157 research outputs found
Exponential Lifetime Improvement in Topological Quantum Memories
We propose a simple yet efficient mechanism for passive error correction in
topological quantum memories. Our scheme relies on driven-dissipative ancilla
systems which couple to local excitations (anyons) and make them "sink" in
energy, with no required interaction among ancillae or anyons. Through this
process, anyons created by some thermal environment end up trapped in potential
"trenches" that they themselves generate, which can be interpreted as a "memory
foam" for anyons. This self-trapping mechanism provides an energy barrier for
anyon propagation, and removes entropy from the memory by favoring anyon
recombination over anyon separation (responsible for memory errors). We
demonstrate that our scheme leads to an exponential increase of the
memory-coherence time with system size , up to an upper bound
which can increase exponentially with , where is
the temperature and is some energy scale defined by potential
trenches. This results in a double exponential increase of the memory time with
, which greatly improves over the Arrhenius (single-exponential)
scaling found in typical quantum memories.Comment: 18 pages including appendices; 8 figure
Shortcuts to nonabelian braiding
Topological quantum information processing relies on adiabatic braiding of
nonabelian quasiparticles. Performing the braiding operations in finite time
introduces transitions out of the ground-state manifold and deviations from the
nonabelian Berry phase. We show that these errors can be eliminated by suitably
designed counterdiabatic correction terms in the Hamiltonian. We implement the
resulting shortcuts to adiabaticity for simple protocols of nonabelian braiding
and show that the error suppression can be substantial even for approximate
realizations of the counterdiabatic terms.Comment: 5 pages, 3 figures plus supplementary materia
Topological Polaritons
The interaction between light and matter can give rise to novel topological
states. This principle was recently exemplified in Floquet topological
insulators, where \emph{classical} light was used to induce a topological
electronic band structure. Here, in contrast, we show that mixing \emph{single}
photons with excitons can result in new topological polaritonic states --- or
"topolaritons". Taken separately, the underlying photons and excitons are
topologically trivial. Combined appropriately, however, they give rise to
non-trivial polaritonic bands with chiral edge modes allowing for
unidirectional polariton propagation. The main ingredient in our construction
is an exciton-photon coupling with a phase that winds in momentum space. We
demonstrate how this winding emerges from spin-orbit coupling in the electronic
system and an applied Zeeman field. We discuss the requirements for obtaining a
sizable topological gap in the polariton spectrum, and propose practical ways
to realize topolaritons in semiconductor quantum wells and monolayer transition
metal dichalcogenides.Comment: For Supplementary Information and Video see source files; v3: updated
to published versio
Interaction effects in superconductor/quantum spin Hall devices: universal transport signatures and fractional Coulomb blockade
Interfacing s-wave superconductors and quantum spin Hall edges produces
time-reversal-invariant topological superconductivity of a type that can not
arise in strictly 1D systems. With the aim of establishing sharp fingerprints
of this novel phase, we use renormalization group methods to extract universal
transport characteristics of superconductor/quantum spin Hall heterostructures
where the native edge states serve as leads. We determine scaling forms for the
conductance through a grounded superconductor and show that the results depend
sensitively on the interaction strength in the leads, the size of the
superconducting region, and the presence or absence of time-reversal-breaking
perturbations. We also study transport across a floating superconducting island
isolated by magnetic barriers. Here we predict e-periodic Coulomb-blockade
peaks, as recently observed in nanowire devices [Albrecht et al., Nature 531,
206 (2016)], with the added feature that the island can support fractional
charge tunable via the relative orientation of the barrier magnetizations. As
an interesting corollary, when the magnetic barriers arise from strong
interactions at the edge that spontaneously break time-reversal symmetry, the
Coulomb-blockade periodicity changes from e to e/2. These findings suggest
several future experiments that probe unique characteristics of topological
superconductivity at the quantum spin Hall edge.Comment: 18 pages, 7 figure
A geometric protocol for a robust Majorana magic gate
A universal quantum computer requires a full set of basic quantum gates. With
Majorana bound states one can form all necessary quantum gates in a
topologically protected way, bar one. In this manuscript we present a protocol
that achieves the missing, so called, 'magic' phase gate. The protocol
is based on the manipulation of geometric phases in a universal manner, and
does not require fine tuning for distinct physical realizations. The protocol
converges exponentially with the number of steps in the geometric path.
Furthermore, the magic gate protocol relies on the most basic hardware
previously suggested for topologically protected gates, and can be extended to
any-phase-gate, where is substituted by any .Comment: 14 pages, 8 figures (including appendices), v3: simplified
derivation, more explicit connection between topological protection and
exponential convergenc
Topological polaritons from photonic Dirac cones coupled to excitons in a magnetic field
We introduce an alternative scheme for creating topological polaritons (topolaritons) by exploiting the presence of photonic Dirac cones in photonic crystals with triangular lattice symmetry. As recently proposed, topolariton states can emerge from a coupling between photons and excitons combined with a periodic exciton potential and a magnetic field to open up a topological gap. We show that in photonic crystals the opening of the gap can be substantially simplified close to photonic Dirac points. Coupling to Zeeman-split excitons breaks time reversal symmetry and allows to gap out the Dirac cones in a nontrival way, leading to a topological gap similar to the strength of the periodic exciton potential. Compared to the original topolariton proposal [T. Karzig et al., Phys. Rev. X 5, 031001 (2015)], this scheme significantly increases the size of the topological gap over a wide range of parameters. Moreover, the gap opening mechanism highlights an interesting connection between topolaritons and the scheme of [F. D. M. Haldane and S. Raghu, Phys. Rev. Lett. 100, 013904 (2008)] to create topological photons in magneto-optically active materials
Chiral Bogoliubons in Nonlinear Bosonic Systems
We present a versatile scheme for creating topological Bogoliubov excitations
in weakly interacting bosonic systems. Our proposal relies on a background
stationary field that consists of a Kagome vortex lattice, which breaks
time-reversal symmetry and induces a periodic potential for Bogoliubov
excitations. In analogy to the Haldane model, no external magnetic field or net
flux is required. We construct a generic model based on the two-dimensional
(2D) nonlinear Schr\"odinger equation and demonstrate the emergence of
topological gaps crossed by chiral Bogoliubov edge modes. Our scheme can be
realized in a wide variety of physical systems ranging from nonlinear optical
systems to exciton-polariton condensates.Comment: 6 pages, 3 figures; with Supplemental Material (5 pages; in source
Scattering theory of adiabatic reaction forces due to out-of-equilibrium quantum environments
The Landauer-Buettiker theory of mesoscopic conductors was recently extended
to nanoelectromechanical systems. In this extension, the adiabatic reaction
forces exerted by the electronic degrees of freedom on the mechanical modes
were expressed in terms of the electronic S-matrix and its first non-adiabatic
correction, the A-matrix. Here, we provide a more natural and efficient
derivation of these results within the setting and solely with the methods of
scattering theory. Our derivation is based on a generic model of a slow
classical degree of freedom coupled to a quantum-mechanical scattering system,
extending previous work on adiabatic reaction forces for closed quantum
systems.Comment: Minor typos fixed, published versio
Topological Polaritons and Excitons in Garden Variety Systems
Topological polaritons (aka topolaritons) present a new frontier for
topological behavior in solid-state systems. They combine light and matter,
which allows to probe and manipulate them in a variety of ways. They can also
be made strongly interacting, due to their excitonic component. So far,
however, their realization was deemed rather challenging. Here we present a
scheme which allows to realize topolaritons in garden variety zinc-blende
quantum wells. Our proposal requires a moderate magnetic field and a potential
landscape which can be implemented, e.g., via surface acoustic waves or
patterning. We identify indirect excitons in double quantum wells as a
particularly appealing alternative for topological states in exciton-based
systems. Indirect excitons are robust and long lived (with lifetimes up to
milliseconds), and, therefore, provide a flexible platform for the realization,
probing, and utilization of topological coupled light-matter states.Comment: 6 pages, 4 figures; v2: improved figures and text, with added details
regarding achievable topological gap
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