20 research outputs found
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
Decoherence in the dynamical quantum phase transition of the transverse Ising chain
For the prototypical example of the Ising chain in a transverse field, we
study the impact of decoherence on the sweep through a second-order quantum
phase transition. Apart from the advance in the general understanding of the
dynamics of quantum phase transitions, these findings are relevant for
adiabatic quantum algorithms due to the similarities between them. It turns out
that (in contrast to first-order transitions studied previously) the impact of
decoherence caused by a weak coupling to a rather general environment increases
with system size (i.e., number of spins/qubits), which might limit the
scalability of the system.Comment: 4 pages, 1 figure, minor clarification
Quantum simulator for the Ising model with electrons floating on a helium film
We propose a physical setup that can be used to simulate the quantum dynamics
of the Ising model with present-day technology. Our scheme consists of
electrons floating on superfluid helium which interact via Coulomb forces. In
the limit of low temperatures, the system will stay near the ground state where
its Hamiltonian is equivalent to the Ising model and thus shows phenomena such
as quantum criticality. Furthermore, the proposed design could be generalized
in order to study interacting field theories (e.g., ) and
adiabatic quantum computers.Comment: 4 page
Efficient quantum circuits for diagonal unitaries without ancillas
The accurate evaluation of diagonal unitary operators is often the most resource-intensive element of quantum algorithms such as real-space quantum simulation and Grover search. Efficient circuits have been demonstrated in some cases but generally require ancilla registers, which can dominate the qubit resources. In this paper, we point out a correspondence between Walsh functions and basis for diagonal operators that gives a simple way to construct efficient circuits for diagonal unitaries without ancillas. This correspondence reduces the problem of constructing the minimal-depth circuit within a given error tolerance, for an arbitrary diagonal unitary eif(ˆx) in the |xi basis, to that of finding the minimal-length Walsh-series approximation to the function f(x). We apply this approach to the quantum simulation of the classical Eckart barrier problem of quantum chemistry, demonstrating that high-fidelity quantum simulations can be achieved with few qubits and low depth.Chemistry and Chemical BiologyPhysic
Decoherence in a dynamical quantum phase transition
Motivated by the similarity between adiabatic quantum algorithms and quantum
phase transitions, we study the impact of decoherence on the sweep through a
second-order quantum phase transition for the prototypical example of the Ising
chain in a transverse field and compare it to the adiabatic version of Grovers
search algorithm, which displays a first order quantum phase transition. For
site-independent and site-dependent coupling strengths as well as different
operator couplings, the results show that (in contrast to first-order
transitions) the impact of decoherence caused by a weak coupling to a rather
general environment increases with system size (i.e., number of spins/qubits).
This might limit the scalability of the corresponding adiabatic quantum
algorithm.Comment: 14 pages, 9 figure
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Quantum Simulator of an Open Quantum System Using Superconducting Qubits: Exciton Transport in Photosynthetic Complexes
Open quantum system approaches are widely used in the description of physical, chemical and biological systems. A famous example is electronic excitation transfer in the initial stage of photosynthesis, where harvested energy is transferred with remarkably high efficiency to a reaction center. This transport is affected by the motion of a structured vibrational environment, which makes simulations on a classical computer very demanding. Here we propose an analog quantum simulator of complex open system dynamics with a precisely engineered quantum environment. Our setup is based on superconducting circuits, a well established technology. As an example, we demonstrate that it is feasible to simulate exciton transport in the Fenna–Matthews–Olson photosynthetic complex. Our approach allows for a controllable single-molecule simulation and the investigation of energy transfer pathways as well as non-Markovian noise-correlation effects.Chemistry and Chemical Biolog
General error estimate for adiabatic quantum computing
Most investigations devoted to the conditions for adiabatic quantum computing
are based on the first-order correction . However, it is
demonstrated that this first-order correction does not yield a good estimate
for the computational error. Therefore, a more general criterion is proposed,
which includes higher-order corrections as well and shows that the
computational error can be made exponentially small -- which facilitates
significantly shorter evolution times than the above first-order estimate in
certain situations. Based on this criterion and rather general arguments and
assumptions, it can be demonstrated that a run-time of order of the inverse
minimum energy gap is sufficient and necessary, i.e.,
T=\ord(\Delta E_{\rm min}^{-1}). For some examples, these analytical
investigations are confirmed by numerical simulations. PACS: 03.67.Lx,
03.67.-a.Comment: 8 pages, 6 figures, several modification