411 research outputs found
Two-excitation routing via linear quantum channels
Routing quantum information among different nodes in a network is a
fundamental prerequisite for a quantum internet. While single-qubit routing has
been largely addressed, many-qubit routing protocols have not been intensively
investigated so far. Building on the many-excitation transfer protocol in
arXiv:1911.12211, we apply the perturbative transfer scheme to a two-excitation
routing protocol on a network where multiple two-receivers block are coupled to
a linear chain. We address both the case of switchable and permanent couplings
between the receivers and the chain. We find that the protocol allows for
efficient two-excitation routing on a fermionic network, although for a
spin- network only a limited region of the network is suitable for
high-quality routing.Comment: 12 pages, 7 figure
Coherent state transfer via highly mixed quantum spin chains
Spin chains have been proposed as quantum wires in many quantum information
processing architectures. Coherent transmission of quantum information over
short distances is enabled by their internal dynamics, which drives the
transport of single-spin excitations in perfectly polarized chains. Given the
practical challenge of preparing the chain in a pure state, we propose to use a
chain that is initially in the maximally mixed state. We compare the transport
properties of pure and mixed-state chains, finding similarities that enable the
experimental study of pure-state transfer by its simulation via mixed-state
chains, and demonstrate protocols for the perfect transfer of quantum
information in these chains. Remarkably, mixed-state chains allow the use of
Hamiltonians which do not preserve the total number of excitations, and which
are more readily obtainable from the naturally occurring magnetic dipolar
interaction. We propose experimental implementations using solid-state nuclear
magnetic resonance and defect centers in diamond.Comment: 9 page
Chaos and Complexity of quantum motion
The problem of characterizing complexity of quantum dynamics - in particular
of locally interacting chains of quantum particles - will be reviewed and
discussed from several different perspectives: (i) stability of motion against
external perturbations and decoherence, (ii) efficiency of quantum simulation
in terms of classical computation and entanglement production in operator
spaces, (iii) quantum transport, relaxation to equilibrium and quantum mixing,
and (iv) computation of quantum dynamical entropies. Discussions of all these
criteria will be confronted with the established criteria of integrability or
quantum chaos, and sometimes quite surprising conclusions are found. Some
conjectures and interesting open problems in ergodic theory of the quantum many
problem are suggested.Comment: 45 pages, 22 figures, final version, at press in J. Phys. A, special
issue on Quantum Informatio
Studies of quantum spin chain dynamics and their potential applications in quantum information
Ph.DDOCTOR OF PHILOSOPH
Many-body localization, thermalization, and entanglement
Thermalizing quantum systems are conventionally described by statistical
mechanics at equilibrium. However, not all systems fall into this category,
with many body localization providing a generic mechanism for thermalization to
fail in strongly disordered systems. Many-body localized (MBL) systems remain
perfect insulators at non-zero temperature, which do not thermalize and
therefore cannot be described using statistical mechanics. In this Colloquium
we review recent theoretical and experimental advances in studies of MBL
systems, focusing on the new perspective provided by entanglement and
non-equilibrium experimental probes such as quantum quenches. Theoretically,
MBL systems exhibit a new kind of robust integrability: an extensive set of
quasi-local integrals of motion emerges, which provides an intuitive
explanation of the breakdown of thermalization. A description based on
quasi-local integrals of motion is used to predict dynamical properties of MBL
systems, such as the spreading of quantum entanglement, the behavior of local
observables, and the response to external dissipative processes. Furthermore,
MBL systems can exhibit eigenstate transitions and quantum orders forbidden in
thermodynamic equilibrium. We outline the current theoretical understanding of
the quantum-to-classical transition between many-body localized and ergodic
phases, and anomalous transport in the vicinity of that transition.
Experimentally, synthetic quantum systems, which are well-isolated from an
external thermal reservoir, provide natural platforms for realizing the MBL
phase. We review recent experiments with ultracold atoms, trapped ions,
superconducting qubits, and quantum materials, in which different signatures of
many-body localization have been observed. We conclude by listing outstanding
challenges and promising future research directions.Comment: (v2) minor changes, added one figure and expanded bibliography; (v1)
colloquium-style review on many-body localization; 29 pages, 11 figures;
comments are welcom
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