Magnetic detection under high pressures using designed silicon vacancy centres in silicon carbide.

Pressure-induced magnetic phase transitions are attracting interest as a means to detect superconducting behaviour at high pressures in diamond anvil cells, but determining the local magnetic properties of samples is a challenge due to the small volumes of sample chambers. Optically detected magnetic resonance of nitrogen vacancy centres in diamond has recently been used for the in situ detection of pressure-induced phase transitions. However, owing to their four orientation axes and temperature-dependent zero-field splitting, interpreting these optically detected magnetic resonance spectra remains challenging. Here we study the optical and spin properties of implanted silicon vacancy defects in 4H-silicon carbide that exhibit single-axis and temperature-independent zero-field splitting. Using this technique, we observe the magnetic phase transition of Nd2Fe14B at about 7 GPa and map the critical temperature-pressure phase diagram of the superconductor YBa2Cu3O6.6. These results highlight the potential of silicon vacancy-based quantum sensors for in situ magnetic detection at high pressures.

Arc discharge method to fabricate large concave structures for open-access fiber Fabry-Pérot cavities.

We present a novel micro-fabrication technique for creating concave surfaces on the endfacets of photonic crystal fibers. A fiber fusion splicer is used to generate arc discharges to melt and reshape the fiber endfacet. This technique can produce large spherical concave surfaces with roughness as low as 0.12 nm in various types of photonic crystal fibers. The deviation of fabricated surface and a spherical profile in the region of 70 µm in diameter is less than 50 nm. The center of the concave surface and the fiber mode field are highly coincident with a deviation less than 500 nm. Finesse measurements have shown that a Fabry-Pérot cavity composed of the fiber fabricated using this method and a plane mirror maintains finesse of 20000. This method is easy to replicate, making it a practical and efficient approach to fabricate concave surface on fibers for open-access fiber Fabry-Pérot cavities.

Characterizing Biphoton Spatial Wave Function Dynamics with Quantum Wavefront Sensing.

With an extremely high dimensionality, the spatial degree of freedom of entangled photons is a key tool for quantum foundation and applied quantum techniques. To fully utilize the feature, the essential task is to experimentally characterize the multiphoton spatial wave function including the entangled amplitude and phase information at different evolutionary stages. However, there is no effective method to measure it. Quantum state tomography is costly, and quantum holography requires additional references. Here, we introduce quantum Shack-Hartmann wavefront sensing to perform efficient and reference-free measurement of the biphoton spatial wave function. The joint probability distribution of photon pairs at the back focal plane of a microlens array is measured and used for amplitude extraction and phase reconstruction. In the experiment, we observe that the biphoton amplitude correlation becomes weak while phase correlation shows up during free-space propagation. Our work is a crucial step in quantum physical and adaptive optics and paves the way for characterizing quantum optical fields with high-order correlations or topological patterns.

Coherent Control and Magnetic Detection of Divacancy Spins in Silicon Carbide at High Pressures.

Spin defects in silicon carbide appear to be a promising tool for various quantum technologies, especially for quantum sensing. However, this technique has been used only at ambient pressure until now. Here, by combining this technique with diamond anvil cell, we systematically study the optical and spin properties of divacancy defects created at the surface of SiC at pressures up to 40 GPa. The zero-field-splitting of the divacancy spins increases linearly with pressure with a slope of 25.1 MHz/GPa, which is almost two-times larger than that of nitrogen-vacancy centers in diamond. The corresponding pressure sensing sensitivity is about 0.28 MPa/Hz-1/2. The coherent control of divacancy demonstrates that coherence time decreases as pressure increases. Based on these, the pressure-induced magnetic phase transition of Nd2Fe14B sample at high pressures was detected. These experiments pave the way to use divacancy in quantum technologies such as pressure sensing and magnetic detection at high pressures.

A versatile multi-tone laser system for manipulating atomic qubits based on a fiber Mach-Zehnder modulator and second harmonic generation.

Stimulated Raman transition is a fundamental method to coherently manipulate quantum states in different physical systems. Phase-coherent dichromatic radiation fields matching the energy level splitting are the key to realizing stimulated Raman transition. Here we demonstrate a flexible-tuning, spectrum-clean and fiber-compatible method to generate a highly phase-coherent and high-power multi-tone laser. This method features the utilization of a broadband fiber Mach-Zehnder modulator working at carrier suppression condition and second harmonic generation. We generate a multi-tone continuous-wave 532 nm laser with a power of 1.5 Watts and utilize it to manipulate the spin and motional states of a trapped 171Yb+ ion via stimulated Raman transition. For spin state manipulation, we acquire an effective Rabi frequency of 2π × 662.3 kHz. Due to the broad bandwidth of the fiber modulator and nonlinear crystal, the frequency gap between tones can be flexibly tuned. Benefiting from the features above, this method can manipulate 171Yb+ and 137Ba+ simultaneously in the multi-species ion trap and has potential to be widely applied in atomic, molecular and optical physics.

Large-tuning-range frequency stabilization of an ultraviolet laser by an open-loop piezoelectric ceramic controlled Fabry-Pérot cavity.

We demonstrate a laser frequency stabilization method with large tuning range to stabilize a UV laser by installing piezoelectric ceramic actuators into a Fabry-Pérot cavity with an ultra-low expansion spacer. To suppress piezoelectric drift, a two-layer symmetrical structure is adopted for the piezoelectric actuator, and a 14.7 GHz tuning range is achieved. The short-term drift of the piezoelectric ceramics caused by temperature and creep is eliminated, and the long-term drift is 0.268 MHz/h when the Fabry-Pérot cavity is sealed in a chamber without a vacuum environment. The long-term frequency drift is mainly caused by stress release and is eliminated by compensating the cavity voltage with an open loop. Without the need for an external reference or a vacuum environment, the laser frequency stabilization system is greatly simplified, and it can be extended to wavelengths ranging from ultraviolet to infrared. Owing to its simplicity, stability, and large tuning range, it is applicable in cold atom and trapped ion experiments.

Quantum frequency conversion from ultraviolet to visible band through waveguides in a period-poled MgO:LiTaO3 crystal.

In research on hybrid quantum networks, visible or near-infrared frequency conversion has been realized. However, technical limitations mean that there have been few studies involving the ultraviolet band, and unfortunately the wavelengths of the rare-earth or alkaline-earth metal atoms or ions that are used widely in research on quantum information are often in the UV band. Therefore, frequency conversion of the ultraviolet band is very important. In this paper, we demonstrate a quantum frequency conversion between ultraviolet and visible wavelengths by fabricating waveguides in a period-poled MgO:LiTaO3 crystal with a laser writing system, which will be used to connect the wavelength of the dipole transition of 171Yb+ at 369.5 nm and the absorption wavelength of Eu3+ at 580 nm in a solid-state quantum memory system. An external conversion efficiency of 0.85% and a signal-to-noise ratio of greater than 500 are realized with a pumping power of 3.28 W at 1018 nm. Furthermore, we complete frequency conversion of the classical polarization state by means of a symmetric optical setup based on the fabricated waveguide, and the process fidelity of the conversion is (96.13 ± 0.021)%. This converter paves the way for constructing a hybrid quantum network and realizing a quantum router in the ultraviolet band in the future.

Super-resolved Imaging of a Single Cold Atom on a Nanosecond Timescale.

In cold atomic systems, fast and high-resolution microscopy of individual atoms is crucial, since it can provide direct information on the dynamics and correlations of the system. Here, we demonstrate nanosecond-scale two-dimensional stroboscopic pictures of a single trapped ion beyond the optical diffraction limit, by combining the main idea of ground-state depletion microscopy with quantum-state transition control in cold atoms. We achieve a spatial resolution up to 175 nm using a NA=0.1 objective in the experiment, which represents a more than tenfold improvement compared with direct fluorescence imaging. To show the potential of this method, we apply it to observe the secular motion of the trapped ion; we demonstrate a temporal resolution up to 50 ns with a displacement detection sensitivity of 10 nm. Our method provides a powerful tool for probing particle positions, momenta, and correlations, as well as their dynamics in cold atomic systems.

Experimental Transmission of Quantum Information Using a Superposition of Causal Orders.

Communication in a network generally takes place through a sequence of intermediate nodes connected by communication channels. In the standard theory of communication, it is assumed that the communication network is embedded in a classical spacetime, where the relative order of different nodes is well defined. In principle, a quantum theory of spacetime could allow the order of the intermediate points between sender and receiver to be in a coherent superposition. Here we experimentally realize a tabletop simulation of this exotic possibility on a photonic system, demonstrating high-fidelity transmission of quantum information over two noisy channels arranged in a superposition of two alternative causal orders.

Coherent Control of Nitrogen-Vacancy Center Spins in Silicon Carbide at Room Temperature.

Solid-state color centers with manipulatable spin qubits and telecom-ranged fluorescence are ideal platforms for quantum communications and distributed quantum computations. In this work, we coherently control the nitrogen-vacancy (NV) center spins in silicon carbide at room temperature, in which telecom-wavelength emission is detected. We increase the NV concentration sixfold through optimization of implantation conditions. Hence, coherent control of NV center spins is achieved at room temperature, and the coherence time T_{2} can be reached to around 17.1 µs. Furthermore, an investigation of fluorescence properties of single NV centers shows that they are room-temperature photostable single-photon sources at telecom range. Taking advantage of technologically mature materials, the experiment demonstrates that the NV centers in silicon carbide are promising platforms for large-scale integrated quantum photonics and long-distance quantum networks.

An Acoustic Sensor Based on Active Fiber Fabry-Pérot Microcavities.

We demonstrate an active acoustic sensor based on a high-finesse fiber Fabry-Pérot micro-cavity with a gain medium. The sensor is a compacted device lasing around 1535 nm by external optical pumping. The acoustic pressure acting on the sensor disturbs the emitted laser frequency, which is subsequently transformed to beat signals through a delay-arm interferometer, and directly detected by a photo-detector. In this configuration, the sensing device exhibits a high sensitivity of 2.6 V/Pa and a noise equivalent acoustic signal level of 230 µPa/Hz1/2 at a frequency of 4 kHz. Experimental results provide a wide frequency response from 100 Hz to 18 kHz. As the sensor works at communication wavelength and the output laser can be electrically tuned in the 10 nm range, a multi-sensor network can be easily constructed with the dense wavelength division multiplexing devices. Extra lasers or demodulators are unnecessary thus the proposed sensor is low cost and easy fabrication. The proposed sensor shows broad applications prospect in remote oil and gas leakage exploration, photo-acoustic spectrum detection, and sound source location.

Comparison of gal-lac operons in wild-type galactose-positive and -negative Streptococcus thermophilus by genomics and transcription analysis.

Streptococcus thermophilus is one of the most important homo-fermentative thermophilic bacteria, which is widely used as a starter culture in dairy industry. Both wild-type galactose-negative (Gal-) S. thermophilus AR333 and galactose-positive (Gal+) S. thermophilus S-3 in this study were isolated from Chinese traditional dairy products. Here, to access the mechanism of the difference of galactose utilization between strains AR333 and S-3, the expression of gal-lac operons was examined using real-time qPCR in the presence of different sugars, and the gene organization of gal-lac operons was characterized using comparative genomics analysis. As compared with medium containing glucose, the expression of gal-lac operons in AR333 and S-3 was significantly activated (> 5-fold) in the presence of galactose or lactose in the medium. More importantly, the expression of gal operon in S-3 was higher than that of AR333, suggesting that the strength of gal promoter in AR333 and S-3 may be different. The genomes of AR333 and S-3 were the first time sequenced to provide insight into the difference of gal-lac operons in these two strains. Comparative genomics analysis showed that gene order and individual gene size of gal-lac operons are conserved in AR333 and S-3. The DNA sequence of gal operon responsible for galactose utilization between AR333 and S-3 is almost identical except that galK promoter of S-3 possesses single base pair mutation (G to A substitution) at -9 box galK region. Moreover, the expression of red fluorescent protein can be activated by galK promoter of S-3, but cannot by galK promoter of AR333 in galactose medium, suggesting that gal operon is silent in AR333 and active in S-3 under galactose-containing medium. Overall, our results indicated that single point mutation at -9 box in the galK promoter can significantly affect the expression of gal operon and is largely responsible for the Gal+ phenotype of S. thermophilus.

Experimental observation of quantum state-independent contextuality under no-signaling conditions.

Contextuality, the impossibility of assigning context-independent measurement outcomes, is a critical resource for quantum computation and communication. No-signaling between successive measurements is an essential requirement that should be accomplished in any test of quantum contextuality and that is difficult to achieve in practice. Here, we introduce an optimal quantum state-independent contextuality inequality in which the deviation from the classical bound is maximal. We then experimentally test it using single photons generated from a defect in a bulk silicon carbide, while satisfying the requirement of no-signaling within the experimental error. Our results shed new light on the study of quantum contextuality under no-signaling conditions.

Experimental implementation of generalized transitionless quantum driving.

It is known that high intensity fields are usually required to implement shortcuts to adiabaticity via transitionless quantum driving (TQD). Here, we show that this requirement can be relaxed by exploiting the gauge freedom of generalized TQD, which is expressed in terms of an arbitrary phase when mimicking the adiabatic evolution. We experimentally investigate the performance of generalized TQD in comparison to both traditional TQD and adiabatic dynamics. By using a Yb+171 trapped ion hyperfine qubit, we implement a Landau-Zener adiabatic Hamiltonian and its (traditional and generalized) TQD counterparts. We show that the generalized theory provides energy-optimal Hamiltonians for TQD, with no additional fields required. In addition, the optimal TQD Hamiltonian for the Landau-Zener model is investigated under dephasing. Even using less intense fields, optimal TQD exhibits fidelities that are more robust against a decohering environment, with performance superior to that provided by the adiabatic dynamics.

Quantum statistical imaging of particles without restriction of the diffraction limit.

A quantum measurement method based on the quantum nature of antibunching photon emission has been developed to detect single particles without the restriction of the diffraction limit. By simultaneously counting the single-photon and two-photon signals with fluorescence microscopy, the images of nearby nitrogen-vacancy centers in diamond at a distance of 8.5±2.4 nm have been successfully reconstructed. Also their axes information was optically obtained. This quantum statistical imaging technique, with a simple experimental setup, can also be easily generalized in the measuring and distinguishing of other physical properties with any overlapping, which shows high potential in future image and study of coupled quantum systems for quantum information techniques.

Controlling deformation in a high quality factor silica microsphere toward single directional emission.

High-Q deformed silica microsphere cavities are fabricated by short CO(2) laser pulses, where the deformation is well controlled by adjusting the intensity and number of pulses. Using this method, directional emission from whispering-gallery mode (WGM) with a high quality factor of 10(7) in these microspheres is achieved, and a transition from two-directional to single-directional emission is observed. Such concentrated directional emission and high-Q of WGMs show high potential for future studies of the chaotic ray dynamics in deformed microcavity and cavity quantum electrodynamics and optomechanics.

An ion trap apparatus with high optical access in multiple directions.

Optical controls provided by lasers are the most important and essential techniques in trapped ion and cold atom systems. It is crucial to increase the optical accessibility of the setup to enhance these optical capabilities. Here, we present the design and construction of a new segmented-blade ion trap integrated with a compact glass vacuum cell, in place of the conventional bulky metal vacuum chamber. The distance between the ion and four outside surfaces of the glass cell is 15 mm, which enables us to install four high-numerical-aperture (NA) lenses (with two NA ⩽ 0.32 lenses and two NA ⩽ 0.66 lenses) in two orthogonal transverse directions, while leaving enough space for laser beams in the oblique and longitudinal directions. The high optical accessibility in multiple directions allows the application of small laser spots for addressable Raman operations, programmable optical tweezer arrays, and efficient fluorescence collection simultaneously. We have successfully loaded and cooled a string of 174Yb+ and 171Yb+ ions in the trap, which verifies the trapping stability. This compact high-optical-access trap setup not only can be used as an extendable module for quantum information processing but also facilitates experimental studies on quantum chemistry in a cold hybrid ion-atom system.

Robust coherent control of solid-state spin qubits using anti-Stokes excitation.

Optically addressable solid-state color center spin qubits have become important platforms for quantum information processing, quantum networks and quantum sensing. The readout of color center spin states with optically detected magnetic resonance (ODMR) technology is traditionally based on Stokes excitation, where the energy of the exciting laser is higher than that of the emission photons. Here, we investigate an unconventional approach using anti-Stokes excitation to detect the ODMR signal of silicon vacancy defect spin in silicon carbide, where the exciting laser has lower energy than the emitted photons. Laser power, microwave power and temperature dependence of the anti-Stokes excited ODMR are systematically studied, in which the behavior of ODMR contrast and linewidth is shown to be similar to that of Stokes excitation. However, the ODMR contrast is several times that of the Stokes excitation. Coherent control of silicon vacancy spin under anti-Stokes excitation is then realized at room temperature. The spin coherence properties are the same as those of Stokes excitation, but with a signal contrast that is around three times greater. To illustrate the enhanced spin readout contrast under anti-Stokes excitation, we also provide a theoretical model. The experiments demonstrate that the current anti-Stokes excitation ODMR approach has promising applications in quantum information processing and quantum sensing.

Anti-bunching and luminescence blinking suppression from plasmon-interacted single CdSe/ZnS quantum dot.

CdSe/ZnS colloidal quantum dots generally exist as blinking phenomena during the luminescence process that remarkably influences its applications. In this work, we used the surface plasmonic effect to effectively modulate single quantum dots. Obvious contrasts have been observed by comparing single quantum dots on silica and gold films. The surface plasmon is shown to obviously suppress the blinking of single quantum dots. With further demonstrated second- order correlation measurements, an anti-bunching effect was observed. The anti-bunching dip gives the smallest value of g(2)(0) = 0.15, and the lifetime of the exciton has been reduced. This method presents the application's potential towards tunable high-emitting-speed single photon sources at room temperature.

Experimental demonstration of suppressing residual geometric dephasing.

The geometric phase is regarded as a promising strategy in fault tolerance quantum information processing (QIP) domain due to its phase only depending on the geometry of the path executed. However, decoherence caused by environmental noise will destroy the geometric phase. Traditional dynamic decoupling sequences can eliminate dynamic dephasing but can not reduce residual geometric dephasing, which is still vital for high-precision quantum manipulation. In this work, we experimentally demonstrate effective suppression of residual geometric dephasing with modified dynamic decoupling schemes, using a single trapped 171Yb+ ion. The experimental results show that the modified schemes can reduce dephasing rate up to more than one order of magnitude compared with traditional dynamic decoupling schemes, where residual geometric dephasing dominates. Besides, we also investigate the impact of intensity and correlation time of the low-frequency noise on coherence of the quantum system. And we confirm these methods can be used in many cases.