arXiv subjects

Cristiano Ciuti

Publications and source records attributed to Cristiano Ciuti.

At least 37 records · Page 2Linked to original sources

arXiv2023

Efficient estimation of trainability for variational quantum circuits

Parameterized quantum circuits used as variational ansätze are emerging as promising tools to tackle complex problems ranging from quantum chemistry to combinatorial optimization. These variational quantum circuits can suffer from the well-known curse of barren plateaus, which is characterized by an exponential vanishing of the cost-function gradient with the system size, making training unfeasible for practical applications. Since a generic quantum circuit cannot be simulated efficiently, the determination of its trainability is an important problem. Here we find an efficient method to compute the gradient of the cost function and its variance for a wide class of variational quantum circuits. Our scheme relies on our proof of an exact mapping from randomly initialized circuits to a set of Clifford circuits that can be efficiently simulated on a classical computer by virtue of the celebrated Gottesmann-Knill theorem. This method is scalable and can be used to certify trainability for variational quantum circuits and explore design strategies that can overcome the barren plateau problem. As illustrative examples, we show results with up to 100 qubits.

quant-ph↗

arXiv2023

Dynamics with autoregressive neural quantum states: application to critical quench dynamics

Despite very promising results, capturing the dynamics of complex quantum systems with neural-network ansätze has been plagued by several problems, one of which being stochastic noise that makes the dynamics unstable and highly dependent on some regularization hyperparameters. We present an alternative general scheme that enables one to capture long-time dynamics of quantum systems in a stable fashion, provided the neural-network ansatz is normalized, which can be ensured by the autoregressive property of the chosen ansatz. We then apply the scheme to time-dependent quench dynamics by investigating the Kibble-Zurek mechanism in the two-dimensional quantum Ising model. We find an excellent agreement with exact dynamics for small systems and are able to recover scaling laws in agreement with other variational methods.

quant-ph↗

arXiv2023

Metrics and properties of optimal gauges in multimode cavity QED

Multi-mode cavity quantum electrodynamics (QED) describes, for example, the coupling between an atom and a multi-mode electromagnetic resonator. The gauge choice is important for practical calculations in truncated Hilbert spaces, because the exact gauge-invariance is recovered only in the whole space. An optimal gauge can be defined as the one predicting the most accurate observables for the same number of atomic levels and modes. Different metrics quantifying the gauge performance can be introduced depending on the observable of interest. In this work we demonstrate that the optimal choice is generally mode-dependent, i.e., a different gauge is needed for each cavity mode. While the choice of gauge becomes more important for increasing light-matter interaction, we also show that the optimal gauge does not correspond to the situation where the entanglement between light and matter is the smallest.

quant-ph↗

arXiv2023

Remote gate control of topological transitions in moiré superlattices via cavity vacuum fields

Placed in cavity resonators with three-dimensionally confined electromagnetic wave, the interaction between quasiparticles in solids can be induced by exchanging virtual cavity photons, which can have a non-local characteristic. Here we investigate the possibility of utilizing this nonlocality to realize the remote control of the topological transition in mesoscopic moiré superlattices at full filling (one electron/hole per supercell) embedded in a split-ring terahertz electromagnetic resonator. We show that gate tuning one moiré superlattice can remotely drive a topological band inversion in another moiré superlattice not in contact but embedded in the same cavity. Our study of remote on/off switching of a topological transition provides a novel paradigm for the control of material properties via cavity vacuum fields.

cond-mat.mtrl-sci↗

arXiv2023

Quantum electron transport controlled by cavity vacuum fields

We explore theoretically how the coupling to cavity vacuum fields affects the electron transport in quantum conductors due to the counter-rotating-wave terms of light-matter interaction. We determine the quantum conductance in terms of the transmission coefficients predicted by an effective electron Hamiltonian. The coupling between bare electronic states is mediated by virtual processes involving intermediate states with one electron (or one hole) on top of the Fermi sea and one virtual cavity photon. We study the behavior of the quantum conductance in the presence of artificial or disordered single-particle potentials, as well as a spatially varying cavity mode. As illustrative examples, we apply our theory to 1D conductors and to disordered 2D quantum Hall systems. We show how the cavity vacuum fields can lead to both large enhancement or suppression of electron conductance in the ballistic regime, as well as modification of the conductance quantization and fluctuations.

cond-mat.mes-hall↗

arXiv2022

Theory of strong down-conversion in multi-mode cavity and circuit QED

We revisit the superstrong coupling regime of multi-mode cavity quantum electrodynamics (QED), defined to occur when the frequency of vacuum Rabi oscillations between the qubit and the nearest cavity mode exceeds the cavity's free spectral range. A novel prediction is made that the cavity's linear spectrum, measured in the vanishing power limit, can acquire an intricate fine structure associated with the qubit-induced cascades of coherent single-photon down-conversion processes. This many-body effect is hard to capture by a brute-force numerics and it is sensitive to the light-matter coupling parameters both in the infra-red and the ultra-violet limits. We focused at the example case of a superconducting fluxonium qubit coupled to a long transmission line section. The conversion rate in such a circuit QED setup can readily exceed a few MHz, which is plenty to overcome the usual decoherence processes. Analytical calculations were made possible by an unconventional gauge choice, in which the qubit circuit interacts with radiation via the flux/charge variable in the low-/high-frequency limits, respectively. Our prediction of the fine spectral structure lays the foundation for the "strong down-conversion" regime in quantum optics, in which a single photon excited in a non-linear medium spontaneously down-converts faster than it is absorbed.

quant-ph↗

arXiv2022

Enhanced Cavity Optomechanics with Quantum-well Exciton Polaritons

Semiconductor microresonators embedding quantum wells can host tightly confined and mutually interacting excitonic, optical and mechanical modes at once. We theoretically investigate the case where the system operates in the strong exciton-photon coupling regime, while the optical and excitonic resonances are parametrically modulated by the interaction with a mechanical mode. Owing to the large exciton-phonon coupling at play in semiconductors, we predict an enhancement of polariton-phonon interactions by two orders of magnitude with respect to mere optomechanical coupling: a near-unity single-polariton quantum cooperativity is within reach for current semiconductor resonator platforms. We further analyze how polariton nonlinearities affect dynamical back-action, modifying the capability to cool or amplify the mechanical motion.

quant-ph↗

arXiv2022

Quantum density matrix theory for a laser without adiabatic elimination of the population inversion: transition to lasing in the class-B limit

Despite the enormous technological interest in micro and nanolasers, surprisingly, no class-B quantum density-matrix model is available to date, capable of accurately describing coherence and photon correlations within a unified theory. In class-B lasers $-$applicable for most solid-state lasers at room temperature$-$, the macroscopic polarization decay rate is larger than the cavity damping rate which, in turn, exceeds the upper level population decay rate. Here we carry out a density-matrix theoretical approach for generic class-B lasers, and provide closed equations for the photonic and atomic reduced density matrix in the Fock basis of photons. Such a relatively simple model can be numerically integrated in a straightforward way, and exhibits all the expected phenomena, from one-atom photon antibunching, to the well-known S-shaped input-output laser emission and super-Poissonian autocorrelation for many atoms ($1\leq g^{(2)}(0)\leq 2$), and from few photons (large spontaneous emission factors, $β\sim1$) to the thermodynamic limit ($N\gg1$ and $β\sim 0$). Based on the analysis of $g^{(2)}(τ)$, we conclude that super-Poissonian fluctuations are clearly related to relaxation oscillations in the photon number. We predict a strong damping of relaxation oscillations with an atom number as small as $N\sim 10$. This model enables the study of few-photon bifurcations and non-classical photon correlations in class-B laser devices, also leveraging quantum descriptions of coherently coupled nanolaser arrays.

quant-ph↗

arXiv2022

Machine learning via relativity-inspired quantum dynamics

We present a machine-learning scheme based on the relativistic dynamics of a quantum system, namely a quantum detector inside a cavity resonator. An equivalent analog model can be realized for example in a circuit QED platform subject to properly modulated driving fields. We consider a reservoir-computing scheme where the input data are embedded in the modulation of the system (equivalent to the acceleration of the relativistic object) and the output data are obtained by linear combinations of measured observables. As an illustrative example, we have simulated such a relativistic quantum machine for a challenging classification task, showing a very large enhancement of the accuracy in the relativistic regime. Using kernel-machine theory, we show that in the relativistic regime the task-independent expressivity is dramatically magnified with respect to the Newtonian regime.

quant-ph↗

arXiv2022

Noisy Quantum Kernel Machines

In the noisy intermediate-scale quantum era, an important goal is the conception of implementable algorithms that exploit the rich dynamics of quantum systems and the high dimensionality of the underlying Hilbert spaces to perform tasks while prescinding from noise-proof physical systems. An emerging class of quantum learning machines is that based on the paradigm of quantum kernels. Here, we study how dissipation and decoherence affect their performance. We address this issue by investigating the expressivity and the generalization capacity of these models within the framework of kernel theory. We introduce and study the effective kernel rank, a figure of merit that quantifies the number of independent features a noisy quantum kernel is able to extract from input data. Moreover, we derive an upper bound on the generalization error of the model that involves the average purity of the encoded states. Thereby we show that decoherence and dissipation can be seen as an implicit regularization for the quantum kernel machines. As an illustrative example, we report exact finite-size simulations of machines based on chains of driven-dissipative quantum spins to perform a classification task, where the input data are encoded into the driving fields and the quantum physical system is fixed. We determine how the performance of noisy kernel machines scales with the number of nodes (chain sites) as a function of decoherence and examine the effect of imperfect measurements.

quant-ph↗

arXiv2022

Down-conversion of a single photon as a probe of many-body localization

Decay of a particle into more particles is a ubiquitous phenomenon to interacting quantum systems, taking place in colliders, nuclear reactors, or solids. In a non-linear medium, even a single photon would decay by down-converting (splitting) into lower frequency photons with the same total energy, at a rate given by Fermi's Golden Rule. However, the energy conservation condition cannot be matched precisely if the medium is finite and only supports quantized modes. In this case, the photon's fate becomes the long-standing question of many-body localization (MBL), originally formulated as a gedanken experiment for the lifetime of a single Fermi-liquid quasiparticle confined to a quantum dot. Here we implement such an experiment using a superconducting multi-mode cavity, the non-linearity of which was tailored to strongly violate the photon number conservation. The resulting interaction attempts to convert a single photon excitation into a shower of low-energy photons, but fails due to the MBL mechanism, which manifests as a striking spectral fine structure of multi-particle resonances at the cavity's standing wave mode frequencies. Each resonance was identified as a many-body state of radiation composed of photons from a broad frequency range, and not obeying the Fermi's Golden Rule theory. Our result introduces a new platform to explore fundamentals of MBL without having to control many atoms or qubits.

quant-ph↗

arXiv2022

Photonic kernel machine learning for ultrafast spectral analysis

We introduce photonic kernel machines, a scheme for ultrafast spectral analysis of noisy radio-frequency signals from single-shot optical intensity measurements. The approach combines the versatility of machine learning and the speed of photonic hardware to reach unprecedented throughput rates. We theoretically describe some of the key underlying principles, and then numerically illustrate the reached performances on a photonic lattice-based implementation. We apply the technique both to picosecond pulsed radio-frequency signals, on energy-spectral-density estimation and a shape classification task, and to continuous signals, on a frequency tracking task. The presented optical computing scheme is resilient to noise while requiring minimal control on the photonic-lattice parameters, making it readily implementable in realistic state-of-the-art photonic platforms.

physics.optics↗

arXiv2021

Continuous-time dynamics and error scaling of noisy highly-entangling quantum circuits

We investigate the continuous-time dynamics of highly-entangling intermediate-scale quantum circuits in the presence of dissipation and decoherence. By compressing the Hilbert space to a time-dependent "corner" subspace that supports faithful representations of the density matrix, we simulate a noisy quantum Fourier transform processor with up to 21 qubits. Our method is efficient to compute with a controllable accuracy the time evolution of intermediate-scale open quantum systems with moderate entropy, while taking into account microscopic dissipative processes rather than relying on digital error models. The circuit size reached in our simulations allows to extract the scaling behaviour of error propagation with the dissipation rates and the number of qubits. Moreover, we show that depending on the dissipative mechanisms at play, the choice of input state has a strong impact on the performance of the quantum algorithm.

quant-ph↗

arXiv2021

Effective bath model for arrays of coupled non-Hermitian nanoresonators

Nanophotonics systems have recently been studied under the perspective of non-Hermitian physics. Given their potential for wavefront control, nonlinear optics and quantum optics, it is crucial to develop predictive tools to assist their design. We present here a simple model relying on the coupling to an effective bath consisting of a continuum of modes to describe systems of coupled resonators, and test it on dielectric nanocylinder chains accessible to experiments. The effective coupling constants, which depend non-trivially on the distance between resonators, are extracted from numerical simulations in the case of just two coupled elements. The model predicts successfully the dispersive and reactive nature of modes for configurations with multiple resonators, as validated by numerical solutions. It can be applied to larger systems, which are hardly solvable with finite-element approaches.

physics.optics↗

arXiv2021

Breakdown of the topological protection by cavity vacuum fields in the integer quantum Hall effect

The control of the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is one of the emerging frontiers of condensed matter physics. We show here that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators dramatically affects arguably one of the most paradigmatic quantum protectorates, namely the quantum Hall electron transport in high-mobility 2D electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling finally result into a finite resistivity induced by the vacuum fluctuations. The present experimental platform can be used for any 2D material and provides new ways to manipulate electron phases in matter thanks to vacuum-field engineering

cond-mat.mes-hall↗

arXiv2021

Cavity-mediated electron hopping in disordered quantum Hall systems

We investigate the emergence of long-range electron hopping mediated by cavity vacuum fields in disordered quantum Hall systems. We show that the counter-rotating (anti-resonant) light-matter interaction produces an effective hopping between disordered eigenstates within the last occupied Landau band. The process involves a number of intermediate states equal to the Landau degeneracy: each of these states consists of a virtual cavity photon and an electron excited in the next Landau band with the same spin. We study such a cavity-mediated hopping mechanism in the dual presence of a random disordered potential and a wall potential near the edges, accounting for both paramagnetic coupling and diamagnetic renormalization. We determine the cavity-mediated scattering rates, showing the impact on both bulk and edge states. The effect for edge states is shown to increase when their energy approaches the disordered bulk band, while for higher energy the edge states become asymptotically free. We determine the scaling properties while increasing the Landau band degeneracy. Consequences on the quantum Hall physics and future perspectives are discussed.

cond-mat.mes-hall↗

arXiv2020

Dissipation-induced antiferromagnetic-like frustration in coupled photonic resonators

We propose a photonic quantum simulator for anti-ferromagnetic spin systems based on reservoir engineering. We consider a scheme where quadratically driven dissipative Kerr cavities are indirectly coupled via lossy ancillary cavities. We show that the ancillary cavities can produce an effective dissipative and Hamiltonian anti-ferromagnetic-like coupling between the cavities. By solving the master equation for a triangular cavity configuration, we demonstrate that the non-equilibrium steady state of the system bears full analogy with the ground state of an antiferromagnetic Ising model, exhibiting key signatures of frustration. We show that when the effective photon hopping amplitude is zero, the engineered non-local dissipation alone is capable of inducing antiferromagnetic interaction and frustration. This simple scheme can be generalised to arbitrary lattice geometries, providing a fully controllable recipe for simulating antiferromagnetism and frustration on a controlled quantum optical platform.

quant-ph↗

arXiv2020

Entanglement dynamics in dissipative photonic Mott insulators

We theoretically investigate the entanglement dynamics in photonic Mott insulators in the presence of particle losses and dephasing. We explore two configurations where entanglement is generated following the injection or extraction of a photon in the central site of a chain of cavity resonators. We study the entanglement negativity of two-site reduced density matrices as a function of time and inter-site distance. Our findings show that in spite of particle losses the quantum entanglement propagation exhibits a ballistic character with propagation speeds related to the differerent quasiparticles that are involved in the dynamics, namely photonic doublons and holons respectively. Our analysis reveals that photon dissipation has a strikingly asymmetric behavior in the two configurations with a much more dramatic role on the holon entanglement propagation than for the doublon case.

quant-ph↗