E ff ects of Radioactivity on Superconducting Quantum Bits Laura Cardani for the DEMETRA Collaboration Istituto Nazionale di Fisica Nucleare - Roma 09/09/2019 TAUP , Toyama, Japan
Superconducting QUBITS Qubit: any two level system (many technologies proposed ) One of the most promising implementation: superconducting circuits = V 0 cos ω 01 t V ( t ) � Very short π -pulse time • Based on simple circuital elements (capacitor, inductor…) • Simple to fabricate and operate • Fast gate time, high fidelity • But poor coherence time J.Gambetta, https://www.nature.com/articles/s41534-016-0004-0 Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 2
Coherence Time Time in which a qubit retains its quantum behaviour • Dielectric two levels system • Paramagnetic molecules at the interface • Vortices trapped in the superconductor • …. • Quasiparticles Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 3
Coherence Time Time in which a qubit retains its quantum behaviour • Dielectric two levels system • Paramagnetic molecules at the interface • Vortices trapped in the superconductor • …. • Quasiparticles • Quasiparticles (“broken Cooper pairs”) can be viewed as free electrons • They are dissipative • They lead to decoherence • Coherence time of 1ms —> 1QP for 10 9 Cooper pairs Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 4
Signatures of Quasiparticles A “steady population” worsen the performance (short coherence, low Q) Non equilibrium QP results in “bursts” Pop 2018 doi:10.1103/PhysRevLett.121.117001 The higher the number of bursts, the shorter the coherence time Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 5
Radioactivity Why do we expect radioactivity to produce quasiparticles? • Qubits are very tiny —> almost radioactive free • But they are deposited on a much wider substrate (Si or sapphire) • Radioactivity hits the substrate producing phonons • Phonons travel in the substrate and hit the qubit • Whey they hit the qubit they produce QP bursts For us rather obvious (N.Casali’s talk on New Techn. 4, Wed), not for qubit Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 6
DEMETRA experiment Funded by INFN (Grant73/2018) Our goals: 1. Prove that qubits su ff er from radioactivity 2. Demonstrate the key role of the substrate 3. Demonstrate that radioactivity suppression is achievable 4. Derive the impact qubit performance Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 7
DEMETRA experiment Funded by INFN (Grant73/2018) Our goals: 1. Prove that qubits su ff er from radioactivity 2. Demonstrate the key role of the substrate 3. Demonstrate that radioactivity suppression is achievable 4. Derive the impact qubit performance Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 8
Do QUBIT see radioactivity? We measured the QP bursts in three superconducting circuits operated as KIDs. We faced a ThO source to the device. • The closer the source, the higher the rate (up to x100) Qubits will see environmental radioactivity! Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 9
DEMETRA experiment Funded by INFN (Grant73/2018) Our goals: 1. Prove that qubits su ff er from radioactivity 2. Demonstrate the key role of the substrate 3. Demonstrate that radioactivity suppression is achievable 4. Derive the impact qubit performance Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 10
• Substrate or qubit itself? • Two hypotheses: • If the radioactive interaction is in the qubit —> single burst • If it is in the substrate —> bursts in all the 3 devices full response 4 1 3 2 Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 11
DEMETRA experiment Funded by INFN (Grant73/2018) Our goals: 1. Prove that qubits su ff er from radioactivity 2. Demonstrate the key role of the substrate 3. Demonstrate that radioactivity suppression is achievable 4. Derive the impact qubit performance Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 12
Radioactivity Suppression • Measurement in the above-ground laboratory of KIT (Germany) • Basic cleaning and repeat measurement in Roma (Italy) • Move Roma sample (and readout) in the Underground LNGS (Italy) A B C 10 2 Γ B (mHz) K R G 10 G, no lead G + Th0 2 • Bursts rate in Roma smaller than KIT (cleaning the sample made something) • Bursts rate in LNGS much smaller than Roma/KIT • Proved a suppression by more than x10 Laura Cardani, INFN - Roma TAUP (Toyama, Japan) 13 �
Impact on Intrinsic Performance • How will the rate correlate with the qubit performance? Amplitude [dB] 2.5 • Measurements of quality factor Q [A.U.] 16 2 15 1.5 • (Proportional to coherence) 14 1 13 0.5 12 0 f = 2365.426 MHz 0 11 − 0.5 Q = 119 k 10 Q = 148 k − 1 c 9 Q = 599 k 1.5 − i 40 20 0 20 40 − − 6.5 6 5.5 5 4.5 4 3.5 3 2.5 − − − − − − − − − Frequency - f [kHz] I [A.U.] 0 A B C K R G 10 5 n ⇡ 1 G, no lead G + Th0 2 Q i @ ¯ • KIT/Roma changed because of the set-up • At LNGS, results better than in any other above-ground attempt! • E ff ect of the source still under investigation Laura Cardani, INFN - Roma TAUP (Toyama, Japan) 14 �
Conclusions and Perspectives Our goals: 1. Prove that qubits su ff er from radioactivity 2. Demonstrate the key role of the substrate 3. Demonstrate that radioactivity suppression is achievable 4. Derive the impact qubit performance What next: 1) Understand the mechanisms relating radioactivity to coherence: 1) The energy distribution matters? 2) The time response matters? 2) Quantify: “how much” radioactivity suppression do we actually need for quantum processors? M. Martinez et al. Phys Rev Apply 2019 Laura Cardani, INFN - Roma TAUP (Toyama, Japan) 15 �
Thanks for the Attention! L. Cardani, N. Casali, M. Clemenza, A. Cruciani, L. Gironi, S. Pirro, C. Rusconi, M. Vignati I. Colantoni M. Martinez T. Charpentier, L. Gruenhaupt, D. Gusenkova, F . Henriques, M. Lagoin, I. Pop, F . Valenti, W. Wernsdorfer, A. Ustinov We welcome new collaborators! – G. Catelani Laura Cardani, INFN - Roma TAUP (Toyama, Japan) � 16
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