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[Fizinfo] Stat Fiz Szeminarium

Chronological Thread 
  • From: StatFizSzeminar <statfiz AT>
  • To: fizinfo AT
  • Subject: [Fizinfo] Stat Fiz Szeminarium
  • Date: Wed, 06 Feb 2019 16:48:45 +0100

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ELTE TTK Fizikai Intézet

2019. február 13.



János Asbóth

Wigner SZFI

"Kitaev's Toric Code Model: Topological Quantum
Excitations and how they hide Quantum Information"

I will give an elementary introduction to Kitaev's Toric
Code model [1], which forms the basis of the "surface
code" version of topological quantum computing [2] that
IBM, Google, and Rigetti are trying to implement. The
Toric Code is an exactly solvable Hamiltonian for
spin-1/2's living on edges of a planar graph, with
n-body-interactions between spins that share a plaquette
or a vertex. The interaction terms all commute, and so
can be interpreted as counting the number (mod 2) of
elementary excitations on each plaquette and at each vertex.
These excitations can be created, moved, and fused, by
acting on the state of the system with unitary operators.
The excitations are topological similarly to vortices in
superfluids: they can be created and destroyed only pairwise
in the bulk, but can be "brought in" or "pushed out" at
suitable boundaries of the planar graph. In this sense,
boundaries of holes can store topological excitations at
no energy cost, which ensures topological degeneracy of
the ground state, and can be used to hide and - to a
certain extent - manipulate quantum bits.

[1]: Alexei Kitaev, Chris Laumann: Topological
phases and quantum computation, Les Houches lecture notes,

[2]: Fowler et al: Surface codes: Towards practical large-
scale quantum computation, Phys. Rev. A 86, 032324 (2012)

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2019. február 13.



András László

Wigner RMI

"General Relativity experiment with spin polarized
particle beams"

In experimental proposals published in the last two
decades, a so called frozen spin storage ring concept
emerged, for setting upper experimental bounds to
electric dipole moment (EDM) of charged elementary
particles with spin. In a recent paper of ours
(Class.Quant.Grav.35(2018)175003), a fully covariant
general relativistic (GR) calculation was presented
on the Earth's gravitational modification effect on
the spin transport inside such a frozen spin storage
ring. It turns out that in certain configurations,
Earth's gravity is expected to produce a similar order
of magnitude effect as the aimed EDM sensitivity, and
thus it becomes kind of realistic to experimentally
see this GR effect. If such an experiment could be
conducted, it could provide a novel test of GR: with
microscopic particles, at relativistic speeds, along
non-geodesic (forced) trajectories, and the tensorial
nature of GR would be at test, not merely the
gravitational drag. In more technical terms: the GR
correction to the so called Thomas precession could be
tested in lab. For details on the experimental idea,
we refer to: arXiv:1901.06217 (Proceedings of
Spin2018 Conference).

1117, Budapest, Pázmány P. sétány 1/A, Északi tömb 2.54

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