Coherence is a fundamental property of light. Recent years have witnessed a growing interest in coherence structuring, yielding a new type of partially coherent source with spatially tailored nonuniform correlations. However, the routine way of producing entangled photons via parametric down-conversion is to use a highly coherent laser source to pump nonlinear crystals. Here, we present a new strategy of exploiting such nonuniformly partially coherent sources to excite parametric down-conversion and thus endow the generated entanglement with a tailored and unusual spatial symmetry. To reveal the spatially varying degree of entanglement, we demonstrate the observation of an unusual Einstein-Podolsky-Rosen (EPR) correlation by transferring the circular structure of the coherence of the pump beam onto that of down-converted photons. For this, we observe both anomalous ghost diffraction and normal ghost images to examine the EPR-type inequalities and showcase the transition from nonentanglement to entanglement. Our work suggests that coherence structuring can offer a new dimension to generate and control quantum entanglement.

We observe blockade of microwave photons in a Tavis-Cummings system comprising a superconducting cavity and up to $N=3$ transmon qubits. The effect is characterized with photon-number-resolving spectroscopy using an additional dispersively coupled transmon “witness” qubit to directly probe the cavity’s photon-number distribution. We first observe polariton formation with splitting proportional to $\sqrt{N}$, confirming the Tavis-Cummings coupling, and subsequently obtain sub-Poissonian cavity photon statistics when the cavity is driven at polariton frequencies.

The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building innovative superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit; however, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times compared to conventional transmons fabricated from aluminum-oxide-based Josephson junctions. Here, we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt-capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1-MHz precision over a range of several gigahertz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design.

Entangled photon pairs are of crucial importance in quantum networks. For the future demands of large-scale and secure quantum communication, integrated photon sources are highly effective solutions. Here, we report entanglement distribution over 30 km in a metropolitan area using an on-chip broadband silicon nanowire biphoton polarization-entangled source based on a silicon-on-insulator platform. This source generates a continuous spectrum spanning the entire C band (4.5 THz), achieving locally detected coincidence counts of about 154 kHz within a 100-GHz bandwidth, making it suitable for long-distance entanglement distribution among multiple users. By combining this source with quantum entanglement, enhanced by high-precision clock synchronization that achieves a standard deviation of 56.8 ps over 600 s, we observe a violation of the Clauser-Horne-Shimony-Holt inequality by 27.8 standard deviations. Our results showcase the potential of silicon photonic technology as a scalable and practical platform for quantum technologies.

Quantum technologies provide many applications for information processing tasks that are impossible to realize within classical physics. These capabilities include such fundamental resources as generating secure, i.e., private and unpredictable random values. Yet, the problem of quantifying the amount of generated randomness is still not fully solved. This work presents a comprehensive analysis of the design and performance optimization of a quantum random number generator (QRNG) based on Bell inequality violations. We investigate key protocol parameters, including the smoothing parameter (${ϵ}_{\mathrm{s}}$), test round probability ($\gamma$), and switching delays, and their effects on the generation rate and quality of randomness. We identify optimal ranges for $\gamma$ and $p_{\mathrm{\Omega }}$ (the protocol’s nonaborting probability) to balance the trade-off between randomness consumption and net randomness generation. We investigate the impact of the laser power on the overall generation rate. We explore the impact of switching delays on the system’s performance, providing strategies to mitigate these effects. The work provides practical guidelines for the efficient and secure design of QRNG systems and other cryptographic protocols.

The quasiparticle density observed in low-temperature superconducting circuits is several orders of magnitude larger than the value expected at thermal equilibrium. The tunneling of this excess of quasiparticles across Josephson junctions is recognized as one of the main loss and decoherence mechanisms in superconducting qubits. Here we present a unified impedance theory that accounts for quasiparticle energy loss in circuit regions both far from and near to (across) junctions. Our theory leverages the recent experimental demonstration that the excess quasiparticles are in quasiequilibrium [Connolly et al., Phys. Rev. Lett. 132, 217001 (2024)] and uses a generalized fluctuation-dissipation theorem to predict the amount of charge and flux noise generated by them. We compute the resulting energy relaxation time $T_1$ in transmon qubits with and without junction asymmetric gap engineering, and show that quasiparticles residing away from junctions can play a dominant role in the former case. They also may provide an upper limit for resonator quality factors if the density of amorphous two-level systems is reduced. In addition, we show that charge noise from quasiparticles leads to flux noise that is logarithmic in frequency, giving rise to a “nearly white” contribution that is comparable to the flux noise observed in flux qubits. This contrasts with amorphous two-level systems, whose associated flux noise is shown to be super-Ohmic. We discuss how this quasiparticle flux noise can limit $T_2^{\ast }$ coherence times in flux-tunable qubits. The final conclusion is that asymmetric gap engineering can greatly reduce noise and increase coherence times in superconducting qubits.

Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms, allowing for quantum processors and memories with reduced error rates and more modes. The complex surface chemistry of niobium, however, makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impact of etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions prior to and during cooldown—in particular, niobium hydride evolution—on single-photon coherence. We demonstrate cavities with quality factors $Q_{\mathrm{int}}\gtrsim 1.4\times {10}^9$ in the single-photon regime, a 15-fold improvement over aluminum cavities of the same geometry. We rigorously quantify the sensitivity of our fabrication process to various loss mechanisms and demonstrate a two- to fourfold reduction in the two-level system loss tangent and a three- to fivefold improvement in the residual resistance over traditional buffered chemical polishing etching techniques. Finally, we demonstrate transmon integration and coherent cavity control while maintaining a cavity coherence of 11.3 ms. The accessibility of our method, which can be easily replicated in academic laboratory settings, together with the demonstration of its performance, mark an advancement in three-dimensional cQED.

We present a superconducting cavity-coupled double quantum dot (DQD) photodiode that achieves a maximum photon-to-electron conversion efficiency of 25% in the microwave domain. With a higher-quality-factor cavity and improved device design to prevent photon leakages through unwanted pathways, our device measures microwave signals down to the 100-aW power level and achieves sensitivity to probe microwave signals with one photon at a time in the cavity. We analyze the photodiode operation using the Jaynes-Cummings input-output theory, identifying the key improvements of stronger cavity-DQD coupling needed to achieve near-unity photodetection efficiency. The results presented in this work represent a crucial advancement toward near-unity microwave photodetection efficiency with single-cavity-photon sensitivity.

Establishing quantum entanglement between pairs of distant quantum memories (QMs) is essential for numerous applications in quantum networks. In this paper, we propose some heralded schemes for entanglement generation between multiple pairs of QMs placed in long-distance separated nodes, where the qubits are encoded on QMs coupled to a one-dimensional waveguide. By distributing a pair of spatial-polarization hyperentangled photons to two distant nodes, where each photon scatters with QMs, we can generate entanglement between multiple pairs of distant QMs. In our schemes, the faulty scattering events can be filtered out and discarded via detecting the polarization states of scattered photon components within each module QM. Moreover, the responses from single-photon detectors placed at the end of each node enable us to obtain direct entangled states between QMs in a heralded manner. Eventually, we theoretically extend the proposed schemes by demonstrating specific regularities that arise when entangling more pairs of QMs. With recent advancements in hyperentanglement generation, our schemes become increasingly feasible for practical implementation.

We present a numerical framework that integrates a modified Langevin noise formalism into multimode Jaynes- and Tavis-Cummings models for first-principles, non-Markovian analysis of atom-field interactions in dissipative electromagnetic environments under weak coupling. This formalism enables the quantization of electric field operators in terms of boundary- and medium-assisted modes with real-valued eigenfrequencies, accounting for both radiative and absorptive losses. To efficiently handle the continuum nature of the boundary- and medium-assisted mode spectrum, we apply adaptive frequency refinement and mode-space coarse-graining based on finite-element simulations to obtain a tractable discrete set of field modes. These modes are then used to construct matrix representations of the Schrödinger equation, enabling direct computation of atomic observables and field quantities—such as single-photon amplitudes and higher-order correlation functions—beyond the Markovian approximation. Importantly, this framework does not rely on any phenomenologically designed memory kernel. Instead, the spectrum of boundary- and medium-assisted modes interacting with atoms inherently and rigorously captures memory effects arising from the open and dissipative electromagnetic environment. We present four numerical examples to validate the proposed framework and demonstrate its ability to model non-Markovian atomic dynamics in open and dissipative electromagnetic environments: (i) a two-level system near the surface of a perfect electric conductor, (ii) dissipative cavity quantum electrodynamics for a single two-level system, (iii) super- and subradiance in two-level system arrays, and (iv) entanglement sudden death between two two-level systems. This framework provides a versatile simulation tool for the design of quantum photonic devices, including nitrogen-vacancy center single-photon sources coupled to plasmonic structures, entangled photon-pair generation near lossy media, and decoherence modeling of solid-state quantum memories based on rare-earth ions and color centers. It is particularly well suited to dissipative quantum electrodynamics in photonic integrated circuits, where accurate modeling of scattering and diffraction is essential.

Measurement-device-independent quantum key distribution (MDI QKD), which eliminates all the attacks from the eavesdropper to the measurement party, has been one of the most promising technologies for the implementation of end-to-end quantum networks. In practice, the asymmetry of both sources and channels is generally inevitable. Therefore, we propose a theory to analyze the performance when any two MDI users in networks communicate using asymmetric sources in distinct single or multiple temporal modes. As a specific application, we model to obtain the key rate of MDI QKD with a weak-coherent pulse source and a spontaneous parametric down-conversion source, and compare the performance to the cases with symmetric (i.e., identical) sources. The result demonstrates that the actual performance does not degrade due to the asymmetry of sources. On the contrary, it maintains at a good level over the entire distance we study. This work provides a theoretical basis for analyzing and optimizing MDI-QKD networks with asymmetric sources, thus paving the way for the practical deployment of completely asymmetric MDI-QKD networks.

The accurate measurement of quantum two-level objects (qubits) is crucial for developing quantum computers. Over the last decade, the measure of choice for benchmarking readout routines for superconducting qubits has been assignment fidelity; however, this method only focuses on the preparation of computational basis states and therefore does not provide a complete characterization of the readout. Here, we expand the focus to the use of detector tomography to fully characterize multiqubit readout of superconducting transmon qubits. The impact of different readout parameters on the rate of information extraction is studied using quantum-state reconstruction infidelity as a proxy. The results are then compared with assignment fidelities, showing good agreement for separable two-qubit states. We therefore propose the rate of infidelity convergence as a validation tool for assignment fidelity and a more comprehensive benchmark for single- and multiqubit readout optimization. To make the best use of experimental resources, we investigate the most efficient distribution of a limited shot budget between detector tomography and state reconstruction within the context of single- and two-qubit experiments. To address the growing interest in three-qubit gates and test the scalability of the validation tool, we perform three-qubit quantum-state tomography that goes beyond conventional readout-error-mitigation methods and find a factor-of-30 reduction in quantum infidelity. Our results demonstrate that qubit readout correlations are not induced by a significantly reduced state distinguishability. Consequently, correlation coefficients can serve as a valuable tool in qubit readout optimization.

Electron spins in semiconductor devices are highly promising building blocks for quantum processors (QPs). Commercial semiconductor foundries can create QPs using the same processes used for conventional chips, once the QP design is suitably specified. There is a vast accessible design space; to identify the most promising options for fabrication, one requires predictive modeling of interacting electrons in real geometries and complex nonideal environments. In this work we explore a modeling method based on real-space grids, an ab initio approach without assumptions relating to device topology and therefore with wide applicability. Given an electrode geometry, we determine the exchange coupling between quantum dot qubits, and model the full evolution of a $\sqrt{\mathrm{SWAP}}$ gate to predict qubit loss and infidelity rates for various voltage profiles. We determine full, three-dimensional solutions and introduce a method that can obtain near-identical predictions using far more efficient two-dimensional computations. Moreover we explore the impact of unwanted charge defects (static and dynamic) in the environment, and test robust pulse sequences. As an example we exhibit a sequence correcting both systematic errors and (unknown) charge defects, observing an order of magnitude boost in fidelity. The technique can thus identify the most promising device designs for fabrication, as well as bespoke control sequences for each such device.

The cooperative effects of coherent and dissipative couplings have demonstrated significant potential for developing nonreciprocal photonic devices. However, beyond photon transmission, directional phonon lasing and coherent amplification of mechanical oscillations remain largely unexamined in the context of these cooperative effects. Here the potential for generating nonreciprocal phonon lasing within an open-cavity magnomechanical system is investigated. It is found that the interplay of coherent and dissipative couplings can lead to robust nonreciprocal mechanical gain, facilitated by the asymmetric dissipation of hybrid magnon-photon supermodes in opposite driving directions. Consequently, the phonon lasing can be selectively induced by driving the system from one direction, while remaining inactive with driving from the other direction. The study presents an experimentally viable approach to explore one-way phonon lasing through dissipation engineering, with potential applications in sound sensing, imaging, and chiral acoustics.

Quantum random-number generators (QRNGs) produce true random numbers, which are guaranteed by the fundamental principles of quantum physics. Miniaturization of QRNGs is crucial for a wide range of communication and cryptography applications. Here, we first report a fully functional QRNG chip based on vacuum-state fluctuations, with dimensions of $16.6\times 7.8{\mathrm{mm}}^2$. The quantum entropy source, which is achieved via hybrid photonic integration with an ${\mathrm{Si}\mathrm{O}}_2$ waveguide, generates raw quantum random numbers. The hybrid photonic and electrical components are assembled into a compact ceramic package using system-in-package technology. A microcontroller unit acquires the raw data and outputs the processed quantum random numbers via a serial peripheral interface. According to the characterization results, the QRNG chip achieves a constant real-time output rate of 5.2 Mbps across the industrial temperature range of $-40$ to $85{}^{\circ }\mathrm{C}$, making it suitable for practical applications.

Quantum nonlinear optics using Rydberg polaritons can enable single-photon transistor and switch, single-photon source, and deterministic quantum information processing. A major hindrance in this study is the fast motional decoherence. Here, we devise a scheme to significantly enhance the coherence of Rydberg polaritons by letting the atoms remember their velocities or, alternatively, by changing the phase of a Rydberg polariton according to its storage time. After the Rydberg polariton is prepared with a Rydberg state $|r_1⟩$, i.e., during the storage time, two laser fields induce a transition between $|r_1⟩$ and a nearby Rydberg state $|r_2⟩$ via a low-lying intermediate state $|f⟩$ which is largely detuned. In particular, we find that either a $2\pi \mathbb{N}$ protocol, a $\pi$-wait-$\pi$ protocol, or a wait-$\pi$ protocol, along with an appropriate choice of $|f⟩$ can lead to a phase-coherent Rydberg polariton upon its retrieval. Importantly, the coherent transition between $|r_1⟩$ and $|r_2⟩$ ensures that the Rydberg polariton can block the Rydberg excitation of nearby atoms as in usual applications of Rydberg polaritons. Numerics show that the theory can nearly completely eliminate the motional dephasing, leaving Rydberg-state decay as the only fundamental channel of decoherence. This sheds light on a broad application of Rydberg-mediated quantum nonlinear optics.