Light-by-Light Scattering Constraint on Born-Infeld Theory

The recent measurement by ATLAS of light-by-light scattering in LHC Pb-Pb collisions is the first direct evidence for this basic process. We find that it requires the mass scale of a nonlinear Born-Infeld extension of QED to be $\gtrsim 100$~GeV, a much stronger constraint than those derived previously. In the case of a Born-Infeld extension of the Standard Model in which the U(1)$_{\rm Y}$ hypercharge gauge symmetry is realized nonlinearly, the limit on the corresponding mass scale is $\gtrsim 90$~GeV, which in turn imposes a lower limit of $\gtrsim 11$~TeV on the magnetic monopole mass in such a U(1)$_{\rm Y}$ Born-Infeld theory.Comment: 11 pages, 4 figures, version accepted for publication in PR

The Price of an Electroweak Monopole

In a recent paper, Cho, Kim and Yoon (CKY) have proposed a version of the SU(2) $\times$ U(1) Standard Model with finite-energy monopole and dyon solutions. The CKY model postulates that the effective U(1) gauge coupling $\to \infty$ very rapidly as the Englert-Brout-Higgs vacuum expectation value $\to 0$, but in a way that is incompatible with LHC measurements of the Higgs boson $H \to \gamma \gamma$ decay rate. We construct generalizations of the CKY model that are compatible with the $H \to \gamma \gamma$ constraint, and calculate the corresponding values of the monopole and dyon masses. We find that the monopole mass could be $< 5.5$ TeV, so that it could be pair-produced at the LHC and accessible to the MoEDAL experiment.Comment: 15 pages; Two clarifying footnotes (3 and 4) added. No effect on conclusion

Quantum correlated light pulses from sequential superradiance of a condensate

We discover an inherent mechanism for entanglement swap associated with sequential superradiance from an atomic Bose-Einstein condensate. Based on careful examinations with both analytical and numerical approaches, we conclude that as a result of the swap mechanism, Einstein-Podolsky-Rosen (EPR)-type quantum correlations can be detected among the scattered light pulses.Comment: 10 pages, 6 figure

Majorana fermions in s-wave noncentrosymmetric superconductor with Rashba and Dresselhaus (110) spin-orbit couplings

The asymmetric spin-orbit (SO) interactions play a crucial role in realizing topological phases in noncentrosymmetric superconductor (NCS).We investigate the edge states and the vortex core states in s-wave NCS with Rashba and Dresselhaus (110) SO couplings by both numerical and analytical methods. In particular, we demonstrate that there exists a novel semimetal phase characterized by the flat Andreev bound states in the phase diagram of the s-wave Dresselhaus NCS which supports the emergence of Majorana fermion (MF). The flat dispersion implies a peak in the density of states which has a clear experimental signature in the tunneling conductance measurements and the MFs proposed here should be experimentally detectable

Tunnelling of condensate magnetization in a double-well potential

We study quantum dynamical properties of a spin-1 atomic Bose-Einstein condensate in a double-well potential. Adopting a mean field theory and single spatial mode approximation, we characterize our model system as two coupled spins. For certain initial states, we find full magnetization oscillations between wells not accompanied by mass (or atom numbers) exchange. We identify dynamic regimes of collective spin variables arising from nonlinear self-interactions that are different from the usual Josephson oscillations. We also discuss magnetization beats and incomplete oscillations of collective spin variables other than the magnetization. Our study points to an alternative approach to observe coherent tunnelling of a condensate through a (spatial) potential barrier.Comment: 5 pages, 5 figures, submitted to Physical Review

Switchable coupling between charge and flux qubits

We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective inter-qubit coupling controlled by the external magnetic flux. This switchable inter-qubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.Comment: 5 pages, 1 figur

Superradiance induced topological vortex phase in a Bose-Einstein condensate

We investigate theoretically a topological vortex phase transition induced by a superradiant phase transition in an atomic Bose-Einstein condensate driven by a Laguerre-Gaussian optical mode. We show that superradiant radiation can either carry zero angular momentum, or be in a rotating Laguerre-Gaussian mode with angular momentum. The conditions leading to these two regimes are determined in terms of the width for the pump laser and the condensate size for the limiting cases where the recoil energy is both much smaller and larger than the atomic interaction energy.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Exactly solvable pairing model for superconductors with a p+ip-wave symmetry

We present the exact Bethe ansatz solution for the two-dimensional BCS pairing Hamiltonian with p_x + i p_y symmetry. Using both mean-field theory and the exact solution we obtain the ground-state phase diagram parameterized by the filling fraction and the coupling constant. It consists of three phases denoted weak coupling BCS, weak pairing, and strong pairing. The first two phases are separated by a topologically protected line where the exact ground state is given by the Moore-Read pfaffian state. In the thermodynamic limit the ground-state energy is discontinuous on this line. The other two phases are separated by the critical line, also topologically protected, previously found by Read and Green. We establish a duality relation between the weak and strong pairing phases, whereby ground states of the weak phase are "dressed" versions of the ground states of the strong phase by zero energy (Moore-Read) pairs and characterized by a topological order parameter.Comment: 4 pages, 2 figures. Discussion on winding numbers added. Accepted in Phys Rev