DEMETRA Suppression of the relaxation induced by radioactivity in superconducting qubits

Resonators

Correlations A-C

• Superconducting circuits: one of the leading technologies for quantum processors • Possible limit: coherence time. Goal: few ms — obtained: 100 μs • Quasiparticles are one of the main source of decoherence

DEMETRA Hypothesis: • Radioactivity creates phonons in the

qubit substrate • Phonons enter the qubit breaking

Cooper pairs into quasiparticles • This phenomenon is actually the

working principle of phonon-mediated Kinetic Inductance Detectors (KIDs)

Prototype with 3 GrAl resonators deposited on sapphire to prove our hypothesis

Prototype exposed to an intense ThO2 γ-source • Rate of quasiparticle bursts increased by x100:

radioactivity matters • Majority of bursts in time-coincidence in all the

resonators: interactions are in the substrate

Prototype operated in deep underground laboratory • Same device operated at LNGS (depth: 3600

m.w.e.) in the HallC R&D Facility • Rate of quasiparticles bursts drops underground • Lead shield around the cryostat: further x2

suppression

*Standard indicates operation above-ground without radiation shields

• The underground operation resulted in an improvement • Adding a source did not result in a worsening. • Tests runs to be done soon to understand this result

We can act on the rate of quasiparticles bursts. Does this change other general features of the resonators?Extract Qi : circle fit example

f (Hz)f (Hz)

Re {S11}

Im {S11}

|S11|(dB)

∠S11 (rads)

Resonator: Af0 = 7.244 GHzn = 0.1Qi = 170kQc = 10kQl = 10k

Focus on some parameters, such as the internal Q, extracted from a fit to the resonance in the real and imaginary plane

• More than one order of magnitude suppression • Still a lot of room for improvement

Extract Qi : circle fit example

f (Hz)f (Hz)

Re {S11}

Im {S11}

|S11|(dB)

∠S11 (rads)

Resonator: Af0 = 7.244 GHzn = 0.1Qi = 170kQc = 10kQl = 10k

L. Cardani (laura.cardani@roma1.infn.it), N. Casali, G. Catelani, T. Charpentier, M. Clemenza, I. Colantoni, A. Cruciani, L.Gironi, L. Gruenhaupt, D. Gusenkova, F. Henriques, M. Lagoin, M. Martinez, S. Pirro, I. Pop, C. Rusconi, A. Ustinov, F. Valenti, M. Vignati, W. Wernsdorfer

A B CResonator

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