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X. Ma

Publications and source records attributed to X. Ma.

108 records · Page 6Linked to original sources
arXiv2017

Faithful conversion of propagating quantum information to mechanical motion

We convert propagating qubits encoded as superpositions of zero and one photons to the motion of a micrometer-sized mechanical resonator. Using quantum state tomography, we determine the density matrix of both the propagating photons and the mechanical resonator. By comparing a sufficient set of states before and after conversion, we determine the average process fidelity to be $F_{\textrm{avg}} = 0.83\substack{+0.03-0.06}$ which exceeds the classical bound for the conversion of an arbitrary qubit state. This conversion ability is necessary for using mechanical resonators in emerging quantum communication and modular quantum computation architectures.

quant-ph↗
arXiv2016

Reversible Vector Ratchets for Skyrmion Systems

We show that ac driven skyrmions interacting with an asymmetric substrate provide a realization of a new class of ratchet system which we call a vector ratchet that arises due to the effect of the Magnus term on the skyrmion dynamics. In a vector ratchet, the dc motion induced by the ac drive can be described as a vector that can be rotated clockwise or counterclockwise relative to the substrate asymmetry direction. Up to a full $360^{\circ}$ rotation is possible for varied ac amplitudes or skyrmion densities. In contrast to overdamped systems, in which ratchet motion is always parallel to the substrate asymmetry direction, vector ratchets allow the ratchet motion to be in any direction relative to the substrate asymmetry. It is also possible to obtain a reversal in the direction of rotation of the vector ratchet, permitting the creation of a reversible vector ratchet. We examine vector ratchets for ac drives applied parallel or perpendicular to the substrate asymmetry direction, and show that reverse ratchet motion can be produced by collective effects. No reversals occur for an isolated skyrmion on an asymmetric substrate. Since a vector ratchet can produce motion in any direction, it could represent a new method for controlling skyrmion motion for spintronic applications.

cond-mat.str-el↗
arXiv2016

Talbot effect for exciton-polaritons

We demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter waves -- exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic 'Talbot carpet' is produced by loading the exciton-polariton condensate into a microstructured one dimensional periodic array of mesa traps, which creates an array of sources for coherent polariton flow in the plane of the quantum wells. The spatial distribution of the Talbot fringes outside the mesas mimics the near-field diffraction of a monochromatic wave on a periodic amplitude and phase grating with the grating period comparable to the wavelength. Despite the lossy nature of the polariton system, the Talbot pattern persists for distances exceeding the size of the mesas by an order of magnitude.

cond-mat.quant-gas↗
arXiv2015

Collective state transitions of exciton-polaritons loaded into a periodic potential

We study the loading of a nonequilibrium, dissipative system of composite bosons -- exciton polaritons -- into a one dimensional periodic lattice potential. Utilizing momentum resolved photoluminescence spectroscopy, we observe a transition between an incoherent Bose gas and a polariton condensate, which undergoes further transitions between different energy states in the band-gap spectrum of the periodic potential with increasing pumping power. We demonstrate controlled loading into distinct energy bands by modifying the size and shape of the excitation beam. The observed effects are comprehensively described in the framework of a nonequilibrium model of polariton condensation. In particular, we implement a stochastic treatment of quantum and thermal fluctuations in the system and confirm that polariton-phonon scattering is a key energy relaxation mechanism enabling transitions from the highly nonequilibrium polariton condensate in the gap to the ground band condensation for large pump powers.

cond-mat.mes-hall↗
arXiv2015

Instability-induced formation and non-equilibrium dynamics of phase defects in polariton condensates

We study, theoretically and numerically, the onset and development of modulational instability in an incoherently pumped spatially homogeneous polariton condensate. Within the framework of mean-field theory, we identify regimes of modulational instability in two cases: 1) Strong feedback between the condensate and reservoir, which may occur in scalar condensates, and 2) Parametric scattering in the presence of polarization splitting in spinor condensates. In both cases we investigate the instability induced textures in space and time including non-equilibrium dynamics of phase dislocations and vortices. In particular we discuss the mechanism of vortex destabilization and formation of spiraling waves. We also identify the presence of topological defects, which take the form of half-vortex pairs in the spinor case, giving an "eyelet" structure in intensity and dipole type structure in the spin polarization. In the modulationally stable parameter domains, we observe formation of the phase defects in the process of condensate formation from an initially spatially incoherent low-density state. In analogy to the Kibble-Zurek type scaling for nonequilibrium phase transitions, we find that the defect density scales with the pumping rate.

cond-mat.mes-hall↗
arXiv2015

Oscillatory dynamics of non-equilibrium dissipative exciton-polariton condensates in weak-contrast lattices

We study nonlinear dynamics of exciton-polaritons in an incoherently pumped semiconductor microcavity with embedded weak-contrast lattice and coupled to an exciton reservoir. We elucidate fundamental features of non-equilibrium exciton-polariton condensate trapped in one-dimensional periodical potential close to zero momentum (so-called "Zero-state") and to the state at the boundary of Brillouin zone ("$π$-state"). Within the framework of the mean-field theory, we identify different regimes of both relaxation and oscillatory dynamics of coherent exciton-polaritons governed by superpositions of Bloch eigenstates within the periodic lattice. In particular, we theoretically demonstrate stable macroscopical oscillations, akin to nonlinear Josephson oscillations, between different spectral components of a polariton condensate in the momenta-space. We elucidate a strong influence of the dissipative effects and the feedback induced by the inhomogeneity of incoherent reservoir on the dynamics of the coherent polaritons.

cond-mat.mes-hall↗
arXiv2014

Information processing with topologically protected vortex memories in exciton-polariton condensates

We show that in a non-equilibrium system of an exciton-polariton condensate, where polaritons are generated from incoherent pumping, a ring-shaped pump allows for stationary vortex memory elements of topological charge $m = 1$ or $m = -1$. Using simple potential guides we can choose whether to copy the same charge or invert it onto another spatially separate ring pump. Such manipulation of binary information opens the possibility of a new type processing using vortices as topologically protected memory components.

cond-mat.quant-gas↗
arXiv2013

Mutual ionization in atomic collisions near the electronic threshold

We study mutual ionization in collisions between atomic hydrogen and helium at impact velocities near the electronic threshold for this process (determined by the condition that kinetic energy of an equivelocity free electron is approximately equal to the sum of binding energies of active electrons in the projectile and target). We show that this process is substantially influenced by the Coulomb repulsion between the emitted electrons and that the atomic nuclei are very strongly involved in the momentum balance along the collision velocity.

physics.atom-ph↗
arXiv2013

Longitudinal Schottky spectra of a bunched Ne10+ ion beam at the CSRe

The longitudinal Schottky spectra of a radio-frequency (RF) bunched and electron cooled 22Ne10+ ion beam at 70 MeV/u have been studied by a newly installed resonant Schottky pick-up at the experimental cooler storage ring (CSRe), at IMP. For an RF-bunched ion beam, a longitudinal momentum spread of has been reached with less than 107 stored ions. The reduction of momentum spread compared with coasting ion beam was observed from Schottky noise signal of the bunched ion beam. In order to prepare the future laser cooling experiment at the CSRe, the RF-bunching power was modulated at 25th, 50th and 75th harmonic of the revolution frequency, effective bunching amplitudes were extracted from the Schottky spectrum analysis. Applications of Schottky noise for measuring beam lifetime with ultra-low intensity of ion beams are presented, and it is relevant to upcoming experiments on laser cooling of relativistic heavy ion beams and nuclear physics at the CSRe.

physics.acc-ph↗
arXiv2012

On the dynamics of transfer-ionization in fast ion-atomic collisions

We consider trasfer-ionization in collisions of fast (3.6 -- 11 MeV/u) protons, alpha-particles and lithium nuclei with helium atoms. There are just a few basic mechanisms contributing to this process which can be grouped into correlated ones, which crucially depend on the electron-electron interaction, and uncorrelated, which do not need this interaction to proceed. We show that by exploring momentum spectra of the emitted electrons the correlated and uncorrelated mechanisms can be cleary separated from each other. This exploration also enables one to get insight into subtle details of the dynamics of transfer-ionization.

physics.atom-ph↗
arXiv2012

Scaling of intrinsic Gilbert damping with spin-orbital coupling strength

We have experimentally and theoretically investigated the dependence of the intrinsic Gilbert damping parameter $α_0$ on the spin-orbital coupling strength $ξ$ by using L1$_{\mathrm{0}}$ ordered FePd$_{\mathrm{1-x}}$Pt$_{\mathrm{x}}$ ternary alloy films with perpendicular magnetic anisotropy. With the time-resolved magneto-optical Kerr effect, $α_0$ is found to increase by more than a factor of ten when $x$ varies from 0 to 1.0. Since changes of other leading parameters are found to be neglected, the $α_0$ has for the first time been proven to be proportional to $ξ^2$.

cond-mat.mtrl-sci↗
arXiv2012

Test of detuning system for dielectronic recombination experiment at CSRm

The storage ring equipped with an electron cooler is an ideal platform for dielectronic recombination (DR) experiments. In order to fulfil the requirement of DR measurements at the main Cooler Storage Ring, a detuning system for the precision control of the relative energy between the ion beam and the electron beam has been installed on the electron cooler device. The test run using 7.0 MeV/u C6+ beam was performed to examine the influence of this system on the performance of the stored ion beam. The Schottky spectra and the ion beam currents were recorded to monitor the beam status. The influence of pulse heights and widths of the detuning voltage on the ion beam was investigated. For the small pulse height, the experimental results from the Schottky spectrum were in good agreement with the theoretical results. The frequency shift in the Schottky spectrum is significantly reduced for the short pulse width. For the large pulse height, an oscillation phenomenon was observed. From the Schottky spectrum, we found the oscillation amplitude is dependent on the pulse width of detuning and the ion beam intensity. The detailed description of the phenomenon and the theoretical model based on the plasma oscillation was discussed in this paper.

physics.acc-ph↗
arXiv2011

Magnetic Sublevel Population and Alignment for the Excitation of H- and He-like Uranium in Relativistic Collisions

We have measured the alignment of the L-shell magnetic-substates following the K-shell excitation of hydrogen- and helium-like uranium in relativistic collisions with a low-Z gaseous target. Within this experiment the population distribution for the L-shell magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The results show a very distinctive behavior for the H- and He-like heavy systems. In particular for $K \rightarrow L$ excitation of He-like uranium, a considerable alignment of the L-shell levels was observed. A comparison of our experimental findings with recent rigorous relativistic predictions provides a good qualitative and a reasonable quantitative agreement, emphasizing the importance of the magnetic-interaction and many-body effects in the strong-field domain of high-Z ions.

physics.atom-ph↗
arXiv2009

Entangled Quantum Key Distribution with a Biased Basis Choice

We investigate a quantum key distribution (QKD) scheme which utilizes a biased basis choice in order to increase the efficiency of the scheme. The optimal bias between the two measurement bases, a more refined error analysis, and finite key size effects are all studied in order to assure the security of the final key generated with the system. We then implement the scheme in a local entangled QKD system that uses polarization entangled photon pairs to securely distribute the key. A 50/50 non-polarizing beamsplitter with different optical attenuators is used to simulate a variable beamsplitter in order to allow us to study the operation of the system for different biases. Over 6 hours of continuous operation with a total bias of 0.9837/0.0163 (Z/X), we were able to generate 0.4567 secure key bits per raw key bit as compared to 0.2550 secure key bits per raw key bit for the unbiased case. This represents an increase in the efficiency of the key generation rate by 79%.

quant-ph↗
arXiv2006

Phase Relations in the Li2O-V2O3-V2O5 System at 700 C: Correlations with Magnetic Defect Concentration in Heavy Fermion LiV2O4

The phase relations in the Li2O-V2O3-V2O5 ternary system at 700 C for compositions in equilibrium with LiV2O4 are reported. This study clarified the synthesis conditions under which low and high magnetic defect concentrations can be obtained within the spinel structure of LiV2O4. We confirmed that the LiV2O4 phase can be obtained containing low (0.006 mol%) to high (0.83 mol%) magnetic defect concentrations n{defect} and with consistently high magnetic defect spin S values between 3 and 6.5. The high n{defect} values were obtained in the LiV2O4 phase in equilibrium with V2O3, Li3VO4, or LiVO2 and the low values in the LiV2O4 phase in equilibrium with V3O5. A model is suggested to explain this correlation.

cond-mat.str-el↗
arXiv2005

Practical Decoy State for Quantum Key Distribution

Decoy states have recently been proposed as a useful method for substantially improving the performance of quantum key distribution. Here, we present a general theory of the decoy state protocol based on only two decoy states and one signal state. We perform optimization on the choice of intensities of the two decoy states and the signal state. Our result shows that a decoy state protocol with only two types of decoy states--the vacuum and a weak decoy state--asymptotically approaches the theoretical limit of the most general type of decoy state protocols (with an infinite number of decoy states). We also present a one-decoy-state protocol. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long distance (larger than 100km) QKD, our two-decoy-state protocol can be implemented with only a few hours of experimental data. In conclusion, decoy state quantum key distribution is highly practical.

quant-ph↗