Ruifang Dong

Demonstration of quantum synchronization based on second-order quantum coherence of entangled photons

Based on the second-order quantum interference between frequency entangled photons that are generated by parametric down conversion, a quantum strategic algorithm for synchronizing two spatially separated clocks has been recently presented. In the reference frame of a Hong-Ou-Mandel (HOM) interferometer, photon correlations are used to define simultaneous events. Once the HOM interferometer is balanced by use of an adjustable optical delay in one arm, arrival times of simulta- neously generated photons are recorded by each clock. The clock offset is determined by correlation measurement of the recorded arrival times. Utilizing this algorithm, we demonstrate a proof-of-principle experiment for synchronizing two clocks separated by 4 km fiber link. A minimum timing stability of 0.44 ps at averaging time of 16000 s is achieved with an absolute time accuracy of 73.2 ps. The timing stability is verified to be limited by the correlation measurement device and ideally can be better than 10 fs. Such results shine a light to the application of quantum clock synchronization in the real high-accuracy timing system.

Two-component dipolar Bose-Einstein condensate in concentrically coupled annular traps

Dipolar Bosonic atoms confined in external potentials open up new avenues for quantum-state manipulation and will contribute to the design and exploration of novel functional materials. Here we investigate the ground-state and rotational properties of a rotating two-component dipolar Bose-Einstein condensate, which consists of both dipolar bosonic atoms with magnetic dipole moments aligned vertically to the condensate and one without dipole moments, confined in concentrically coupled annular traps. For the nonrotational case, it is found that the tunable dipolar interaction can be used to control the location of each component between the inner and outer rings, and to induce the desired ground-state phase. Under finite rotation, it is shown that there exists a critical value of rotational frequency for the nondipolar case, above which vortex state can form at the trap center, and the related vortex structures depend strongly on the rotational frequency. For the dipolar case, it is found that various ground-state phases and the related vortex structures, such as polygonal vortex clusters and vortex necklaces, can be obtained via a proper choice of the dipolar interaction and rotational frequency. Finally, we also study and discuss the formation process of such vortex structures.

Realization of multiform time derivatives of pulses using a Fourier pulse shaping system

The technique of multiform time derivatives of pulse has been shown necessary to achieve various time-space metrology goals with a precision at or beyond the standard quantum limit. However, the efficient generation of the desired time derivatives remains challenging. In this paper, we report on the efficient realization of multiform time derivatives with a programmable 4-f pulse shaping system. The first-order time derivative of the pulse electric field has been achieved with a generation efficiency of 72.12%, which is more than 20 times higher than that of previous methods. Moreover, the first- and second-order time derivatives of the pulse envelope have been achieved with the generation efficiencies being 11.10% and 3.53%, respectively. In comparison, these efficiencies are three times higher than those for previously reported methods. Meanwhile, the measured fidelities of the three time-derived pulses are reasonably high, with values of 99.53%, 98.37% and 97.32% respectively.

Exotic vortex lattices in a rotating binary dipolar Bose-Einstein condensate.

In the last decade, considerable advances have been made in the investigation of dipolar quantum gases. Previous theoretical investigations of a rotating binary dipolar Bose-Einstein condensate, where only one component possesses dipole moment, were mainly focused on two special orientations of the dipoles: perpendicular or parallel to the plane of motion. Here we study the ground-state and rotational properties of such a system for an arbitrary orientation of the dipoles. We demonstrate the ground-state vortex structures depend strongly on the relative strength between dipolar and contact interactions and the rotation frequency, as well as on the orientation of the dipoles. In the absence of rotation, the tunable dipolar interaction can be used to induce the squeezing or expansion of the cloud, and to derive the phase transition between phase coexistence and separation. Under finite rotation, the system is found to exhibit exotic ground-state vortex configurations, such as kernel-shell, vortex necklace, and compensating stripe vortex structures. We also check the validity of the Feynman relation, and find no significant deviations from it. The obtained results open up alternate ways for the quantum control of dipolar quantum gases.

Coherent Transfer of Optical Frequency over 112km with Instability at the 10

We demonstrate optical-carrier transfer over a 112-km single-span urban fiber link. By actively compensating the phase noise induced along the fiber link, a noise suppression of 55 dB at 1 Hz is obtained. A fractional frequency instability of 2.5 × 10?16 at 1s is achieved, and reaching 7.5 × 10?20 at 10000 s. The system is stable and able to run for a long time. This work will contribute to optical frequency distribution and remote comparison among atomic clocks.

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Thermal noise in optical reference cavities sets a severe limit to the frequency instability of ultra-stable lasers. We, for the first time, give an analytic expression of the contribution of the spindle spacer based on Levins formulation of the fluctuation-dissipation-theorem (FDT). The analysis indicates that the thermal noise of spindle cavity has a few different features from cylindrical cavities with similar geometry. The analytic estimate for the spindle spacer by our equation fits better with finite element analysis (FEA) results compared with previous equations.

Level

Abstract. Two diode lasers at 698 nm are separately locked to two independent optical reference cavities with a finesse of about 128,000 by the Pound–Drever–Hall method. The more accurate coefficient between voltage and frequency of the error signal is measured, with which quantitative evaluation of the effect of many noises on the frequency stability can be made much more conveniently. A temperature-insensitive method is taken to reduce the effect of residual amplitude modulation on laser frequency stability. With an active fiber noise cancellation, the optical heterodyne beat between two independent lasers shows that the linewidth of one diode laser reaches 1 Hz. The fractional Allan deviation removed linear frequency shift less than 30  mHz/s is below 2.6×10−15 with 1- to 100-s average time.

New analytic estimate of thermal noise in spindle optical cavities

Along with the rapid development of the optical clocks, optical frequency transfer via communication fibers has shown much higher precision than the conventional methods. In this paper, we report the progress on the optical frequency transfer via communication fibers at NTSC. Over a 71-km spooled fiber, the optical frequency transfer with a residual instability (Allan deviation) of 5.4E-15 at 1s and 5.6E-18 at 104s is demonstrated. Furthermore, for a measurement bandwidth of 100 Hz, the instability is 1.2E-15 at one second and 1.4E-18 at 104 s. This transfer instability is fundamentally limited by the fiber length. The Allan deviation curves fall as τ-1 for short times, which is consistent with the fact that the transferred signal is dominated by white phase noise. This experimental achievement provides the foundation for the upcoming optical transfer experiment over a 300-km urban communication fiber link.

698-nm diode laser with 1-Hz linewidth

Frequency entangled biphotons based on spontaneous parametric down conversion (SPDC) of nonlinear crystal are widely used in Quantum clock synchronization protocols. The time-correlation width of the entangled source determines the accuracy of the attainable synchronization, which relies on the spectral bandwidth of the generated biphotons. We theoretically investigate the generation of frequency anti-correlated biphotons from chirped periodically-poled potassium titanyl phosphate (C-PPKTP). It is demonstrated that an ultra-broadband entangled biphoton source with a width of 857 nm was obtained, by using a 10 mm-long crystal with a chirping of 9.7 × 10−6 μm−2, and a cw pumping source with a wavelength of 792 nm. The corresponding time correlation width is only 3.5 fs, which implies feasible clock synchronization accuracy in femtosecond scale. We further demonstrate that the narrowing of the time-correlation width slows down dramatically by increasing the chirping and the length of the nonlinear crystal, which provides a theoretical instruction for us to trade off between the complexity of the crystal fabrication and the sufficiently narrow time-correlation width.