Entanglement has been recognized as being crucial when implementing various quantum information tasks. Nevertheless, quantifying entanglement for an unknown quantum state requires nonphysical operations or post-processing measurement data. For example, evaluation methods via quantum state tomography require vast amounts of measurement data and likely estimation. Although a direct entanglement determination has been reported for the unknown pure state, it is still tricky for the mixed state. In this work, assisted by weak measurement and deep learning technology, we directly detect the entanglement (namely, the concurrence) of a class of two-photon polarization-entangled mixed states both theoretically and experimentally according to the local photon spatial distributions after weak measurement. In this way, the number of projective bases is much smaller than that required in quantum state tomography.

Phase-coherent multi-tone lasers play a critical role in atomic, molecular, and optical physics. Among them, the Raman opeartion laser for manipulating atomic hyperfine qubits requires gigahertz bandwidth and low phase noise to retain long-term coherence. Raman operation lasers generated by directly modulated and frequency-multipled infrared lasers are compact and stable but lack feedback control to actively suppress the phase noise, which limits their performance in practical applications. In this work, we employ a fiber electro-optical modulator driven by a voltage-controlled oscillator (VCO) to modulate a monochromatic laser and employ a second-harmonic generation process to convert it to the visible domain, where the beat note of the Raman operation laser is stabilized by controlling the output frequency of VCO with a digital phase-locked loop (PLL). The low-frequency phase noise is effectively suppressed compared to the scheme without active feedback and it reaches -80dBc/Hz@5kHz with a 20 kHz loop bandwidth. Furthermore, this compact and robust scheme effectively reduces the system’s complexity and cost, which is promising for extensive application in atomic, molecular, and optical physics.

As a quantum resource, quantum coherence plays an important role in modern physics. Many coherence measures and their relations with entanglement have been proposed, and the dynamics of entanglement has been experimentally studied. However, the knowledge of general results for coherence dynamics in open systems is limited. Here we propose a coherence factorization law that describes the evolution of coherence passing through any noisy channels characterized by genuinely incoherent operations. We use photons to implement the quantum operations and experimentally verify the law for qubits and qutrits. Our work is a step toward understanding of the evolution of coherence when the system interacts with the environment, and will boost the study of more general laws of coherence.

Spin defects in silicon carbide (SiC) have attracted much attentions in various quantum technologies. In this work, we study the optical manipulation of charge state and coherent control of multifold divacancy spins ensemble in SiC under resonant excitation. The results reveal that the resonantly excited divacancy ensemble counts have dozens of enhancements by repumping a higher-energy laser. Moreover, it has a similar optimal repump laser wavelength of around 670 nm for multiple divacancies. On the basis of this, the optically detected magnetic resonance (ODMR) experiment shows that repump lasers with different wavelengths do not affect the ODMR contrast and line width. In addition, the repump lasers also do not change the divacancy spins’ coherence times. The experiments pave the way for using the optimal repump excitation method for SiC-based quantum information processing and quantum sensing.

The class quantum Merlin–Arthur (QMA), as the quantum analog of nondeterministic polynomial time, contains the decision problems whose YES instance can be verified efficiently with a quantum computer. The problem of deciding the group non-membership (GNM) of a group element is conjectured to be a member of QMA. Previous works on the verification of GNM, which still lacks experimental demonstration, required a quantum circuit with O(n5) group oracle calls. Here, we provide an efficient way to verify GNM problems, in which each quantum circuit only contains O(1) group of oracle calls, and the number of qubits in each circuit is reduced by half. Based on this protocol, we then experimentally demonstrate the new verification process with a four-element group in an all-optical circuit. The new protocol is validated experimentally by observing a significant completeness-soundness gap between the probabilities of accepting elements in and outside the subgroup. This work efficiently simplifies the verification of GNM and is helpful in constructing more quantum protocols based on the near-term quantum devices.

A narrow-linewidth laser operating at the telecommunications band combined with both fast and wide-band tuning features will have promising applications. Here we demonstrate a single-mode (both transverse and longitudinal mode) continuous microlaser around 1535 nm based on a fiber Fabry–Pérot microcavity, which achieves wide-band tuning without mode hopping to the 1.3 THz range and fast tuning rate to 60 kHz and yields a frequency scan rate of 1.6×1017Hz/s. Moreover, the linewidth of the laser is measured as narrow as 3.1 MHz. As the microlaser combines all these features into one fiber component, it can serve as the seed laser for versatile applications in optical communication, sensing, frequency-modulated continuous-wave radar, and high-resolution imaging.

Weak measurement has been shown to play important roles in the investigation of both fundamental and practical problems. Anomalous weak values are generally believed to be observed only when post-selection is performed, i.e., only a particular subset of the data is considered. Here, we experimentally demonstrate that an anomalous weak value can be obtained without discarding any data by performing a sequential weak measurement on a single-qubit system. By controlling the blazing density of the hologram on a spatial light modulator, the measurement strength can be conveniently controlled. Such an anomalous phenomenon disappears when the measurement strength of the first observable becomes strong. Moreover, we find that the anomalous weak value cannot be observed without post-selection when the sequential measurement is performed on each of the components of a two-qubit system, which confirms that the observed anomalous weak value is based on sequential weak measurement of two noncommutative operators.

The intrinsic characteristics of single photons became critical issues since the early development of quantum mechanics. Nowadays, acting as flying qubits, single photons are shown to play important roles in the quantum key distribution and quantum networks. Many different single photon sources (SPSs) have been developed. Point defects in silicon carbide (SiC) have been shown to be promising SPS candidates in the telecom range. In this work, we demonstrate a stable SPS in an epitaxial 3C-SiC with the wavelength in the near C-band range, which is very suitable for fiber communications. The observed SPSs show high single photon purity and stable fluorescence at even above 400 K. The lifetimes of the SPSs are found to be almost linearly decreased with the increase of temperature. Since the epitaxial 3C-SiC can be conveniently nanofabricated, these stable near C-band SPSs would find important applications in the integrated photonic devices.

We have demonstrated a mode matching method between two different fibers by a hybrid thermal expanded core technique, which can be applied to match the modes of fiber-based Fabry–Pérot cavities. Experimentally, this method has achieved an expansion of the ultraviolet fiber core by 3.5 times while keeping fundamental mode propagation. With the experiment parameters, the fundamental mode coupling efficiency between the fiber and micro-cavity can reach 95% for a plano-concave cavity with a length of 400 μm. This method can not only have potential in quantum photonics research but also can be applied in classical optical fields.