Quantum Physics (quant-ph)

Abstract: We propose a technique to improve the probability of single-photon emission with an electrically pumped quantum dot in an optical microcavity, by continuously monitoring the energy state of the dot and using feedback to control when to stop pumping. The goal is to boost the probability of single-photon emission while bounding the probability of two or more photons. We model the system by a stochastic master equation that includes post-measurement operations. Ideally, feedback should be based on the entire continuous measurement record, but in practice, it may be difficult to do such processing in real-time. We show that even a simple threshold-based feedback scheme using measurements at a single time can improve performance over deterministic (open-loop) pumping. This technique is particularly useful for strong dot-cavity coupling with lower rates of pumping, as can be the case for electrical pumping. It is also numerically tractable since we can perform ensemble averaging with a single master equation rather than averaging over a large number of quantum trajectories.

Abstract: Information scrambling refers to the propagation of information throughout a quantum system. Its study not only contributes to our understanding of thermalization but also has wide implications in quantum information and black hole physics. Recent studies suggest that information scrambling is mediated by collective modes called scramblons. However, a criterion for the validity of scramblon theory in a specific model is still missing. In this work, we address this issue by investigating the signature of the scramblon effective theory in random spin models with all-to-all interactions. We demonstrate that, in scenarios where the scramblon description holds, the late-time operator size distribution can be predicted from its early-time value, requiring no free parameters. As an illustration, we examine whether Brownian circuits exhibit a scramblon description and obtain a positive confirmation both analytically and numerically. We also discuss the prediction of multiple-quantum coherence when the scramblon description is valid. Our findings provide a concrete experimental framework for unraveling the scramblon field theory in random spin models using quantum simulators.

Abstract: Unsupervised machine learning models build an internal representation of their training data without the need for explicit human guidance or feature engineering. This learned representation provides insights into which features of the data are relevant for the task at hand. In the context of quantum physics, training models to describe quantum states without human intervention offers a promising approach to gaining insight into how machines represent complex quantum states. The ability to interpret the learned representation may offer a new perspective on non-trivial features of quantum systems and their efficient representation. We train a generative model on two-qubit density matrices generated by a parameterized quantum circuit. In a series of computational experiments, we investigate the learned representation of the model and its internal understanding of the data. We observe that the model learns an interpretable representation which relates the quantum states to their underlying entanglement characteristics. In particular, our results demonstrate that the latent representation of the model is directly correlated with the entanglement measure concurrence. The insights from this study represent proof of concept towards interpretable machine learning of quantum states. Our approach offers insight into how machines learn to represent small-scale quantum systems autonomously.

Abstract: Spin-glass graphs are simulated with a novel scheme using exciton-polaritons. Acting as an effective Monte Carlo solver, the ground state is found efficiently. By tuning a parameter, the system either solves XY or Ising problems. Unlike previous proposals, our setup with auxiliary micropillars naturally avoids any bias from amplitute heterogenity. We demonstrate that the simulator is able to find the ground state asymptotically for arbitrary large graphs. These findings show explicitly how polariton simulators could be useful in practice. We furthermore provide strong evidence for the system's ability to harness a quantum speedup.

Abstract: Inspired by the remarkable success of artificial neural networks across a broad spectrum of AI tasks, variational quantum circuits (VQCs) have recently seen an upsurge in quantum machine learning applications. The promising outcomes shown by VQCs, such as improved generalization and reduced parameter training requirements, are attributed to the robust algorithmic capabilities of quantum computing. However, the current gradient-based training approaches for VQCs do not adequately accommodate the fact that trainable parameters (or weights) are typically used as angles in rotational gates. To address this, we extend the concept of weight re-mapping for VQCs, as introduced by K\"olle et al. (2023). This approach unambiguously maps the weights to an interval of length $2\pi$, mirroring data rescaling techniques in conventional machine learning that have proven to be highly beneficial in numerous scenarios. In our study, we employ seven distinct weight re-mapping functions to assess their impact on eight classification datasets, using variational classifiers as a representative example. Our results indicate that weight re-mapping can enhance the convergence speed of the VQC. We assess the efficacy of various re-mapping functions across all datasets and measure their influence on the VQC's average performance. Our findings indicate that weight re-mapping not only consistently accelerates the convergence of VQCs, regardless of the specific re-mapping function employed, but also significantly increases accuracy in certain cases.

Abstract: Large-scale variational quantum algorithms possess an expressive capacity that is beyond the reach of classical computers and is widely regarded as a potential pathway to achieving practical quantum advantages. However, the presence of quantum noise might suppress and undermine these advantages, which blurs the boundaries of classical simulability. To gain further clarity on this matter, we present a novel polynomial-scale method that efficiently approximates quantum mean values in variational quantum algorithms with bounded truncation error in the presence of independent single-qubit depolarizing noise. Our method is based on path integrals in the Pauli basis. We have rigorously proved that, for a fixed noise rate $\lambda$, our method's time and space complexity exhibits a polynomial relationship with the number of qubits $n$, the circuit depth $L$, the inverse truncation error $\frac{1}{\varepsilon}$, and the inverse success probability $\frac{1}{\delta}$. Furthermore, We also prove that computational complexity becomes $\mathrm{Poly}\left(n,L\right)$ when the noise rate $\lambda$ exceeds $\frac{1}{\log{L}}$ and it becomes exponential with $L$ when the noise rate $\lambda$ falls below $\frac{1}{L}$.

Abstract: We experimentally investigate inductively shunted transmon-type artificial atoms as an alternative to address the challenges of low anharmonicity and the need for strong charge dispersion in superconducting quantum systems. We characterize several devices with varying geometries and parameters (Josephson energies and capacitances), and find a good agreement with calculations. Our approach allows us to retain the benefits of transmon qubit engineering and fabrication technology and high coherence, while potentially increasing anharmonicity. The approach offers an alternative platform for the development of scalable multi-qubit systems in quantum computing.

Abstract: Micromotion is detrimental to accurate qubit control of trapped ions, thus measuring and minimizing it is crucial. In this paper, we present a simple method to measure and minimize micromotion of trapped ions by Rabi oscillation combined with direct scanning of dc voltages. The approach utilizes the qubit control scheme itself, and eliminates the need to install additional experimental setups, or compromise the trapping stability by adjusting the intensity or frequency of the trapping lasers or fields. Accordingly, the method enables in-situ measurement of micromotion during qubit controls of the ions, while achieving a comparable level of sensitivity to commonly used techniques.

Abstract: Unitarity violations were recently proposed as a cause of objective quantum state reduction. This complements proposals based on stochastic modifications of Schrodinger's equation, but also differs from them in several aspects. Here, we formalise the description of unitarity violations, and show that they generically imply models of dynamical quantum state reduction (DQSR) driven by colored noise. We present a formalism for exploring such models as well as a prescription for enforcing explicit norm-preservation, and we show that the resulting pure state dynamics is described by a modified von-Neumann Liouville equation which in a particular limit reduces to the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equations. We additionally show adherence to Born's rule emerging in the same limit from a physical constraint relating fluctuating and dissipating components of the model.

Abstract: We explore a nonlinear extension to quantum theory giving rise to deterministic partial disentanglement between pairs of particles. The extension is based on a modified Schr\"{o}dinger equation having an added nonlinear term. To avoid conflicts with the principles of causality and separability, it is postulated that disentanglement is active only during the time when particles interact. A butterfly-like effect is found near highly entangled multipartite vector states.

Abstract: Niobium offers the benefit of increased operating temperatures and frequencies for Josephson junctions, which are the core component of superconducting devices. However existing niobium processes are limited by more complicated fabrication methods and higher losses than now-standard aluminum junctions. Combining recent trilayer fabrication advancements, methods to remove lossy dielectrics and modern superconducting qubit design, we revisit niobium trilayer junctions and fabricate all-niobium transmons using only optical lithography. We characterize devices in the microwave domain, measuring coherence times up to $62~\mu$s and an average qubit quality factor above $10^5$: much closer to state-of-the-art aluminum-junction devices. We find the higher superconducting gap energy also results in reduced quasiparticle sensitivity above $0.16~$K, where aluminum junction performance deteriorates. Our low-loss junction process is readily applied to standard optical-based foundry processes, opening new avenues for direct integration and scalability, and paves the way for higher-temperature and higher-frequency quantum devices.

Abstract: In quantum mechanics, the connection between the operator algebraic realization and the logical models of measurement of state observables has long been an open question. In the approach that is presented here, we introduce a new application of the cubic lattice. We claim that the cubic lattice may be faithfully realized as a subset of the self-adjoint space of a von Neumann algebra. Furthermore, we obtain a unitary representation of the symmetry group of the cubic lattice. In so doing, we re-derive the classic quantum gates and gain a description of how they govern a system of qubits of arbitrary cardinality.

Abstract: We present a generalized framework to adapt universal quantum state approximators, enabling them to satisfy rigorous normalization and autoregressive properties. We also introduce filters as analogues to convolutional layers in neural networks to incorporate translationally symmetrized correlations in arbitrary quantum states. By applying this framework to the Gaussian process state, we enforce autoregressive and/or filter properties, analyzing the impact of the resulting inductive biases on variational flexibility, symmetries, and conserved quantities. In doing so we bring together different autoregressive states under a unified framework for machine learning-inspired ans\"atze. Our results provide insights into how the autoregressive construction influences the ability of a variational model to describe correlations in spin and fermionic lattice models, as well as ab initio electronic structure problems where the choice of representation affects accuracy. We conclude that, while enabling efficient and direct sampling, thus avoiding autocorrelation and loss of ergodicity issues in Metropolis sampling, the autoregressive construction materially constrains the expressivity of the model in many systems.

Abstract: Identical quantum particles exhibit only two types of statistics: bosonic and fermionic. Theoretically, this restriction is commonly established through the symmetrization postulate or (anti)commutation constraints imposed on the algebra of creation and annihilation operators. The physical motivation for these axioms remains poorly understood, leading to various generalizations by modifying the mathematical formalism in somewhat arbitrary ways. In this work, we take an opposing route and classify quantum particle statistics based on operationally well-motivated assumptions. Specifically, we consider that a) the standard (complex) unitary dynamics defines the set of single-particle transformations, and b) phase transformations act locally in the space of multi-particle systems. We develop a complete characterization, which includes bosons and fermions as basic statistics with minimal symmetry. Interestingly, we have discovered whole families of novel statistics (dubbed transtatistics) accompanied by hidden symmetries, generic degeneracy of ground states, and spontaneous symmetry breaking -- effects that are (typically) absent in ordinary statistics.

Abstract: Network nonlocality allows one to demonstrate non-classicality in networks with fixed joint measurements, that is without random measurement settings. The simplest network in a loop, the triangle, with 4 outputs per party is especially intriguing. The "elegant distribution" [N. Gisin, Entropy 21, 325 (2019)] still resists analytic proofs, despite its many symmetries. In particular, this distribution is invariant under any output permutation. The Finner inequality, which holds for all local and quantum distributions, has been conjectured to be also valid for all no-signalling distributions with independent sources (NSI distributions). Here we provide evidence that this conjecture is false by constructing a 4-output network box that violate the Finner inequality and prove that it satisfies all NSI inflations up to the enneagon. As a first step toward the proof of the nonlocality of the elegant distribution, we prove the nonlocality of the distributions that saturates the Finner inequality by using geometrical arguments.

Abstract: We consider the relation between three different approaches to defining quantum states across several times and locations: the pseudo-density matrix (PDM), the process matrix, and the multiple-time state approaches. Previous studies have shown that bipartite two-time states can reproduce the statistics of bipartite process matrices. Here, we show that the operational scenarios underlying two-time states can be represented as PDMs, and thereby construct a mapping from process matrices to PDMs. The existence of this mapping implies that PDMs can, like the process matrix, model processes with indefinite causal orders. We illustrate this ability by showing how negativity of the PDM, a measure of temporal correlations, is activated by creating a quantum-switched order of operators associated with reset channels. The results contribute to the unification of quantum models of spatiotemporal states.

Abstract: The design of the Quantum Internet protocol stack is at its infancy and early-stage conceptualization. And different heterogeneous proposals are currently available in the literature. The underlying assumption of the existing proposals is that they implicitly mimic classical Internet Protocol design principles: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there''. Hence the network nodes are labeled with classical addresses, constituted by classical bits, and these labels aim at reflecting the node location within the network topology. In this paper, we argue that this twofold assumption of classical and location-aware addressing constitutes a restricting design option, which prevents to scale the quantumness to the network functionalities, beyond simple information encoding/decoding. On the contrary, by embracing quantumness within the node addresses, quantum principles and phenomena could be exploited for enabling a quantum native functioning of the entire communication network. This will unleash the ultimate vision and capabilities of the Quantum Internet.

Abstract: We demonstrate that collective vibrational strong coupling of molecules in thermal equilibrium can give rise to significant local electronic polarization effects in the thermodynamic limit. We do so by first showing that the full non-relativistic Pauli-Fierz problem of an ensemble of strongly-coupled molecules in the dilute-gas limit reduces in the cavity Born-Oppenheimer to a cavity-Hartree equation. Consequently, each molecule experiences a self-consistent coupling to the dipoles of all other molecules. In the thermodynamic limit, the sum of all molecular dipoles constitutes the macroscopic polarization field and the self-consistency then accounts for the delicate back-action on its heterogeneous microscopic constituents. The here derived cavity-Hartree equations allow for a computationally efficient implementation in an ab-initio molecular dynamics setting. For a randomly oriented ensemble of slowly rotating model molecules, we observe a red shift of the cavity resonance due to the polarization field, which is in agreement with experiments. We then demonstrate that the back-action on the local polarization takes a non-negligible value in the thermodynamic limit and hence the collective vibrational strong coupling can modify individual molecular properties locally. This is not the case, however, for dilute atomic ensembles, where room temperature does not induce any disorder and local polarization effects are absent. Our findings suggest that the thorough understanding of polaritonic chemistry, e.g. modified chemical reactions, requires self-consistent treatment of the cavity induced polarization and the usually applied restrictions to the displacement field effects may be insufficient.

Abstract: The quantum Ising chain has shortcuts to adiabaticity when operated with weak processes. However, when exactly do the non-equilibrium effects of the Kibble-Zurek mechanism, inherent to the system, appear in the optimal protocols in such a context? I propose here that such contrasting difference occurs due to the manner by which one measures the excitation spent energy of the system. Therefore, in this work, I made a qualitative analysis of a quantum annealing procedure of the time-averaged excess work, where the system acquires as a diverging decorrelation time the heuristic Kibble-Zurek mechanism relaxation time. Four important effects are then observed: the absence of shortcuts to adiabaticity, the pausing effect around the critical point in the optimal protocol when the Kibble-Zurek mechanism holds, the persistence of the time-averaged work to avoid slowly-varying regime even for large switching times, and diverging fluctuations of the time-averaged work. In the end, by comparing the excess and the time-averaged excess works, I conclude that this last one is not useful to measure the excitation spent energy in weak processes, although brings an intuition to what happens in the strong driving case.

Abstract: For potential quantum advantage, Variational Quantum Algorithms (VQAs) need high accuracy beyond the capability of today's NISQ devices, and thus will benefit from error mitigation. In this work we are interested in mitigating measurement errors which occur during qubit measurements after circuit execution and tend to be the most error-prone operations, especially detrimental to VQAs. Prior work, JigSaw, has shown that measuring only small subsets of circuit qubits at a time and collecting results across all such subset circuits can reduce measurement errors. Then, running the entire (global) original circuit and extracting the qubit-qubit measurement correlations can be used in conjunction with the subsets to construct a high-fidelity output distribution of the original circuit. Unfortunately, the execution cost of JigSaw scales polynomially in the number of qubits in the circuit, and when compounded by the number of circuits and iterations in VQAs, the resulting execution cost quickly turns insurmountable. To combat this, we propose VarSaw, which improves JigSaw in an application-tailored manner, by identifying considerable redundancy in the JigSaw approach for VQAs: spatial redundancy across subsets from different VQA circuits and temporal redundancy across globals from different VQA iterations. VarSaw then eliminates these forms of redundancy by commuting the subset circuits and selectively executing the global circuits, reducing computational cost (in terms of the number of circuits executed) over naive JigSaw for VQA by 25x on average and up to 1000x, for the same VQA accuracy. Further, it can recover, on average, 45% of the infidelity from measurement errors in the noisy VQA baseline. Finally, it improves fidelity by 55%, on average, over JigSaw for a fixed computational budget. VarSaw can be accessed here: https://github.com/siddharthdangwal/VarSaw.

Abstract: We study the generation of nearly maximal stationary entanglement between two non-identical quantum emitters embedded in a cavity and coherently excited at the two-photon resonance, i.e., with half of the energy of the doubly excited state. We report a mechanism that arises when the emitters interact forming a dimer and the cavity linewidth is small enough to resolve their internal excitonic structure. This condition gives rise to a frequency-dependent Purcell effect which results into two resonant conditions for the cavity frequency. At each resonance, we observe a nearly maximal steady-state occupation of either the symmetric or antisymmetric combination of one-excitation qubit states. This mechanism is optically tunable and leads to significantly greater and faster stationary entanglement than the resonant excitation of the transition from the ground state to the symmetric or antisymmetric states, reported in previous works. By exploring the parameter space of the system, we show that this phenomenon is one of a family of effects that can generate both stationary and metastable entanglement when driving the emitters at the two-photon resonance. We provide a global perspective of this landscape of mechanisms and contribute analytical descriptions and insights into these phenomena, establishing connections with previous reports in the literature and discussing how some of these effects can be optically detected.

Abstract: We investigate a family of $N\times N$ Euclidean random matrices $S$, whose entries are $\operatorname{sinc}$ functions of the distance between points independently sampled from a Gaussian distribution in three dimensions. This random matrix model arises in the study of cooperative photon emission rates of a random atomic cloud, initially excited by a laser in the linear regime. The spectral properties of $S$, in the large-$N$ limit, strongly depend on the atomic cloud density. We show that in the low-density regime the eigenvalue density of $S$ has a nontrivial limit that only depends on the so-called cooperativeness parameter $b_0$, the only parameter of the model. For small values $b_0\ll1$, we find that the limit eigenvalue density is approximatively triangular. We also study the nearest-neighbour spacing distribution and the eigenvector statistics. We find that, although $S$ is a Euclidean random matrix, the bulk of its spectrum is described by classical random matrix theory. In particular, in the bulk there is level repulsion and the eigenvectors are delocalized. Therefore, the bulk of the spectrum of $S$ exhibits the universal behaviour of chaotic quantum systems.

Abstract: This paper introduces a construction of quantum CSS codes from a tuple of component CSS codes and two collections of subsets. The resulting codes have parallelizable encoding and syndrome measurement circuits and built-in redundancy in the syndrome measurements. In a certain subfamily of the general construction, the resulting codes are related to a natural generalization of classical Reed-Muller codes, and this leads to a formula for the quantum code distance. The paper gives a number of examples of codes with block size $2^m, m=3,\dots,9$, and with syndrome measurements involving 2, 4 or 8 qubits. These include codes for which the distance exceeds the syndrome measurement weight, as well as codes which provide asymmetric protection against bit flip and phase flip errors.