arXiv subjects

Jun Chen

Publications and source records attributed to Jun Chen.

At least 505 records · Page 28Linked to original sources

arXiv2012

Proximity effect at superconducting Sn-Bi2Se3 interface

We have investigated the conductance spectra of Sn-Bi2Se3 interface junctions down to 250 mK and in different magnetic fields. A number of conductance anomalies were observed below the superconducting transition temperature of Sn, including a small gap different from that of Sn, and a zero-bias conductance peak growing up at lower temperatures. We discussed the possible origins of the smaller gap and the zero-bias conductance peak. These phenomena support that a proximity-effect-induced chiral superconducting phase is formed at the interface between the superconducting Sn and the strong spin-orbit coupling material Bi2Se3.

cond-mat.supr-con↗

arXiv2011

Strong Superconducting Proximity Effect in Pb-Bi2Te3 Hybrid Structures

Majarona fermions (MFs) were predicted more than seven decades ago but are yet to be identified [1]. Recently, much attention has been paid to search for MFs in condensed matter systems [2-10]. One of the seaching schemes is to create MF at the interface between an s-wave superconductor (SC) and a 3D topological insulator (TI) [11-13]. Experimentally, progresses have been achieved in the observations of a proximity-effect-induced supercurrent [14-16], a perfect Andreev reflection [17] and a conductance peak at the Fermi level [18]. However, further characterizations are still needed to clarify the nature of the SC-TI interface. In this Letter, we report on a strong proximity effect in Pb-Bi2Te3 hybrid structures, based on which Josephson junctions and superconducting quantum interference devices (SQUIDs) can be constructed. Josephson devices of this type would provide a test-bed for exploring novel phenomena such as MFs in the future.

cond-mat.mes-hall↗

arXiv2011

The methods to detect vacuum polarization by evanescent modes

We propose the evanescent-mode-sensing methods to probe the electrodynamics (QED) vacuum polarization. From our methods, high-sensitivity can be achieved even though the external field is much smaller than the Schwinger critical field and may be realizable in contemporary experimental conditions. The methods are based on the effect of phase change and time delay of evanescent wave which is transmitted in QED vacuum. These methods can also be widely used in sensitive probing of tiny dissipation in other fields.

cond-mat.mes-hall↗

arXiv2011

Aharonov-Casher effect in Bi$_{\rm 2}$Se$_{\rm 3}$ square-ring interferometers

Electrical control of spin dynamics in Bi$_{\rm 2}$Se$_{\rm 3}$ was investigated in ring-type interferometers. Aharonov-Bohm and Altshuler-Aronov-Spivak resistance oscillations against magnetic field, and Aharorov-Casher resistance oscillations against gate voltage were observed in the presence of a Berry phase of $π$. A very large tunability of spin precession angle by gate voltage has been obtained, indicating that Bi$_{\rm 2}$Se$_{\rm 3}$-related materials with strong spin-orbit coupling are promising candidates for constructing novel spintronic devices.

cond-mat.mes-hall↗

arXiv2011

Backward Pulling Force from a Forward Propagating Beam

A photon carries a momentum of, so one may anticipate light to "push" on any object standing in its path via the scattering force. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. Here, we show that it is possible to realize a backward scattering force which pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle and if the projection of the total photon momentum along the propagation direction is small (as in some propagation invariant beams), attractive optical force is possible. This possibility adds "pulling" as an additional degree of freedom to optical micromanipulation.

physics.optics↗

arXiv2010

Geometric kernel smoothing of tensor fields

In this paper, we study a kernel smoothing approach for denoising a tensor field. Particularly, both simulation studies and theoretical analysis are conducted to understand the effects of the noise structure and the structure of the tensor field on the performance of different smoothers arising from using different metrics, viz., Euclidean, log-Euclidean and affine invariant metrics. We also study the Rician noise model and compare two regression estimators of diffusion tensors based on raw diffusion weighted imaging data at each voxel.

stat.ME↗

arXiv2009

Mode expansion and Bragg filtering enable a high-fidelity fiber-based photon-pair source

We report the development of a fiber-based single spatial mode source of photon-pairs where the efficiency of extracting photon-pairs is improved over a previous source [18] through the use of fiber-end expansion and Bragg filters. This improvement in efficiency enabled a spectrally bright and pure photon-pair source having a small second-order correlation function (0.03) and a raw spectral brightness of 44,700 pairs/(s nm mW). The source can be configured to generate entangled photon-pairs,characterized via optimal and minimal quantum state tomography, to have a fidelity of 97% and tangle of 92%, without subtracting any background.

quant-ph↗

arXiv2009

A Random Variable Substitution Lemma With Applications to Multiple Description Coding

We establish a random variable substitution lemma and use it to investigate the role of refinement layer in multiple description coding, which clarifies the relationship among several existing achievable multiple description rate-distortion regions. Specifically, it is shown that the El Gamal-Cover (EGC) region is equivalent to the EGC* region (an antecedent version of the EGC region) while the Venkataramani-Kramer-Goyal (VKG) region (when specialized to the 2-description case) is equivalent to the Zhang-Berger (ZB) region. Moreover, we prove that for multiple description coding with individual and hierarchical distortion constraints, the number of layers in the VKG scheme can be significantly reduced when only certain weighted sum rates are concerned. The role of refinement layer in scalable coding (a special case of multiple description coding) is also studied.

cs.IT↗

arXiv2009

Resolution and sensitivity of a Fabry-Perot interferometer with a photon-number-resolving detector

With photon-number resolving detectors, we show compression of interference fringes with increasing photon numbers for a Fabry-Perot interferometer. This feature provides a higher precision in determining the position of the interference maxima compared to a classical detection strategy. We also theoretically show supersensitivity if N-photon states are sent into the interferometer and a photon-number resolving measurement is performed.

quant-ph↗

arXiv2009

30S RI Beam Production and X-ray Bursts

The present work reports the results of 30S radioactive beam development for a future experiment directly measuring data to extrapolate the 30S(alpha,p) stellar reaction rate in Type I X-ray bursts, a phenomena where nuclear explosions occur repeatedly on the surface of accreting neutron stars. We produce the radioactive ion 30S via the 3He(28Si,30S)n reaction, by bombarding a cryogenically cooled target of 3He at 400 Torr and 80 K with 28Si beams of 6.9 and 7.54 MeV/u. In order to perform a successful future experiment which allows us to calculate the stellar 30S(alpha, p) reaction rate, Hauser-Feshbach calculations indicate we require a 30S beam of ~10^5 particles per second at ~32 MeV. Based on our recent beam development experiments in 2006 and 2008, it is believed that such a beam may be fabricated in 2009 according to the results presented. We plan to measure the 4He(30S,p) cross-section at astrophysical energies in 2009, and some brief remarks on the planned (alpha,p) technique are also elucidated.

nucl-ex↗

arXiv2009

Mixing in stratified gravity currents: Prandtl mixing length

Shear-induced vertical mixing in a stratified flow is a key ingredient of thermohaline circulation. We experimentally determine the vertical flux of momentum and density of a forced gravity current using high-resolution velocity and density measurements. A constant eddy viscosity model provides a poor description of the physics of mixing, but a Prandtl mixing length model relating momentum and density fluxes to mean velocity and density gradients works well. For $ \approx 0.08$ and $Re_λ\approx 100$, the mixing lengths are fairly constant, about the same magnitude, comparable to the turbulent shear length.

physics.flu-dyn↗

arXiv2008

New Bounds for the Capacity Region of the Finite-State Multiple Access Channel

The capacity region of the Finite-State Multiple Access Channel (FS-MAC) with feedback that may be an arbitrary time-invariant function of the channel output samples is considered. We provided a sequence of inner and outer bounds for this region. These bounds are shown to coincide, and hence yield the capacity region, of FS-MACs where the state process is stationary and ergodic and not affected by the inputs, and for indecomposable FS-MAC when feedback is not allowed. Though the capacity region is `multi-letter' in general, our results yield explicit conclusions when applied to specific scenarios of interest.

cs.IT↗

arXiv2008

Demonstration of a Quantum Controlled-NOT Gate in the Telecom Band

We present the first quantum controlled-NOT (CNOT) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 $μ$m telecommunications band. Using this free-space CNOT gate, all four Bell states are produced and fully characterized by performing quantum state tomography, demonstrating the gate's unambiguous entangling capability and high fidelity. Telecom-band operation makes this CNOT gate particularly suitable for quantum information processing tasks that are at the interface of quantum communication and linear optical quantum computing.

quant-ph↗

arXiv2007

Multiuser Successive Refinement and Multiple Description Coding

We consider the multiuser successive refinement (MSR) problem, where the users are connected to a central server via links with different noiseless capacities, and each user wishes to reconstruct in a successive-refinement fashion. An achievable region is given for the two-user two-layer case and it provides the complete rate-distortion region for the Gaussian source under the MSE distortion measure. The key observation is that this problem includes the multiple description (MD) problem (with two descriptions) as a subsystem, and the techniques useful in the MD problem can be extended to this case. We show that the coding scheme based on the universality of random binning is sub-optimal, because multiple Gaussian side informations only at the decoders do incur performance loss, in contrast to the case of single side information at the decoder. We further show that unlike the single user case, when there are multiple users, the loss of performance by a multistage coding approach can be unbounded for the Gaussian source. The result suggests that in such a setting, the benefit of using successive refinement is not likely to justify the accompanying performance loss. The MSR problem is also related to the source coding problem where each decoder has its individual side information, while the encoder has the complete set of the side informations. The MSR problem further includes several variations of the MD problem, for which the specialization of the general result is investigated and the implication is discussed.

cs.IT↗

arXiv2007

Partially coherent matter wave and its evolution

The evolution and propagation of a partially coherent matter wave (PCMW) is investigated theoretically by the correlation function method. The ABCD matrix formalism previously used for a fully coherent matter wave is extended to the PCMW domain. A new ABCD law is derived, using a tensor method to describe the evolution of a PCMW. An analytical solution of the first-order correlation function is obtained that makes the propagation and evolution of a PCMW very simple and clear. As an example, the propagation of a PCMW in a gravitational field is calculated numerically.

physics.atom-ph↗

arXiv2007

Quantum theory of degenerate $χ^{(3)}$ two-photon state

We developed a quantum theory for degenerate $χ^{(3)}$ two-photon state generated from optical fiber, and compared the theory predictions with an experimental result which exhibits a Hong-Ou-Mandel dip visibility of around 94%. Excellent agreement between theory and experiment has been achieved, and we attribute the missing 6% visibility mainly to spatial mode mismatch between signal and idler photons at the beamsplitter.

quant-ph↗

arXiv2006

Slepian-Wolf Code Design via Source-Channel Correspondence

We consider Slepian-Wolf code design based on LDPC (low-density parity-check) coset codes for memoryless source-side information pairs. A density evolution formula, equipped with a concentration theorem, is derived for Slepian- Wolf coding based on LDPC coset codes. As a consequence, an intimate connection between Slepian-Wolf coding and channel coding is established. Specifically we show that, under density evolution, design of binary LDPC coset codes for Slepian-Wolf coding of an arbitrary memoryless source-side information pair reduces to design of binary LDPC codes for binary-input output-symmetric channels without loss of optimality. With this connection, many classic results in channel coding can be easily translated into the Slepian-Wolf setting.

cs.IT↗

arXiv2006

Capacity Results for Block-Stationary Gaussian Fading Channels with a Peak Power Constraint

We consider a peak-power-limited single-antenna block-stationary Gaussian fading channel where neither the transmitter nor the receiver knows the channel state information, but both know the channel statistics. This model subsumes most previously studied Gaussian fading models. We first compute the asymptotic channel capacity in the high SNR regime and show that the behavior of channel capacity depends critically on the channel model. For the special case where the fading process is symbol-by-symbol stationary, we also reveal a fundamental interplay between the codeword length, communication rate, and decoding error probability. Specifically, we show that the codeword length must scale with SNR in order to guarantee that the communication rate can grow logarithmically with SNR with bounded decoding error probability, and we find a necessary condition for the growth rate of the codeword length. We also derive an expression for the capacity per unit energy. Furthermore, we show that the capacity per unit energy is achievable using temporal ON-OFF signaling with optimally allocated ON symbols, where the optimal ON-symbol allocation scheme may depend on the peak power constraint.

cs.IT↗