arXiv subjects

Wuming Liu

Publications and source records attributed to Wuming Liu.

28 records · Page 2Linked to original sources

arXiv2019

Twin-beam-enhanced displacement measurement of a membrane in a cavity

Ultrasensitive measurement of a small displacement is an essential goal in various applications of science and technology, ranging from large-scale laser interferometric gravitational wave detectors to micro-electro-mechanical-systems-based force microscopy. The least measurable displacement is ultimately limited by the quantum nature of light in a classical optical sensor. Here we use the bright quantum correlated light, i.e., twin beams, generated by a coherent atomic medium to surpass the shot-noise limit (SNL) of the displacement measurement of a membrane in an optical cavity. The sensitivity of 200 $am/\sqrt {Hz}$ is achieved, which is more than two orders of magnitude better than a standard Michelson interferometer. An improvement of 3 $dB$ in the signal-to-noise ratio (SNR) beyond the SNL is realized at an equivalent optical power, by using quantum correlated light with noise squeezed 7 $dB$ below the vacuum level. Moreover, the frequency correlation of twin beams is directly measured by using optical cavities, and this relation is utilized to reduce the excess classical noise. Additionally, the displacement measurement sensitivity is further substantially enhanced by the cavity mediated dispersion and the SNR is increased by one order of magnitude compared to the free space case. These results provide a novel strategy for the world of precision measurement as well as to control cavity optomechanical systems with non-classical light.

physics.optics↗

arXiv2019

Closed-loop Control of Compensation Point in the K-Rb-$^{21}$Ne Comagnetometer

We investigate the real-time closed-loop control of compensation point in the K-Rb-$^{21}$Ne comagnetometer operated in the spin-exchange relaxation-free regime. By locking the electron resonance, the alkali metal electrons are free from the fluctuations of the longitudinal ambient magnetic field and nuclear magnetization, which could improve the systematic stability, enlarge the linear measuring range, and suppress the cross-talk error of the comagnetometer. This is the first demonstration of closed-loop control of magnetic field in the single nuclear species comagnetometer, which will be of great significance for rotation sensing as gyroscopes and other high precision metrology applications of the comagnetometer.

physics.atom-ph↗

arXiv2018

Expansion dynamics of a spherical Bose-Einstein condensate

We experimentally and theoretically observe the expansion behaviors of a spherical Bose-Einstein condensate. A rubidium condensate is produced in an isotropic optical dipole trap with an asphericity of 0.037. We measure the variation of the condensate size during the expansion process. The free expansion of the condensate is isotropic, which is different from that of the condensate usually produced in the anisotropic trap. The expansion in the short time is speeding and then after a long time the expansion velocity asymptotically approaches a constant value. We derive an analytic solution of the expansion behavior based on the spherical symmetry, allowing a quantitative comparison with the experimental measurement. The interaction energy of the condensate is gradually converted into the kinetic energy at the beginning of the expansion and the kinetic energy dominates after a long-time expansion. We obtain the interaction energy of the condensate in the trap by probing the expansion velocity, which is consistent with the theoretical prediction.

cond-mat.quant-gas↗

arXiv2015

Optimization of broadband omnidirectional antireflection coatings for solar cells

Broadband and omnidirectional antireflection coating is a generally effective way to improve solar cell efficiency, because the destructive interference between the reflected and input waves could maximize transmission light in the absorption layer. Several theoretical calculations have been developed to optimize the anti-reflective coating to maximize the average transmittance. However, the solar irradiances of the clear sky spectral direct beam on a receiver plane at different positions and times are variable greatly. Here we report a new theoretical calculation of anti-reflective coating with incident quantum efficiency ηin as evaluation function for practical application. The two-layer and three-layer anti-reflective coatings are optimized over λ = [300, 1100] nm and θ = [0°, 90°] for cities of Quito, Beijing and Moscow. The ηin of two-layer anti-reflective coating increases by 0.26%, 1.37% and 4.24% for these 3 cities, respectively, compared with that other theoretical calculations due to better match between the local actual solar spectrum and quantum efficiency spectrum. Our numerical simulation and comparison data with other optimization methods suggest that this optimization method combining ant colony algorithm method with SPCTRL2 solar spectral irradiance can effectively push the efficient solar cell toward higher quantum efficiency, thus enabling high utilization efficiency of solar irradiance.

physics.optics↗

arXiv2015

High-responsivity vertical-illumination Si/Ge uni-traveling-carrier photodiodes based on silicon-on-insulator substrate

Si/Ge uni-traveling carrier photodiodes exhibit higher output current when space-charge effects are overcome and thermal effects are suppressed, which is highly beneficial for increasing the dynamic range of various microwave photonic systems and simplifying high-bit-rate digital receivers in different applications. From the point of view of packaging, detectors with vertical-illumination configuration can be easily handled by pick-and-place tools and are a popular choice for making photo-receiver modules. However, vertical-illumination Si/Ge uni-traveling carrier (UTC) devices suffer from inter-constraint between high speed and high responsivity. Here, we report a high responsivity vertical-illumination Si/Ge UTC photodiode based on a silicon-on-insulator substrate. The maximum absorption efficiency of the devices was 2.4 times greater than the silicon substrate owing to constructive interference. The Si/Ge UTC photodiode was successfully fabricated and had a dominant responsivity at 1550 nm of 0.18 A/W, a 50% improvement even with a 25% thinner Ge absorption layer.

physics.ins-det↗

arXiv2015

Mechanism and modulation of terahertz generation from a semimetal - graphite

Semi-metals might offer a stronger interaction and a better confinement for terahertz wave than semiconductors, while preserve tunability. Particularly, graphene-based materials are envisioned as terahertz modulators, filters and ultra-broadband sources. However, the understanding of terahertz generation from those materials is still not clear, thus limits us recognizing the potential and improving device performances. Graphite, the mother material of graphene and a typical bulk semi-metal, is a good system to study semi-metals and graphene-based materials. Here we experimentally modulate and maximize the terahertz signal from graphite surface, thus reveal the mechanism - surface field driving photon induced carriers into transient current to radiate terahertz wave. We also discuss the differences between graphite and semiconductors; particularly graphite shows no temperature dependency from room temperature to 80C. Above knowledge will help us understand terahertz generations, achieve maximum output and electric modulation, in semi-metal or graphene based devices.

physics.optics↗

arXiv2013

Fidelity susceptibility and topological phase transition of a two dimensional spin-orbit coupled Fermi superfluid

We investigate the fidelity susceptibility (FS) of a two-dimensional spin-orbit coupled (SOC) Fermi superfluid and the topological phase transition driven by a Zeeman field in the perspective of its ground-state wave-function. Without Zeeman coupling, FS shows new features characterizing the BCS-BEC crossover induced by SOC. In the presence of a Zeeman field, the topological phase transition is explored using both FS and the topological invariant. In particular, we obtain the analytical result of the topological invariant which explicitly demonstrates that the topological phase transition corresponds to a sudden change of the ground state wave-function. Consequently, FS diverges at the phase transition point with its critical behavior being: $χ\propto ln|h-h_{c}|$ . Based on this observation, we conclude that the topological phase transition can be detected by measuring the momentum distribution in cold atoms experiment.

cond-mat.supr-con↗

arXiv2013

Goldstone and Higgs modes of photons inside a cavity

Goldstone and Higgs modes have been detected in various condensed matter, cold atom and particle physics experiments. Here, we demonstrate that the two modes can also be observed in optical systems with only a few (artificial) atoms inside a cavity. We establish this connection by studying the $ U(1)/Z_2 $ Dicke model where $ N $ qubits (atoms) coupled to a single photon mode. We determine the Goldstone and Higgs modes inside the super-radiant phase and their corresponding spectral weights by performing both $ 1/J=2/N $ expansion and exact diagonization (ED) study at a finite $ N $. We find nearly perfect agreements between the results achieved by the two approaches when $ N $ gets down even to $ N = 2 $. The quantum finite size effects at a few qubits make the two modes quite robust against an effectively small counter-rotating wave term. We present a few schemes to reduce the critical coupling strength, so the two modes can be observed in several experimental systems of (artificial) atoms inside a cavity by just conventional optical measurements.

quant-ph↗

arXiv2012

Exciton correlations and input-output relations in non-equilibrium exciton superfluids

The Photoluminescent (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron-hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg-Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous Greater, Lesser, Advanced, Retarded, and Time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input-output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton-polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron-electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor $ ν_T= 1 $. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems.

cond-mat.str-el↗

arXiv2011

An unified minimum effective model of magnetism in iron-based superconductors

Since 2008, many new families of iron-based high temperature (high-$T_c$) superconductors have been discovered \cite{Hosono,ChenXH,FeTe,ChenXL}. Unlike all parent compounds of cuprates that share a common antiferromagnetically (AF) ordered ground state, those of iron-based superconductors exhibit many different AF ordered ground states, including collinear-AF (CAF) state in ferropnictides \cite{caf}, bicollinear-AF (BCAF) state in 11-ferrochalcogenide $FeTe$ \cite{bcaf,bcaf2}, and block-AF (BAF) state in 122-ferrochalcogenide $K_{0.8}Fe_{1.6}Se_2$ \cite{baf}. While the universal presence of antiferromagnetism suggests that superconductivity is strongly interrelated with magnetism, the diversity of the AF ordered states obscures their interplay. Here we show that all magnetic phases can be unified within an effective magnetic model. This model captures three incommensurate magnetic phases, two of which have been observed experimentally. The model characterizes the nature of phase transitions between the different magnetic phases and explains a variety of magnetic properties, such as spin-wave spectra and electronic nematism. Most importantly, by unifying the understanding of magnetism, we cast new insight on the key ingredients of magnetic interactions which are critical to the occurrence of superconductivity.

cond-mat.supr-con↗