolution with random unitaries, as a source of barren plateaus and characterize them in terms of many-body entanglement dynamics, detailing their formation as a function of system size, circuit depth, and circuit connectivity. Using this comprehension of entanglement, we propose and demonstrate a number of barren plateau ameliorating techniques, including initial partitioning of cost function and non-cost function registers, meta-learning of low-entanglement circuit initializations, selective inter-register interaction, entanglement regularization, the addition of Langevin noise, and rotation into preferred cost function eigenbases. We find that entanglement limiting, both automatic and engineered, is a hallmark of high-accuracy training and emphasize that, because learning is an iterative organization process whereas barren plateaus are a consequence of randomization, they are not necessarily unavoidable or inescapable. Our work forms both a theoretical characterization and a practical toolbox; first defining barren plateaus in terms of random entanglement and then employing this expertise to strategically combat them.

We determine the global renormalization group (RG) flow of the Sachdev-Ye-Kitaev (SYK) model. From a controlled truncation of the infinite hierarchy of the exact functional RG flow equations, we identify several fixed points. Apart from a stable fixed point, associated with the celebrated non-Fermi liquid state of the model, we find another stable fixed point related to an integer-valence state. These stable fixed points are separated by a discontinuity fixed point with one relevant direction, describing a quantum first-order transition. Most notably, the fermionic spectrum continues to be quantum critical even at the discontinuity fixed point. This rules out a description of the transition in terms of a local effective Ising variable as is established for classical transitions. We propose an entangled quantum state at phase coexistence as a possible physical origin of this critical behavior.

X-ray radiation from laser wakefield accelerated (LWFA) electrons provides a collimated broadband femtosecond x-ray source with micron-scale source size. However, the photon flux is still inadequate for a range of applications. By interaction of LWFA relativistic electrons with the second and the third harmonic of the laser driving the wakefield, we report more than an order of magnitude enhancement of photon flux emitted by the electrons undergoing betatron oscillations resonant with the harmonics. This phenomenon is demonstrated with analytical and numerical models, as well as particle-in-cell simulations.

We consider the fractional quantum Hall effect at the filling $\nu =6/17$, where experiments have observed features of incompressibility in the form of a minimum in the longitudinal resistance. We propose a parton state denoted as “$3\overline{2}{1}^3$” and show it to be a feasible candidate to capture the ground state at $\nu =6/17$. We work out the low-energy effective theory of the $3\overline{2}{1}^3$ edge and make several predictions for experimentally measurable properties of the state which can help detect its underlying topological order. Intriguingly, we find that the $3\overline{2}{1}^3$ state likely lies in the same universality class as the state obtained from composite-fermionizing the $1+1/5$ Laughlin state.

In this paper we propose a Rydberg entangling gate scheme which we demonstrate theoretically to have an order-of-magnitude improvement in fidelities and speed over existing cold atom protocols. It requires a large Rabi frequency compared to the interaction strength, which is difficult in cold atoms, but natural in donors in silicon, where it could help overcome the strenuous requirements on atomic precision donor placement and substantial gate tuning, which so far has hampered scaling. Furthermore, the gate operation would be ultrafast, on the order of picoseconds. We calculate multivalley van der Waals, induced electric dipole and total Rydberg interactions for several donor species using the finite-element method and show that they are important even for low-lying excited states. We show that Rydberg gate operation is possible within the lifetime of donor excited states with 99.9% fidelity for the creation of a Bell state in the presence of decoherence.

Cylindrical magnetic nanowires with large transversal magnetocrystalline anisotropy have been shown to sustain nontrivial magnetic configurations resulting from the interplay of spatial confinement, exchange, and anisotropies. Exploiting these peculiar three-dimensional (3D) spin configurations and their solitonic inhomogeneities is expected to improve magnetization switching in future spintronics, such as power-saving magnetic memory and logic applications. Here we employ holographic vector-field electron tomography to reconstruct the remanent magnetic states in CoNi nanowires with 10 nm resolution in 3D, with a particular focus on domain walls between remanent states and ubiquitous real-structure effects stemming from irregular morphology and anisotropy variations. By tuning the applied magnetic field direction, both longitudinal and transverse multivortex states of different chiralities and peculiar 3D features such as shifted vortex cores are stabilized. The chiral domain wall between the longitudinal vortices of opposite chiralities exhibits a complex 3D shape characterized by a push out of the central vortex line and a gain in exchange and anisotropy energy. A similar complex 3D texture, including bent vortex lines, forms at the domain boundary between transverse-vortex states and longitudinal configurations. Micromagnetic simulations allow an understanding of the origin of the observed complex magnetic states.