Qubit-based variational quantum algorithms have undergone rapid development in recent years but still face several challenges. Here, we introduce a symmetry-based enhancement to digitized counterdiabatic quantum algorithms, applicable for qudits of any dimension. This approach offers three types of compression compared to conventional variational circuits. First, compression in the circuit depth is achieved by counterdiabatic protocols. Second, information about the problem is compressed by replacing qubits with qudits, allowing for a more efficient representation of the problem. Finally, the number of parameters is reduced by employing the symmetries of the system. We illustrate this approach by tackling a graph-based optimization problem Max-3-Cut, a highly entangled state preparation, the qutrit $W$ state, and a two-body only antiferromagnetic Ising problem. As our numerical results show, we achieve a better convergence with a lower circuit depth and less measurement overhead, albeit with some identified limitations for which we propose a work-around. This work leads to a better design of shallow variational quantum circuits, improving the feasibility of their implementation on near-term qudit devices.

Floquet theory is a widely used framework to describe the dynamics of periodically driven quantum systems. The usual scenario to describe such systems is to consider the effect of an external control with a definite period in time that can act either locally or globally on the system of interest. However, apart from periodicity, such drives typically lack classical correlations or additional structure. In this work, we consider drives with intrinsic dynamics that undergo self-organization, leading to periodic steady states with emergent symmetries. To substantiate our results, we consider two examples of one-dimensional quantum spin chains coupled to a classical Kuramoto model. First, we investigate a Kuramoto model with all-to-all coupling driving a one-dimensional quantum Ising chain into a time-periodic steady state with an emergent translational symmetry. Next, we consider a Kuramoto model in a zigzag lattice driving an XX spin chain. The dynamics of traveling waves in the Kuramoto model trimerizes the lattice, effectively inducing topological behavior that can be exploited to perform topological pumping. Our results can be experimentally implemented in digital and analog near-term quantum devices.

Quantum computing has changed the paradigm of computer science, where quantum technologies have promised to outperform classical ones. Such an advantage was only demonstrated for tasks with no application or out of reach for the state-of-the-art quantum technologies. In this context, a promising strategy to find practical use of quantum computers exploits hybrid models, where a quantum device estimates a hard-to-compute quantity, while a classical optimizer trains the parameters. In this work, we demonstrate that single photons and linear optical networks are sufficient for implementing variational quantum algorithms, when the problem specification (ansatz) is tailored to this specific platform. We show this by a proof-of-principle demonstration to tackle an instance of a factorization task, whose solution is encoded in the ground state of a suitable Hamiltonian. This work, which combines Variational Quantum Algorithms with hardware efficient ansätze for linear optics, showcases a promising pathway toward practical applications for photonic quantum platforms.

A Berry phase of odd multiples of $\pi$ inferred from quantum oscillations (QOs) has often been treated as evidence for nontrivial reciprocal space topology. However, disentangling the Berry phase values from the Zeeman effect and the orbital magnetic moment is often challenging. In centrosymmetric compounds, the case is simpler as the orbital magnetic moment contribution is negligible. Although the Zeeman effect can be significant, it is usually overlooked in most studies of QOs in centrosymmetric compounds. Here, we present a detailed study on the nonmagnetic centrosymmetric ${\mathrm{SrGa}}_2$ and ${\mathrm{BaGa}}_2$, which are predicted to be Dirac nodal line semimetals based on density functional theory (DFT) calculations. Evidence of the nontrivial topology is found in magnetotransport measurements. The Fermi surface topology and band structure are carefully studied through a combination of angle-dependent QOs, angle-resolved photoemission spectroscopy (ARPES), and DFT calculations, where the nodal line is observed in the vicinity of the Fermi level. Strong de Haas–van Alphen fundamental oscillations associated with higher harmonics are observed in both compounds, which are well fitted by the Lifshitz-Kosevich (LK) formula. However, even with the inclusion of higher harmonics in the fitting, we found that the Berry phases cannot be unambiguously determined when the Zeeman effect is included. We revisit the LK formula and analyze the phenomena and outcomes that were associated with the Zeeman effect in previous studies. Our experimental results confirm that ${\mathrm{SrGa}}_2$ and ${\mathrm{BaGa}}_2$ are Dirac nodal line semimetals. Additionally, we highlight the often overlooked role of spin-damping terms in Berry phase analysis.

The initial stage of zebrafish morphogenesis is characterized by a synchronous to asynchronous transition (SAT) in cell divisions. The cells divide in unison in the synchronous phase, unlike in the asynchronous phase. Despite the widespread observation of SAT in multiple organisms, there is no theoretical framework to predict the critical number of cell cycles $n^{*}$ that mark the beginning of asynchronous division. Here, by probabilistically modeling cell cycle progression under the assumption that the distribution of cell division times is broadened, we predict the value of $n^{*}$ and the time at which the SAT occurs. The theory, supplemented by agent-based simulations, supports the hypothesis that the SAT emerges as a consequence of biomechanical feedback on cell division. Our results are in excellent agreement with experiments while also explaining the cell cycle lengthening that arises as a result of biomechanical feedback. The emergence of the asynchronous phase is due to increasing fluctuations in the cell cycle times with each round of cell division. We also make several testable predictions that further shed light on the role of biomechanical feedback in growing multicellular systems, such as during tissue and tumor growth.

The fractional quantum Hall effect provides the first example of multiple quantum phases hosting quasiparticles not being fermions or bosons, but anyons. It is therefore important to investigate whether all these phases can be practical to detect these anyons. Here, we propose revisiting the intriguing physics of hole-conjugate Fractional Quantum Hall (FQH) phases with counter-propagating edge channels in the filling factor range $1/2\le \nu \le 1$ whose properties may prevent anyon interference detection. In the seminal KFP model by Kane, Fisher, and Polchinski, upstream neutral and downstream charge modes, along with random incoherent interchannel tunneling, are considered essential for explaining the observed conductance at $\nu =2/3$. In this work, we introduce a minimal model that qualitatively reproduces the edge physics of the $\nu =2/3$ FQH phase without invoking neutral and charge modes. By inserting $N$ fictive reservoirs—either Landauer reservoirs (LRs) or Energy-Preserving Reservoirs (EPRs)—along the counter-propagating channels to simulate edge channel mixing, we show that the conductance rapidly approaches the KFP fixed point of $2/3\times (e^2/h)$ for both EPRs and LRs. While LRs produce equal charge and thermal relaxation lengths, EPRs exhibit an infinite thermal relaxation length, consistent with recent observations. Our model also predicts current noise in small Hall bars that varies linearly with voltage at large bias, with an apparent Fano factor $\simeq 0.19/\sqrt{N+1}$ for $N$ LRs or an exponentially vanishing Fano factor for EPRs. Moreover, with a Quantum Point Contact at the center of the Hall bar, the model predicts the recently observed $0.5e^2/h$ conductance plateau. Finally, early experiments reporting upstream excitations are found to be conveniently explained by upstream diffusive heat flow and delta-$T$ noise, rather than by specific collective neutral modes. The results obtained in this study can be generalized to more complex hole-conjugate fractional quantum Hall phases.

We present a new decoder for the surface code, which combines the accuracy of the tensor-network decoders with the efficiency and parallelism of the belief-propagation algorithm. Our main idea is to replace the expensive tensor-network contraction step in the tensor-network decoders with the blockBP algorithm—a recent approximate contraction algorithm, based on belief propagation. Our decoder is therefore a belief-propagation decoder that works in the degenerate maximal likelihood decoding framework. Unlike conventional tensor-network decoders, our algorithm can run efficiently in parallel, and may therefore be suitable for real-time decoding. We numerically test our decoder and show that for a large range of lattice sizes and noise levels it delivers a logical error probability that outperforms the Minimal Weight Perfect Matching decoder, sometimes by more than an order of magnitude.

The simulation of quantum many-body systems, relevant for quantum chemistry and condensed matter physics, is one of the most promising applications of near-term quantum computers before fault-tolerance. However, since the vast majority of quantum computing technologies are built around qubits and discrete gate-based operations, the translation of the physical problem into this framework is a crucial step. This translation will often be device specific, and a suboptimal implementation will be punished by the exponential compounding of errors on real devices. The importance of an efficient mapping is already revealed for models of spinful fermions in two or three dimensions, which naturally arise when the relevant physics relates to electrons. Using the most direct and well-known mapping, the Jordan-Wigner transformation, leads to a nonlocal representation of local degrees of freedom, and necessities efficient decompositions of nonlocal unitary gates into a sequence of hardware accessible local gates. In this paper, we provide a step-by-step recipe for simulating the paradigmatic two-dimensional Fermi-Hubbard model on a quantum computer using only local operations. To provide the ingredients for such a recipe, we briefly review the plethora of different approaches that have emerged recently but focus on the Derby-Klassen compact fermion mapping to make our discussion concrete. We provide a detailed recipe for an end-to-end simulation including embedding on a physical device, preparing initial states such as ground states, simulation of unitary time evolution, and measurement of observables and spectral functions. We explicitly compute the resource requirements for simulating a global quantum quench and conclude by discussing the challenges and future directions for simulating strongly correlated fermionic matter on quantum computers.

Aberration correction is crucial for achieving atomic resolution in electron microscopy. While conventional correctors use multipolar electron lenses, our work explores a fundamentally different approach tailored specifically for ultrafast electron microscopes operating in pulsed mode. We propose using spatially structured intense laser pulses to manipulate electron wave fronts via free-space electron-photon interactions. Our simulations demonstrate that such laser fields can effectively compensate for the spherical and chromatic aberrations of subsequent magnetic lenses, resulting in substantial improvements in probe size. These findings pave the way for optical elements designed for ultrafast electron microscopy.

Rashba-type spin-orbit coupling gives rise to distinctive surface and interface phenomena, such as spin-momentum locking and spin splitting. In nonequilibrium settings, one of the key manifestations is the (Rashba-)Edelstein effect, where an electric current generates a net spin or orbital polarization perpendicular to the current direction. While the steady-state behavior of these effects is well studied, their dynamics on ultrafast timescales remain largely unexplored. In this work, we present a theoretical investigation of the ultrafast spin and orbital Edelstein effects on an Au(001) surface, triggered by excitation with a femtosecond laser pulse. These effects are intrinsic and inherently nonlinear. Using a real-space tight-binding model combined with time evolution governed by the von Neumann equation, we simulate the electron dynamics in response to the pulse. Our results reveal pronounced differences between the spin and orbital responses, offering detailed insights into their distinct temporal profiles and magnitudes. We further explore the associated charge, spin, and orbital currents, including the emergence of laser-induced spin and orbital Hall effects. Finally, we quantify the angular momentum transfer mediated by the light-matter interaction. These findings shed light on the intricate ultrafast dynamics driven by spin-orbit coupling and offer guidance for the design of next-generation spintronic and orbitronic devices.

We introduce bipartite projected ensembles (BPEs) for quantum many-body wave functions, which consist of pure states supported on two local subsystems, with each state associated with the outcome of a projective measurement of the complementary subsystem in a fixed local basis. We demonstrate that the corresponding ensemble-averaged entanglements (EAEs) between two subsystems can effectively identify entanglement phases. In volume-law entangled states, EAE converges to a nonzero value with increasing distance between subsystems. For critical systems, EAE exhibits power-law decay, and it decays exponentially for area-law systems. Thus, entanglement phase transitions can be viewed as a disordered-ordered phase transition. We also apply BPE and EAE to measured random Clifford circuits to probe measurement-induced phase transitions. We show that EAE not only serves as a witness to phase transitions, but also unveils additional critical phenomena properties, including dynamical scaling and surface critical exponents. Our findings provide an alternative approach to diagnosing entanglement laws, thus enhancing the understanding of entanglement phase transitions. Moreover, given the accessibility of measuring EAE in quantum simulators, our results hold promise for impacting quantum simulations.

A quantum walk on a lattice is a paradigm of a quantum search in a database. The database qubit strings are the lattice sites, qubit rotations are tunneling events, and the target site is tagged by an energy shift. For quantum walks on a continuous time, the walker diffuses across the lattice and the search ends when it localizes at the target site. The search time $T$ can exhibit Grover's optimal scaling with the lattice size $N$, namely, $T\sim \sqrt{N}$, on an all-connected, complete lattice. For finite-range tunneling between sites, instead, Grover's optimal scaling is warranted when the lattice is a hypercube of $d>4$ dimensions. Here, we show that Grover's optimum can be reached in lower dimensions on lattices of long-range interacting particles, when the interaction strength scales algebraically with the distance $r$ as $1/r^{\alpha }$ and $0<\alpha <3d/2$. For $\alpha <d$ the dynamics mimics the one of a globally connected graph. For $d<\alpha <d+2$, the quantum search on the graph can be mapped to a short-range model on a hypercube with spatial dimension $d_s=2d/(\alpha -d)$, indicating that the search is optimal for $d_s>4$. Our work identifies an exact relation between criticality of long-range and short-range systems, it provides a quantitative demonstration of the resources that long-range interactions provide for quantum technologies, and indicates when existing experimental platforms can implement efficient analog quantum search algorithms.

It is known that the same physical system can be described by different effective theories depending on the scale at which it is observed. In this work, we formulate a prescription for finding the unitary that best approximates the large-scale dynamics of a quantum system evolving discretely in time, as it is the case for digital quantum simulators. We consider the situation in which the degrees of freedom of the system can be divided between an IR part that we can observe and a UV part that we cannot observe. Following a principle of minimal dissipation, our goal is to find the unitary dynamics that best approximates the (generally nonunitary) time evolution of the IR degrees of freedom. We first prove that when the IR and UV degrees of freedom are weakly coupled, the unitary that maximizes the fidelity is given by a mean-field dynamics and the error is given by a sum of energy variances. We then apply our results to a one-dimensional quantum walk, which is known to reproduce the Dirac equation in the small-mass and -momentum limit. We find that in this limit the effective IR dynamics is obtained by a mass redefinition.

We investigate the entropy production in the Diósi-Penrose (DP) model, one of the most extensively studied gravity-related collapse mechanisms, and one of its dissipative extensions. To this end, we analyze the behavior of a single harmonic oscillator, subjected to such collapse mechanisms, focusing on its phase-space dynamics and the time evolution of the entropy production rate—a central quantity in nonequilibrium thermodynamics. Our findings reveal that the original DP model induces unbounded heating, producing dynamics consistent with the second law of thermodynamics only under the assumption of an infinite-temperature noise field. In contrast, its dissipative extension achieves physically consistent thermalization in the regime of low dissipation strength. Using a short-time approach, we further our study to address the complete dynamics of the dissipative extension, thus including explicitly non-Gaussian features in the state of the system that lack from the low-dissipation regime.