Papers

We propose and analyze a novel quantum random-number generator based on a tripartite quantum system in which two subsystems act as detectors. Within a quantum non-demolition measurement scheme, one detector is used to certify the presence of genuine quantum effects in the system's evolution, while the second generates random numbers from a distribution that can be optimized to maximize their entropy. Using one two-level system and two three-level systems, we generate random numbers from a nearly uniform three-outcome distribution, yielding close-to-maximal entropy and therefore near-optimal randomness generation. A key feature of the protocol is that randomness generation and certification occur simultaneously. Moreover, certification does not rely on spacelike separation between detectors, removing a major constraint of device-independent approaches. This property enables practical implementation and facilitates the miniaturization of the device, making the protocol a promising candidate for scalable quantum technologies.

In this paper, we introduce a novel mixed-order Rényi divergence and investigate its fundamental properties. Using this divergence, we define a family of mixed-order order-two Rényi mutual information and Rényi conditional entropy. We derive exact reliability functions of quantum soft covering and privacy amplification under the sandwiched Rényi divergence with order $\alpha\in[2,\infty)$. The former is jointly characterized by the sandwiched and mixed-order order-two Rényi mutual information quantities, while the latter is characterized by the corresponding conditional entropies. These results provide operational interpretations of the proposed mixed-order Rényi divergence. To the best of our knowledge, this is the first exact characterization of the reliability function for quantum soft covering.

We calculate the probabilities $p_{\{n\}}$ of the measured bitstrings $\{n\}$ for the vacuum of Rydberg ladders with $N_q$ atoms. As $N_q$ increases, the $p_{\{n\}}$ decrease but become more dense in the low $p$ region raising the possibility that their smallness could be compensated by their large number. The importance of the low probability states can be estimated from the cumulative probability distribution $\Sigma(p_{\Lambda},N_q)$, which is the probability to observe any state having a probability $p\leq p_{\Lambda}$. For not too large values of $p_{\Lambda}$, it is possible to approximately collapse the $\Sigma(p_{\Lambda},N_q)$ for successive $N_q$ into a function resembling the Fermi function when plotted as a function of $-\ln(p_{\Lambda})$. We show that the number of shots necessary to reduce $\Sigma(p_{\Lambda},N_q)$ to some low enough value grows exponentially with $N_q$. We discuss the implications for calculating observables associated with the vacuum.

Predictive simulations of catalytic interfaces require correlated electronic-structure treatments that describe localized chemical transformations while retaining the influence of the extended metallic environment. We introduce QC-DFET, a quantum-computing density-functional embedding framework that maps surface-reaction active spaces to compact, environment-aware qubit Hamiltonians. A reaction-consistent active-space protocol preserves orbital continuity along reaction coordinates, while quantum-selected configuration interaction based on measurements from the Zuchongzhi superconducting quantum processor and strongly contracted perturbation theory capture static and dynamic correlation. On Cu(111), QC-DFET treats active spaces up to 28 qubits and is validated through a hierarchy of experimentally constrained surface-chemistry challenges. H2 dissociation/desorption tests balanced bond breaking and recombination barriers, CO adsorption tests site selectivity and metal-adsorbate bonding, and formate hydrogenation tests competing hydrogenation branches with different kinetic and thermodynamic signatures. Across these cases, QC-DFET reproduces bidirectional H2 barriers, recovers the observed top-site preference and adsorption strength of CO, and reconciles the experimentally benchmarked H2COO* reverse barrier with the lower forward barrier to HCOOH*. These results establish embedded quantum computing as a practical route to correlated surface-reaction energetics.

Conditioning on both past and future assigns intermediate-time properties a causal observer does not; these time-symmetric assignments obey exact symmetry theorems and are measurable from trajectories. We use two-state generator extended dynamic mode decomposition (gEDMD): because weak values obey $dA_w/dt=i\langle[H,A]\rangle_w$ exactly, generator extraction, with an exact-derivative baseline, applies unchanged to them. First, a reflection involution on the pre-/post-selected ensemble splits every window-fitted friction uniquely as $\gamma_{fwd}=\gamma_A+\gamma_S$: $\gamma_A$, antisymmetric about the midpoint, carries the modes' boundary-condition physics; $\gamma_S$, symmetric, comes from the differencing scheme; both follow from the same data as $(\gamma_{fwd}\pm\gamma_{bwd})/2$. At a fixed inference resolution the arrow of time has two layers: the coherent-mode arrow reverses at the midpoint, the fluctuation-level one does not, $\gamma_S$ dominating $\gamma_A$ at every size and class. The difference is one of degree: $\gamma_S$ is 34 times larger there than at the mode layer, and with the exact derivative both layers reverse: immunity belongs to the inference, not the ensemble. Second, in a lattice interferometer conditioned only at its ports, the quantum Cheshire-cat structure emerges unimposed: particle and polarization obey separate continuity equations, and a local field in the polarization-carrying arm rotates that phase alone, at exactly twice the field strength, entering the generator as a rigid imaginary shift, while the particle's weak density stays invariant to machine precision. We verify the sample-level identity and the two layers from $2^8$ to $2^{20}$ dimensions: $|\gamma_A|/\gamma_S=0.09$ to $0.27$ across five classes; self-averaging makes it insensitive to class among those sharing a boundary modulation, removing the $2^{-N/2}$ overlap obstruction for $N$ qubits.

Arrays of circular Rydberg atoms provide a promising platform for quantum simulation and computation; however, their preparation in the presence of interatomic interactions remains a major challenge. While optimal control methods have enabled the design of fast and accurate radio-frequency pulses for the circularization of a single atom and of an atom pair, the extension to more atoms is fundamentally limited by the exponential growth of the Hilbert space, which renders numerical simulations computationally infeasible. Here, we introduce an effective model that treats interactions within a mean-field approximation, thereby enabling the simulation of large atomic systems. Our model further enables the adaptation of pulses optimized for non-interacting atoms to interacting systems, based on the computation of a single time evolution. For two interacting $^{87}\mathrm{Rb}$ atoms, we demonstrate that the error of our method remains below $1 \, \%$ and that our adapted pulses recover the initial performance of optimal pulses in the regime of weak to moderate interaction strengths.

In this study, we investigate as-grown InAsP/InP quantum dots emitting in the third telecommunication window under detuned quasi-resonant excitation. A large excitation-emission detuning of 32 meV enables efficient suppression of scattered laser light while retaining several advantages of near-resonant excitation. The single-photon nature of the emission is confirmed by a Hanbury Brown and Twiss experiment, yielding a raw second-order autocorrelation value of $g_{\mathrm{raw}}^{(2)}(0)=0.076(6)$. Hong-Ou-Mandel measurement is used to determine the degree of indistinguishability of single photons and reveal as measured visibilities of $V=0.094(4)$ and $V=0.106(5)$ for excitation pulse separations of 13.1 ns and 5.3 ns, respectively. These results demonstrate the potential of as-grown InAsP/InP quantum dots grown via molecular beam epitaxy under not experimentally demanding detuned excitation for generating indistinguishable telecom single photons. Further improvements are to be achieved through Purcell enhancement in optical cavities.

We propose a sparse computational-basis encoding of voxelized molecular geometries that converts molecular reconstruction from full-state tomography into support recovery by computational-basis sampling. To realize the encoding scheme, the molecular space is discretized into a 3D grid, and each atom's position and chemical species is mapped to a single computational basis state. This discretization introduces spatial quantization at the voxel-resolution scale. The molecule is then encoded as an equal superposition over this sparse set of occupied states, where we assume that a suitable state preparation method exists. In contrast to full state tomography, which requires on the order of $\mathcal{O}(3^n \times 10^{2\text{--}3})$ measurement shots, where $n$ is the number of qubits, our proposed encoding scheme reduces to a coupon-collector sampling problem in the computational basis. Complete recovery of an $A$-atom molecule requires $\mathcal{O}(A\log A)$ shots on noise-free hardware. On noisy hardware, the required number of shots increases. We demonstrate the method on the 156-qubit IBM Kingston device using 8-qubit circuits to reconstruct the discretized geometry of a 10-atom ethylamine molecule with high mean reconstruction recall using only $\mathcal{O}(10^2)$ shots despite substantial hardware noise. These results demonstrate that our proposed encoding scheme is a practical, readout-efficient representation for molecular geometries on near-term devices.

The quantum Fisher information (QFI) of the vacuum of three quantum field theories is evaluated with respect to the mass parameter of each theory. All field theories are considered on a $(d+1)$-dimensional Euclidean spacetime. We consider the Klein-Gordon field and the free Dirac field. For the free KG case, we find a $m^{d-2}$ dependence of the QFI, and thus no dependence for $d=2$, in agreement with the holographic duality characterizing the theory. The vacuum QFI with respect to the free Dirac field mass is shown to be UV-divergent for $d=2$ and $d=3$, mass-dependent for $d=1$, and zero for $d=0$.

The coexistence of quantum and classical signals in the same optical fiber is a critical challenge for the deployment of quantum networks. Indeed, selecting an optimal channel for quantum signal transmission is crucial to minimize noise arising from co-propagating classical signals. This work experimentally investigates spontaneous Raman scattering (SpRS), a major source of noise in signals transmitted along the same fiber. Unlike most previous studies relying on narrow-linewidth laboratory lasers or architectures based on spatial or temporal multiplexing of quantum and classical signals, we employ commercial SFP optical transceivers and standard single-core single-mode fiber for the transmission of quantum and classical signals in the same fiber, reflecting conditions typical of deployed urban fiber infrastructures. Building on these measurements, we derive a compact and predictive model that captures the Raman scattering profile, enabling accurate estimation of SpRS noise as a function of source power, wavelength, and fiber length. A key outcome of this work is that the proposed model is independent of the specific optical source used, demonstrating its generality and robustness. The model can therefore be used for the identification of optimal C-band channels for quantum signal allocation, namely those least affected by SpRS noise generated by co-propagating O-band classical traffic. These results pave the way for a parameter-robust description of Raman scattering applicable to diverse fiber-based systems.

Structural asymmetry combined with optical nonlinearity often leads to nonreciprocal light transport. We explore the mechanism by which 1-photon interference effects can revive reciprocity in such nonlinear models. To this end, we study correlated 2-photon scattering where an artificial atom is asymmetrically (a) side-coupled to an infinite waveguide at two spatially separated points, and (b) direct-coupled to two semi-infinite waveguides. The setup (a) gives robust reciprocal transport for the two photons. However, the setup (b) shows a transition from a nonreciprocal to a reciprocal regime by tuning the interference effect via an additional tunneling path for photons between the two waveguides.

We construct a pressure-consistent operator-and-readout interface for Born, adjoint, and Gauss–Newton actions in constant-density acoustic full-waveform inversion (FWI) using Schrödingerised propagation. The energy variables $\pi=c^{-1}\partial_t u$ and $q=\nabla u$ yield an auxiliary-space Hamiltonian, while physical pressure $p=c\pi$ depends explicitly on wavespeed. Its derivative $D(c\pi)[c_0](\delta c)=c_0\delta\pi+\delta c\,\pi_0$ combines propagated wavefield sensitivity with a direct receiver-calibration term. Duhamel and receiver-row differentiation retain both contributions in the Born map, its adjoint, and the Gauss–Newton normal action. We prove a conditional consistency estimate with a periodic second-order finite-difference specialization and give a resource model for state preparation, normalization, quadrature, and selected-output measurement. A compiled nine-qubit instance realizes structured preparation, product-formula propagation, a derivative-LCU block, and calibrated pressure-overlap measurements. Bernoulli samples from ideal-circuit probabilities drive a four-parameter hybrid inversion. A two-qubit VQLS circuit represents the normalized update direction, while normal-system assembly, line search, and model refresh remain classical. Finite differences, tangent and reverse-adjoint recurrences, autodiff JVP/VJP evaluations, and explicit Jacobians verify the discrete Born, adjoint, and normal actions. Smooth periodic refinement confirms second-order convergence, whereas omitting receiver calibration leaves an order-one Born error and substantially changes the regularized Gauss–Newton direction. All ten predeclared finite-shot runs reduce the initial model error. These results specify the physical-pressure derivative and selected-output measurements needed to connect Schrödingerised propagation to a local FWI update.

Gradient-based quantum optimal control and the theory of operator evolution are rarely discussed together. In this Letter, we bridge this gap by showing that the gradient of the time-evolved propagator involves the Heisenberg evolution of local operators. We present a unifying framework by deriving from first principles the formal solution for the gradient under an arbitrary pulse parameterization, and show that the same construction extends to derivatives of any order. We obtain a series of nested, time-independent commutators weighted by time-dependent scalar coefficients. The commutators are thus computed once, and each optimization step only updates the scalars. The method is particularly suited for simulating optimal control tasks in quantum systems with local interactions, which is a common situation in large multi-qubit platforms. In this setting, light-cone arguments and sparse-Pauli truncation heuristics may be used to keep the number of relevant terms in the series small. We compare the computational cost required for the series with the Gradient Optimization of Analytic conTrols (GOAT) method, and, focusing on the problem of preparation of a GHZ state, demonstrate more than an order of magnitude speedup for a qubit ladder and a chain geometry.

The coherent one-way (COW) protocol is a quantum key distribution scheme that has attracted significant attention, leading to the development and commercialization of practical implementations. Despite this progress, the security of the COW protocol has remained a fundamental challenge since its introduction. Numerous studies have investigated its security, and several security proofs have been proposed over the years. More recently, a number of works have questioned the security of this protocol. In particular, one of the latest studies introduced an attack that severely limits the security of COW-QKD and reported a maximum secure distance of less than 20km. In this work, we introduce minimal alteration to the COW protocol that can enhance its security. Specifically, instead of monitoring the coherence between successive pulses, we propose to monitor quantum correlations through the violation of Bell inequalities. This approach enables the detection of a broader class of potential attacks. Our simulation results indicate that, by employing this method, the maximum secure distance of the protocol can be extended to approximately 259km.

Many security proofs for quantum key distribution (QKD) require Bob's measurement to have a tensor-product structure across protocol rounds, with some techniques requiring the stronger independent-and-identically-distributed (IID) condition. Time-bin encoded protocols often rely on interferometers whose detector outcomes depend on the interference between optical modes from neighbouring rounds, obstructing the direct application of such proofs. We show that classical post-processing of Bob's measurement data — specifically, discarding the outcomes of detectors sensitive to inter-round coherence — is sufficient to recover a product measurement positive operator-valued measure (POVM) (which is IID when the same single-round setup is used in every round). Applied to the Mach-Zehnder interferometer and the IID variant of the COW detection setup, this removes the need for the additional vacuum pulse introduced in prior analyses to establish tensor product structure of the measurement POVM, recovering better key rates without placing any restriction on Eve's attack.

We demonstrate an end-to-end hybrid quantum-classical optimisation framework based on Benders decomposition, capable of solving mixed-integer linear programming (MILP) problems. The framework builds on a previously presented hybrid quantum-classical end-to-end pipeline based on Multiple Cuts via Multiple Solutions (MCMS) Benders decomposition where the cut selection step was performed on quantum annealing hardware. We extend this with gate-based QAOA implementations for both tensor network emulators and superconducting quantum hardware. The Vehicle Routing Problem (VRP) is used as a representative case study and we run the pipeline end-to-end on 10 permutations of a standardised benchmarking instance (20 customers and 4 vehicles from QOptLib) with a classical solver performing the cut selection step. We find that for our instances, only a small fraction of the compute in classical MCMS Benders decomposition is spent on the cut selection step. For a full hybrid end-to-end assessment, we run the pipeline for a toy problem with MPS-JuliQAOA, a powerful tensor network emulator, to execute QAOA. Here, the majority of the time is spent on the cut selection step, deeming quantum advantage of this framework unlikely at problems of this size. This highlights the need for more large-scale benchmarking research when more powerful (QPU) QUBO solvers are available.

This thesis explores the role of modes, states, and symmetries in quantum optics, within the context of quantum information and quantum metrology. It proposes a unified framework to analyze how the modal structure of photonic fields, the statistical nature of states, and their symmetry properties determine the physical resources that can be exploited for quantum information processing and quantum parameter estimation. A first line of investigation develops a description of time-frequency degrees of freedom as continuous quantum variables, highlighting their richness for encoding and manipulating information. A second axis studies entanglement and collective variables, clarifying the link between physical resources and metrological gains, particularly in reaching ultimate precision limits. Interferometric scenarios of the Hong-Ou-Mandel type are then analyzed, and a general formalism centered on the notion of symmetry is developed. This framework enables the analysis of a broad range of situations and leads to several generalizations. Finally, the thesis examines the symmetries imposed by optical superselection rules and their consequences for the structure of quantum states and their operational performance, with the aim of providing a deeper understanding of the foundations of quantum optics.

Randomized compiling converts a coherent gate error into a stochastic one by dressing each gate with a randomly chosen Pauli operator, and the standard view is that the coherent information is lost once outcomes are averaged over these random choices. This is incorrect: an exact Fisher-information identity shows that the averaged data and the correlation between the outcome and the random choice, the twirl label, together conserve the full information, and only the label is normally discarded. Retaining it turns an unmodified randomized-compiling run into an unbiased estimator for the coherent error, at no added circuit cost, valid under realistic decoherence and at any circuit depth. The same construction extends to coupling between qubits through two further mechanisms: a neighboring qubit prepared in a known, randomized state probes static coupling, and randomizing the pulse count used to prepare that state isolates a second, physically distinct leakage effect found unexpectedly on real hardware during this work. All three results are derived exactly, checked against closed-form oracles, and tested on a 127-qubit IBM processor. The single-qubit case is confirmed cleanly: an injected phase is recovered to 0.006~rad while the same data without labels shows no signal. The two-qubit extensions are validated at the level of the recovery circuit, with clean positive controls on hardware, though the coupling strengths themselves remain below the resolution of the shot budgets used here.

We propose a rapid adiabatic passage (RAP) scheme based on level-crossing-free pulses for deterministic generation of multiqubit entangled states in Rydberg atom systems. Unlike conventional RAP protocols that rely on level crossings, our approach uses an antisymmetric Rabi frequency and an even-symmetric detuning, enabling robust population transfer without passing through any level crossing. By exploiting the Rydberg blockade effect, the protocol prepares entangled states directly from an initial product state. Specifically, two sequential RAP pulses separated by a pi_g pulse generate two-qubit Bell states, three-qubit W states, four-qubit GHZ states, and six-qubit honeycomb W states. Numerical simulations show that the fidelities exceed 0.9997 for the Bell and three-qubit W states, reach 0.997 for the four-qubit GHZ state, and surpass 0.9995 for the six-qubit honeycomb W state. The scheme demonstrates excellent robustness against pulse parameter fluctuations, with fidelities remaining above 0.99 under +/-5% parameter variations. This work provides a simple, efficient, and robust method for entangled-state preparation in neutral-atom quantum information processing.

Spins in condensed matter, especially well-isolated nuclear spins, offer attractive quantum degrees of freedom for computing, sensing, and networking because of their long coherence times. The possibility of electric-field control is an important feature for practical scalable quantum technologies, but, typically, nuclear spins couple only weakly to electric fields in conventional semiconductor hosts, limiting operation efficiency. Here we show that a choice of a highly polarizable oxide host can overcome this bottleneck. In Mn2+ doped ZnO, electric-field modulation of the spin Hamiltonian is amplified by hyperfine-coupled electron spins, and offers efficient electric-field manipulation of an I = 5/2 nuclear spin qudit, in a manner analogous to the hyperfine enhancement of conventional nuclear magnetic resonance. We demonstrate both resonant and non-resonant coherent manipulation using a single uniaxial electric field applied along the crystalline c-axis, the polarization axis of ZnO. This approach allows universal single-qudit gate operations with efficiencies comparable to or exceeding those of conventional magnetic-field driving. These results support the deployment of doped oxides as active host materials for electrically controllable spin qubits, highlighting the importance of materials design in developing scalable quantum technologies.

Quantum generative modelling casts sampling as a generative task: a parametrised quantum circuit is trained such that sampling reproduces a target probability distribution. Instantaneous Quantum Polynomial-time (IQP) circuits combine structural simplicity with complexity-theoretic evidence for quantum advantage. Yet their practical value depends not only on expressivity, but on how efficiently they consume genuinely quantum resources. We study this question through the lens of magic, or non-stabiliserness, as a resource for quantum generative modelling. We show that established fidelity- and geodesic-based notions of computational progress in a projective Hilbert space are ill-suited to generative models, since operational performance is determined by output probability distributions rather than quantum states themselves. We evaluate magic-consumption directly on the probability simplex, using changes in Jensen-Shannon divergence to quantify progress. Applying this framework to trained random {\gamma}-sparse IQP circuits shows signatures of efficient magic use, with the dominant contribution arising from two-qubit gates. As IQP circuits produce remarkably low intermediate magic relative to phase-randomised states with the same sampling distributions, this renders IQP-based quantum generative models as promising candidates for resource-efficient demonstrations of quantum advantage on early fault-tolerant architectures.

We investigate microscopic mechanisms underlying decoherence-induced transitions between topologically ordered states. As a first case study, we analyze the color code using topological entanglement negativity (TEN) and a disorder parameter associated with 1-form symmetry. We interpret these quantities as first- and zeroth-dimensional simplicial homological objects, respectively, and show that the transition can be understood in terms of decoherence percolation. To resolve its local structure, we introduce quasi-local TEN (QLTEN), which visualizes the spatial distribution of local topological properties and the growth of decohered regions. We further introduce explosive percolation (EP), corresponding here to biased decoherence that suppresses the formation of large decohered clusters. As a second case study, we consider the toric code on a triangular lattice under external-field-type decoherence. Numerical results show that QLTEN faithfully captures the emergence of Higgs regions and the survival of logical qubits. Although global TEN, QLTEN clusters, and string operators are strongly correlated in both models, the color code and toric code respond differently to EP patterns. This difference indicates that a universal microscopic description of decoherence-induced topological phase transitions remains challenging even for closely related systems.

Relational quantum causal processes formulate finite operational contexts as normal positive functionals on local completely positive maps. Response differences generate an influence algebra, and its central projections define jointly readable Boolean events. We develop this starting point through a sequence of exact and controlled models. Fresh-environment unitary collision circuits produce dephasing-exchange kinetics with an exact charge-center fixed algebra, a uniform finite-step limit at fixed response order, and graph-controlled metastable Markov dynamics. An absorbing-state model exhibits a sharp transition between non-Abelian quantum memory and Boolean records. A reversal-covariant defect dynamics generates a locally finite partial order on a restricted graph family without assuming a Lyapunov time. Conditional on a certified order, a positive additive record measure, compactness, and identifiability, we prove subsequential convergence to a Lorentzian metric-measure space, finite reconstruction bounds, and uniqueness of admissible smooth limits. Complementary finite regulators provide controlled tests of modular-to-boost response, null tomography, same-update variational identities, induced quadratic gravity, and compatible common-refinement limits. These results are exact or controlled within their stated models, but they do not yet constitute a single background-independent microscopic law that jointly generates adjacency, time, volume normalization, dimension, signature, nonlinear Einstein constraints, and quantum matter. We therefore present RQCP-QG as a theorem-indexed framework that separates established mechanisms, conditional compositions, and open assumptions.

We propose a novel approach to numerically derive approximate reduced dynamical models for Markovian quantum open systems without perturbative iterations, projecting the evolution to the subspace associated to their slowest degrees of freedom. The two algorithms we develop are based on Oja's continuous-time principal component flow: the first returns the optimal reduction to the slowest decaying operator-subspace, and is extended to time-dependent dynamics, while the second one is designed to reduce the dynamics on a subspace of the system's Hilbert space, and thus preserve conditional complete positivity. The methods represent a non-perturbative alternative to well-established Adiabatic Elimination (AE) methods, and the second can be used to find noise-protected subspace codes for quantum information processing. Both are tested on a paradigmatic central spin model.

Using a non-perturbative quantum kinetic framework, we develop a unified description of the far-from-equilibrium dynamics of three-dimensional Bose gases following cooling quenches across the Bose-Einstein condensation transition. By tracking the spatio-temporal evolution of the momentum distribution, we show that the equilibrium condensation threshold simultaneously acts as a dynamical critical point, separating distinct far-from-equilibrium universality classes governed by different non-equilibrium attractors. While quenches above the transition exhibit a single-timescale relaxation toward a thermal fixed point, quenches below the transition display a crossover from a transient weak-turbulence regime to a coarsening fixed point governed by the diffusive recombination of vortex lines. Quenches directly to the condensation threshold, finally, are controlled by a previously unidentified critical fixed point characterized by the superdiffusive spreading of critical fluctuations and a distinct set of dynamical exponents. Together, these dynamical scaling laws establish a far-from-equilibrium counterpart of the condensation phase transition, in which the equilibrium critical point also organizes the long-time non-equilibrium dynamics.

Quantum Noise Stream Cipher (QNSC) has emerged as a physical-layer encryption technique that exploits quantum noise and non-orthogonal coherent-state modulation to secure optical communication. However, the security of QNSC relies exceedingly on the secrecy and freshness of its seed key. Quantum Key Distribution (QKD), on the other hand, provides information-theoretically secure key exchange rooted in the laws of quantum mechanics. The convergence of these two paradigms, i.e., integrated QKD-QNSC architectures, offers a compelling solution to each of their limitations. In such integrated systems, QKD continuously supplies and refreshes the secret seed key that governs QNSC modulation. Thus, governing a unified security framework that couples provably secure key establishment with high-speed quantum-enhanced physical-layer encryption. This work presents a comprehensive review of QNSC systems, examining their operating principles, security models under various attacks, and their integration with QKD systems. We analyze the security interplay between the key generation and encryption layers and survey experimental demonstrations and architectural progress toward practical deployment. Furthermore, we identify the open challenges and future research directions that must be addressed to realize fully integrated, quantum-secured optical communication networks at a practical scale.

Erasures are more favorable for quantum sensing than unflagged errors such as Pauli errors. However, realistic sensing noise does not usually appear as erasures; it often acts within the same sensing Hilbert space as the signal, making it difficult to identify and mitigate. For such noise, we establish a noise-model-agnostic necessary and sufficient condition for erasure conversion, identifying the noise components that can be converted into erasures and removed without damaging the signal. For components satisfying the condition, conversion can be realized by a passive dimension-lifted scheme requiring neither detailed noise knowledge nor active control. Theoretically, the protocol remains effective over a broad range of noise strengths and approaches the corresponding precision limit. Experimentally, in single-photon phase sensing, we recover standard-quantum-limit precision in a Pauli-noise channel with erasure-convertible weight 0.5, using orbital angular momentum as the ancilla. These results provide a practical route to robust quantum sensing under realistic noise.

Generating high-speed, verifiable random numbers is a fundamental requirement for cryptography, large-scale stochastic simulations, and secure quantum communication. Here, we present a robust quantum random number generator that utilizes the spatial distribution of quantum fluctuations captured by an electron-multiplying charge-coupled device operated in high-speed kinetic mode. Through rigorous detector calibration and shot-noise analysis, we characterize the spatial quantum noise obtained from the spatial intensity fluctuations of the coherent states of light. Such quantum noise serves as a high-entropy source, enabling an instantaneous random bit generation rate of 5.92 Gbps without algorithmic randomness extraction in the present configuration. The sustained output rate is, however, limited to 7.5 Mbps by the bandwidth of the serial electronic readout. The generated sequences successfully pass the NIST SP 800-22 and Marsaglia Diehard statistical test suites, confirming the high quality and unpredictability of the entropy source.

Fast measurement, signal processing, and accurate estimation of Hamiltonian parameters are essential for feedback control in quantum-classical interface circuitry. However, existing frequentist and Bayesian inference methods typically require a large number of measurements to achieve the accuracy needed to mitigate qubit decoherence. Consequently, feedback control of semiconductor qubits has largely been limited to single-qubit frequency stabilization, whereas two-qubit parameter stabilization remains experimentally unexplored. Here, we demonstrate a real-time feedback framework based on sequential Monte Carlo estimation using one bit of data from a single-shot measurement. Using a four-qubit semiconductor quantum dot device, we rapidly estimate individual qubit frequencies, yielding an approximately twofold increase in coherence time compared with a conventional Bayesian strategy. Moreover, sequential two-qubit parameter estimation using two bits of data enables stabilization of qubit-qubit coupling, allowing both quasi-static frequency drift and exchange-interaction noise to be estimated and suppressed. By shortening the time required for precise parameter estimation, these results demonstrate the importance of the synergistic development of classical and quantum electronics for building robust and scalable quantum technologies in fluctuating environments.

The unitary synthesis problem (Aaronson-Kuperberg, CCC 2007) asks whether every $n$-qubit unitary $U$ is computable by efficient quantum circuits relative to some classical oracle $f = f_U$ depending on $U$. Recently, Lombardi-Ma-Wright (STOC 2024) proved that Haar-random unitaries cannot be efficiently synthesized by algorithms that make 1 query (or poly$(n)$ parallel queries) to an arbitrary classical oracle. In this work, we prove several results about the hardness (and easiness!) of variants of unitary synthesis. Our results include: (1) 1-query vs. 2-query unitary synthesis: we prove 1-query lower bounds for synthesizing random permutation unitaries $P\lvert x\rangle = \lvert \pi(x)\rangle$, as well as random alternating-basis phase unitaries $F_2 \cdot H^{\otimes n} \cdot F_1$. This gives 1-query lower bounds for "explicit" families of unitaries that have efficient (even 2-query) synthesis algorithms. (2) Upper bound for complex phase unitaries: we also consider complex phase unitaries $\lvert x\rangle\mapsto \alpha_x \lvert x\rangle$, which have a clean 2-query synthesis algorithm with no obvious 1-query algorithm. In this case, we prove an upper bound: there are 1-query algorithms (relative to binary phase oracles) that constant-approximate these unitaries in diamond distance. In order to prove our lower bounds, we introduce and analyze two new cryptographic games: the oracle state search game and the oracle Choi state game. Compared to prior work, our framework is mathematically simple, more flexible in what it can prove, and more accurately captures the hardness of synthesizing unitaries that are not "fully random". Finally, we also use the search game to prove a new hardness-of-approximation result for quantum programs (synthesizing unitaries relative to quantum advice) for phase unitaries, giving a sharper separation between 1-query unitary synthesis and quantum programs.