Coherent oscillations in superconducting flux qubit without microwave pulse

1
Coherent oscillations insuperconducting flux
qubit without microwave pulse

• S. Poletto1, J. Lisenfeld1, A. Lukashenko1
• M.G. Castellano2, F. Chiarello2,
• C. Cosmelli3, P. Carelli4, A.V. Ustinov1

1 Physikalisches Institut III, Universität
Erlangen-Nürnberg - Germany 2 Istituto di
Fotonica e Nanotecnologie del CNR Italy 3 INFN
and Università di Roma la Sapienza - Italy 4
Università degli Studi dellAquila - Italy
2
Outline
Outline

• Circuit description
• Observation of coherent oscillations without
  microwaves
• Theoretical interpretation
• Summary and conclusions

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Circuit description
4
Circuit description
For Fx F0/2 the potential is a symmetric double
well
Qubit parameters
Fully controllable system
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Circuit description
The system is fully gradiometric, realized in Nb,
designed by IFN-CNR, fabricated by Hypres (100
A/cm2)
Flux bias Fc
1/100 coupling
Readout SQUID
flux bias Fx
junctions
100mm
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Coherent oscillations without microwaves
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Coherent oscillations without microwaves
Main idea (energy potential view)
E2
E1
E0
system preparation
evolution
readout
Population of the ground and exited states is
determined by the potential symmetry and barrier
modulation rate
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Coherent oscillations without microwaves
Main idea (fluxes view)
?x
?c
Readout
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Coherent oscillations without microwaves
Experimental results

• Oscillations for preparation of the left L? and
  right R? states

• Frequency changes depending on pulse amplitude

?c
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Theoretical interpretation
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Theoretical interpretation
Symmetric double-well potential (Fx F0/2 ) ?
description in the base L?, R?
L?
R?
It is possible to describe the system in the
energy base 0?, 1? as well
1?
0?
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Theoretical interpretation
?
expected oscillation frequency of up to 35 GHz
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Theoretical interpretation
Frequency dependence on pulse amplitude (?Fc)
Green dots experimental data Blue line
theoretical curve
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Theoretical interpretation
Note In the case of asymmetric potential one
should take into account a non-adiabatic
population of the states 0?, 1?
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Conclusions
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Summary and conclusions
Advantages of the demonstrated approach

• Oscillations are obtained without using
  microwave pulses
• Due to large energy level spacing the system
  can evolve at high temperature (up to h?/kB ?
  1.1K)
• High frequency of coherent oscillations (up to
  35 GHz) allow for high speed quantum gates
• A qubit coherence time of 500 ns should be
  sufficient to implement an error correction
  algorithm
• (required 104 operations during the
  coherence time.
• See e.g. arXivquant-ph/0110143)