5 research outputs found
Fast and reliable entanglement distribution with quantum repeaters: principles for improving protocols using reinforcement learning
Future quantum technologies such as quantum communication, quantum sensing,
and distributed quantum computation, will rely on networks of shared
entanglement between spatially separated nodes. In this work, we provide
improved protocols/policies for entanglement distribution along a linear chain
of nodes, both homogeneous and inhomogeneous, that take practical limitations
such as photon losses, non-ideal measurements, and quantum memories with short
coherence times into account. For a wide range of parameters, our policies
improve upon previously known policies, such as the
``swap-as-soon-as-possible'' policy, with respect to both the waiting time and
the fidelity of the end-to-end entanglement. This improvement is greatest for
the most practically relevant cases, namely, for short coherence times, high
link losses, and highly asymmetric links. To obtain our results, we model
entanglement distribution using a Markov decision process, and then we use the
Q-learning reinforcement learning (RL) algorithm to discover new policies.
These new policies are characterized by dynamic, state-dependent memory cutoffs
and collaboration between the nodes. In particular, we quantify this
collaboration between the nodes. Our quantifiers tell us how much ``global''
knowledge of the network every node has. Finally, our understanding of the
performance of large quantum networks is currently limited by the computational
inefficiency of simulating them using RL or other optimization methods. Thus,
in this work, we present a method for nesting policies in order to obtain
policies for large repeater chains. By nesting our RL-based policies for small
repeater chains, we obtain policies for large repeater chains that improve upon
the swap-as-soon-as-possible policy, and thus we pave the way for a scalable
method for obtaining policies for long-distance entanglement distribution.Comment: Version 2, title changed, some typos fixed. 27 pages, 18 figures and
3 tables. Comments are welcom
Multipartite entanglement at dynamical quantum phase transitions with non-uniformly spaced criticalities
We report dynamical quantum phase transition portrait in the alternating
field transverse XY spin chain with Dzyaloshinskii-Moriya interaction by
investigating singularities in the Loschmidt echo and the corresponding rate
function after a sudden quench of system parameters. Unlike the Ising model,
the analysis of Loschmidt echo yields non-uniformly spaced transition times in
this model. Comparative study between the equilibrium and the dynamical quantum
phase transitions in this case reveals that there are quenches where one occurs
without the other, and the regimes where they co-exist. However, such
transitions happen only when quenching is performed across at least a single
gapless or critical line. Contrary to equilibrium phase transitions, bipartite
entanglement measures do not turn out to be useful for the detection, while
multipartite entanglement emerges as a good identifier of this transition when
the quench is done from a disordered phase of this model.Comment: 10 pages, 6 figures; close to published versio