Lorentz transformations of spin density matrices for a particle with positive mass and spin 1/2 are described by maps of the kind used in open quantum dynamics. They show how the Lorentz transformations of the spin depend on the momentum. Since the spin and momentum generally are entangled, the maps generally are not completely positive and act in limited domains. States with two momentum value...

For a class of frustrated spin lattices including the Kagomé lattice we construct exact eigenstates consisting of several independent, localized one-magnon states and argue that they are ground states for high magnetic fields. If the maximal number of local magnons scales with the number of spins in the system, which is the case for the Kagomé lattice, the effect persists in the thermodynamic l...

We present an implementation of a robust quantum random number generator based on the quantum fluctuations of the collective spin of an alkali-metal vapor. The achieved bit rate is limited by the spin relaxation rate of the alkali-metal atoms 1 /T2 to about 1 kbit/s. However, the same physical scheme, which is impervious to limitations posed by single-photon detectors used in current implementa...

Strong interactions between single spins and photons are essential for quantum networks 1 and distributed quantum computation 2. They provide the necessary interface for entanglement distribution 3,4 , non-destructive quantum measurements 5-7 , and strong photon-photon interactions 8-10. Achieving a spin-photon interface in a solid-state device could enable compact chip-integrated quantum circu...

7.1 Quantum spin Let us consider a simple example of how Wess-Zumino effective Lagrangian appears from the “current algebra”. To simplify the story we take an example of quantum spin S. This is a quantum mechanical system with an SU(2) spin algebra playing the role of “current algebra” of quantum field theory. We have standard spin commutation relations [S, S] = i!S, (7.1) where a, b, c take va...

Sharp quantum phase transitions typically require a large system with many particles. Here we show that, for a frustrated fully connected Ising spin network represented by trapped atomic ions, the competition between different spin orders leads to rich phase transitions whose sharpness scales exponentially with the number of spins. This unusual finite-size scaling behavior opens up the possibil...

Linear Paul traps have been used recently to simulate the transverse-field Isingmodel with long-range spin-spin couplings.We study the intrinsic effects of phonon creation (from the initial phonon ground state) on the spin-state probability and spin entanglement for such quantum spin simulators.While it has often been assumed that phonon effects are benign because they play no role in the pure ...

The purpose of this paper is to exhibit a quantum network phenomenon—the anti-core—that goes against the classical network concept of congestion core. Classical networks idealized as infinite, Gromov hyperbolic spaces with least-cost path routing (and subject to a technical condition on the Gromov boundary) have a congestion core, defined as a subnetwork that routing paths have a high probabili...