Materials Science in Quantum Computing

1. Materials Science in Quantum Computing

2. Materials scientist view of qubit • Materials • SiOx sub substrate • Superconductor (Al,Nb) • SiOx dielectric • Al0x tunnel barrier

3. The quantum computing challenge superconductors

4. Superconducting Josephson junction phase qubit principle Top superconductor Cooper pair wavefunction Ytop= Y0 Tunnel Junction ~1.5 nm I Ybot= Y0eid Bottom superconductor • I depends on d • Voltage only when phase is changing Josephson relations

5. Quantum behavior - potential that phase qubit lives in • Increase bias => cubic potential lifts degeneracy • Use the |0> and |1> states for information

6. Qubit has SiO2 Cap in || with J.J. Insert qubit pic here SiO2 AlOx Stripline (C-SiO2 ) Qubit L => Measure “Q” of LC resonators Josephson Junction (L&C)

7. Parallel plate capacitor resonators w/SiO dielectric Power dependence to parallel plate capacitor resonators wave resonator C Pin lowering L L Pout [mW] Q of the resonator goes down as power decreases! f [GHz]

8. Interdigitated capacitor resonators Room-temperature deposited SiO2 over the capacitor Data with and without SiO2 on Cap C/2 C/2 L Dissipation is in SiO2 dielectric of the capacitor! ~Pout

9. Power dependence of QLC for parallel plate capacitors Q decreases with at very low power (where we run qubits) QLC C L Nphotons HUGE Dissipation Explains small T1!

10. Theory: Why 1/E Dependence? Schickfus and Hunklinger, 1977 Uncompensated spins in SiOx E d • Spin (TLS) bath: saturates at • high power, decreasing loss 1/loss tangent SiOx (amorphous) high power 1 10 102 104 |E| [V/m] • Loss “saturates” from each TLS • Pabs ~ number saturated • ~ d*E • Solve Bloch eqn’s for bath rn:

11. Temperature Dependence of Q Q also decreases at low temperature! ~T RSiO2=2.1kW

12. dissipation increases, by 10 – 1000! Problem - amorphous SiO2Why short T1’s in phase Josephson qubits? • Dissipation: Idea - Nature: • At low temperatures (& low powers) • environment “freezes out”: • dissipation lowers Change the qubit design: • single crystal sapphire substrates • SiN dielectric & minimize dielectric in design

13. 1 1 SiN/sapphire: Significant improvement in T1, T2* 1 0.4 P(1) P(1) 0 Time (ms) 2 0 Time (ms) 1 • T1 increased to nearly 600 ns • T2* nearly 300 ns • Still need to deal with low fidelity => junctions • Do spectroscopy on qubits

14. Qubit spectroscopy • Increase the bias voltage (tilt) • Frequency of |0> => |1> transition decreases Increase bias Resonances • Resonance density increases with junction size

15. Microscopic two-state fluctuators in junction Amorphous AlO tunnel barrier • Continuum of • metastable vacancies • Changes on thermal cycling • Origin? • uncompensated spins in barrier • O atoms tunneling between sites

16. Off resonance On resonance |1e> |1e> |1g> |1g> DE DE=0 Anti-crossing, splitting S Resonators must be 2 level, coherent with qubit! qubit - |0> or |1> res. - |g> or |e> |0e> 0 |0g>

17. off on off on Qubit-resonator coupled interaction • Off resonance - |1> decays • On resonance, put qubit in |1> • Wait some time, tint/2 • Qubit goes into |0> • =>Wait tint • Qubit goes back to |1> with enhanced amplitude! • States oscillate |1g> <=> |0e> Resonator has longer coherence time than qubit

18. Re-design tunnel junctions Existing technology: What we need: Amorphous Aluminum oxide barrier Spurious resonators in junctions Fluctuations in barrier No spurious resonators Stable barrier Top electrode Poly - Al Crystalline barrier a-Al2O3 ? Interfaces: Smooth Stable No dangling bonds Amorphous tunnel barrier a -AlOx Rough interfaces Unstable at room temp. Dangling bonds SC bottom electrode Poly- Al amorphous SiO2 dangling bonds at interface Low loss substrate/dielectric : SiN Silicon

19. Q: Can we prepare crystalline Al2O3 on Al? 68 Metallic aluminum 10 Å AlOx on Al (300 K + anneal) 10 Å AlOx on Al (exposed at elevated temp.) AES Energy of Reacted Al (eV) Aluminum Melts Al in sapphire Al203 Annealing Temp (K) • Anneal the natural oxides • Oxidize at elevated temp. Binding energy of Al AES peak in oxide A: No

20. Chose bottom superconducting electrode to stabilize crystalline Al2O3 or MgOtunnel barrier Elements with high melting temperature

21. Elements with TC > 1K

22. Elements that lattice match sapphire (Al203)

23. Elements that form weaker bond with oxygen than Al

24. Elements that are not radioactive

25. UHV system: Pbase< 5x10-10 Torr Load Lock Al2O3 growth: Al thermal deposition under O2 exposure on top of base Epi Re. LEED, RHEED, AES Re Sputtering STM Ex-situ AFM O2 Al Oxygen

26. 0.5 x 0.5 mm 100 nm Re Base layer @ 850 C on sapphire • 1.5 nm RMS roughness • 1-2 atomic layer steps • Screw dislocations on mesas • Stranski-Krastanov growth • Initial wetting of substrate • Formation of 3-d islands • Islands fill in gradually

27. Single crystal Al2O3 on Re(0001) Al2O3 Re(0001) Epi Re Grow Al2O3 @ RT + Anneal @ 800 ◦C 4x10-6 Torr O2 3m

28. Al Re Fabricate test junctions with epi-Al2O3 barrier Al Al2O3 Re(0001) • First high quality junctions made with epitaxial barrier!! I-V curve at 20 mK 1/R vs. Area at 300 K V(mV)

29. 1 Conclusions • Amorphous dielectrics can have HUGE loss due to two level system (spin bath) • Problem with phase qubit: Loss in dielectric • Fix by using SiN dielectric • Tunnel junctions have coherent two state systems that are detrimental to the fidelity of the measurements • Materials optimization is critical to long coherence times • Status • Testing qubits w/epi-barriers • Eliminating/improving dielectrics around qubit