Photonic lattices facilitate band structure engineering, supporting both localized and extended modes through their geometric design. However, greater control over these modes can be achieved by taking advantage of the interference effect between external drives with precisely tuned phases and photonic modes within the lattice. In this work, we build on this principle to demonstrate optical switching, directed light propagation, and site-specific localization in a one-dimensional photonic lattice of coupled microresonators by resonantly driving the system with a coherent field of controlled phase. Importantly, our experimental results provide direct evidence that increased driving power acts as a tuning parameter enabling nonlinear localization at frequencies previously inaccessible in the linear regime. These findings open different avenues for controlling light propagation and localization in lattices with more elaborate band structures.

The directed movement of microorganisms toward light is called phototaxis—a fundamental process shown by many uni- and multicellular photosynthetic microorganisms for their survival and growth. Here, we determine the time-dependent response of a population of model phototactic single-celled algae Chlamydomonas reinhardtii to changing light conditions. When the condition changes from dark to light, we show that the response of a dark-adapted isotropic population depends on not just the light intensity but also the cell number density. On the other hand, when conditions change from light to dark, the time dependence of the recovery from an aligned state to an isotropic state is independent of cell density and light intensity. We find that such kinetics emerges from the coupling between flagellar dynamics, cell speed, and incident photon flux. We further show that existing minimal noninteracting models, which robustly describe the steady-state properties of populations of Chlamydomonas under given light conditions, capture this time-dependent phenomenon only for dilute suspensions.

We investigate the phase transitions from chaotic to nonchaotic dynamics in a quantum spin chain with a local non-Hermitian disorder, which can be realized with a Rydberg atom array setting. As the disorder strength increases, the emergence of nonchaotic dynamics is qualitatively captured through the suppressed growth of Krylov complexity and quantitatively identified through the reciprocity breaking of Krylov space. We further find that the localization in Krylov space generates another transition in the weak disorder regime, suggesting a weak ergodicity breaking. Our results closely align with conventional methods, such as the entanglement entropy and complex level spacing statistics, and pave the way to explore non-Hermitian phase transitions using Krylov complexity and associated metrics.

Number-conserved subspace encoding reduces resources needed for quantum simulations, but scalable complexity trade-off bounds for $M$ modes and $N$ particles with $\mathcal{O}(NlogM)$ qubits have remained unknown. We study qubit-gate-measurement trade-offs through the lens of classical/quantum error correction complexity and develop a framework of fermionic gate and measurement complexity based on classical encoder/decoder appearing in the error correction framework. We demonstrate optimal encoding with random classical parity check code and propose the Fermionic Expectation Decoder for scalable probability decoding in $\mathcal{O}\left(M^4\right)$ bases. The protocol is tested with variational quantum eigensolver on LiH in the STO-3G and 6-31G bases, and ${\text{H}}_2$ potential energy curve in the 6-311G* basis.

Wigner’s friend experiment and its modern extensions display the ambiguity of the quantum mechanical description regarding the assignment of quantum states. While the friend applies the state-update rule to the system upon observing an outcome of her measurement in a quantum system, Wigner describes the friend’s measurement as a unitary evolution, resulting in an entangled state for the composite system of the friend and the system. In this respect, Wigner is often referred to as a “superobserver,” who has the supreme technological ability to keep the friend’s laboratory coherent. As such, it is often argued that he has the “correct” description of the state. Here, we show that the situation is more symmetrical than is usually thought: There are different types of information that each of the observers has that the other fundamentally cannot have—they reside in different “bubbles” (in Cavalcanti’s terminology). While this can explain why the objectivity of the state assignment is only relative to the bubble, we consider more elaborated situations in the form of a game in which the players can switch between bubbles. We find that, in certain circumstances, observers may be entitled to adopt and verify the state assignment from another bubble if they condition their predictions on all information that is in principle available to them.

Causal models capture cause-effect relations both qualitatively—via the graphical causal structure—and quantitatively—via the model parameters. They offer a powerful framework for analyzing and constructing processes. Here, we introduce a tool—the flow of causal structures—to visualize and explore the dynamical aspect of classical-deterministic processes, arguably like those present in general relativity. The flow describes all possible ways in which the causal structure of a process can evolve. We also present an algorithm to construct its supergraph—the superflow—from the causal structure only, without invoking the model parameters. As an application, we show that if all leaves of a flow are trivial, then the corresponding process produces causal correlations only, i.e., correlations where future data cannot influence past events. This strengthens the result that processes, where every directed cycle in their causal structure is chordless, establish causal correlations only. We also discuss the main difficulties for the quantum generalization of the present algorithms.

The characterization of the phase space of laser-driven proton beams—particularly across a broad energy spectrum—in spite of the recent interest in laser-driven particle accelerators, remains a significant challenge due to the complex interplay between transverse dynamics and spectral properties. This work presents a detailed spectral study of the lateral emittance of laser-driven proton beams, providing energy-resolved, single-shot measurements of the beam emittance by means of a magnetically streaked pepper-pot method. Experimental results from a laser-driven proton source demonstrate the ability to resolve the beam’s transverse trace space at various proton energies, yielding precise emittance estimations and highlighting the variation of beam quality with energy. The study emphasizes the necessity of high spectral resolution for accurate emittance characterization. The findings offer valuable implications for optimizing beamline design and improving the performance of future tabletop laser accelerators.

We show that the Schrödinger equation describes the ensemble mean dynamics of solitons in a Galilean invariant field theory where we interpret solitons as particles. On a zero background, solitons move classically, following Newton's second law; however, on a nonzero amplitude chaotic background, their momentum and position fluctuate fulfilling an exact uncertainty relation, which gives rise to the emergence of quantum phenomena. The Schrödinger equation for the ensemble of solitons is obtained from this exact uncertainty relation, and the amplitude of the background fluctuations is what corresponds to the value of $\hslash$. We confirm our analytical results running simulations of solitons moving against a potential barrier and comparing the ensemble probabilities with the predictions of the time-dependent Schrödinger equation, providing a deterministic version of the quantum tunneling effect. We conclude with a discussion of how our theory does not present statistical independence between measurement and experiment outcomes.

Designing efficient fault-tolerance schemes is crucial for building useful quantum computers. Most standard schemes assume no knowledge of the underlying device noise and rely on general-purpose quantum error-correcting (QEC) codes capable of handling arbitrary errors. Biased-noise alternatives focus on only correcting a subset of some generic error basis (e.g., Pauli error basis) and lower resource needs by channeling the redundancy to dealing only with that subset. Yet, the most resource-efficient codes are expected to be those that directly target the specific noise process that afflicts the quantum device, rather than using a generic error-basis description. However, the question of whether such noise-adapted QEC protocols are amenable to fault-tolerant implementations remains largely unexplored. Here, we design a fault-tolerance scheme based on the Bacon-Shor codes that can protect against amplitude-damping noise in the device. We construct a universal set of logical gadgets tolerant to multiple damping errors and estimate the fault-tolerance threshold of our scheme. Our work thus establishes the possibility of achieving fault tolerance against amplitude-damping noise using noise-adapted quantum codes, while highlighting some of the unique challenges that arise in this context.

The existence of exceptional points (EPs) in the spectrum of non-Hermitian Hamiltonians leads to a nontrivial spectral topology that can be exploited to generate topological operations. However, their implementation has remained elusive due to the difficulty of generating a true adiabatic evolution. Here, we develop fast, robust control protocols that generate a desired topological operation. Our strategy relies on shortcuts to adiabaticity, but is not a trivial extension. The presence of EPs renders the strategy developed for Hermitian Hamiltonians impractical as it will lead to faulty control protocols. Moreover, due to the dynamics sensitivity to parameter uncertainties, not all shortcuts to adiabaticity can be used in a realistic setting. We illustrate our method in the context of a two-mode non-Hermitian Hamiltonian and discuss why in general celebrated shortcuts to adiabaticity like transitionless driving and superadiabatic transitionless are not appropriate control protocols for non-Hermitian systems.

We present a statistical study of the transmission and dwell-time matrices in disordered media composed of resonators, focusing on how frequency detuning influences their eigenvalue distributions. Our analysis reveals that the distribution of transmission eigenvalues undergoes a transition from a monomodal to a bimodal profile, and back to monomodal, as the frequency approaches the resonant frequency of the particles. Moreover, the distribution of dwell-time eigenvalues broadens significantly near resonance, with the longest lifetimes exceeding the median by several orders of magnitude. These results are explained by examining how frequency $\omega$ affects the transport mean free path of light $\ell \left(\omega \right)$ and the energy transport velocity $v_E\left(\omega \right)$, which in turn shape the observed distributions. We demonstrate the strong potential of wavefront shaping to synthesize wavefronts associated with eigenstates that enhance transmission and energy storage (or radiation trapping) in resonant disordered media. In the diffusive regime, where the system thickness $L$ exceeds the mean free path, both transmission and dwell time can be enhanced by a factor $\propto L/\ell \left(\omega \right)\gg 1$ when using wavefronts associated with the largest eigenvalues instead of plane waves. In the localized regime, the enhancements become $\propto N{e}^{2L/\xi }$ for transmission and $\propto N\xi /L$ for dwell time, where $\xi$ is the localization length and $N$ is the number of controlled scattering channels. Finally, we show that employing high-$Q$ resonators instead of low-$Q$ ones increases energy storage within the medium by a factor of $\propto Q/k\ell \left(\omega \right)$ in both the diffusive and localized regimes.

The generation of high Fock states of light pulses and their superpositions with provable quantum non-Gaussian features is still very challenging, although the power of conditional methods to herald the approximate state from the available Gaussian states is growing. The atom-light interaction in the high-Q cavity has been considered a viable alternative to the heralded Fock states of the light pulses from nonlinear optics limited to three-photon Fock states for the last decade. Here, by optimizing the realistic protocol combining it with available optical delay elements, we conclusively predict filtering of Fock states up to ten photons with a high success rate of $20\%$ using a hierarchy of quantum non-Gaussian criteria. Moreover, the filtering protocol enables the preparation of superposition of Fock states, and we analyze this emerging case up to two photons with provable quantum non-Gaussian coherence and high success rate of $50\%$. To demonstrate their quality for applications, we evaluate the robustness of such features, the bunching capability in a linear network, and the sensing capability to estimate the magnitude of unknown force, noise, and phase. These assessments outline the essential conditions and application criteria for realistic optical cavity QED interaction on light pulses to outperform photon detection methods.

Quantum computers promise to solve computational problems significantly faster than classical computers. These “speedups” are achieved by utilizing a resource known as magic resource. Measuring the amount of magic resource used by a device allows us to quantify its potential computational power. Without this property, quantum computers are no faster than classical computers. Whether magic resource can be accurately measured on large-scale quantum computers has remained an open problem. To address this question, we introduce Pauli instability as a measure of magic resource and experimentally measure it on the IBM Eagle quantum processor. We show that sufficiently small quantities of magic resource can be measured efficiently. We also prove that measuring large (i.e., extensive) quantities of magic resource with our measure is intractable. Our results lead us to surmise that magic resource may be difficult to measure when a quantum computer provides a speedup. We find that one of the most sought-after properties in quantum computation is only partially accessible to experiments. We support our conclusions with both theoretical and experimental evidence. Our work illustrates the capabilities and limitations encountered when studying the power of quantum computers.

We investigate non-Markovian transport dynamics and signatures of the Kondo effect in a single quantum dot (QD) model. The QD is coupled to a left lead non-Markovian bath and weakly coupled to a right lead, acting as a detector. We calculate the waiting time distribution (WTD) of electrons tunneling into the detector using a combination of the hierarchical equations of motion (HEOM) approach and the Born-Markov (BM) approximation. Oscillations emerge in the short-time WTD, becoming more pronounced with stronger left-lead coupling. Fourier analysis reveals a blueshift in the oscillation frequency as coupling increases, indicating enhanced system-bath hybridization. Crucially, comparison with a master equation confirms that these oscillations are a direct consequence of non-Markovian system-bath correlations. By introducing a toy model, we analyze the role of different parameters, such as coupling strength and bandwidth, in significantly influencing the oscillatory behavior in WTD. In addition, we examine the Kondo effect's influence on these oscillations by varying the coupling strength and the bandwidth of the non-Markovian bath. Decreasing the bandwidth and increasing the coupling strength enhance the WTD oscillations, while also enhancing the Kondo resonance in the quantum dot's density of states. Our results demonstrate that WTD oscillations offer a valuable tool for probing non-Markovian system-bath interactions and the emergence of Kondo correlations within QD systems.