Coherent oscillations in
superconducting 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 dell’Aquila - Italy
Outline
Outline
• Circuit description
• Observation of coherent oscillations
without microwaves
• Theoretical interpretation
• Summary and conclusions
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Circuit description
Circuit description
For Φx = Φ0/2 the potential
is a symmetric double well
Qubit
parameters
JJ  8 μA
L  85 pH
l  6 pH
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
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Coherent oscillations without
microwaves
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)
Fx
Fc
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
Fc
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Theoretical interpretation
Theoretical interpretation
Symmetric double-well potential
(Φx = Φ0/2 )
 description in the base {|L, |R}
|L
|R
It is possible to describe the
system in the energy base
{|0, |1} as well
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|1
|0
L  0  1

 0  1 
2
R
2
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Theoretical interpretation
L 0  1 
2
i
  E1 t dt
  i  E0 t dt

|    e
| 0  e 
| 1 


|1
2
|0
PL t    L |  t  
2
 t   
t
0
?
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1  cos t 
2
E1  E0   dt
expected oscillation frequency of
up to 35 GHz
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Theoretical interpretation
Frequency dependence on pulse amplitude (Φc)
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
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.: arXiv:quant-ph/0110143)
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