arXiv subjects

Shougang Zhang

Publications and source records attributed to Shougang Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2023

Self-driven Hybrid Atomic Spin Oscillator

A self-driven hybrid atomic spin oscillator is demonstrated in theory and experiment with a vapor Rb-Xe dual-spin system. The raw signal of Rb spin oscillation is amplified, phase-shifted and sent back to drive the Xe spins coherently. By fine tuning the driving field strength and phase, a self-sustaining spin oscillation signal with zero frequency shift is obtained. The effective coherence time is infinitely prolonged beyond the intrinsic coherence time of Xe spins, forming a hybrid atomic spin oscillator. Spectral analysis indicates that a frequency resolution of 13.1 nHz is achieved, enhancing the detection sensitivity for magnetic field. Allan deviation analysis shows that the spin oscillator can operate in continuous wave mode like a spin maser. The prototype spin oscillator can be easily implanted into other hybrid spin systems and enhance the detection sensitivity of alkali metal-noble gas comagnetometers.

quant-ph↗

arXiv2023

Response Dynamics of Alkali Metal-Noble Gas Hybrid Trispin System

With numerical calculation of coupled Bloch equations, we have simulated the spin dynamics of nuclear magnetic resonance gyroscope based on alkali metal-noble gas hybrid trispin system. From the perspective of damping harmonic oscillator, a thorough analysis of the response dynamics is demonstrated. The simulation results shows a linear increasing response of gyroscope signal while the noblge gas nuclear spin magnetization and alkali atomic spin lifetime parameters are at the over damping condition. An upper limit of response is imposed on the NMR gyroscope signal due to the inherent dynamics of the hybrid trispin system. The results agrees with present available experimental results and provide useful guidings for future experiments.

physics.atom-ph↗

arXiv2022

Spectrally resolved two-photon interference in a modified Hong-Ou-Mandel interferometer

A modified Hong--Ou--Mandel (HOM) interference reveals that the two-photon interference phenomenon can be explained only by the concept of a two-photon wave packet rather than a single-photon one. However, the temporal interferogram in the modified HOM interferometer becomes flat in some cases so that no useful information can be extracted from time-domain measurement. Here, we theoretically explore such temporal interferogram from the frequency domain and obtain the spectrally resolved interference with high visibility. The result represents a modulation of the joint spectral intensity along both the frequency sum and the frequency difference directions. This is quite different from the cases of the spectrally resolved HOM interference and N00N state interference where the modulations happened only in one direction. Moreover, we have shown that such modulations have a potential application in the generation and characterization of high-dimensional frequency entanglement.

quant-ph↗

arXiv2022

Quantum two-way time transfer over a hybrid free-space and fiber link

As the superiority of quantum two-way time transfer (Q-TWTT) has been proved convincingly over fiber links, its implementation on free-space links becomes an urgent need for remote time transfer expanding to the transcontinental distance. In this paper, the first Q-TWTT experimental demonstration over a hybrid link of 2 km-long turbulent free space and 7 km-long field fiber is reported. Despite the significant loss of more than 25 dB and atmospheric turbulence, reliable time transfer performance lasting for overnights has been realized with time stability in terms of time deviation far below 1 picosecond. This achievement shows the good feasibility of quantum-enhanced time transfer in the space-ground integrated optical links and nicely certifies the capability of Q-TWTT in comparing and synchronizing the state-of-the-art space microwave atomic clocks.

quant-ph↗

arXiv2022

Entanglement-enhanced Synchronous differential comparison

The quantum entanglement enables the precision measurement and frequency metrology beyond the standard quantum limit that is imposed by the quantum projection noise and photon shot noise. Here we propose employing the entangled atoms in the synchronous differential measurement to enhance the sensitivity of the spatial-shift detection. Two ways of engineering the entangled atoms are studied. The synchronous comparison between two pixels within an entangled atomic cloud leads to a sensitivity enhancement factor of 1.4 over the standard quantum limit. Increasing the atom number hardly further improves the sensitivity. In contrast, the synchronous comparison between two independent pixels that are individually composed of entangled atoms allows for a strong sensitivity enhancement by a factor of, for example, 9.7 with $10^{3}$ entangled atoms in each pixel, corresponding to a reduction of the averaging time by a factor of about $10^{2}$. A large atom number may further elevate the sensitivity. Our work paves the way towards the entanglement-enhanced detection of the gravitational redshift by means of the \emph{in situ} imaging spectroscopy.

quant-ph↗

arXiv2022

Surpassing the classical limit of microwave photonic frequency fading effect by quantum microwave photonics

With energy-time entangled biphoton sources as the optical carrier and time-correlated single-photon detection for high-speed radio frequency (RF) signal recovery, the method of quantum microwave photonics (QMWP) has presented the unprecedented potential of nonlocal RF signal encoding and efficient RF signal distilling from the dispersion interference associated with ultrashort pulse carriers. In this letter, its capability in microwave signal processing and prospective superiority is further demonstrated. Both the QMWP RF phase shifting and transversal filtering functionality, which are the fundamental building blocks of microwave signal processing, are realized. Besides the perfect immunity to the dispersion-induced frequency fading effect associated with the broadband carrier in classical microwave photonics, a native two-dimensional parallel microwave signal processor is provided. These demonstrations fully prove the superiority of QMWP over classical MWP and open the door to new application fields of MWP involving encrypted processing.

physics.optics↗

arXiv2022

A proof-of-principle demonstration of quantum microwave photonics

With the rapid development of microwave photonics, which has expanded to numerous applications of commercial importance, eliminating the emerging bottlenecks becomes of vital importance. For example, as the main branch of microwave photonics, radio-over-fiber technology provides high bandwidth, low-loss, and long-distance propagation capability, facilitating wide applications ranging from telecommunication to wireless networks. With ultrashort pulses as the optical carrier, huge capacity is further endowed. However, the wide bandwidth of ultrashort pulses results in the severe vulnerability of high-frequency RF signals to fiber dispersion. With a time-energy entangled biphoton source as the optical carrier and combined with the single-photon detection technique, a quantum microwave photonics method is proposed and demonstrated experimentally. The results show that it not only realizes unprecedented nonlocal RF signal modulation with strong resistance to the dispersion associated with ultrashort pulse carriers but provides an alternative mechanism to effectively distill the RF signal out from the dispersion. Furthermore, the spurious-free dynamic range of both the nonlocally modulated and distilled RF signals has been significantly improved. With the ultra-weak detection and high-speed processing advantages endowed by the low-timing-jitter single-photon detection, the quantum microwave photonics method opens up new possibilities in modern communication and networks.

quant-ph↗

arXiv2021

Implementation of field two-way quantum synchronization of distant clocks across a 7 km deployed fiber link

The two-way quantum clock synchronization has been shown not only providing femtosecond-level synchronization capability but also security against symmetric delay attacks, thus becoming a prospective method to compare and synchronize distant clocks with both enhanced precision and security. In this letter, a field test of two-way quantum synchronization between a H-maser and a Rb clock linked by a 7 km-long deployed fiber was implemented. Limited by the frequency stability of the Rb clock, the achieved time stability at 30 s was measured as 32 ps. By applying a fiber-optic microwave frequency transfer technology, the stability was improved by more than one-magnitude to 1.9 ps, even though the number of acquired photon pairs was only 1440 in 30 s due to the low sampling rate of the utilized coincidence measurement system. Such implementation demonstrates the high practicability of two-way quantum clock synchronization method for promoting the field applications.

quant-ph↗

arXiv2021

Demonstration of 50 km Fiber-optic two-way quantum time transfer at femtosecond-scale precision

The two-way quantum time transfer method has been proposed and experimentally demonstrated for its potential enhancements in precision and better guarantee of security. To further testify its advantage in practical applications, the applicable direct transmission distance as well as the achievable synchronization precision between independent time scales is of great interest. In this paper, an experiment on two-way quantum time transfer has been carried out over a 50 km long fiber link. With the common clock reference, a short-term stability of 2.6 ps at an averaging time of 7 s and a long-term stability of 54.6 fs at 57300 s were obtained. With independent clock references, assisted by microwave frequency transfer technology, the achieved synchronization showed almost equal performance and reached a stability of 89.5 fs at 57300 s. Furthermore, the spectral consistency of the utilized entangled photon pair sources has been studied concerning its effect on the transfer accuracy and long-term stability. The results obtained have promised a bright future of the two-way quantum time transfer for realizing high-precision time synchronization on metropolitan area fiber links.

quant-ph↗

arXiv2021

Visualization of magnetic fields with cylindrical vector beams in a warm atomic vapor

We propose and demonstrate an experimental implementation for the observation of magnetic fields from spatial features of absorption profiles in a warm atomic vapor. A radially polarized vector beam that traverses an atomic vapor will generate an absorption pattern with petal-like structure by the mediation of a transverse magnetic field (TMF). The spatial absorption pattern rotates when the azimuthal angle of the TMF is changed, while its contrast decreases when the longitudinal component of the magnetic field increases. By analyzing the intensity distribution of the transmitted pattern we can determine the magnetic field strength. Our work provides a framework for investigating three-dimensional magnetic field distributions based on atoms.

physics.atom-ph↗

arXiv2021

Phase gradient protection of stored spatially multimode perfect optical vortex beams in a diffused rubidium vapor

We experimentally investigate the optical storage of perfect optical vortex (POV) and spatially multimode perfect optical vortex (MPOV) beams via electromagnetically induced transparency (EIT) in a hot vapor cell. In particular, we study the role that phase gradients and phase singularities play in reducing the blurring of the retrieved images due to atomic diffusion. Three kinds of manifestations are enumerated to demonstrate such effect. Firstly, the suppression of the ring width broadening is more prominent for POVs with larger orbital angular momentum (OAM). Secondly, the retrieved double-ring MPOV beams' profiles present regular dark singularity distributions that are related to their vortex charge difference. Thirdly, the storage fidelities of the triple-ring MPOVs are substantially improved by designing line phase singularities between multi-ring MPOVs with the same OAM number but $π$ offset phases between adjacent rings. Our experimental demonstration of MPOV storage opens new opportunities for increasing data capacity in quantum memories by spatial multiplexing, as well as the generation and manipulation of complex optical vortex arrays.

quant-ph↗

arXiv2021

Implementation of quantum synchronization over a 20-km fiber distance based on frequency-correlated photon pairs and HOM interference

The quantum synchronization based on frequency-correlated photon pairs and HOM interference has shown femtosecond-level precision and great application prospect in numerous fields depending on high-precision timefrequency signals. Due to the difficulty of achieving stable HOM interference fringe after long-distance fiber transmission, this quantum synchronization is hampered from long-haul field application. Utilizing segmented fibers instead of a single long-length fiber, we successfully achieved the stable observation of the two-photon interference of the lab-developed broadband frequency-correlated photon pairs after 20 km-long fiber transmission, without employing auxiliary phase stabilization method. Referenced to this interference fringe, the balance of the two fiber arms is successfully achieved with a long-term stability of 20 fs. The HOM-interference-based synchronization over a 20-km fiber link is thus demonstrated and a minimum stability of 74 fs has been reached at 48,000 s. This result not only provides a simple way to stabilize the fiber-optic two-photon interferometer for long-distance quantum communication systems, but also makes a great stride forward in extending the quantum-interference-based synchronization scheme to the long-haul field applications.

quant-ph↗

arXiv2021

Robust optical frequency transfer in a noisy urban fiber network

Optical fibers have been recognized as one of the most promising host material for high phase coherence optical frequency transfer over thousands of kilometers. In the pioneering work, the active phase noise cancellation (ANC) technique has been widely used for suppressing the fiber phase noise introduced by the environmental perturbations, in which an ideal phase detector with high resolution and unlimited detection range is needed to extract the fiber phase noise, in particular for noisy fiber links. We demonstrate the passive phase noise cancellation (PNC) technique without the need of phase detector could be preferable for noisy fiber links. To avoid the effect of the radio frequency (RF) from the time base at the local site in the conventional active or passive phase noise cancellation techniques, here we introduce a fiber-pigtailed acousto-optic modulator (AOM) with two diffraction order outputs (0 and +1 order) with properly allocating the AOM-driving frequencies allowing to cancel the time base effect. Using this technique, we demonstrate transfer of coherent light through a 260 km noisy urban fiber link. The results show the effect of the RF reference can be successfully removed. After being passively compensated, {we demonstrate a fractional frequency instability of $4.9\times10^{-14}$ at the integration time of 1 s and scales down to $10^{-20}$ level at 10,000 s in terms of modified Allan deviation over the 260 km noisy urban fiber link}. The frequency uncertainty of the retrieved light after transferring through this noise-compensated fiber link relative to that of the input light achieves $(0.41\pm4.7)\times10^{-18}$. The proposed technique opens a way to a broad distribution of an ultrastable frequency reference with high coherence without any effects coming from the RF reference and enables a wide range of applications beyond metrology over fiber networks.

physics.ins-det↗

arXiv2021

A low phase noise microwave source for high performance CPT Rb atomic clock

Phase noise of the frequency synthesizer is one of the main limitations to the short-term stability of microwave atomic clocks. In this work, we demonstrated a low-noise, simple-architecture microwave frequency synthesizer for a coherent population trapping (CPT) clock. The synthesizer is mainly composed of a 100 MHz oven controlled crystal oscillator (OCXO), a microwave comb generator and a direct digital synthesizer (DDS). The absolute phase noises of 3.417 GHz signal are measured to be -55 dBc/Hz, -81 dBc/Hz, -111 dBc/Hz and -134 dBc/Hz, respectively, for 1 Hz, 10 Hz, 100 Hz and 1 kHz offset frequencies, which shows only 1 dB deterioration at the second harmonic of the modulation frequency of the atomic clock. The estimated frequency stability of intermodulation effect is 4.7*10^{-14} at 1s averaging time, which is about half order of magnitude lower than that of the state-of-the-art CPT Rb clock. Our work offers an alternative microwave synthesizer for high-performance CPT Rb atomic clock.

physics.ins-det↗

arXiv2021

High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser

The coherent population trapping (CPT) atomic clock is very promising for use in next-generation spaceborne applications owing to its compactness and high performance. In this paper, we propose and implement a CPT atomic clock based on the direct modulation of a large-modulation-bandwidth and narrow-linewidth distributed Bragg reflector laser, which replaces the usually used external bulk modulator in the high-performance CPT clock. Our method retains the high performance while significantly reducing the size. Using this highly compact bichromatic light source and simplest CPT configuration, in which a circularly polarized bichromatic laser interrogates the ^{87}Rb atom system, a CPT signal of clock transition with a narrow linewidth and high contrast is observed. We then lock the local oscillator frequency to the CPT error signal and demonstrate a short-term frequency stability of 3.6 \times 10^{-13} τ^{-1/2} (4 s \le τ \le 200 s). We attribute it to the ultralow laser frequency and intensity noise as well as to the high-quality-factor CPT signal. This study can pave the way for the development of compact high-performance CPT clocks based on our scheme.

physics.atom-ph↗

arXiv2021

Quantum microwave photonics

By harnessing quantum superposition and entanglement, remarkable progress has sprouted over the past three decades from different areas of research in communication computation and simulation. To further improve the processing ability of microwave pho-tonics, here, we have demonstrated a quantum microwave photonic processing system using a low jitter superconducting nanowire single photon detector (SNSPD) and a time-correlated single-photon counting (TCSPC) module. This method uniquely combines extreme optical sensitivity, down to a single-photon level (below -100 dBm), and wide processing bandwidth, twice higher than the transmission bandwidth of the cable. Moreover, benefitted from the trigger, the system can selectively process the desired RF signal and attenuates the other in-tense noise and undesired RF components even the power is 15dB greater than the desired signal power. Using this method we show microwave phase shifting and frequency filtering for the desired RF signal on the single-photon level. Besides its applications in space and under-water communications and testing and qualification of pre-packaged photonic modulators and detectors. This RF signal processing capability at the single-photon level can lead to significant development in the high-speed quantum processing method.

physics.app-ph↗

arXiv2020

High-precision nonlocal temporal correlation identification of entangled photon pairs for quantum clock synchronization

High-precision nonlocal temporal correlation identification in the entangled photon pairs is critical to measure the time offset between remote independent time scales for many quantum information applications. The first nonlocal correlation identification was reported in 2009, which extracts the time offset via the algorithm of iterative fast Fourier transformations (FFTs) and their inverse. The least identification resolution is restricted by the peak identification threshold of the algorithm, and thus the time offset calculation precision is limited. In this paper, an improvement for the identification is presented both in the resolution and precision via a modified algorithm of direct cross correlation extraction. A flexible resolution down to 1 ps is realized, which is only dependent on the Least Significant Bit (LSB) resolution of the time-tagging device. The attainable precision is shown mainly determined by the inherent timing jitter of the single photon detectors, the acquired pair rate and acquisition time, and a sub picosecond precision (0.72 ps) has been achieved at an acquisition time of 4.5 s. This high-precision nonlocal measurement realization provides a solid foundation for the field applications of entanglement-based quantum clock synchronization, ranging and communications.

quant-ph↗

arXiv2019

Possibility to construct a clock on non 0-0 transition

In this letter, we discuss the possibility of non 0-0 atomic lines as the reference transitions to design a new atomic clock. The proposed clock operates in high C-field regime offering an interesting alternative to conventional clock based on 0-0 transition as the former can also eliminate the influence of the first-order Zeeman effect. Using the Breit-Rabi formula, we theoretically calculate the magnetically insensitive frequency and the corresponding C-field value for different alkali atom species. We also provide an estimate of the uncertainty of frequency shift to change of C-field. We show that a high C-field clock based on Cs: 3,-1 - 4,0, 3,0 - 4,-1, and 3,-1 - 4,-1, can achieve a bias of 10-14 with a C-field of 10-6 uncertainty. The free first-order Zeeman effect C-field for 3,-1 - 4,0 or 3,0 - 4,-1 transition is about one half that for 3,-1 - 4,-1 transition.

physics.atom-ph↗