14 research outputs found
Magnetic Monopoles in Spin Ice
Electrically charged particles, such as the electron, are ubiquitous. By
contrast, no elementary particles with a net magnetic charge have ever been
observed, despite intensive and prolonged searches. We pursue an alternative
strategy, namely that of realising them not as elementary but rather as
emergent particles, i.e., as manifestations of the correlations present in a
strongly interacting many-body system. The most prominent examples of emergent
quasiparticles are the ones with fractional electric charge e/3 in quantum Hall
physics. Here we show that magnetic monopoles do emerge in a class of exotic
magnets known collectively as spin ice: the dipole moment of the underlying
electronic degrees of freedom fractionalises into monopoles. This enables us to
account for a mysterious phase transition observed experimentally in spin ice
in a magnetic field, which is a liquid-gas transition of the magnetic
monopoles. These monopoles can also be detected by other means, e.g., in an
experiment modelled after the celebrated Stanford magnetic monopole search.Comment: (6 pages, 6 figures) v2: fig 3 replaced with colour version. For the
high-definition version of the paper click
http://www-thphys.physics.ox.ac.uk/user/ClaudioCastelnovo/Publications/papersub.pd
Operator hydrodynamics, OTOCs, and entanglement growth in systems without conservation laws
Thermalization and scrambling are the subject of much recent study from the
perspective of many-body quantum systems with locally bounded Hilbert spaces
(`spin chains'), quantum field theory and holography. We tackle this problem in
1D spin-chains evolving under random local unitary circuits and prove a number
of exact results on the behavior of out-of-time-ordered commutators (OTOCs),
and entanglement growth in this setting. These results follow from the
observation that the spreading of operators in random circuits is described by
a `hydrodynamical' equation of motion, despite the fact that random unitary
circuits do not have locally conserved quantities (e.g., no conserved energy).
In this hydrodynamic picture quantum information travels in a front with a
`butterfly velocity' that is smaller than the light cone
velocity of the system, while the front itself broadens diffusively in time.
The OTOC increases sharply after the arrival of the light cone, but we do
\emph{not} observe a prolonged exponential regime of the form for a fixed Lyapunov exponent .
We find that the diffusive broadening of the front has important consequences
for entanglement growth, leading to an entanglement velocity that can be
significantly smaller than the butterfly velocity. We conjecture that the
hydrodynamical description applies to more generic ergodic systems and support
this by verifying numerically that the diffusive broadening of the operator
wavefront also holds in a more traditional non-random Floquet spin-chain. We
also compare our results to Clifford circuits, which have less rich
hydrodynamics and consequently trivial OTOC behavior, but which can
nevertheless exhibit linear entanglement growth and thermalization.Comment: 11+6 pages, 9 figure
Tunable non-equilibrium dynamics: field quenches in spin ice
We present non-equilibrium physics in spin ice as a novel setting which
combines kinematic constraints, emergent topological defects, and magnetic long
range Coulomb interactions. In spin ice, magnetic frustration leads to highly
degenerate yet locally constrained ground states. Together, they form a highly
unusual magnetic state -- a "Coulomb phase" -- whose excitations are pointlike
defects -- magnetic monopoles -- in the absence of which effectively no
dynamics is possible. Hence, when they are sparse at low temperature, dynamics
becomes very sluggish. When quenching the system from a monopole-rich to a
monopole-poor state, a wealth of dynamical phenomena occur the exposition of
which is the subject of this article. Most notably, we find reaction diffusion
behaviour, slow dynamics due to kinematic constraints, as well as a regime
corresponding to the deposition of interacting dimers on a honeycomb lattice.
We also identify new potential avenues for detecting the magnetic monopoles in
a regime of slow-moving monopoles. The interest in this model system is further
enhanced by its large degree of tunability, and the ease of probing it in
experiment: with varying magnetic fields at different temperatures, geometric
properties -- including even the effective dimensionality of the system -- can
be varied. By monitoring magnetisation, spin correlations or zero-field Nuclear
Magnetic Resonance, the dynamical properties of the system can be extracted in
considerable detail. This establishes spin ice as a laboratory of choice for
the study of tunable, slow dynamics.Comment: (16 pages, 13 figures
50 years of quantum spin liquids
In 1973, Philip Anderson published a paper introducing the resonating valence
bond state, which can be recognized in retrospect as a topologically ordered
phase of matter - one that cannot be classified in the conventional way
according to its patterns of spontaneously broken symmetry. Steven Kivelson and
Shivaji Sondhi reflect on the impact of this paper over the past 50 years.Comment: This is a historical perspective solicited by Nature Reviews Physic
Quantum tasks assisted by quantum noise
We introduce a notion of expected utility for quantum tasks and discuss some
general conditions under which this is increased by the presence of quantum
noise in the underlying resource states. We apply the resulting formalism to
the specific problem of playing the parity game with ground states of the
random transverse-field Ising model. This demonstrates a separation in the
ground-state phase diagram between regions where rational players will be
``risk-seeking'' or ``risk-averse'', depending on whether they win the game
more or less often in the presence of disorder. The boundary between these
regions depends non-universally on the correlation length of the disorder.
Strikingly, we find that adding zero-mean, uncorrelated disorder to the
transverse fields can generate a weak quantum advantage that would not exist in
the absence of noise.Comment: 18 pages, 6 figure