Adiabatic reverse annealing (ARA) has been proposed as an improvement to conventional quantum annealing for solving optimization problems, in which one takes advantage of an initial guess at the solution to suppress problematic phase transitions. Here we interpret the performance of ARA through its effects on the free-energy landscape and use the intuition gained to introduce a classical analog to ARA termed “simulated reverse annealing” (SRA). The close similarity between the two means that they should be compared side by side in applications since SRA could potentially give comparable results. As a solvable example, we analyze how both protocols behave in a family of infinite-range (nondisordered) $p$-spin models. Through both the thermodynamic phase diagrams and explicit dynamical behavior, we establish that the quantum algorithm has no advantage over its classical counterpart (in the range of models considered): SRA succeeds at avoiding discontinuous transitions not only in every case where ARA does, but even in a narrow window of parameters where ARA fails to do so.

The Grover lower bound for the unstructured search problem can be surpassed when some information about the data structure is available. Here, we numerically observe that reinforcement can exponentially reduce the number of required evolution layers from $\sqrt{D}$ to $lnD$ in a $D$-dimensional system, by exploiting the information provided by the quantum state. Therefore, a reinforced quantum search is expected to exhibit a larger noise threshold compared to a standard search algorithm in a noisy environment. We use numerical simulations to characterize the level of noise tolerance via reinforcement in the presence of both coherent and incoherent noise, considering a system of $N$ qubits and a single $D$-level (qudit) system. Our results show that reinforcement significantly enhances the algorithm's success probability and improves the scaling of the number of reinforced evolution layers with system size. These findings indicate that reinforcement offers a promising strategy for error mitigation, especially when a precise noise model is unavailable.

Random circuits giving rise to unitary designs are key tools in quantum information science and many-body physics. In this work, we investigate a class of random quantum circuits with a specific gate structure. Within this framework, we prove that one-dimensional structured random circuits with non-Haar-random local gates can exhibit substantially more global randomness compared to Haar-random circuits with the same underlying circuit architecture. In particular, under a solvability condition, we exactly diagonalize the second-moment operators for these structured random circuits by establishing a link to free-fermionic models, and we show that their spectral gaps can exceed those of Haar-random circuits. While solvable structured random circuits exhibit free-fermionic behavior, the underlying fixed gate need not be a matchgate. Our findings have applications in improving circuit depth bounds for randomized benchmarking, in generating approximate unitary 2-designs from shallow random circuits, and in enabling exact analyses of quantum many-body dynamics.

Trapped-ion lattice geometries, which determine the interactions between trapped-ion qubits, are typically governed by the balance of Coulomb repulsion forces with the external trapping potential. Here we demonstrate how the effective ion lattice geometry and resulting qubit-qubit interactions may be reconfigured in situ, by shelving specific ions in metastable states outside the qubit subspace. Using a triangular lattice of three ${}^{171}\mathrm{Yb}^{+}$ ions, we optically pump selected ions into the long-lived ${}^{2}F_{7/2}$ state. We then apply a global Ising-like Hamiltonian to the system and verify that the shelved qubits are fully removed from participation in the quantum dynamics. We characterize the metastable state lifetime in the presence of laser-driven ion-ion interactions, finding a deshelving rate that is orders of magnitude slower than the spin-spin interaction rate and scales quadratically with applied laser intensity.

It is often argued that two linearly coupled quantum harmonic oscillators, even when cooled to their ground state, display no inherently quantum features beyond quantized energy levels. Here, we challenge this view by showing that their classical observables encode genuinely quantum features. In particular, we demonstrate that the characteristic frequency shifts of the normal modes observed in coupled oscillators signal nonclassical correlations and ground-state entanglement at zero temperature, corresponding to two-mode squeezing between the uncoupled modes. From a complementary perspective, these two effects, frequency shifts and squeezing, represent the same underlying phenomenon but expressed in different mode bases. What appears as a spectral renormalization in one description manifests itself as entanglement in the other. Frequency shifts therefore constitute an entanglement witness accessible via standard spectroscopy. When the underlying squeezing is not directly measurable, it can be still exploited to enhance the signal-to-noise ratio in precision frequency measurements of individual oscillators without requiring squeezed quantum noise. This uncovers an alternative route to quantum-enhanced sensing within systems traditionally regarded as classical, offering insight into how signatures of quantumness persists across the quantum-to-classical boundary.

Finite local Hilbert-space truncations arise naturally in quantum simulations of lattice field theories and motivate qudit encodings, but their fault-tolerant advantage over qubit encodings remains unclear. We compare the non-Clifford cost of implementing quadratic diagonal evolutions, exemplified by $U=e^{-it{\varphi }_x^2}$ in a uniform field-amplitude discretization of a real scalar field, using either one logical $d$-level qudit or $n_b=\lceil {log}_{2}d\rceil$ logical qubits. We analyze two standard settings: product-formula simulation and linear combination of unitaries (LCU) per block encoding, taking the resource metric to be the number of non-Clifford gates after synthesis into a discrete logical gate set. Because tight synthesis bounds for general single-qudit rotations are not known, we express the qudit constructions in terms of embedded two-level $\text{SU}\left(2\right)$ rotations and derive explicit finite-$d$ break-even conditions for their synthesis cost; these serve as compiler targets for when qudit encodings can outperform the qubit baseline. Within the constructive models studied here, product-formula implementations would require an exponentially stronger per-primitive synthesis advantage for qudits to win asymptotically, while in the LCU setting the qubit encoding is asymptotically cheaper in $d$. Nevertheless, the finite-$d$ threshold analysis identifies low-dimensional regions in which qudits can yield meaningful constant-factor savings, particularly for LCU-based implementations. As a secondary analysis of the LCU construction, we use an idealized negligible-overhead qubit-qudit code-switching model to give an absolute $T$-count comparison and reinterpret the savings as an allowable per-switch overhead budget.

The quantum Fisher information of a quantum state with respect to a certain parameter quantifies the sensitivity of the quantum state to changes in that parameter. Maximizing the quantum Fisher information is essential for achieving the optimal estimation precision of quantum sensors. A typical quantum sensor involves a qubit (e.g., a spin-$\frac{1}{2}$) probe undergoing an unknown rotation, here the unknown rotation angle is the parameter to be estimated. A well-known limitation is that if the rotation axis is unknown beforehand, the maximal quantum Fisher information is impossible to attain. This limitation has been lifted recently by leveraging entanglement between the probe qubit and an ancilla qubit. Namely, through measurement of the ancilla after the axis is revealed, one can prepare the probe that is optimal for any unknown rotation axis. This protocol, however, works only for a spin-$\frac{1}{2}$. Considering large spin probes can achieve a larger quantum Fisher information, offering enhanced metrological advantage, we here utilize the entanglement between a large spin probe and an ancilla to achieve optimal quantum Fisher information for estimating the rotation angle, without prior knowledge of the rotation axis. Different from the previous spin-$\frac{1}{2}$ case, achieving the optimal precision with large spins generally requires postselection, resulting in a success probability dependent on the dimension of the Hilbert space. Furthermore, we extend the encoding state from the maximally entangled case to general entangled states, showing that optimal metrology can still be achieved with a certain success probability.

High-fidelity two-qubit gates leverage qubit-qubit interactions and are essential for scalable quantum computation. However, environmental noise and imperfect control of the interactions are inevitable, which degrade the quality of gate operations. Here we introduce local microwave driving in conventional exchange-coupled semiconductor spin qubits and find that this can provide sufficient degrees of freedom to suppress various errors. Specifically, under specific driving configurations, we find that the two-qubit quantum dynamics can be mapped to lower-dimensional $\mathfrak{su}\left(2\right)$ or $\mathfrak{su}\left(2\right)\oplus \mathfrak{su}\left(2\right)$ Lie algebraic structures. Thus, combining this mapping with filter-function theory and truncated Fourier pulse shaping, the robust quantum control of two qubits can be simplified as a pulse optimization problem for a quantum two-level system. Numerical simulations show that our optimized controlled phase and $\sqrt{\mathrm{iSWAP}}$ gates maintain high fidelities, with typical errors under realistic conditions. Therefore, our protocol offers a promising alternative for robust quantum control.

In the computational chemistry of molecular materials, strong static correlation effects appear when electronic states, often involving the ground state, become quasidegenerate, as occurs, for example, in bond-breaking processes. Such situations present significant challenges for accurate theoretical treatment. In these regimes, many-body methods involving a single-determinant description, such as Hartree-Fock theory and its time-dependent extension, fail to reproduce the correct topology of the ground and excited state potential energy surfaces (e.g., near conical intersections). When strongly correlated electronic systems are further strongly coupled to a quantized radiation field within the framework of nonrelativistic cavity quantum electrodynamics, an additional photonic degree of freedom introduces both new complexity and new opportunities for control. Excited cavity photons can modify bond-breaking processes and enable tunability of geometrical and spin-phase transitions, for instance, in organometallic complexes. To overcome this bottleneck, in this work, we extend the well-studied spin-flip configuration interaction singles (SF-CIS) approach to explicitly include quantized cavity photons leading to the QED-SF-CIS method. We derive the spin-flip Hamiltonian and find that the double excitation subspace of the hybrid system (i.e., mixed electron-photon single-single excitations) must be included in the configurations to properly describe singlet electronic states interacting with cavity photons. We then illustrate, through representative molecular examples, how cavity coupling can provide additional tunability in bond-breaking processes. We finally generalize this approach to include higher numbers of photonic excitations, which are required in the strong coupling regime and at higher-excitation frequencies.

Strong-field ionization in two-color ($\omega +2\omega$) elliptically polarized laser fields highlights subcycle electron dynamics in the angular distribution of photoelectrons. In these distributions, a characteristic alternating angular pattern is observed, in which successive above-threshold ionization peaks and sidebands exhibit opposite angular shifts and are controllable through the relative phase between the two driving fields. This pattern originates from subcycle interference between adjacent ionization bursts, where a weak-field-induced phase modulation changes sign from one subcycle to the next, thereby directly controlling the emission angle. We further identify abrupt sideband shifts arising from the sensitivity to the relative phase between the two fields. Time-dependent Schrödinger equation simulations show that the long-range Coulomb interaction selectively suppresses electron emission counterrotating with respect to the driving field, with a pronounced dependence on the field helicity. These results establish angular shifts as a direct observable of subcycle phase modulation and demonstrate their utility as a sensitive interferometric probe of attosecond electron wave-packet dynamics.

We present a comprehensive study of Compton scattering of 20-keV photons by bound electrons over a wide range of binding energies ($\mathrm{Ne}2p, \mathrm{Ne}2s, \mathrm{C}1s, \mathrm{O}1s$, and $\mathrm{Ne}1s$ orbitals). In particular, we have measured fully differential cross sections, i.e., electron angular distributions with respect to the direction of the photon momentum transfer for fixed electron energy. Our data show strong deviations from the predictions of the “quasi-free” electron approximation. The binding energy and the shape of the Coulomb potential from which the electron escapes both play a significant role in its kinematics. We find that Compton electrons are scattered and even backreflected by the ionic potential. The strength of this effect depends on the orbital from which the electron is ejected.

Floquet engineering offers powerful tools to manipulate quantum states by periodically driving physical parameters. In this work, we investigate the quantum transport through two periodically driven impurities in a mesoscopic one-dimensional channel. By mapping the time-dependent Hamiltonian into an effective multichannel scattering problem, we unveil a rich landscape of transport phenomena arising from the interplay between Fabry-Pérot cavity modes and Fano interference. We demonstrate that the inter-impurity distance acts as a critical control parameter, allowing for the formation of bound states in the continuum (BICs). Furthermore, we identify quasi-BICs—extremely narrow resonances with finite lifetimes—that can be dynamically tuned by the drive amplitude. We show that these states enable a robust coherent trapping mechanism, allowing the system to switch from perfect transparency or reflection to strong localization with giant Wigner time delays. Our results suggest possible applications for tunable delay lines and quantum memories, with feasible experimental realizations in the context of cold atoms.

Optomechanical generation of entangled optical beams is usually hindered by thermal noise. We present a theoretical study of low-frequency entanglement generation between two nondegenerate optical harmonics emitted from a cavity optomechanical system operating in the resolved-sideband regime. The system comprises three nearly equidistant optical modes in a high-finesse Fabry-Perot cavity, with the central mode coherently driven by a continuous wave classical signal. This configuration enables radiation-pressure interactions that generate strong quantum correlations between the two undriven sideband modes. Remarkably, these correlations persist even at large numbers of thermal phonons and a broad range of system parameters if the optical pump is strong enough. Entanglement is verified using a full quantum Langevin model that accounts for thermal noise and optical losses. Our findings demonstrate the feasibility of robust entanglement under ambient conditions, opening new avenues for hybrid quantum technologies based on mechanical interfaces and continuous-variable quantum information processing.

We extend the nonlocality transfer matrix framework to quantum phases exhibiting spontaneous translational symmetry breaking. We show that for a broad family of these phases, the standard assumption—wherein the spatial periodicity of the transfer matrix ($u_T$) matches that of the ground state ($u$)—is insufficient to capture the maximal violation of Bell-type inequalities. Instead, we establish that an optimal construction with $u_T=2u$ is physically required to fully detect multipartite nonlocality in these regimes. Applying this generalized framework to the axial next-nearest-neighbor Ising model and the Kitaev-Heisenberg chain, we demonstrate that the extended nonlocality spectrum reveals multipartite nonlocality hidden by the conventional approach and provides sensitive signatures of quantum phase transitions.