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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

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

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Circuit description

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

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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

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Coherent oscillations without microwaves

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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

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EuroSQIPS.Poletto8 Main idea (fluxes view) ? ? cc xx Readout Coherent oscillations without microwaves

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

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Theoretical interpretation

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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

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

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

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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 }

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Conclusions

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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

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