Radical pairs and the dynamics they undergo are prevalent in many chemical and biological systems. Specifically, it has been proposed that the radical-pair mechanism results from a relatively strong hyperfine interaction with its intrinsic nuclear spin environment. While the existence of this mechanism is undisputed, the nanoscale details remain to be experimentally shown. Here, we analyze the role of a quantum sensor in detecting the spin dynamics (non-Markovian) of individual radical pairs in the presence of a weak magnetic field. We show how quantum control methods can be used to set apart the dynamics of the radical-pair mechanism at various stages of the evolution. We expect these findings to have implications to the understanding of the physical mechanism in magnetoreception and other biochemical processes with a microscopic detail.

Optically active spin centers associated with atomic scale defects in $\mathrm{Si}\mathrm{C}$ are promising candidates for quantum technology, owing to their outstanding optical and spin properties. Photoluminescence as a mature optical investigating tool is widely used for the identification of spin defects and exploration of their properties. However, in the case of spectrally overlapping contributions from different types of defects, traditional photoluminescence measurements cannot be used to separately obtain their optical and vibrational properties, such as the local phonon energy and the Debye-Waller factor. Here, we apply spin-resonant microwave-assisted spectroscopy to investigate the optical and vibrational properties of silicon vacancies in 6H-$\mathrm{Si}\mathrm{C}$ and divacancies in 4H- and 6H-$\mathrm{Si}\mathrm{C}$. We isolate contributions from each type of defect, investigate their local vibrational modes, and obtain the Debye-Waller factor. This work proves that microwave-assisted spectroscopy is a suitable tool for the investigation of optical and vibrational properties of a large variety of spin defects.

Quantum platforms based on trapped ions are the main candidates to build a quantum hardware with computational capacities that largely surpass those of classical devices. Among the available control techniques in these setups, pulsed dynamical decoupling (pulsed DD) has been revealed as a useful method to process the information encoded in ion registers, whilst minimizing the environmental noise over them. In this work, we incorporate a pulsed DD technique that uses random pulse phases, or correlated pulse phases, to significantly enhance the robustness of entangling spin-spin dynamics in trapped ions. This procedure was originally conceived in the context of nuclear magnetic resonance for nuclear spin detection purposes, and here we demonstrate that the same principles apply for robust quantum-information processing in trapped-ion settings.

Detector blinding attacks have been proposed in the last few years, and they could potentially threaten the security of quantum key distribution systems. Even though such attacks are technically challenging to implement, it is important to consider countermeasures to avoid information leakage. In this paper, we present a countermeasure against these kinds of attacks based on the use of multipixel detectors. We show that with this method, we are able to estimate an upper bound on the information an eavesdropper could have on the key exchanged. Finally, we test a multipixel detector based on superconducting nanowire single-photon detectors to show it can fulfill all the requirements for our countermeasure to be effective.

Quantum key distribution (QKD) can generate secure key bits between remote users with quantum mechanics. However, the gap between the theoretical model and practical realizations gives eavesdroppers opportunities to intercept the secret key. The most insidious attacks, known as quantum hacking, are the ones with no significant discrepancy of the measurement results using side-channel loopholes of QKD systems. Depicting full-time-scale characteristics of the quantum signals, the quantum channel, and the QKD system can provide legitimate users extra capabilities to defeat malicious attacks. Here, we propose the method exploring a temporal ghost-imaging (TGI) scheme to perceive quantum hacking with temporal fingerprints and experimentally verify its validity. The scheme presents a common approach to promote QKD’s practical security from a unique perspective of signals and systems.

Randomness is a fundamental feature of quantum mechanics, which is an invaluable resource for both classical and quantum technologies. Practical quantum random-number generators (QRNGs) usually need to trust their devices, but their security can be jeopardized in the case of imperfections or malicious external actions. In this work, we present a robust implementation of a semi-device-independent QRNG that guarantees both security and fast generation rates. The system works in a prepare-and-measure scenario, where measurement and source are uncharacterized, but a bound on the energy of the prepared states is assumed. Our implementation exploits heterodyne detection, which offers increased generation rate and improved long-term stability compared to alternative measurement strategies. In particular, due to the tomographic properties of heterodyne measurement, we can compensate for fast phase fluctuations via postprocessing, avoiding complex active phase-stabilization systems. As a result, our scheme combines high security and speed with a simple setup featuring only commercial-off-the-shelf components, making it an attractive solution in many practical scenarios.

Nitrogen-vacancy (N-V) centers, optically active atomic defects in diamond, have been widely applied to emerging quantum sensing, imaging, and network efforts, showing unprecedented field sensitivity and nanoscale spatial resolution. Many of these advantages derive from their excellent quantum-coherence, single-spin addressability, and remarkable functionality over a broad temperature range, enabling opportunities to outperform their classical counterparts. To date, N-V measurements of spin fluctuations have mainly focused on systems with in-plane magnetization, but applications to material systems with a spontaneous out-of-plane magnetization remain largely unexplored despite their technological importance. Here, we report N-V sensing of intrinsic spin fluctuations of magnetic insulator ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ thin films with perpendicular magnetic anisotropy. The measured field dependence of N-V relaxation rates is well correlated to the variation of magnon density and band structure of the magnetic samples, which are challenging to access by the conventional magnetometry methods. Our results highlight the significant opportunities offered by N-V centers in diagnosing the noise environment of functional magnetic elements, providing valuable information to develop next-generation, high-density, and scalable spintronic devices.

Solid-state atomlike systems have great promise for linear optic quantum computing and quantum communication but are burdened by phonon sidebands and broadening due to surface charges. Nevertheless, coupling to a small-mode-volume cavity would allow high rates of extraction from even highly dephased emitters. We consider the nitrogen-vacancy center in diamond, a system understood to have a poor quantum optics interface with highly distinguishable photons, and design a silicon nitride cavity that allows 99% efficient extraction of photons at 200 K with an indistinguishability of $>$ 50%, improvable by external filtering. We analyze our design using Lumerical fdtd simulations and treat optical emission using a cavity-QED master equation valid at and beyond strong coupling and that includes both zero-phonon line broadening and sideband emission. The simulated design is compact ($<10\mu \mathrm{m}$) and, owing to its planar geometry, can be fabricated using standard silicon processes. Our work therefore points toward scalable fabrication of noncryogenic atomlike efficient sources of indistinguishable photons.

Quantum amplitude estimation (QAE) can achieve a quadratic speedup for applications classically solved by Monte Carlo simulation. A key requirement to realize this advantage is efficient state preparation. If state preparation is too expensive, it can diminish the quantum advantage. Preparing arbitrary quantum states has exponential complexity with respect to the number of qubits, and thus, is not applicable. Currently known efficient techniques require problems based on log-concave probability distributions, involve learning an unknown distribution from empirical data, or fully rely on quantum arithmetic. In this paper, we introduce an approach to simplify state preparation, together with a circuit optimization technique, both of which can help reduce the circuit complexity for QAE state preparation significantly. We demonstrate the introduced techniques for a numerical integration example on real quantum hardware, as well as for option pricing under the Heston model, i.e., based on a stochastic volatility process, using simulation.

Quantum error mitigation (QEM) is vital for noisy intermediate-scale quantum (NISQ) devices. While most conventional QEM schemes assume discrete gate-based circuits with noise appearing either before or after each gate, the assumptions are inappropriate for describing realistic noise that may have strong gate dependence and complicated nonlocal effects, and general computing models such as analog quantum simulators. To address these challenges, we first extend the scenario, where each computation process, being either digital or analog, is described by a continuous time evolution. For noise from imperfections of the engineered Hamiltonian or additional noise operators, we show it can be effectively suppressed by a stochastic QEM method. Since our method assumes only accurate single qubit controls, it is applicable to all digital quantum computers and various analog simulators. Meanwhile, errors in the mitigation procedure can be suppressed by leveraging the Richardson extrapolation method. As we numerically test our method with various Hamiltonians under energy relaxation and dephasing noise and digital quantum circuits with additional two-qubit crosstalk, we show an improvement of simulation accuracy by 2 orders. We assess the resource cost of our scheme and conclude the feasibility of accurate quantum computing with NISQ devices.

The orbital angular momentum (OAM) of photons presents a degree of freedom for enhancing the secure key rate of free-space quantum key distribution (QKD) through mode-division multiplexing (MDM). However, atmospheric turbulence can lead to substantial modal crosstalk, which is a long-standing challenge to MDM for free-space QKD. Here, we show that the digital generation of time-reversed wavefronts for multiple probe beams is an effective method for mitigating atmospheric turbulence. We experimentally characterize seven OAM modes after propagation through a 340-m outdoor free-space link and observe an average modal crosstalk as low as 13.2% by implementing real-time time reversal. The crosstalk can be further reduced to 3.4% when adopting a mode spacing $\mathrm{\Delta }\ell$ of 2. We implement a classical MDM system as a proof-of-principle demonstration, and the bit error rate is reduced from $3.6\times {10}^{-3}$ to be less than $1.3\times {10}^{-7}$ through the use of time reversal. We also propose a practical and scalable scheme for high-speed, mode-multiplexed QKD through a turbulent link. The modal crosstalk can be further reduced by using faster equipment. Our method can be useful to various free-space applications that require crosstalk suppression.

High-dimensional entanglement promises to increase the information capacity of photons and is now routinely generated, exploiting spatiotemporal degrees of freedom of single photons. A curious feature of these systems is the possibility of certifying entanglement despite strong noise in the data. We show that it is also possible to exploit this noisy high-dimensional entanglement for quantum key distribution by introducing a protocol that uses multiple subspaces of the high-dimensional system simultaneously. Our protocol can be used to establish a secret key even in extremely noisy experimental conditions, where qubit protocols fail. To show that, we analyze the performance of our protocol for noise models that apply to the two most commonly used sources of high-dimensional entanglement: time bins and spatial modes.

Developing compact, low-dissipation, cryogenic-compatible microwave electronics is essential for scaling up low-temperature quantum computing systems. In this paper, we demonstrate an ultracompact microwave directional forward coupler based on high-impedance slow-wave superconducting-nanowire transmission lines. The coupling section of the fabricated device has a footprint of $416\mu {\mathrm{m}}^2$. At 4.753 GHz, the input signal couples equally to the through port and forward-coupling port (50:50) at $-6.7\mathrm{dB}$ with $-13.5\mathrm{dB}$ isolation. The coupling ratio can be controlled with dc bias current or temperature by exploiting the dependence of the kinetic inductance on these quantities. The material and fabrication processes are suitable for direct integration with superconducting circuits, providing a practical solution to the signal distribution bottlenecks in developing large-scale quantum computers.

Ultralow-dissipation mechanical systems play an important role in metrology and exploring macroscopic quantum phenomena. Here we report a diamagnetic levitated micromechanical oscillator operating at 3 K with measured dissipation down to $0.59\mu \mathrm{Hz}$ and a quality factor up to $2\times {10}^7$. This mechanical system achieves the lowest dissipation among the state-of-the-art microscale and nanoscale mechanical systems reported to date, with orders-of-magnitude reduction over other systems based on different principles. The cryogenic diamagnetic levitated oscillator is applicable on a wide range of mass, making it a good candidate system for ultrasensitive measurements of both force and acceleration. By virtue of the strong magnetic gradient, this system is potentially capable of studying quantum spin mechanics.

Atmospheric free space and fiber have been widely exploited as the channels for quantum communication, and have enabled intercontinent and intercity applications. An air-sea free-space channel, being capable of linking the satellite-based quantum resource and underwater vehicle, has now become the last piece of the puzzle in building a global quantum communication network. Here, we present an experimental demonstration of air-water decoy-state quantum key distribution against high loss, meanwhile keeping a low quantum bit error rate less than 2.5% for different distances. We are able to reach a long-distance quantum key distribution in water channel that can enable real-life air-sea quantum-communication tasks. The demonstrated distance, even in coastal water of Jerlov type III, is up to 30 m, about one-order improvement over the proof-in-principle demonstrations in the previous experiments, and the channel loss is equivalent to 345-m-long clean seawater of Jerlov type I, representing a key step forward to practical air-sea secure communication.

The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling-gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of such a fast entangling-gate operation presents a big technical challenge, as such the required laser source is not available off the shelf. For this, we engineer an ultrafast entangling-gate source based on a frequency comb. The source generates bursts of several hundred mode-locked pulses with pulse energy approximately 800 pJ at 5-GHz repetition rate at 393.3 nm and complies with all requirements for implementing a fast two-qubit gate operation. Using a single, chirped ultraviolet pulse, we demonstrate a rapid adiabatic passage in a ${\mathrm{Ca}}^{+}$ ion. To verify the applicability and projected performance of the laser system for inducing entangling gates we run simulations based on our source parameters. The gate time can be faster than a trap period with an error approaching ${10}^{-4}$.

We show how to implement stationary one-way quantum steering with strong entanglement in a cavity magnonic system that consists of two magnon modes and a microwave cavity. The cavity is driven by a squeezed vacuum field generated by a flux-driven Josephson parameter amplifier and coupled to two Kittel modes via magnetic dipole interaction. We find that the steering directivity only depends on the ratio of two coupling rates (i.e., the ratio of coherent information exchange frequencies) and is barely affected by the dissipation of the system. Meanwhile, the entanglement and steering can be significantly enhanced due to the squeezed vacuum field and thus are more robust against thermal noises. This provides an active method to manipulate the steering directivity instead of adding asymmetric losses or noises to subsystems at the cost of reducing steerability.