arXiv subjects
Publications and source records attributed to Ying Li.
We present a protocol for generating entanglement over long distances in a two-dimensional quantum network based on the surface error-correction code. This protocol requires a fixed number of quantum memories at each node of the network and tolerates error rates of up to 1.67% in the quantum channels.
In this work, within the QCD factorization approach, we investigate the branching fractions and CP asymmetries of decays $B \to K_0^*(1430)ρ$ and $B \to K_0^*(1430)ϕ$ under two different scenarios both in the standard model and the family nonuniversal $Z^\prime$ model. We find that the annihilation terms play crucial roles in these decays and lead to the main uncertainties. For decays $B^- \to \bar K_0^{*-} (1430)ρ^0 (ω)$, the new $Z^\prime$ boson could change branching fractions remarkably. However, for other decays, its contribution might be clouded by large uncertainties from annihilations. Unfortunately, neither the standard model nor $Z^\prime$ model can reproduce all experimental data under one certain scenario. We also noted that the CP asymmetries of $B^-\to \bar K_0^{*-}(1430) ρ^0 (ω)$ could be used to identify the$K_0^*(1430)$ meson and search for the new physics contribution.
We present the design of a femtogram L3-nanobeam photonic crystal cavity for optomechanical studies. Two symmetric nanobeams are created by placing three air slots in a silicon photonic crystal slab where three holes are removed. The optical quality factor (Q) is optimized up to 52,000. The nanobeams' mechanical frequencies are higher than 600 MHz due to their femtogram effective modal masses. The optical and mechanical modes are dispersively coupled with a vacuum optomechanical coupling rate g0/2pi exceeding 200 kHz. The anchor-loss-limited mechanical Q of the differential beam mode is evaluated to be greater than 10,000 for structures with ideally symmetric beams. The influence of variations on the air slot width and position is also investigated. The devices can be used as ultrasensitive sensors of mass, force, and displacement.
We experimentally demonstrate an optomechanical cavity based on an air-slot photonic crystal cavity with optical quality factor Qo=4.2\times10^4 and a small modal volume of 0.05 cubic wavelengths. The optical mode is coupled with the in-plane mechanical modes with frequencies up to hundreds of MHz. The fundamental mechanical mode shows a frequency of 65 MHz and a mechanical quality factor of 376. The optical spring effect, optical damping and amplification are observed with a large experimental optomechanical coupling rate gom/2pi of 154 GHz/nm, corresponding to a vacuum optomechanical coupling rate g*/2pi of 707 kHz. With sub-mW or less input power levels, the cavity exhibits strong parametric oscillations. The phase noise of the photonic crystal optomechanical oscillator is also measured.
In the distributed quantum computing paradigm, well-controlled few-qubit `nodes' are networked together by connections which are relatively noisy and failure prone. A practical scheme must offer high tolerance to errors while requiring only simple (i.e. few-qubit) nodes. Here we show that relatively modest, three-qubit nodes can support advanced purification techniques and so offer robust scalability: the infidelity in the entanglement channel may be permitted to approach 10% if the infidelity in local operations is of order 0.1%. Our tolerance of network noise is therefore a order of magnitude beyond prior schemes, and our architecture remains robust even in the presence of considerable decoherence rates (memory errors). We compare the performance with that of schemes involving nodes of lower and higher complexity. Ion traps, and NV- centres in diamond, are two highly relevant emerging technologies.
Within the QCD factorization formalism, we study the possible impacts of the nonuniversal $Z^\prime$ model, which provides a flavor-changing neutral current at the tree level, on rare decays $B \to K_0^*π$. Under two different scenarios (S1 and S2) for identifying the scalar meson $K_0^*(1430)$, the branching ratios, CP asymmetries, and isospin asymmetries are calculated in both the standard model (SM) and the family nonuniversal $Z^\prime$ model. We find that the branching ratios and CP asymmetries are sensitive to weak annihilation. In the SM, with $ρ_A=1$ and $ϕ_A\in[-30^\circ, 30^\circ]$, the branching ratios of S1 (S2) are smaller (larger) than the experimental data. Adding the contribution of the $Z^\prime$ boson in two different cases (Case-I and Case-II), for S1, the branching ratios are still far away from experiment. For S2, in Case-II, the branching ratios become smaller and can accommodate the data; in Case-I, although the center values are enhanced, they can also explain the data with large uncertainties. Similar conclusions are also reached for CP asymmetries. Our results indicate that S2 is more favored than S1, even after considering new physics effects. Moreover, if there exists a nonuniversal $Z^\prime$ boson, Case-II is preferred. All results can be tested in the LHC-b experiment and forthcoming super-B factory.
The implications of the family non-universal $Z^\prime$ model in the $B\to K_{1}(1270,1400)\ell^{+}\ell^{-}(\ell=e\,,μ\,,τ)$ decays are explored, where the mass eigenstates $K_{1}(1270,1400)$ are the mixtures of $^{1}{P}_{1}$ and $^{3}{P}_{1}$ states with the mixing angle $θ$. In this work, considering the $Z^\prime$ boson and setting the mixing angle $θ=(-34\pm13)^{\circ}$, we analyze the branching ratio, the dilepton invariant mass spectrum, the normalized forward-backward asymmetry and lepton polarization asymmetries of each decay mode. We find that all observables of $B\to K_{1}(1270)μ^{+}μ^{-}$ are sensitive to the $Z^{\prime}$ contribution. Moreover, the observables of $B\to K_{1}(1400)μ^{+}μ^{-}$ are relatively strong $θ$-dependence; thus, the $Z^{\prime}$ contribution will be buried by the uncertainty of the mixing angle $θ$. Furthermore, the zero crossing position in the FBA spectrum of $B\to K_{1}(1270)μ^{+}μ^{-}$ at low dilepton mass will move to the positive direction with $Z^\prime$ contribution. For the tau modes, the effects of $Z^\prime$ are not remarkable due to the small phase space. These results could be tested in the running LHC-b experiment and Super-B factory.
Within the perturbative QCD approach and ignoring the contributions of long distance and subleading penguin loops, we investigate $\bar B^0 \to D^0 μ^+μ^-$ decay in the large recoiling kinematic region in the Standard Model. At the tree level, $\bar B^0$ decays to $D^0$ by exchanging a $W$ boson accompanied by a virtual photon emission from the valence quarks of $\bar B^0$ and $D^0$ meson, then the virtual photon decays to the lepton pair. Numerically, we find that the branching ratio decreases rapidly as the $q^2$ increases, and the branching ratio of $ \bar B^0\to D^0μ^+μ^-$ is $(9.7_{-3.2}^{+4.2})\times 10^{-6}$ in the region $q^2 \in [1,5] \mathrm{GeV}^2$. The order of the branching ratio shows a possibility to study this interesting channel in the current $B$ factories and the Large Hadron Collider. The precise experimental data will help us to test the factorization approach and the QCD theory, in general.
Spin valves incorporating perpendicularly magnetized materials are promising structures for memory elements and high-frequency generators. We report the angular dependence of the spin-transfer torque in spin valves with perpendicular equilibrium magnetization computed by first-principles circuit theory and compare results with experiments by W.H. Rippard c.s. [Phys. Rev. B 81, 014426 (2010)] on the CoFe|Cu|CoNi system. Furthermore, we predict a non-monotonous ("wavy") spin-transfer torque when the Cu spacer is replaced by a Ru layer.
Fiber-based remote comparison of $^{87}$Sr lattice clocks in 24 km distant laboratories is demonstrated. The instability of the comparison reaches $5\times10^{-16}$ over an averaging time of 1000 s, which is two orders of magnitude shorter than that of conventional satellite links and is limited by the instabilities of the optical clocks. By correcting the systematic shifts that are predominated by the differential gravitational redshift, the residual fractional difference is found to be $(1.0\pm7.3)\times10^{-16}$, confirming the coincidence between the two clocks. The accurate and speedy comparison of distant optical clocks paves the way for a future optical redefinition of the second.
Measurement-based quantum computation utilizes an initial entangled resource state and proceeds with subsequent single-qubit measurements. It is implicitly assumed that the interactions between qubits can be switched off so that the dynamics of the measured qubits do not affect the computation. By proposing a model spin Hamiltonian, we demonstrate that measurement-based quantum computation can be achieved on a thermal state with always-on interactions. Moreover, computational errors induced by thermal fluctuations can be corrected and thus the computation can be executed fault-tolerantly if the temperature is below a threshold value.
Utilizing form factors calculated within the light-cone sum rules, we have evaluated the decay branching ratios of $B_s\to ϕγ$ and $B_s\to ϕ\ell^+\ell^-$ in a single universal extra dimension model (UED), which is viewed as one of the alternative theories beyond the standard model (SM). For the decay $B_s \to ϕ\ell^+\ell^-$, the dilepton invariant mass spectra, the forward-backward asymmetry, and double lepton polarization are also calculated. For each case, we compared the obtained results with predictions of the SM. In lower values of the compactification factor 1/R, the only parameter in this model, we see the considerable discrepancy between the UED and SM models. However, when 1/R increases, the results of UED tend to diminish and at $1/R = 1000 \mathrm{GeV}$, two models have approximately the same predictions. Compared with data from CDF of $B_s \to ϕμ^+ μ^-$, the 1/R tends to be larger than $350 \mathrm{GeV}$. We also note that the zero crossing point of the forward-backward asymmetry is become smaller, which will be an important plat to prob the contribution from the extra dimension model. The results obtained in this work will be very useful in searching new physics beyond SM. Moreover, the order of magnitude for branching ratios shows a possibility to study these channels at the Large Hadron Collider (LHC), CDF and the future super-B factory.
We propose a protocol for the creation of photonic Greenberger-Horne-Zeilinger and linear cluster states emitted from a single atom---or ion---coupled to an optical cavity field. The method is based on laser pulses with different polarizations and exploits the atomic transition amplitudes to state-selectively achieve the desired transitions. The scheme lies within reach of current technology.
We propose a method to improve traditional character-based PPM text compression algorithms. Consider a text file as a sequence of alternating words and non-words, the basic idea of our algorithm is to encode non-words and prefixes of words using character-based context models and encode suffixes of words using dictionary models. By using dictionary models, the algorithm can encode multiple characters as a whole, and thus enhance the compression efficiency. The advantages of the proposed algorithm are: 1) it does not require any text preprocessing; 2) it does not need any explicit codeword to identify switch between context and dictionary models; 3) it can be applied to any character-based PPM algorithms without incurring much additional computational cost. Test results show that significant improvements can be obtained over character-based PPM, especially in low order cases.
We describe the strong optomechanical dynamical interactions in ultrahigh-Q/V slot-type photonic crystal cavities. The dispersive coupling is based on a mode-gap photonic crystal cavities with light localization in an air mode with 0.02(lambda/n)3 modal volumes while preserving optical cavity Q up to 5 x 106. The mechanical mode is modeled to have fundamental resonance omega_m/2pi of 460 MHz and a quality factor Qm estimated at 12,000. For this slot-type optomechanical cavity, the dispersive coupling gom is numerically computed at up to 940 GHz/nm (Lom of 202 nm) for the fundamental optomechanical mode. Dynamical parametric oscillations for both cooling and amplification, in the resolved and unresolved sideband limit, are examined numerically, along with the displacement spectral density and cooling rates for the various operating parameters.
In certain approaches to quantum computing the operations between qubits are non-deterministic and likely to fail. For example, a distributed quantum processor would achieve scalability by networking together many small components; operations between components should assumed to be failure prone. In the logical limit of this architecture each component contains only one qubit. Here we derive thresholds for fault tolerant quantum computation under such extreme paradigms. We find that computation is supported for remarkably high failure rates (exceeding 90%) providing that failures are heralded, meanwhile the rate of unknown errors should not exceed 2 in 10^4 operations.
We revisited the extracting $\left|V_{ub}/V_{cb}\right|$ and $|V_{ub}|$ through calculating the ratios ${\cal B}(B^0\to D_s^{(*)+}(π^-,ρ^-)/ {\cal B}(B^0\to D_s^{(*)+} D^{(*)-})$ in the perturbative QCD approach, which is regarded as an effective theory in dealing with non-leptonic charmed $B$ decays. Utilizing this approach, we could calculate the form factors effectively as well as non-factorizable and annihilation contributions. Within the updated distribution amplitudes and the latest experimental data, we get $\left|{V_{ub}}/{V_{cb}}\right|=0.083\pm0.007$, which favors a bit smaller $|V_{ub}|$ compared with the averaged PDG value, but agrees well the exclusively measured values. Furthermore, we predict the branching ratio of $\bar B^0 \to D_s^-ρ^+$ $\sim$ $(2.7\pm 1.2)\times 10^{-5}$, which could be measured in $B$ factories near future. In our calculation, the major uncertainty is due to our poor knowledge of heavy meson wave functions. We also comment that it is not trivial to generalize this approach to $B_s$ system, primarily because both $\bar B_s^0 $ and $B_s^0$ can decay into the same final states $D_s^+K^-$ and $D_s^+D_s^-$.
This paper considers a distributed interference avoidance problem employing frequency assignment in the Gaussian interference channel (IC). We divide the common channel into several subchannels and each user chooses the subchannel with less amount of interference from other users as the transmit channel. This mechanism named interference avoidance in this paper can be modeled as a competitive game model. And a completely autonomous distributed iterative algorithm called Tdistributed interference avoidance algorithm (DIA) is adopted to achieve the Nash equilibriumT (NE) of the game. Due to the self-optimum, DIA is a sub-optimal algorithm. Therefore, through introducing an optimal compensation into the competitive game model, we successfully develop a compensation-based game model to approximate the optimal interference avoidance problem. Moreover, an optimal algorithm called iterative optimal interference avoidance algorithm (IOIA) is proposed to reach the optimality of the interference avoidance scheme. We analyze the implementation complexities of the two algorithms. We also give the proof on the convergence of the proposed algorithms. The performance upper bound and lower bound are also derived for the proposed algorithms. The simulation results show that IOIA does reach the optimality under condition of interference avoidance mechanism.