Coherence in Superconducting Materials for Quantum Computing

1. Coherence in Superconducting Materials for Quantum Computing David P. Pappas Jeffrey S. Kline, Fabio da Silva, David Wisbey National Institute of Standards & Technology, Electronics & Electrical Engineering Laboratory, Boulder, CO Collaborators Will Oliver, Paul Welander – MIT/LL Ray Simmonds, Kat Cicak, Josh Strong NIST, Boulder Matthias Steffen, IBM Watson Kevin Osborn, LPS, MD John Martinis, Haohua Wang, UCSB Rob McDermott, U of W Sponsors

2. The quantum computing challenge Implementations Photons Ion traps Neutral atoms NMR ~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~ Spins in semiconductors Quantum dots ~~~~~~~~~~~~~~ Superconducting: ~~~~~~~~~~~~~~~ Charge Flux Phase Coupling Isolation Prepare Prepare Prepare Qubit Qubit Qubit Measure Measure Measure interact interact Decoherence: external – radiation, heat, acoustic… internal – materials, crosstalk…

3. Superconducting qubit measurement setup • Ante • Dilution Refrigerator • Low temperature, < 50 mK • RF measurement • Low power ~ 1 photon of energy in cavity • Improves coherence • Removes quasiparticles in superconductor • Reduces thermal radiation • Hurts coherence: • Low-energy, two-level excitations in amorphous materials

4. The Josephson Junction • Building block of superconducting quantum bits (qubit) • Josephson relations (’62, ‘73) 1.7 nm Al amorphous AlOX Al TEM photo • Not ohmic = > I periodic in d • Voltage only when phase is changing • System is nonlinear for high I

5. Logic Non-linear oscillator Excited |1> vs Ground state |0> Island Charged vs. Not charged Current circulation Left vs Right Types of qubits “Phase” “Charge” “Flux” You & Nori, Physics Today, November (2005)

6. Anatomy of a conventional superconducting circuitMaterials perspective Tunnel barrier Wiring Insulator Substrate Traditional

7. Conventional materials are usedfor a lot of really good reasons… • Si substrate with thermal amorphous a-SiOX on top • Smooth, standard lithography, inexpensive • a-SiOX insulators – CVD • Smooth (no pinholes), low T, easy • a-AlOX tunnel barrier – thermal or plasma oxidation • Smooth, no pinholes, low T, easy, self-limiting • Nb or Al wiring – sputter deposit, polycrystalline • Low temperature, smooth, relatively high TC Need strong motivations for change …

8. Short lifetimes of quantum information in solid state superconducting qubits Lifetime 0.5 T1 = 23 ns 0.25 Prob. |1> state 0 “Rabi” oscillation 200 100 0 Meas. delay (ns) • Relatively short lifetimes and operation cycles • Need lifetime/gate operation time > 1000

9. Outline • Electrical model of a phase qubit • Two Level Systems (TLS) as loss mechanism • substrate & insulators a-SiOX • tunnel barrier a-AlOX • Test structures for materials analysis • New directions in materials • Improved substrates a-Si & removal • Crystalline barriers Al2O3 • Recent progress

10. LCR electrical model for phase qubit LJ~sinf CJ~1-100 x10-12 Rjunction – non-linear QP tunneling - ? Rdielectric – bound dipole relaxation ~ ? Junction & insulators = G(V) Intensity What can we easily measure & optimize? • Quality factor – Energy stored/Energy lost/cycle • Q = = w0/Dw • T1 = Q/w0 • Delectric loss tangent: • tand = Im(e)/Re(e) • = 1/Q frequency

11. Loss in amorphous materials (SiOX-OH-) Schickfuss & Hunklinger, (1974) • Low energy displacements of dipoles, saturate at high T, P • Lose energy through phonon creation • tand = 3x10-3, Q ~333, T1~40 ns • Approaches: 1) Reduce or eliminate dielectrics 2) Optimize mtls. – e.gSiN, a-Si… ++++ ++++ E d _ _ _ _ _ _ _ _

12. Minimize & optimize dielectric - qubit Rabi oscillations Rabi oscillations > 600 ns !! Sapphire substrate + SiN insulator:

13. Optimized SiNx for coherent quantum circuits Kevin Osborn Group Loss Tangent for SiNx films SiNX pillar from high-stress film Qi=1,400 200 nm other labs: NIST, UCSB SiNX Qi=25,000 Al film smooth etch profile from HDP CVD film precursor ratio: N2/SiH4 = 1.8 x-ray reveals polycrystalline order stress = 600 MPa compressive Tgrowth = 300 C The loss tangent is sensitive to PECVD growth!

14. Optimize dielectrics with simple L-C circuits O’Connell, APL (2008) LC – parallel plate C CPW C Substrate L insulator Predicts:

15. Other approach – remove dielectricsSimmonds, Strong, Cicak et. al, NIST Boulder (2008) • Vacuum gap capacitor with an inductor => Flexible circuit - allows us to test the loss in a junction under identical conditions

16. Add a 1.5 nm, 10 mm2 a-AlOX JJ to the circuit • Generally understand dielectric problem – Improve & Reduce • Significant loss in the amorphous AlOX junction • 1.5 nm thick – very strong coupling • Focus on tunnel barriers

17. Tunnel barrier material characterizationQubit spectroscopy • Increase the bias voltage (tilt) • Frequency of |0> => |1> transition goes down Increase I bias Splittings

18. Splittingsin charge qubit - Cooper-Pair Box Z Kim et al., Physical Review B 78, 144506 (2008). 1 mm Al/AlOx/Al gate island junction Vg B gate island (Ec,EJ) Cg junction Vg B

19. Effects of splittings • Quench Rabi Oscilations – strong coupling to qubit • Reduces the measurement fidelity Spectroscopy Rabi oscillations

20. Origin of spectroscopy splittings • Individual, strongly coupled TLS’s in barrier • Distribution of excitation energies - amorphous AlOX 13 um2 junction 70 um2 junction • More splittings, small gaps • weak coupling • Fewer splittings, large gaps • stronger coupling Density of splittings~ 1/GHz/mm2 in 1.5 nm thick junction Reduce materials where possible Improve materials by eliminating TLS’s

21. 1) Reduce materials where possible • Reduce size of junctions in qubits • increases f0 due to smaller capacitance • Add high quality external capacitor to bring f0 down (SiN, a-Si) Steffen, et. al PRL (2006) • T1 ~ 170 ns (SiN) & 600 ns (a-Si:H) • Factor of 2 shorter than expected - Still have a-AlOX in barrier

22. Epitaxial Re/Al2O3 Amorphous AlOX Epitaxial Al2O3 @ 800 C @ RT @ 850 C Al Re Al 10-6 Torr O2 4×10-6 Torr O2, Growth of single-crystal Al2O3 (sapphire) tunnel barrier • Rhenium bottom electrode: • Superconducting – TC ~1 K • hcp - lattice match Al2O3 • high melting T Polycrystalline Al @ RT

23. Al Re (2) Improve JJ’s with crystalline barriers - Al2O3 & MgO Al Al2O3 • Good - High sub-gap resistance • First high quality junctions made with epitaxial barrier • Fabricate into qubit Re(0001) 20 mK I-V curve V(mV)

24. Qubit with 25 mm2 epitaxial Al2O3 junction Kline, et. al, Supercond. Sci. Tech. 22, 015004 (2008) • T1 > 500 ns • best for SiO2 insulator & large junction • No external capacitance • Splitting density reduced • ~3-5 times lower than amorphous barrier of same area

25. Summary & OutlookMaterials in superconducting qubits

26. 12 Qubit Test Die Layout Qubit loop Bias coil DC-SQUID

27. Two level systems in junction Amorphous AlO tunnel barrier • Continuum of • metastable vacancies • Changes on thermal cycling • Resonators must be 2 level, • coherent with qubit! I

28. Design of 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 Amorphous tunnel barrier a –AlOx – OH- SC bottom electrode Poly- Al amorphous SiO2 Low loss substrate Silicon

29. 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 – need high temperature bottom wiring layer

30. Motivations – New wiring materials • Conventional Al, Nb: • Surface oxides with spin polarized traps • 1/f flux noise, dephasing times, density ~ 1017/m2 • Alternative materials: • Re: resists oxidation, high melting T, hcp lattice => Al2O3, • Al passivated with Re or Ru => resists oxidation e- traps Kondo traps Coupled TLS Koch, Clark, di Vincenzo (PRL 2007) Faoro, Ioffe PRB (2007) McDermott, et. al (2007)

31. Improvement of junctionsseen in spectroscopy of 01 transition Epitaxial barrier 70 m2 Amorphous barrier 70 m2 T = 25 mK • Density of coherent splittings reduced by ~5 in epitaxial barrier qubits

32. Source of Residual TLFs: Al-Al2O3 interface? Al2O3 White is oxygen Oxygen content Distance (μm) • Electron Energy Loss Spectroscopy (EELS) from TEM shows • Sharp interface between Al2O3 and Re • Noticeable oxygen diffusion into Al from Al2O3 • Indicates presence of a-AlOx at interface • Will “heal” pinholes

33. Need to improve top barrier interface! • Interfacial effect • ~1 in 5 oxygens at Al interface • Agrees with reduced splitting density Al non-epi Al interface Oxygen ~1.5 nm a-AlOx epi-Re interface Re

34. Top electrode mattersAl top electrode always gives good I/V Al/a-AlO/Al Re/c-AlO/Al Re/c-MgO/Al Al top electrode Tunnel barrier Bottom electrode a: Amorphous c: Crystalline substrate Supports conclusion that Al top electrode “heals” pinholes

35. Re on top makes JJ leaky substrate Re/c-AlO/Re Re top electrode Tunnel barrier Bottom electrode => Pinholes in tunnel barrier

36. Electrical Testing Summary & ComparisonPhase qubits

37. Goals • Inter-laboratory compatibility • Infrastructure - 6”-wafer chamber for epitaxial trilayers • Develop 6” substrate capability • Re/Al2O3/Al, Re/Al2O3/Re • Supply samples to flux qubit, 6” wafer fabrication facililty • Extend work on epitaxial tunnel barriers to flux qubits • Continue on barriers at chip level • Chip level • Develop JJ and qubit circuits compatible w/flux qubits • study fully epitaxial systems • Study new materials for wiring layers • Al/Ru capping with anneal • Push to understand flux noise and wiring surfaces

38. Other potential new insulators – from VLSI world? • “Medium” K dielectrics? • Si • SiN • Al2O3 • MgO • Diamond • ZrSiO • CaO • SiC • Need to use thicker insulators • “low” K dielectrics? • doped SiOx (F, C • Porous SiOx • Spin-on polymers (HSQ) new Probably not

39. tunnel barrier insulator wiring substrate New directions Materials Difficulties CMOS

40. Two-level systems in a-SiO2 Low E High E Low E SiO2 - Bridge bond Schickfus and Hunklinger, 1975 E d • TLS bath saturates at • high E (power), decreasing loss Amorphous material has all barrier heights present

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

42. 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: • find better substrates • find better dielectric & minimize insulators in design

43. Common insulator/substrate materials • SiOX • Bridge bond, unstable • Amorphous films have uncompensated O- , H, OH- • Si3N4 • N has three bonds – more stable • Amorphous films, still have uncompensated charges, H • 20% H for low T films, ~ 2% H in high T films • Al2O3 • Amorphous – high loss, similar to a-SiO2, has H, OH- in film • Single crystal (sapphire) - Very low loss system

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

45. Tunnel barrier materials Found improvements due to optimized materials in insulators Superconductor - Aluminum I Tunnel junction a- AlOx-OH-