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SUMMARY:Ultrastrong Coupling of matter and radiation: detection of virtua
l photons and multiqubit quantum gates
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UID:indico-contribution-41-215@cern.ch
DESCRIPTION:Speakers: Giuseppe Falci (University of Catania)\nUltrastrong
coupling (USC) between light and matter has been recently achieved in erch
itectures of solid state artificial atoms coupled to cavities. They may pr
ovide new building blocks for quantum state processing\, where ultrafast q
uantum gates can be performed thus meeting the requirements for fault-tole
rant quantum computation.\nHowever it has been shown [1] that in the USC r
egime the dynamical Casimir effect (DCE)\, may pose limits on the fidelity
of quantum operations\, as for protocols based on quantum Rabi oscillatio
ns [1]. These latter are used for processing in strongly coupled (SC) circ
uit-QED systems where only single excitations are manipulated across the s
ystem the cavity working as a quantum bus. In the USC\nmultiphoton generat
ion deteriorates the fidelity of such quantum operations [1] even in absen
ce of decoherence.\n\nTo overcome this problem we propose a communication
channel based on an adiabatic protocol similar to STIRAP [2]. Ideally the
cavity is never populated\, operating as a virtual bus\, thus it is expect
ed to greatly reduce the impact of DCE. Indeed we show that high fidelity
operations can be performed for moderate couplings in the USC regime [3] t
hus allowing to operate faster than in SC. Moreover properly crafted contr
ol exends the high fidelity region to even larger couplings. The protocol
is extremely robust agaist DCE\, in the absence of decoherence yields almo
st 100% fidelity for remote population [3] and state trensfer. It is also
resilient to decay due to leakage from the cavity\, which is the main deco
herence mechanism for present USC architectures [3]. In this more realisti
c scenario it is seen that for larger coupling (entering the deep strong c
oupling regime) the fidelity decreases due to the interplay between decohe
rence and DCE.\n\nThe communication channel we address being a prototype o
f family of adiabatic protocols for state transfer and multiqubit gate\, o
ur results suggest that adiabatic manipulations\, which has been recently
proposed for detecting\ndynamically USC [4]\, may be a promising tool for
quantum state processing in the USC regime.\n\n[1] G. Benenti\, A. DArrigo
\, S. Siccardi\, and G. Strini\, Phys. Rev. A 90\, 052313 (2014).\n\n[2] N
. V. Vitanov\, A. A. Rangelov\, B. W. Shore\, and K. Bergmann\, Rev. Mod.
Phys. 89\, 015006 (2017).\n\n[3] M. Stramacchia\, A. Ridolfo\, G. Benenti\
, E. Paladino\, F. M. D. Pellegrino\, G. D. Maccarrone\, G. Falci\, arXiv:
1904.04141\n\n[4] G. Falci\, A. Ridolfo\, P.G. Di Stefano\, and E. Paladin
o\, arXiv 1708.00906.\n\nhttps://indico.unina.it/event/18/contributions/21
5/
LOCATION:Strand Hotel Terme Delfini
URL:https://indico.unina.it/event/18/contributions/215/
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SUMMARY:Adiabatic quantum computation in a dissipative environment
DTSTART;VALUE=DATE-TIME:20190622T073000Z
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UID:indico-contribution-41-223@cern.ch
DESCRIPTION:Speakers: Procolo Lucignano (CNR-SPIN Napoli)\nQuantum anneali
ng (QA) is getting more and more relevant as powerful solver of optimizati
on problems. Thanks to the availability of“commercial”adiabatic qua
ntum computers\, based on QA\, there is plenty of proposals both on the fu
ndamental and on the applied side. In QA a target ground state encodes the
solution of a computationally hard problem. Such ground state is approach
ed exploring the energy landscape\, employing quantum flutcuations that ar
e adiabatically decreased towards to zero.\nAnnealing machines are made by
arrays of superconducting quantum interferometric devices embedded in cla
ssical circuits. They act as a dissipative environment\, that is well know
n to modify the dynamics of any quantum two level system\, affecting in a
detrimental way the annealing performances.\nI will review some recent res
ults regarding the role of dissipation in adiabatic quantum computation\,
discussing a simple model i.e. the ferromagnetic p-spin model that\, in th
e large p limit\, encodes in its ground state the solution to the Grover
’s algorithm for searching in unsorted databases (that is known to prov
ide a quadratic speed-up with respect to its best classical counterpart).
Unexpectedly\, under some particular conditions\, the system-bath coupling
can improve the annealing performances. I will discuss the role of pausin
g as well as the problem of the embedding on real devices.\nTo conclude\,
in the presence of a dissipative bath\, the question whether the dynamics
has driven the system to the target state through a quantum or a classical
path\, up to now\, remained unanswered and only partially addressed in th
e literature\, and the question is still controversial.\nHence I will desc
ribe a new method that we have proposed to assess the quantumness of the s
ystem during its adiabatic dynamics\, based on the Leggett Gargs inequalit
ies (LGI)\, evaluated in the framework of weak measurements.\n\nG. Passare
lli\, G. De Filippis\, V. Cataudella\, P. Lucignano PRA 97\, 022319 (2018)
\nL. M. Cangemi\, G. Passarelli\, V. Cataudella\, P. Lucignano\, G. De Fil
ippis PRB 98 184306 (2018)\nG. Passarelli\, V. Cataudella\, P. Lucignano a
rXiv:1902.06788 \nV. Vitale\, G. De Filippis\, A. De Candia\, A. Tagliaco
zzo\, V. Cataudella\, P. Lucignano arXiv:1902.08257\n\nhttps://indico.uni
na.it/event/18/contributions/223/
LOCATION:Strand Hotel Terme Delfini
URL:https://indico.unina.it/event/18/contributions/223/
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SUMMARY:Universal Scaling of Quantum Geometric Tensor in Disordered Metals
DTSTART;VALUE=DATE-TIME:20190622T070000Z
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UID:indico-contribution-41-237@cern.ch
DESCRIPTION:Speakers: Gergely Zarand (Exotic Quantum Phases “Momentum”
Research Group\, Department of Theoretical Physics\, Budapest University
of Technology and Economics\, 1111 Budapest\, Budafoki ut 8\, Hungary)\nTh
e geometrical structure of the Hilbert space continues to receive a lot of
attention. The Fubini-Study metric tensor of the Hilbert space\, also ref
erred to as Fisher information metric\, provides a natural measure of dist
ance in the Hilbert space\, related to quantum fidelity – a fundamental
concept in quantum information science. Interestingly\, the concepts of th
e Fubini-Study metric tensor and the Berry phase can be unified through th
e so-called quantum geometric tensor (QGT).\n\nIn the work to be presented
[1]\, we demonstrate that the quantum geometric tensor offers deep insigh
t into a long-standing problem in condensed matter physics\, Anderson’s
disorder-driven metal insulator (MI) transition in small external magnetic
fields. In particular\, the structure of the QGT reflects the universalit
y class of the Anderson transition. Elements of the QGT display universal
finite size scaling close to the metal-insulator transition\, and capture
the flow between the orthogonal (B = 0) and unitary (B 6=.. 0) universalit
y classes. At the transition\, the elements of the QGT have universal dist
ributions\, characteristic of the underlying symmetry of the transition\,
but\, surprisingly\, independent of the direction of the external field. W
e predict that these universal fluctuations show up as universal and isotr
opic Hall conductance fluctuations at the metal-insulator transition.\n\n[
1] Miklós Antal Werner\, Arne Brataas\, Felix von Oppen\, Gergely Zaránd
\, Phys. Rev. Lett. 122 (2019) 106601.\n\nhttps://indico.unina.it/event/18
/contributions/237/
LOCATION:Strand Hotel Terme Delfini
URL:https://indico.unina.it/event/18/contributions/237/
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