# All

Topological defects (kinks) in a relativistic $\phi^{4}$ scalar field theory

in $D=(1+1)$ are studied using the matrix product state tensor network. The one

kink state is approximated as a matrix product state and the kink mass is

calculated. The approach used is quite general and can be applied to a variety

of theories and tensor networks. Additionally, the contribution of

kink-antikink excitations to the ground state is examined and a general method

to estimate the scalar mass from equal time ground state observables is

We demonstrate optical spin polarization of the neutrally-charged

silicon-vacancy defect in diamond ($\mathrm{SiV^{0}}$), an $S=1$ defect which

emits with a zero-phonon line at 946 nm. The spin polarization is found to be

most efficient under resonant excitation, but non-zero at below-resonant

energies. We measure an ensemble spin coherence time $T_2>100~\mathrm{\mu s}$

at low-temperature, and a spin relaxation limit of $T_1>25~\mathrm{s}$. Optical

Phase-squeezed light can enhance the precision of optical phase estimation.

The larger the photon numbers are and the stronger the squeezing is, the better

the precision will be. We propose an experimental scheme for generating

phase-squeezed light pulses with large coherent amplitudes. In our scheme, one

arm of a single-photon Mach-Zehnder interferometer interacts with coherent

light via a non-linear optical Kerr medium to generate a coherent superposition

We give an exposition of the Horn inequalities and their triple role

characterizing tensor product invariants, eigenvalues of sums of Hermitian

matrices, and intersections of Schubert varieties. We follow Belkale's

geometric method, but assume only basic representation theory and algebraic

geometry, aiming for self-contained, concrete proofs. In particular, we do not

assume the Littlewood-Richardson rule nor an a priori relation between

intersections of Schubert cells and tensor product invariants. Our motivation

Global quantum quench with a finite quench rate which crosses critical points

is known to lead to universal scaling of correlation functions as functions of

the quench rate. In this work, we explore scaling properties of the

entanglement entropy of a subsystem in a harmonic chain during a mass quench

which asymptotes to finite constant values at early and late times and for

which the dynamics is exactly solvable. When the initial state is the ground

state, we find that for large enough subsystem sizes the entanglement entropy

There exist zero-temperature states in quantum many-body systems that are

fully factorized, thereby possessing vanishing entanglement, and hence being of

no use as resource in quantum information processing tasks. Such states can

become useful for quantum protocols when the temperature of the system is

increased, and when the system is allowed to evolve under either the influence

of an external environment, or a closed unitary evolution driven by its own

Hamiltonian due to a sudden change in the system parameters. Using the

In this work we develop an experimental procedure to interrogate the single-

and multi-photon scattering matrices of an unknown quantum object interacting

with propagating photons. Our proposal requires coherent state laser or

microwave inputs and homodyne detection at the scatterer's output, and provides

simultaneous information about multiple ---elastic and inelastic--- segments of

the scattering matrix. The method is resilient to detector noise and its errors

The measurement device independent (MDI) Quantum Key Distribution (QKD) is a

practically implementable method for transmitting secret keys between

respective partners performing quantum communication. SARG04

(Scarani-Ac\`{i}n-Ribordy-Gisin 2004) is a protocol tailored to struggle

against photon number splitting (PNS) attacks by eavesdroppers and its MDI-QKD

version is reviewed and optimized from secret key bitrate versus communication

distance point of view. We consider the effect of several important factors

The high index contrast of the silicon-on-insulator (SOI) platform allows the

realization of ultra-compact photonic circuits. However, this high contrast

hinders the implementation of narrow-band Bragg filters. These typically

require corrugations widths of a few nanometers or double-etch geometries,

hampering device fabrication. Here we report, for the first time, on the

realization of SOI Bragg filters based on sub-wavelength index engineering in a

differential corrugation width configuration. The proposed double periodicity

Starting from the symmetric gauge and the zero modes of the monolayer

graphene Dirac equation, we develop a mechanism to construct the eigenvalues

and its eigenfunctions. Specifically, we show that for positive configurations

of magnetic fields, we can construct the successive eigenstates from the zero

energy state with negative chirality. We also show that for negative magnetic

fields, the eigenstates may be constructed form the zero mode with positive

quirality. In addition, we discuss the implications that the Aharonov-Casher