32,704 research outputs found
Quantum Quenches in Free Field Theory: Universal Scaling at Any Rate
Quantum quenches display universal scaling in several regimes. For quenches
which start from a gapped phase and cross a critical point, with a rate slow
compared to the initial gap, many systems obey Kibble-Zurek scaling. More
recently, a different scaling behaviour has been shown to occur when the quench
rate is fast compared to all other physical scales, but still slow compared to
the UV cutoff. We investigate the passage from fast to slow quenches in scalar
and fermionic free field theories with time dependent masses for which the
dynamics can be solved exactly for all quench rates. We find that renormalized
one point functions smoothly cross over between the regimes.Comment: 40 pages; v2: a bit late, but it includes minor modifications to
match published versio
Smooth and fast versus instantaneous quenches in quantum field theory
We examine in detail the relationship between smooth fast quantum quenches,
characterized by a time scale , and {\em instantaneous quenches},
within the framework of exactly solvable mass quenches in free scalar field
theory. Our earlier studies \cite{dgm1,dgm2} highlighted that the two protocols
remain distinct in the limit because of the relation
of the quench rate to the UV cut-off, i.e., always holds
in the fast smooth quenches while for instantaneous
quenches. Here we study UV finite quantities like correlators at finite spatial
distances and the excess energy produced above the final ground state energy.
We show that at late times and large distances (compared to the quench time
scale) the smooth quench correlator approaches that for the instantaneous
quench. At early times, we find that for small spatial separation and small
, the correlator scales universally with , exactly as in
the scaling of renormalized one point functions found in earlier work. At
larger separation, the dependence on drops out. The excess energy
density is finite (for finite ) and scales in a universal fashion
for all . However, the scaling behaviour produces a divergent result in the
limit for , just as in an instantaneous
quench, where it is UV divergent for . We argue that similar results
hold for arbitrary interacting theories: the excess energy density produced is
expected to diverge for scaling dimensions .Comment: 52 pages; v2: minor modifications to match published versio
Electron spin interferometry using a semiconductor ring structure
A ring structure fabricated from GaAs is used to achieve interference of the
net spin polarization of conduction band electrons. Optically polarized spins
are split into two packets by passing through two arms of the ring in the
diffusive transport regime. Optical pumping with circularly polarized light on
one arm establishes dynamic nuclear polarization which acts as a local
effective magnetic field on electron spins due to the hyperfine interaction.
This local field causes one spin packet to precess faster than the other,
thereby controlling the spin interference when the two packets are combined.Comment: 4 pages, 2 figure
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