We study the transfer of energy through a network of coupled oscillators, which represents a minimal microscopic power grid connecting multiple active quantum machines. We evaluate the resulting energy currents in the macroscopic, thermal, and quantum regime and describe how transport is affected by the competition between coherent and incoherent processes and nonlinear saturation effects. Specifically, we show that the transfer of energy through such networks is strongly influenced by a nonequilibrium phase transition between a noise-dominated and a coherent transport regime. This transition is associated with the formation and breaking of spatial symmetries and is identified as a generic feature of active networks. Therefore, these findings have important practical consequences for the distribution of energy over coherent microwave, optical, or phononic channels, in particular close to or at the quantum limit.

Adiabatic evolution is a common strategy for manipulating quantum states and has been employed in diverse fields such as quantum simulation, computation, and annealing. However, adiabatic evolution is inherently slow and therefore susceptible to decoherence. Existing methods for speeding up adiabatic evolution require complex many-body operators or are difficult to construct for multilevel systems. Using the tools of Floquet engineering, we design a scheme for high-fidelity quantum state manipulation, utilizing only the interactions available in the original Hamiltonian. We apply this approach to a qubit and experimentally demonstrate its performance with the electronic spin of a nitrogen-vacancy center in diamond. Our Floquet-engineered protocol achieves state preparation fidelity of $0.994\pm 0.004$, on the same level as the conventional fast-forward protocol, but is more robust to external noise acting on the qubit. Floquet engineering provides a powerful platform for high-fidelity quantum state manipulation in complex and noisy quantum systems.

A source that can generate atom-photon quantum correlations or entanglement based on a quantum memory is a basic building block of quantum repeaters (QRs). To achieve high entanglement generation rates in ensemble-based QRs, spatial-, temporal-, and spectral-multimode memories are needed. Previous temporal-multimode memories are based on rephasing mechanisms in inhomogeneously broadened media. Here, by applying a train of write pulses in time, with each pulse coming from a different direction, to a homogeneously broadened atomic ensemble to induce Duan-Lukin-Cirac-Zoller-like Raman processes, we prepare up to 19 pairs of modes, namely, one spin-wave mode and one photonic time bin. Spin-wave-photon (i.e., atom-photon) entanglement is probabilistically produced in these mode pairs. Based on the stored spin-wave modes together with feedforward-controlled readout, we build a temporally multiplexed source and then demonstrate an 18.8-fold increase in the probability of the generation of spin-wave-photon entanglement compared to the sources that use individual modes. The measured Bell parameter for the multiplexed source is $2.30\pm 0.02$, and the memory lifetime is 30 μs.

Implementation of a discrete-time quantum walk (DTQW) with superconducting qubits is difficult since on-chip superconducting qubits cannot hop between lattice sites. We propose an efficient protocol for the implementation of DTQW in circuit quantum electrodynamics (QED), in which only $N+1$ qutrits, $N$ cavities, and other assistant devices are needed for an $N$-step DTQW. The operation of each DTQW step is very quick because only resonant processes are adopted. The numerical simulations show that high-similarity DTQW with a number of steps up to 20 is feasible with present-day circuit QED techniques. This protocol can help to study properties and applications of many-step DTQW in experiments, and can be extended to implement multidimensional DTQW in circuit QED or other quantum systems.

We show how nonclassical correlations in local bipartite states can act as a resource for quantum information processing. Considering the task of quantum random access codes (RACs) through separable Bell-diagonal states, we demonstrate the advantage of superunsteerability over classical protocols assisted with two bits of shared randomness. We propose a measure of superunsteerability which quantifies nonclassicality beyond quantum steering and obtain its analytical expression for Bell-diagonal states in the context of two- and three-setting steering scenarios that are directly related to the quantum $2\to 1$ and $3\to 1$ RAC protocols, respectively. The maximal values of our quantifier yield the optimal quantum efficiency for both of the above protocols, thus showing that superunsteerability provides a precise characterization of the nonclassical resource for implementing RACs with separable Bell-diagonal class of states.

According to the Hohenberg-Kohn theorem of density-functional theory (DFT), all observable quantities of systems of interacting electrons can be expressed as functionals of the ground-state density. This includes, in principle, the spin polarization (magnetization) of open-shell systems; the explicit form of the magnetization as a functional of the total density is however unknown. In practice, open-shell systems are always treated with spin-DFT, where the basic variables are the spin densities. Here the relation between DFT and spin-DFT for open-shell systems is illustrated and the exact magnetization density functional is obtained for the half-filled Hubbard trimer. Errors arising from spin-restricted and -unrestricted exact-exchange Kohn-Sham calculations are analyzed and partially resolved via the exact magnetization functional.

Combined experimental and theoretical studies on electron scattering by propane were performed over a wide energy and angular range. The experimental values were compared to the results calculated using currently available theoretical and semiempirical approaches. The widely used independent atom model with screening corrected additivity rule coarsely reproduces the angular-dependent differential elastic cross sections of propane in the measured energy between 20 eV and 1 keV. For electron energies above 40 eV, the modified independent atom model, which takes into account multiple scattering within the molecule, yields better agreement with the experimental results. Total, integral elastic and integral inelastic electron scattering cross sections of propane were calculated using the spherical complex optical potential model and the variable phase approach. The suitability of the theoretical model is discussed by means of the experimental data.

We propose an experimental quantum state engineering scheme for the high-fidelity conditional generation of various nonclassical states of practical relevance in traveling optical fields. It contains a single measurement, thereby achieving a high success probability. The generated state is encoded in the optimal choice of the physically controllable parameters of the arrangement. These parameter values are determined via numerical optimization.

Dense ensembles of nitrogen-vacancy (NV) centers in diamond are of interest for various applications, including magnetometry, masers, hyperpolarization, and quantum memory. All of the applications above may benefit from a nonlinear response of the ensemble, and hence multiphoton processes are important. An enhancement of the multiphoton response is demonstrated by coupling the NV ensemble to a superconducting cavity. Moreover, the measured multiphoton response of the NV ensemble exhibits a regular pattern, which suggests that a dipolar coupling to nitrogen-14 substitutional defects (P1) plays a role in this process. As an example of an application, increased responsivity to magnetic field is demonstrated.

We show that a one-dimensional chain of trapped ions can be engineered to produce a quantum mechanical system with discrete scale invariance and fractal-like time dependence. By discrete scale invariance we mean a system that replicates itself under a rescaling of distance for some scale factor, and a time fractal is a signal that is invariant under the rescaling of time. These features are reminiscent of the Efimov effect, which has been predicted and observed in bound states of three-body systems. We demonstrate that discrete scale invariance in the trapped ion system can be controlled with two independently tunable parameters. We also discuss the extension to $n$-body states where the discrete scaling symmetry has an exotic heterogeneous structure. The results we present can be realized using currently available technologies developed for trapped ion quantum systems.

Typically, time-dependent thermodynamic protocols need to run asymptotically slowly in order to avoid dissipative losses. By adapting ideas from counterdiabatic driving and Floquet engineering to open systems, we develop fast-forward protocols for swiftly thermalizing a system oscillator locally coupled to an optical phonon bath. These protocols control the system frequency and the system-bath coupling to induce a resonant state exchange between the system and the bath. We apply the fast-forward protocols to realize a fast approximate Otto engine operating at high power near the Carnot efficiency. Our results suggest design principles for swift cooling protocols in coupled many-body systems.

We consider a periodically driven quantum system described by a Hamiltonian which is the product of a slowly varying Hermitian operator $V(\mathit{\lambda }\left(t\right))$ and a dimensionless periodic function with zero average. We demonstrate that the adiabatic evolution of the system within a fully degenerate Floquet band is accompanied by non-Abelian (noncommuting) geometric phases appearing when the slowly varying parameter $\mathit{\lambda }=\mathit{\lambda }\left(t\right)$ completes a closed loop. The geometric phases can have significant values even after completing a single cycle of the slow variable. Furthermore, there are no dynamical phases masking the non-Abelian Floquet geometric phases, as the former average to zero over an oscillation period. This can be used to precisely control the evolution of quantum systems, in particular for performing qubit operations. The general formalism is illustrated by analyzing a spin in an oscillating magnetic field with arbitrary strength and a slowly changing direction.

Here we introduce the all-creation and all-annihilation time-dependent (TD) $\mathcal{PT}$-symmetric bosonic Hamiltonians, which in the interaction picture are described only by creation or annihilation operators. These Hamiltonians are defined from the most general TD $\mathcal{PT}$-symmetric quadratic bosonic Hamiltonian, describing a cavity mode under linear and parametric amplifications. After presenting a general ansatz for the derivation of the TD Dyson map and metric operators, we solve the Schrödinger equations for both the $\mathcal{PT}$-symmetric Hamiltonian and its Hermitian counterpart. We then compute analytically the squeezing degree coming from the all-annihilation Hamiltonian and compare the result with that coming from an ordinary Hermitian Hamiltonian, showing a crucial result for interferometric procedures: instead of the asymptotic divergence of the squeezing degree that takes place for a Hermitian parametric pumping, the all-annihilation amplification leads to a divergence of that quantity at a finite controllable time.

Majorana phase-space representations for fermions map fermionic many-body physics into a distribution over one of the Cartan symmetric spaces of Lie group theory. The representation is in terms of $2M\times 2M$ complex antisymmetric matrices, which generate the Gaussian Majorana operators. Here we show how this expansion can be utilized to calculate quantities arising in quantum thermodynamics and quantum information. Purity and the linear entropy are calculated, as well as the quantum fidelity between two general fermionic states, with numerical examples for pure states. We describe the geometrical properties of the phase space, and show that the overlap between two Gaussian Majorana states depends on the product of their antisymmetric matrices. Fermionic phase space is divided up into two orthogonal subspaces of different number parity, whose matrix representations differ by an orthogonal reflection in the phase space.

The surface code is currently the primary proposed method for performing quantum error correction. However, despite its many advantages, it has no native method to fault-tolerantly apply non-Clifford gates. Additional techniques are therefore required to achieve universal quantum computation. Here we propose a hybrid scheme which uses small islands of a qudit variant of the surface code to enhance the computational power of the standard surface code. This allows the non-trivial action of the non-Abelian anyons in the former to process information stored in the latter. Specifically, we show that a nonstabilizer state can be prepared, which allows universality to be achieved.

We explore an intriguing alternative for a fast and high-fidelity generation of steady-state entanglement. By exponentially enhancing the atom-cavity interaction, we obtain an exponentially enhanced effective cooperativity of the system, which results in a high fidelity of the state generation. Meanwhile, we modulate the amplitudes of the driving fields to accelerate the population transfer to a target state, e.g., a Bell state. An exponentially shortened stabilization time is thus predicted. Specifically, when the cooperativity of the system is $C=30$, the fidelity of the acceleration scheme reaches $98.5\%$, and the stabilization time is about ten times shorter than that without acceleration. Moreover, we find from the numerical simulation that the acceleration scheme is robust against systematic and stochastic (amplitude-noise) errors.

We study the quantum to classical transition in boson sampling by analyzing how $N$-boson interference is affected by inevitable noise in an experimental setup. We adopt the Gaussian noise model of Kalai and Kindler for boson sampling and show that it appears from some realistic experimental imperfections. We reveal a connection between noise in boson sampling and partial distinguishability of bosons, which allows us to prove efficient classical simulatability of noisy no-collision boson sampling with finite noise amplitude $\epsilon$, i.e., $\epsilon =\mathrm{\Omega }\left(1\right)$ as $N\to \infty$. On the other hand, using an equivalent representation of network noise as losses of bosons compensated by random (dark) counts of detectors, it is proven that for noise amplitude inversely proportional to total number of bosons, i.e., $\epsilon =O(1/N)$, noisy no-collision boson sampling is as hard to simulate classically as in the noiseless case. Moreover, the ratio of “noise clicks” (lost bosons compensated by dark counts) to the total number of bosons $N$ vanishes as $N\to \infty$ for arbitrarily small noise amplitude, i.e., $\epsilon =o\left(1\right)$ as $N\to \infty$; hence, we conjecture that such a noisy boson sampling is also hard to simulate classically. The results significantly relax the sufficient condition on noise in network components, such as two-mode beam splitters, for classical hardness of experimental boson sampling

$\left[{\mathrm{M}}^{+*}\right]nl$ core-excited Rydberg states of the alkaline-earth-metal atoms can decay by electronic autoionization ($\left[{\mathrm{M}}^{+*}\right]nl\to {\mathrm{M}}^{+}+\varepsilon l^{\prime }$) or by fluorescence of the core ($\left[{\mathrm{M}}^{+*}\right]nl\to \left[{\mathrm{M}}^{+}\right]nl+h\nu$). The two processes can be detected separately by monitoring the prompt ${\mathrm{M}}^{+}$ autoionization yield and the delayed pulsed-field-ionization yield of the $\left[{\mathrm{M}}^{+}\right]nl$ Rydberg states. Whereas the former process strongly depends on the principal ($n$) and orbital angular-momentum ($l$) quantum numbers, the latter is independent of $n$ and $l$ because it is essentially a property of the ion core. We present an experimental study of the competition between autoionization and core fluorescence in $\left[\mathrm{Ne}\right]3p_{3/2}nl$ core-excited Rydberg states of Mg atoms and of its dependence on $n$ and weak electric fields. We analyze the experimental results using a kinetic model, from which we extract the electronic-autoionization rates by using the ${\mathrm{Mg}}^{+}3p_{3/2}–3s_{1/2}$ core fluorescence (${\tau }_{\mathrm{fl}}=3.8$ ns) as an internal clock. The autoionization rate of the $3p_{3/2}n{d}_{3/2}$ ($J=3$) and of the fully Stark-mixed $3p_{3/2}nk(m=0,\pm 2)$ series are $[1.9\times {10}^{16}/{n^{*}}^3]{\mathrm{s}}^{-1}$ and $[8\left(3\right)\times {10}^{15}/n^4]{\mathrm{s}}^{-1}$, which would translate for the lowest Rydberg states ($3d$ with $n^{*}=2.62$) to autoionization rates of $\sim {10}^{15}$ and $\sim {10}^{14}{\mathrm{s}}^{-1}$, respectively, as fast as Auger decay processes.

We study a heavy-heavy-light three-body system confined to one space dimension. Both binding energies and corresponding wave functions are obtained for (i) the zero-range and (ii) two finite-range attractive heavy-light interaction potentials. In the case of the zero-range potential, we apply the method of Skorniakov and Ter-Martirosian to explore the accuracy of the Born-Oppenheimer approach. For the finite-range potentials, we solve the Schrödinger equation numerically using a pseudospectral method. We demonstrate that when the two-body ground-state energy approaches zero, the three-body bound states display a universal behavior, independent of the shape of the interaction potential.