Exceptional points (EPs) in non-Hermitian photonics offer singular sensitivity enhancements but have thus far been realized almost exclusively in spatially engineered platforms with fixed geometries and limited tunability. Here we extend EP physics into the temporal domain by introducing balanced gain-loss modulation in a photonic time crystal (PTC). A time-periodic refractive-index modulation $n(t)=n_0+\delta n\cos (\mathrm{\Omega }t)$ generates an effective non-Hermitian Floquet Hamiltonian that supports coalescence of quasieigenmodes in frequency space, constituting a genuine temporal exceptional point. Using a reduced two-mode model for the dominant frequency sidebands, we derive a non-Hermitian dimer Hamiltonian $H_{\mathcal{P}\mathcal{T}}(\mathrm{\Delta },\gamma ,\kappa )$ that is strictly $\mathcal{P}\mathcal{T}$ symmetric for $\mathrm{\Delta }=0$ and identify the exact EP condition. Numerical analysis reveals the associated Riemann-sheet topology, mode exchange and Berry-phase accumulation upon encirclement of the EP, and the characteristic $\sqrt{\epsilon }$ perturbation response indicative of enhanced sensing. We further construct a non-Hermitian transmission model that is exact within the reduced two-mode description, compute the Cramér-Rao bound (CRB) for temperature estimation under an explicit noise model, and show that EP-enhanced sensitivity persists when compared to a linewidth-matched Hermitian reference under identical resource constraints. Monte Carlo simulations confirm that the CRB is saturable using spectral measurements. These results establish temporal non-Hermiticity as an alternative paradigm for dynamically reconfigurable, broadband, and geometry-independent exceptional-point photonics.

Recently, cladding-free waveguide systems without nearest-neighbor crosstalk have opened a path toward the miniaturization of photonic integrated circuits. However, there is still a prominent chance of crosstalk between next-nearest-neighbor channels, which severely degrades signal integrity and limits device performance. To overcome this challenge, we propose a deep-learning-based design strategy that further suppresses next-nearest-neighbor crosstalk below −20 dB across the operational bandwidth, thereby maintaining good signal isolation across almost all channels in the system. This approach offers a scalable solution for high-density photonic circuits and phased arrays, facilitating potential applications including optical interconnects, quantum simulations, and radar antennas.

We show that purely lossy defects in one- and two-dimensional non-Hermitian photonic crystals can induce transmission matrix singularities not accessible with lossless defects. These singularities in turn can enable dramatic enhancement of overall system gain not accessible through conventional means. We further show that the underlying mechanism behind the loss-induced gain enhancement is due to the resonances being located specifically at topological branch-cut singularities in the reflection coefficient with nontrivial winding numbers. The resulting resonances can exhibit exceptionally high-quality factors in excess of approximately ${10}^4$. Our work highlights the counterintuitive role of loss in engineering singularities in the gain response in non-Hermitian systems and its connection to topological phenomena in photonic systems.

Photonic Ising machines represent an emerging computational paradigm for solving combinatorial optimization problems, characterized by ultralow power consumption and accelerated processing capabilities. Spatial photonic Ising machines (SPIMs), in particular, have attracted substantial research interest due to their native architectural scalability and parallel processing topology. However, existing spatial photonic Ising machines employing simulated annealing (SA) face challenges in attaining optimal or near-optimal solutions for large-scale problems due to the complexity of the energy landscape for the Ising models. This paper proposes a Dammann grating–enabled spatial-temporal photonic Ising machine for gradient-inspired parallel simulated annealing with tunable spin-cluster size. Using this novel architecture, we experimentally generated a near-optimal solution for the Max-Cut problem on a 1600-node weighted random graph. Compared with an SA-based SPIM, our codesigned photonic framework achieves a 20-fold acceleration in convergence speed and a 2.6-fold improvement in cut value after 400 iterations. Our proposed system effectively unleashes the scalability and parallel processing capabilities of spatial optical computing, enabling accelerated optimization and improved solution quality, thus demonstrating the potential for superior efficiency for large-scale combinatorial optimization problems.

We demonstrate a practical countermeasure against a well-known class of attacks on quantum key distribution (QKD) systems that exploit detection-efficiency mismatch, where the receiver’s detectors do not exhibit identical responses to incoming photons across all degrees of freedom. This class of quantum hacking strategies is broad and significantly includes the time-shift attack, which targets an arrival-time-dependent side-channel at the receiver. The four-state countermeasure, previously only proven to be secure in theory, is implemented here on a gigahertz-clocked prototype QKD system and evaluated for its security and performance. We show that its presence enables almost complete recovery of the system’s ideal secret key rate. Our results provide strong justification for adopting this countermeasure as a standard component in future scalable and practical QKD systems.

For the ever-growing field of quantum information processing, large-scale, efficient multiport interferometers serving as photonic processors are required. In this context, the suitability of quantum walks as the interferometric base for universal computation has been theoretically proven. In this work, we bridge the gap between theoretical proposals and state-of-the-art experimental capabilities by providing the recipe for the implementation of a universal photonic processor in discrete-time quantum walks. Specifically, we present the protocol for translating arbitrary linear transformations into the coin and step operator of a quantum walk and map these to the experimental parameters of the established time-multiplexed platform [A. Schreiber et al., Phys. Rev. Lett. 104, 050502 (2010)]. We show that our interface is highly scalable and resource efficient due to the hybrid encoding consisting of multiple degrees of freedom. Finally, we prove that our system is highly resilient against experimental imperfections and show that it compares favorably against existing architectures.

The negatively charged tin-vacancy center in diamond (${\mathrm{Sn}\mathrm{V}}^{-}$) is an excellent solid-state qubit with optically addressable transitions and a long electron spin-coherence time at elevated temperatures (approximately 1.7 K). However, implementing scalable quantum nodes with high-fidelity optical readout of the electron spin state requires efficient photon emission and collection from the system. In this manuscript, we report a quantum photonic interface for ${\mathrm{Sn}\mathrm{V}}^{-}$ centers based on one-dimensional photonic crystal cavities fabricated in diamond thin films. Furthermore, we provide a rigorous description of the spontaneous emission dynamics of our system, taking into account individual contributions from both the C and D transitions of the emitter. This allows for the determination of Purcell factors per transition and, by extension, the C/D branching ratio ${\mathrm{Sn}\mathrm{V}}^{-}$ zero-phonon line. We observe quality factors of up to approximately 6000 across this sample, and we measure up to a 12-fold lifetime reduction, which translates into a Purcell factor of $F_{\mathrm{C}}=26.2\pm 1.5$ for a targeted C transition. By considering the cavity-mode polarization alignment with the C- and D-transition dipole moments, we validate the C/D branching ratio to be ${\eta }_{\mathrm{BR}}=0.75\pm 0.01$, in line with previous theoretical and experimental findings.

We systematically study the linewidths of multilevel exciton-polariton modes as a function of the detuning in a planar hybrid microcavity (MC) with low quality factor (Q ∼ 300) operating in the linear optical response regime. Using optical reflectivity, we observe that, counterintuitively, the linewidths of the polariton modes undergo a pronounced spectral narrowing as detuning is reduced. By benchmarking the experimental results against three commonly used constant-loss theoretical descriptions, we find that this behavior is only partially reproduced, highlighting limitations of conventional strong-coupling models when applied to nonstandard MC architectures. Our results suggest that frequency-dependent self-energy effects or correlated dissipation mechanisms, typically neglected in simplified treatments, may play an important role. These observations indicate that strong light-matter coupling with narrow polariton lines can be achieved in hybrid low-Q MCs through appropriate loss engineering, providing an alternative design pathway complementary to conventional high-Q approaches.

We describe a general quantum receiver protocol that maps laser-light-modulated classical communications signals into quantum processors for decoding with quantum logic gates. The quantum gates enable joint quantum measurements over a code word to achieve the quantum limit of communication capacity. Our receiver design requires qubit resources that only logarithmically increase with the size of the code word and accommodates practically relevant coherent-state modulation containing multiple photons per pulse. Focusing on classical-quantum polar codes, we outline the necessary quality of quantum operations and code-word lengths to demonstrate a quantum-processing-enhanced communication rate surpassing that of any known classical optical receiver-decoder pair. Specifically, we show that a small quantum receiver of four qubits with operational errors of approximately $0.2\mathrm{\%}$ can already provide a 5% gain in the communication rate in the weak-signal limit. Additionally, we outline a possible hardware implementation of the receiver where efficient spin-photon interfaces such as cavity-coupled diamond color centers or atomic qubits are used to input the received photonic signal to a small-scale quantum processor for decoding. Our results outline a promising route for potential quantum advantage in classical communication with near-term, small-scale quantum computers.

The coherence properties of optical emitters in crystals are crucial for quantum technologies and optical frequency metrology. Cooling to subkelvin temperatures can markedly enhance coherence, making it useful to identify the parameters governing emitter and host crystal behavior in this regime. We investigate a Czochralski-grown europium-doped yttrium orthosilicate crystal, reporting measurements of its heat capacity and optical coherence. Heat capacity not only informs thermal noise limits in metrology schemes but can also reveal two-level systems (TLS) arising from crystal imperfections via a linear-in-temperature term. Below 1 K, where phonon contributions are suppressed, TLS can drive decoherence, leading to a linear broadening of the homogeneous linewidth. From our data, we place an upper bound on the TLS contribution. This, together with constant optical linewidths between 300 mK and 2 K measured via photon-echo lifetimes, is consistent with minimal TLS effects in our sample. A low level of TLS is particularly useful for the performance of optical quantum devices based on doped crystals, since their presence could otherwise limit further improvements in coherence at subkelvin temperatures.

Unlike other quantum hardware, photonic quantum architectures can produce millions of qubits from a single device. However, controlling photonic qubits remains challenging, even at small scales, due to their weak interactions, making nondeterministic gates in linear optics unavoidable. Nevertheless, a single photon can readily spread over multiple modes and create entanglement within the multiple modes deterministically. Rudolph’s concept of third quantization leverages this feature by evolving multiple single photons into multiple modes, distributing them uniformly and randomly to different parties, and creating multipartite entanglement without interactions between photons or nondeterministic gates. This method requires only classical communication and deterministic entanglement within multimode single-photon states and enables universal quantum computing. The multipartite entanglement generated within the third-quantization framework is nearly deterministic, where “deterministic” is achieved in the asymptotic limit of a large system size. In this work, we propose a near-term experiment using antimony donor in a silicon chip to realize third quantization. Utilizing the eight energy levels of antimony, one can generate two eight-mode single-photon states independently and distribute them to parties. This enables a random multipartite Bell-state experiment, achieving a Bell state with an upper-bound efficiency of 87.5% among 56 random pairs without nondeterministic entangling gates. This approach opens alternative pathways for silicon-based photonic quantum computing.

We present a practical design framework for high-fidelity quantum control in coupled Kerr-nonlinear oscillators, directly addressing the challenge of spectral crowding. We show that systematic spectral degeneracies, which hinder selective addressing, are a direct consequence of rational Kerr-nonlinearity ratios ($K_1/K_2$). Our solution is a universal architectural principle: engineer this ratio to be a complex rational value, approximating an incommensurate number to systematically eliminate parasitic resonances. Using a Magnus expansion, we derive a complete effective Hamiltonian, including all Stark-shift corrections, to accurately target transitions. We numerically validate this framework by demonstrating protocols for the deterministic synthesis of NOON states, and high-photon-number Fock states, achieving ideal fidelities exceeding $\mathcal{F}>99.7\mathrm{\%}$. The protocols are shown to be robust against environmental decay and thermal effects. This work provides an architectural blueprint for bosonic processors in circuit quantum electrodynamics and establishes foundational principles that could inform future designs of multimode quantum systems.

Reconfigurable photonics have rapidly become an invaluable tool for information processing. Light-based computing accelerators are promising for boosting neural-network learning and inference [X. Xiao et al., APL Photonics 6, 126107 (2021); X. Meng et al., Nat. Commun. 14, 3000 (2023); J. Cheng et al., Nat. Commun. 15, 6189 (2024)] and optical interconnects are foreseen as a solution to the information transfer bottleneck in high-performance computing [Y. Li et al., in 2021 58th ACM/IEEE Design Automation Conference (DAC) (IEEE, 2021), pp. 931–936; A. Rizzo et al., IEEE J. Sel. Top. Quantum Electron. 29, 1 (2022); A. Netherton et al., Photonics Res. 12, A69 (2024)]. In this study, we demonstrate the successful programming of a transformation implemented using a reconfigurable photonic circuit with a nonconventional architecture. The core of most photonic processors is an MZI-based architecture [M. Reck et al., Phys. Rev. Lett. 73, 58 (1994); W. R. Clements et al., Optica 3, 1460 (2016)] that establishes an analytical connection between controllable parameters and circuit transformation. However, several architectures [R. Tang et al., IEEE Photonics Technol. Lett. 29, 971 (2017); M. Y. Saygin et al., Phys. Rev. Lett. 124, 010501 (2020)] that are substantially more difficult to program have improved robustness to fabrication defects. We use two algorithms that rely on different initial datasets to reconstruct the circuit model of a complex interferometer, and then program the required unitary transformation. The first method is based on the global fitting of the experimental calibration data, while the second method is an ML-based approach introduced in Kuzmin et al. [Opt. Express 29, 38429 (2021)]. Both methods performed accurate circuit programming with an average fidelity greater than 99% and 97%, respectively. Our results provide a strong foundation for the introduction of nonconventional interferometric architectures for photonic information processing.

With the rapid advancement of generative artificial intelligence, efficiently training complex neural networks has emerged as a central challenge in deep learning. Ising machines, as specialized quantum-inspired hardware, show considerable promise for solving combinatorial optimization problems and training neural networks. However, current existing approaches are often limited to shallow architectures due to inefficient sampling strategies. In this study, we introduce a deep belief network (DBN) architecture that utilizes the sampling mechanism of a coherent Ising machine integrated with a spiking neural network (SNN-CIM). By stacking two restricted Boltzmann machines (RBMs), the model supports hierarchical feature learning while significantly enhancing training efficiency. The SNN-CIM mitigates amplitude heterogeneity through dissipative pulse injection and employs Gaussian noise to drive the system toward thermal equilibrium, enabling effective deep network training. Experimental results demonstrate that a single-layer RBM achieves $96.4\mathrm{\%}$ accuracy on the MNIST dataset, while the full DBN reaches $98.9\mathrm{\%}$ on the test set. Compared to Metropolis-Hastings-based MCMC sampling, the SNN-CIM exhibits broader sampling coverage and higher efficiency, achieving orders-of-magnitude performance improvements on large-scale problems. These findings underscore the potential of SNN-CIMs for scalable deep generative modeling and offer alternative pathways for enhancing performance in quantum-inspired computing and deep learning.

Photon acceleration (PA) driven by ultrarelativistic electron beams offers a promising approach to generating high-power, high-frequency coherent radiation sources. While current methods typically rely on external optical laser pulses injected into beam-driven plasma wakefields, they face significant challenges in synchronization and alignment between electron accelerators and laser systems. We propose to utilize transition radiation (TR) generated by the drive electron bunch traversing the vacuum-gas interface as the seed photons for PA. Using a 1 GeV electron bunch, we demonstrate acceleration of TR from 4.4 $\text{μ}\mathrm{m}$ to 184 nm in 1.6 mm of two-stage uniform plasma, achieving more than a 20-fold frequency boost. Moreover, such techniques as the tailored density profile can further boost the wavelength to 137 nm. By increasing beam and plasma densities, this scheme can be scaled to produce coherent extreme ultraviolet radiation. This work addresses the synchronization and alignment issues occurring in previous approaches, providing a practical path toward beam-driven photon acceleration.