Coherent oscillations in superconducting flux qubit without microwave pulse S. Poletto 1, J. Lisenfeld 1, A. Lukashenko 1 M.G. Castellano 2, F. Chiarello 2, C. Cosmelli 3, P. Carelli 4, A.V. Ustinov 1 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

EuroSQIPS.Poletto2 Outline Circuit description Observation of coherent oscillations without microwaves Theoretical interpretation Summary and conclusions

Circuit description

EuroSQIPS.Poletto4 For Φ x = Φ 0 /2 the potential is a symmetric double well Fully controllable system Qubit parameters Circuit description

EuroSQIPS.Poletto5 100  m Flux bias  c flux bias  x junctions Readout SQUID 1/100 coupling The system is fully gradiometric, realized in Nb, designed by IFN-CNR, fabricated by Hypres (100 A/cm 2 ) Circuit description

Coherent oscillations without microwaves

EuroSQIPS.Poletto7 Main idea (energy potential view) Coherent oscillations without microwaves system preparationevolutionreadout Population of the ground and exited states is determined by the potential symmetry and barrier modulation rate ? ? E0E0 E1E1 E2E2

EuroSQIPS.Poletto8 Main idea (fluxes view) ? ? cc xx Readout Coherent oscillations without microwaves

EuroSQIPS.Poletto9 Experimental results Oscillations for preparation of the left |L  and right |R  states cc Frequency changes depending on pulse amplitude Coherent oscillations without microwaves

Theoretical interpretation

EuroSQIPS.Poletto11 Symmetric double-well potential (Φ x = Φ 0 /2 )  description in the base {|L , |R  } It is possible to describe the system in the energy base {|0 , |1  } as well |L  |R  |0  |1  Theoretical interpretation

EuroSQIPS.Poletto12 |0  |1  ? expected oscillation frequency of up to 35 GHz Theoretical interpretation

EuroSQIPS.Poletto13 Frequency dependence on pulse amplitude (  Φ c ) Green dots: experimental data Blue line: theoretical curve Theoretical interpretation

EuroSQIPS.Poletto14 Theoretical interpretation Note: In the case of asymmetric potential one should take into account a non-adiabatic population of the states {|0 , |1  }

Conclusions

EuroSQIPS.Poletto16 Summary and conclusions Oscillations are obtained without using microwave pulses Due to large energy level spacing the system can evolve at high temperature (up to h /k B  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 ~10 4 operations during the coherence time. See e.g.: arXiv:quant-ph/ ) Advantages of the demonstrated approach