Papers

Quantum key distribution (QKD) is a cryptographic solution that leverages the properties of quantum mechanics to be resistant and secure even against an attacker with unlimited computational power. Satellite-based links are important in QKD because they can reach distances that the best fiber systems cannot. However, links between satellites in low Earth orbit (LEO) and ground stations have a duration of only a few minutes, resulting in the generation of a small amount of secure keys. In this context, we investigate the optimization of the information reconciliation step of the QKD post-processing in order to generate as much secure key as possible. As a first step, we build an accurate model of the downlink signal and quantum bit error rate (QBER) during a complete satellite pass, which are time-varying due to three effects: (i) the varying link geometry over time, (ii) the scintillation effect, and (iii) the different signal intensities adopted in the Decoy-State protocol. Leveraging the a-priori information on the instantaneous QBER, we improve the efficiency of information reconciliation (IR) (i.e., the error correction phase) in the Decoy-State BB84 protocol, resulting in a secure key that is almost 3% longer for realistic scenarios.

This paper considers a multiple-input multiple-output (MIMO) wireless system wherein two legitimate users attempt to exchange secret keys over free-space optical (FSO) channels. Novel frameworks for the use of the one- and two-way discrete-variable quantum key distribution (DV-QKD) protocols, employing weak coherent pulses and decoy states, are presented. Focusing on the case where a photon-number-splitting attack is adopted by the eavesdropper and the legitimate multi-antenna receiver using threshold detection for the key extraction, novel expressions for the secret key rate and quantum bit error rate for both one- and two-way protocols are derived. The performance gain with larger MIMO configurations and the tradeoff between the performances with the one- and the two-way protocols with respect to the transmission distance of the legitimate FSO link are numerically assessed.

We present a novel scheme for coherent manipulation of populations and robust creation of arbitrary coherent superposition of metastable states of a quantum system with lambda configuration of operating energy levels using laser pulses with a quadratically chirped carrier frequency. The case of a single broadband laser pulse is considered, when the frequency spectrum of the pulse exceeds the frequency distance between the metastable energy levels of the system. The results of the dressed state analysis and numerical simulation are presented, demonstrating reliable and robust creation of an arbitrary coherent superposition of metastable states by changing the parameters of laser pulses.

This study presents an improved quantum teleportation protocol designed to enhance fidelity in noisy environments by combining weak measurements (WMs) with flip and reversal operations. In our scheme, Alice prepares a four-qubit entangled state and shares one of the entangled qubits with Bob, which serves as the quantum channel for teleporting an arbitrary single-qubit state. Since the communication channel is subject to noise, Alice performs a weak measurement on the shared qubit before transmission to reduce the impact of decoherence. Building upon existing WM-flipreversal frameworks, we propose a modified weak measurement and reversal (WMR) protocol tailored for different noises in a four-qubit entangled system. The approach applies WM and flip operations prior to transmission to enhance resilience against noise, followed by corresponding reversal operations after transmission to recover the original quantum state. We systematically compare the performance of our proposed WMR protocol with the previously proposed WM-flip-reversal method under three common noise models: amplitude damping channel (ADC), phase flip channel (PFC), and bit flip channel (BFC). Our analysis reveals that the modified WMR scheme achieves significantly higher teleportation fidelity and improved robustness, particularly in bit flip noise environments. These findings highlight the potential of optimized weak measurement strategies for developing more reliable and noise-tolerant quantum communication protocols

In quantum illumination, the probe photon is entangled with an ancilla photon, and both are jointly measured at the end. The entanglement between the probe and ancilla photons enhances the detection performance per unit average photon number in the probe mode, particularly in low-reflectivity and high-noise scenarios. However, photon loss severely limits the practical advantage of such protocols. To address this, we employ a channel superposition framework, which encompasses two kinds of channel superposition protocols: indefinite causal order (ICO) and path superposition with disjoint environment (PS-DE). Our analytical and numerical analysis based on the quantum Chernoff bound shows that both ICO and PS-DE can, in principle, achieve an advantage. The advantage persists as long as non-zero interference remains, reverting to the performance of standard quantum illumination once the interference is completely suppressed. Crucially, the ICO protocol is significantly more robust, maintaining a tighter upper bound on the error probability than standard quantum illumination and the PS-DE approach. This performance hierarchy is rooted in their fundamental structures: ICO exploits a shared environment to generate stronger quantum interference, while PS-DE, relying on disjoint environments, offers a more experimentally tractable albeit less potent alternative.

Quasiparticle hybridization remains a major challenge to realizing and controlling exotic states of matter in existing quantum simulation platforms. We report the absence of hybridization for compact localized states (CLS) emerging in the chiral spin liquid described by the Yao-Kivelson model. The CLS form due to destructive quantum interference at fine-tuned coupling constants and populate perfectly flat quasiparticle bands on an effective kagome lattice. Using a formalism for general Majorana-hopping Hamiltonians, we derive exact expressions for CLS for various flux configurations and both for the topological and trivial phases of the model. In addition to finite-energy matter fermions with characteristic spin-spin correlations, we construct compact localized Majorana zero modes attached to $\pi$-flux excitations, which enable non-Abelian braiding of Ising anyons with minimal separation. Our results inform the quantum simulation of topologically ordered states of matter and open avenues for exploring flat-band physics in quantum spin liquids.

The polarization entanglement of photons emitted by semiconductor quantum dots is unavoidably limited by the spin fluctuations of the host lattice nuclei. To overcome this limitation, we develop a theory of entangled photon pair generation by a symmetric colloidal quantum dot mediated by a triplet exciton. We derive general analytical expressions for the concurrence as a function of the hyperfine interaction strength and show that it is intrinsically higher than that in conventional doublet-exciton systems such as self-assembled quantum dots. The concurrence sensitively depends on the shape anisotropy and the strain applied to a nanocrystal. In particular, we uncover a possibility of completely suppressing the detrimental effect of the hyperfine interaction due to the interplay between nanocrystal anisotropy and electron-hole exchange interaction. We argue that this represents the ultimate limit for the generation of entangled photon pairs by semiconductor quantum dots.

Uncertainty relations represent a foundational principle in quantum mechanics, imposing inherent limits on the precision with which mechanically conjugate variables such as position and momentum can be simultaneously determined. This work establishes analogous relations for thermodynamically conjugate variables – specifically, a classical intensive parameter $\theta$ and its corresponding extensive quantum operator $\hat{O}$ – in equilibrium states. We develop a framework to derive a rigorous thermodynamic uncertainty relation for such pairs, where the uncertainty of the classical parameter $\theta$ is quantified by its quantum Fisher information $\mathcal{F}_\theta$. The framework is based on an exact integral representation that relates $\mathcal{F}_{\theta}$ to the autocorrelation function of operator $\hat{O}$. From this representation, we derive a tight upper bound for the quantum Fisher information, which yields a thermodynamic uncertainty relation: $\Delta\theta\,\overline{\Delta O} \ge k_\text{B}T$ with $\overline{\Delta O}\equiv\partial_\theta\langle\hat{O}\rangle\,\Delta\theta$ and $T$ is the system temperature. The result establishes a fundamental precision limit for quantum sensing and metrology in thermal systems, directly connecting it to the thermodynamic properties of linear response and fluctuations.

Superconducting circuits are a leading platform for scalable quantum computing, where parametric modulation is a widely used technique for implementing high-fidelity multi-qubit operations. A critical challenge, however, is that this modulation can induce a dense landscape of parasitic couplings, leading to detrimental frequency collisions that constrain processor performance. In this work, we develop a comprehensive numerical framework, grounded in Floquet theory, to systematically analyze and mitigate these collisions. Our approach integrates this numerical analysis with newly derived analytical models for both qubit-modulated and coupler-modulated schemes, allowing us to characterize the complete map of parasitic sideband interactions and their distinct error budgets. This analysis forms the basis of a constraint-based optimization methodology designed to identify parameter configurations that satisfy the derived physical constraints, thereby avoiding detrimental parasitic interactions. We illustrate the utility of this framework with applications to analog quantum simulation and gate design. Our work provides a predictive tool for co-engineering device parameters and control protocols, enabling the systematic suppression of crosstalk and paving the way for large-scale, high-performance quantum processors.

Although multiple-input multiple-output (MIMO) terahertz (THz) continuous-variable quantum key distribution (CVQKD) is theoretically secure, practical vulnerabilities may arise due to detector imperfections. This paper explores a CV measurement-device-independent (MDI) QKD system operating at THz frequencies within a MIMO framework. In this system, measurement is delegated to an untrusted third party, Charlie, rather than the receiver, eliminating all detector attacks and significantly enhancing the system's practical security. Using transmit-receive beamforming techniques, the system transforms MIMO channels into multiple parallel lossy quantum channels, enabling robust key distribution between Alice and Bob. This study examines entanglement-based and prepare-and-measure protocols, deriving secret key rates for both asymptotic and finite code scenarios. Simulations reveal the critical role of multiple antenna configurations and efficient homodyne detection in mitigating free-space path loss and maximizing key rates. Results indicate that system performance is optimized at lower THz frequencies for long-range transmissions and higher frequencies for short-range applications. The proposed protocol offers a scalable solution for secure quantum communications in next-generation wireless networks, demonstrating potential for deployment in both indoor and outdoor environments.

Magnetic skyrmions are topologically protected spin textures that are promising candidates for low-power spintronic memory and logic devices. Realizing skyrmion-based devices requires an understanding of how structural disorder affects their stability and transport properties. This study uses Ne$^{+}$ ion irradiation at fluences from $10^{11}$ to $10^{14}$ ions-cm$^{-2}$ to systematically vary defect densities in 80 nm epitaxial FeGe films and quantify the resulting modifications to magnetic phase boundaries and electronic scattering. Temperature- and field-dependent Hall measurements reveal that increasing disorder progressively extends the topological Hall signal from a narrow window near 200K̃ in pristine films down to 4K̃ at the highest fluence, with peak amplitude more than doubling. Simultaneously, the anomalous Hall effect transitions from quadratic Berry curvature scaling to linear skew scattering behavior, with the skew coefficient increasing threefold. These results establish quantitative correlations between defect concentration, skyrmion phase space, and transport mechanisms in a chiral magnet. It demonstrates that ion-beam modification provides systematic control over both topological texture stability and electrical detectability.

Quantum steering is a crucial quantum resource that lies intermediate between entanglement and Bell nonlocality. Gaussian channels, meanwhile, play a foundational role in diverse quantum protocols, secure communication, and related fields. In this paper, we focus on several classes of Gaussian channels associated with quantum steering: Gaussian steering-annihilating channels, Gaussian steering-breaking channels, Gaussian unsteerable channels, and maximal Gaussian unsteerable channels. We give the concepts of these channels, derive the necessary and sufficient conditions for a Gaussian channel to belong to each class, and explore the intrinsic relationships among them. Additionally, since quantifying the steering capability of Gaussian channels in continuous-variable systems requires an understanding of the structure of free superchannels, we also provide a detailed characterization of Gaussian unsteerable superchannels and maximal Gaussian unsteerable superchannels.

In an accompanying paper [arxiv:2511.04488], we introduced an approach to interface trapped-ion quantum processors with ensemble-based quantum memories by matching a spontaneous parametric down conversion source to both the ions and the memories. This enables rapid entanglement generation between single trapped ions separated by distances of hundreds of kilometers. In this article, we extend the protocol and provide additional details of the analysis. Particularly, we compare a double-click and single-click approaches for the ion edge nodes. The double-click approach relaxes the phase stability requirement but is strongly affected by finite efficiencies. Choosing the optimal protocol thus depends on the access to the phase stabilization as well as the efficiencies of the interfaces of the ions and ensemble-based memories.

Reinforcement learning-based Quantum Architecture Search (QAS) offers a promising avenue for automating the design of variational quantum circuits, but existing methods typically decouple discrete structure search from continuous parameter optimization, resulting in inefficient or brittle solutions. We propose HyRLQAS (Hybrid-Action Reinforcement Learning for Quantum Architecture Search), a unified reinforcement learning framework that jointly learns gate placement and parameter initialization within a hybrid discrete-continuous action space, while enabling dynamic refinement of previously placed gates. Trained in a variational quantum eigensolver setting, the agent constructs circuits that directly optimize molecular ground-state energies. Across multiple molecular benchmarks, HyRLQAS demonstrates strong and competitive performance against state-of-the-art QAS methods, achieving lower energy errors with fewer gates. Notably, HyRLQAS reaches chemical-accuracy-level convergence down to 1e-8 energy error after classical optimization, and policy-guided initialization reduces the iteration count of downstream classical optimizers. These results demonstrate that hybrid-action reinforcement learning provides a principled and effective mechanism for coupling circuit topology design with optimization-aware parameterization.

In this article, we propose a distributed quantum algorithm for solving counting problem using Grover operator and a classical post-processing procedure. We apply the proposed algorithm to estimate inner products and Hamming distances. Simulations are conducted on the Qisikit platform, further demonstrating the effectiveness of our algorithm and its suitability for the NISQ era. Compared to existing counting algorithms, the proposed algorithm has advantages in terms of the number of qubits, circuit depth, and the number of quantum gates.

The solution of option-pricing problems may turn out to be computationally demanding due to non-linear and path-dependent payoffs, the high dimensionality arising from multiple underlying assets, and sophisticated models of price dynamics. In this context, quantum computing has been proposed as a means to address these challenges efficiently. Prevailing approaches either simulate the stochastic differential equations governing the forward dynamics of underlying asset prices or directly solve the backward pricing partial differential equation. Here, we present an end-to-end quantum algorithmic framework that solves the Kolmogorov forward (Fokker-Planck) partial differential equation for local-volatility models by mapping it to a Hamiltonian-simulation problem via the Schrödingerisation technique. The algorithm specifies how to prepare the initial quantum state, perform Hamiltonian simulation, and how to efficiently recover the option price via a swap test. In particular, the efficiency of the final solution recovery is an important advantage of solving the forward versus the backward partial differential equation. Thus, our end-to-end framework offers a potential route toward quantum advantage for challenging option-pricing tasks. In particular, we obtain a polynomial advantage in grid size for the discretization of a single dimension. Nevertheless, the true power of our methodology lies in pricing high-dimensional systems, such as baskets of options, because the quantum framework admits an exponential speedup with respect to dimension, overcoming the classical curse of dimensionality.

Neural network quantum states emerge as a promising tool for solving quantum many-body problems. However, its successes and limitations are still not well-understood in particular for Fermions with complex sign structures. Based on our recent work [J. Chem. Theory Comput. 21, 10252-10262 (2025)], we generalizes the restricted Boltzmann machine to a more general class of states for Fermions, formed by product of `neurons' and hence will be referred to as neuron product states (NPS). NPS builds correlation in a very different way, compared with the closely related correlator product states (CPS) [H. J. Changlani, et al. Phys. Rev. B, 80, 245116 (2009)], which use full-rank local correlators. In constrast, each correlator in NPS contains long-range correlations across all the sites, with its representational power constrained by the simple function form. We prove that products of such simple nonlocal correlators can approximate any wavefunction arbitrarily well under certain mild conditions on the form of activation functions. In addition, we also provide elementary proofs for the universal approximation capabilities of feedforward neural network (FNN) and neural network backflow (NNBF) in second quantization. Together, these results provide a deeper insight into the neural network representation of many-body wavefunctions in second quantization.

In a system where two identical two-level atoms interact with their common one-mode cavity field, it is shown that entanglement can become abruptly frozen in time, remaining at a constant value for a period of time until it begins to thaw from this value from the entanglement sharing perspective [Ding et al., Phys. Rev. A 103, 032418 (2021)]. We generalize this exotic behavior of entanglement sharing dynamics to more general systems with arbitrary N qubits, instead of restricting to the atom-cavity mode interaction system. We also demonstrate methods to control the entanglement freezing time and freezing value, and we discover a nontrivial dynamics where entanglement is frozen permanently. In addition, we show that this phenomenon is not a coincidence but a universal feature in a variety of systems with a geometric explanation of the mechanisms.

The introduction of an asymmetric term into the quantum Rabi model generally lifts energy-level degeneracies. However, when the asymmetry parameter takes specific multiples of the bosonic mode frequency, level degeneracies reappear$-$a phenomenon referred to as the hidden symmetry in the asymmetric quantum Rabi model. Identifying the origin of this hidden symmetry and its explicit operator form constitutes two central tasks in studying this system. Here, we investigate the origin of this hidden symmetry using the method of two successive diagonalizations, with a focus on physics in the regime where the ratio between the two-level splitting $\Delta$ and the mode frequency $\omega$ satisfies $\Delta/\omega \gg 1$. We find that the hidden symmetry stems from energy-level matching within the asymmetric double-well potential, a picture strongly supported by the wavefunctions of both the ground and excited states. Moreover, the emergence of an excited-state quantum phase transition is identified and qualitatively discussed, which arises from the breaking and restoration of this hidden symmetry across different coupling regimes. Our results provide deeper insight into the physics of the asymmetric quantum Rabi model, particularly in the previously less-explored strong-coupling regime where $\Delta/\omega \gg 1$.

Reservoir computing is a promising neuromorphic paradigm, and its quantum implementation using spin networks has shown some advantage when entanglement is present. Here, we consider a distributed scenario in which two distinct input time series are injected into separate qubits of a spin-network reservoir. We investigate how the overall entanglement, as well as its localization in the system, influence the performance of the reservoir. Focusing on bilinear memory tasks that require computing the product of the two inputs, we evaluate the short-term memory capacity and correlate it with logarithmic negativity as a measure of bipartite entanglement. We find that short-term memory capacity reaches its maximum at relatively small coupling strengths. In contrast, average entanglement peaks at larger couplings. Analyzing entanglement across all bipartitions, we find that the entanglement between the two input qubits is consistently the strongest and most relevant for task performance. In the small coupling strength regime where the short-term memory capacity is maximized, the reservoir exhibits an extended memory tail: performance remains high for a long time. Finally, a pronounced dip in performance at zero time delay, observed across frequencies, indicates that information requires a finite propagation time through the reservoir before it can be effectively recalled. In summary, our results show that moderate entanglement, particularly between the two input qubits, plays a key role in enhancing short-term memory performance.

Scalable quantum gates remain a central challenge for trapped-ion quantum computing. Ultrafast gates driven by spin-dependent kicks (SDKs) provide a promising approach. However, current protocols rely on mode-locked lasers, suffering from inflexible timing control and limited single-SDK fidelity. To overcome this, we propose a scheme using rapid adiabatic passage (RAP) pulses modulated from a continuous-wave laser. We demonstrate that this RAP-based approach suppresses the sensitivity of SDKs to fluctuations in optical intensity, thereby enabling the construction of robust entangling gates. Furthermore, the programmable nature of these modulated pulses allows for precise control over pulse sequences, further optimizing gate performance.

Noise suppression is of paramount importance for reliable quantum information processing and computation. We show that for any single-mode bosonic code (qumode) corrupted by thermal~noise at rate~$\eta$ and mean \mbox{excitation}~$\bar{n}$, a hybrid continuous-discrete-variable~(CV-DV) interferometer using only a single qubit ancilla~(DV) and two controlled~Fourier~(CF) gates sandwiching the noise channel suppresses its effects to $\mathcal{O}(\eta^2)$ without any active error correction or destructive measurements of the encoded state and with high success probabilities~$>0.5$ if~$\eta(1+\bar{n})<0.5$. This suppression scheme works by conditionally monitoring the photon-number parities after the interferometer. Bosonic codes with two logical states of the same photon-number parity (like-parity codes) are completely resilient to DV amplitude- and phase-damping ancilla noise. For such codes, the interferometer simplifies to the use of a qumode rotation gate and a single CF~gate. This presents a clear advantage of our CF-gate-based error suppression scheme over previously-proposed “bypass” protocols, where qubit information transferred to the DV mode is readily corrupted by damping~noise. Finally, we present a simple extension to direct communication of qumode states between two parties over a noisy channel using a preshared DV entangled state, by implementing a CF gate in the first laboratory and its inverse in the other. Such a communication protocol achieves a similar fidelity performance at the same success rate as the single-party case, but with greater resilience to the ancilla noise than DV~teleportation. Resource-efficient multi-qubit codes that depend on a few essential long-range interactions can benefit from it.

We introduce theoretically grounded Continuous Semi-Quantum Boltzmann Machines (CSQBMs) that supports continuous-action reinforcement learning. By combining exponential-family priors over visible units with quantum Boltzmann distributions over hidden units, CSQBMs yield a hybrid quantum-classical model that reduces qubit requirements while retaining strong expressiveness. Crucially, gradients with respect to continuous variables can be computed analytically, enabling direct integration into Actor-Critic algorithms. Building on this, we propose a continuous Q-learning framework that replaces global maximization by efficient sampling from the CSQBM distribution, thereby overcoming instability issues in continuous control.

Quantum circuits are the fundamental representation of quantum algorithms and constitute valuable intellectual property (IP). Multiple quantum circuit obfuscation (QCO) techniques have been proposed in prior research to protect quantum circuit IP against malicious compilers. However, there has not been a thorough security evaluation of these schemes. In this work, we investigate the resilience of split compilation against an oracle-guided attack. Split compilation is one of the most studied QCO techniques, where the circuit to be compiled is split into two disjoint partitions. Each split circuit is known to the compiler, but the interconnections between them are hidden. We propose an oracle-guided security evaluation framework in which candidate connections are systematically tested against input-output observations, with iteratively pruned inconsistent mappings. This hierarchical matching process exploits the reversibility of quantum gates and reduces the search space compared to brute-force enumeration. Experimental evaluation in the RevLib benchmark suite shows that only a small number of I/O pairs are sufficient to recover the correct inter-split connections and reconstruct the entire circuits. Our study marks the first thorough security evaluations in quantum IP protection and highlights the necessity of such evaluations in the development of new protection schemes.

We demonstrate a trapped-ion protocol in which a nearby, dedicated "monitor" qubit tracks magnetic-field drifts in real time without interrupting data-qubit operations. Using two $^{40}\mathrm{Ca}^+$ ions and the optical–metastable–ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum ($1/f^{2}$), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of ${\sim}\sqrt{2}$ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.

Understanding how quantum systems transition from integrable to fully chaotic behavior remains a central open problem in physics. The Sachdev–Ye–Kitaev (SYK) model provides a paradigmatic framework for studying many-body chaos and holography, yet it captures only the strongly correlated limit, leaving intermediate regimes unexplored. Here, we investigate the Yukawa–SYK (YSYK) model, where bosonic fields mediate random fermionic interactions, and demonstrate that it naturally bridges single-particle and many-body chaos. Using spectral and dynamical chaos markers, we perform a comprehensive finite-size characterization of the YSYK model. We show that the interaction strength acts as a tunable control parameter interpolating between the SYK$_2$ and SYK$_4$ limits, and introduce a framework enabling direct and quantitative comparison with these benchmark models. In the intermediate regimes, we uncover distinct dynamical regimes marked by partial ergodicity breaking, prethermalization plateaus, and incomplete scrambling. Finally, we propose a feasible optical-cavity implementation of the YSYK model using ultra-cold atoms. Our results establish the YSYK model as a unifying platform connecting single-particle and many-body chaos, paving the way for experimental observation of these phenomena.

We address the prevailing theoretical explanation of the recently observed breakdown of the integer quantum Hall effect in a two-dimensional electron gas embedded in a metallic split-ring resonator. Within the same single-particle description of quantized Hall conductance, we compare previously proposed vacuum-induced transport modifications against an alternative mechanism that explains this breakdown in terms of non-chiral edge channels arising solely from electrostatic boundary effects. This direct comparison shows that for experimentally relevant parameters, the electrostatic contribution exceeds that of any vacuum-induced conductance modifications by many orders of magnitude and yields characteristic transport signatures and energy scales that align well with experimental observations. This finding sheds new light on this puzzling phenomenon, supporting an electrostatic rather than a vacuum-related interpretation that can be directly tested in experiments.

In the search with wildcards problem [Ambainis, Montanaro, Quantum Inf.~Comput.'14], one's goal is to learn an unknown bit-string $x \in \{-1,1\}^n$. An algorithm may, at unit cost, test equality of any subset of the hidden string with a string of its choice. Ambainis and Montanaro showed a quantum algorithm of cost $O(\sqrt{n} \log n)$ and a near-matching lower bound of $\Omega(\sqrt{n})$. Belovs [Comput.~Comp.'15] subsequently showed a tight $O(\sqrt{n})$ upper bound. We consider a natural generalization of this problem, parametrized by a subset $\cal{Q} \subseteq 2^{[n]}$, where an algorithm may test whether $x_S = b$ for an arbitrary $S \in \cal{Q}$ and $b \in \{-1,1\}^S$ of its choice, at unit cost. We show near-tight bounds when $\cal{Q}$ is any of the following collections: bounded-size sets, contiguous blocks, prefixes, and only the full set. All of these results are derived using a framework that we develop. Using symmetries of the task at hand we show that the quantum query complexity of learning $x$ is characterized, up to a constant factor, by an optimization program, which is succinctly described as follows: `maximize over all odd functions $f : \{-1,1\}^n \to \mathbb{R}$ the ratio of the maximum value of $f$ to the maximum (over $T \in \cal{Q}$) standard deviation of $f$ on a subcube whose free variables are exactly $T$.' To the best of our knowledge, ours is the first work to use the primal version of the negative-weight adversary bound (which is a maximization program typically used to show lower bounds) to show new quantum query upper bounds without explicitly resorting to SDP duality.

The Whittaker-Shannon decomposition provides a temporally localized description of squeezed light, making it applicable in the CW limit and leading to a definition of squeezing strength based on the number of photon pairs at a time. We show examples of its usefulness by calculating quadrature variance in a homodyne detection scheme, coincidence detection probabilities in the continuous-wave limit, and analyzing the Hong-Ou-Mandel effect for strongly squeezed light. Quadrature uncertainty falls farther below the shot noise limit when squeezing is strong, but effects due to correlations between photon pairs are most significant with weak squeezing. Our analysis extends previous results to more general scenarios, and we leverage the Whittaker-Shannon formalism to interpret them based on the temporal properties of photon pairs.

Interactions between quantum systems enable quantum gates, the building blocks of quantum information processing. In photonics, direct photon-photon interactions are too weak to be practically useful, so effective interactions are engineered with linear optics and measurement. A central challenge is to realize such interactions non-locally, i.e., between photons that remain spatially separated. We present experimental proposals for several essential non-local multiphoton quantum gates that act on spatially separated photons, in both qubit and high-dimensional qudit systems. All solutions were discovered by the AI-driven discovery system called PyTheus. Rather than using pre-shared entanglement or Bell state measurements, our gates use as a resource quantum indistinguishability by path identity - a technique that exploits coherent superpositions of the photon pair origins. While analyzing these solutions, we uncovered a new mechanism that mimics much of the properties of quantum teleportation, without shared entanglement or Bell state measurements. Technically, our results establish path indistinguishability as a practical resource for distributed quantum information processing; conceptually, they demonstrate how automated discovery systems can contribute new ideas and techniques in physics.