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

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 1.7 nm Building block of superconducting quantum bits (qubit) Josephson relations (’62, ‘73) Al amorphous AlO X Not ohmic = > I periodic in  Voltage only when phase is changing System is nonlinear for high I TEM photo

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

6. Anatomy of a conventional superconducting circuit Materials perspective Tunnel barrier Wiring Insulator Substrate MaterialPreparation method Tunnel BarrierAlO X Thermal WiringNb or AlSputtered InsulatorSiO X CVD SubstrateSi/SiO X Thermal Traditional

7. Conventional materials are used for a lot of really good reasons… Si substrate with thermal amorphous a-SiO X on top –Smooth, standard lithography, inexpensive a-SiO X insulators – CVD –Smooth (no pinholes), low T, easy a-AlO X 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 T C Need strong motivations for change …

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

9. Outline Electrical model of a phase qubit Two Level Systems (TLS) as loss mechanism –substrate & insulators a-SiO X –tunnel barrier a-AlO X Test structures for materials analysis New directions in materials – Improved substrates a-Si & removal – Crystalline barriers Al 2 O 3 Recent progress

10. LCR electrical model for phase qubit = C J ~1-100 x10 -12 L J ~sin  Intensity G(V) Quality factor – Energy stored/Energy lost/cycle Q =  0 /     Q/   Delectric loss tangent: tan  = Im(  )/Re(  ) = 1/Q R junction – non-linear QP tunneling - ? R dielectric – bound dipole relaxation ~ ? Junction & insulators What can we easily measure & optimize? frequency

11. Loss in amorphous materials (SiO X -OH - ) Low energy displacements of dipoles, saturate at high T, P Lose energy through phonon creation –tan  3x10 -3, Q ~333, T 1 ~40 ns Approaches: 1) Reduce or eliminate dielectrics 2) Optimize mtls. – e.g SiN, a-Si… Schickfuss & Hunklinger, (1974) E d ++++ _ _ _ _

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

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

14. Optimize dielectrics with simple L-C circuits L C LC – parallel plate C CPW Material Q=1/tan  Si(111)200,000 Sapphire – Al 2 O 3 160,000 a-Si:H45,000 a-SiN10,000 a-SiO X 3,300 O’Connell, APL (2008) Predicts: Substrate insulator

15. Other approach – remove dielectrics Simmonds, Strong, Cicak et. al, NIST Boulder (2008) Vacuum gap capacitor with an inductor Q Before dielectric removed SiN, 100 nm x 8000  m 2 600 After dielectric is removed40,000 => Flexible circuit - allows us to test the loss in a junction under identical conditions

16. Add a 1.5 nm, 10  m 2 a-AlO X JJ to the circuit Q Before dielectric removed SiN 100 nm x 8000  m 2 600 After dielectric is removed40,000 With Josephson Junction a-AlO X, 1.5 nm x 10  m 2 400 Generally understand dielectric problem – Improve & Reduce Significant loss in the amorphous AlO X junction 1.5 nm thick – very strong coupling Focus on tunnel barriers

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

18. Splittings in charge qubit - Cooper-Pair Box VgVg CgCg (E c,E J ) 1  m B Al/AlO x /Al island B VgVg gate island gate junction Z Kim et al., Physical Review B 78, 144506 (2008).

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

20. Origin of spectroscopy splittings Individual, strongly coupled TLS’s in barrier Distribution of excitation energies - amorphous AlO X Density of splittings ~ 1/GHz/  m 2 in 1.5 nm thick junction (1)Reduce materials where possible (2)Improve materials by eliminating TLS’s 13 um 2 junction Fewer splittings, large gaps stronger coupling 70 um 2 junction More splittings, small gaps weak coupling

21. 1) Reduce materials where possible Steffen, et. al PRL (2006) Reduce size of junctions in qubits increases f 0 due to smaller capacitance Add high quality external capacitor to bring f 0 down (SiN, a-Si) T 1 ~ 170 ns (SiN) & 600 ns (a-Si:H) Factor of 2 shorter than expected - Still have a-AlO X in barrier

22. Growth of single-crystal Al 2 O 3 (sapphire) tunnel barrier 4×10 -6 Torr O2,Al10 -6 Torr O2 Epitaxial Re/Al 2 O 3 Re @ 850  C Al Amorphous AlO X @ RT Epitaxial Al 2 O 3 @ 800  C Polycrystalline Al @ RT Rhenium bottom electrode: Superconducting – T C ~1 K hcp - lattice match Al2O3 high melting T

23. (2) Improve JJ’s with crystalline barriers - Al 2 O 3 & MgO Good - High sub-gap resistance First high quality junctions made with epitaxial barrier Fabricate into qubit Re(0001) Al 2 O 3 Al Re Al I-V curve 20 mK V(mV)

24. T 1 > 500 ns –best for SiO 2 insulator & large junction –No external capacitance Splitting density reduced –~3-5 times lower than amorphous barrier of same area Qubit with 25  m 2 epitaxial Al 2 O 3 junction Kline, et. al, Supercond. Sci. Tech. 22, 015004 (2008)

25. Summary & Outlook Materials in superconducting qubits

26. 12 Qubit Test Die Layout Bias coil Qubit loop 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. What we need: Crystalline barrier  -Al 2 O 3 Poly - Al Existing technology: Amorphous tunnel barrier a –AlO x – OH - No spurious resonators Stable barrier Amorphous Aluminum oxide barrier Spurious resonators in junctions Fluctuations in barrier Silicon amorphous SiO 2 Low loss substrate Design of tunnel junctions SC bottom electrode Top electrode

29. Q: Can we prepare crystalline Al 2 O 3 on Al? Binding energy of Al AES peak in oxide Annealing Temp (K) AES Energy of Reacted Al (eV) Al in sapphire Al 2 0 3 Metallic aluminum Aluminum Melts 68 10 Å AlO x on Al (300 K + anneal) 10 Å AlO x on Al (exposed at elevated temp.)  Anneal the natural oxides  Oxidize at elevated temp. 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 ~ 10 17 /m 2 Alternative materials: –Re: resists oxidation, high melting T, hcp lattice => Al 2 O 3, –Al passivated with Re or Ru => resists oxidation Koch, Clark, di Vincenzo (PRL 2007) e - traps Kondo traps Faoro, Ioffe PRB (2007) Coupled TLS McDermott, et. al (2007)

31. Improvement of junctions seen in spectroscopy of 0  1 transition T = 25 mK Amorphous barrier 70  m 2 Epitaxial barrier 70  m 2 Density of coherent splittings reduced by ~5 in epitaxial barrier qubits

32. Source of Residual TLFs: Al-Al 2 O 3 interface? Electron Energy Loss Spectroscopy (EELS) from TEM shows 1.Sharp interface between Al 2 O 3 and Re 2.Noticeable oxygen diffusion into Al from Al 2 O 3 1.Indicates presence of a-AlO x at interface 2.Will “heal” pinholes Distance (μm) Oxygen content Al 2 O 3 White is oxygen

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

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

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

36. Electrical Testing Summary & Comparison Phase qubits Materials Wiring & barrier InsulatorT 1 (ns) T 2 * (ns) Splitting density (N/GHz/mm2) Reference Al/AlO x /Al 1  m 2 w/shunting C min-SiN x 11090 (160) 1Steffen - tomography PRL 97 050502 Al/AlO x /Al 13  m 2 min-SiN x 5001501Martinis Dielectric loss PRL 95 210503 Al/AlO x /Almin-SiO 2 170*1Simmonds 2005 Re/Al 2 O 3 /Al epi-junctionmax-SiO 2 150900.2PRB 74 100502 12 qubit - Re/Al 2 O 3 /Al 49  m 2 max-SiO 2 200-400*0.2Submitted APS08 12 qubit - Re/Al 2 O 3 /Al 49  m 2 min-SiO 2 5001400.2Submitted APS08 12 qubit – Re/MgO/Al80500.4New results

37. Goals 1.Inter-laboratory compatibility –Infrastructure - 6”-wafer chamber for epitaxial trilayers Develop 6” substrate capability Re/Al 2 O 3 /Al, Re/Al 2 O 3 /Re –Supply samples to flux qubit, 6” wafer fabrication facililty 2.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 3.Study new materials for wiring layers –Al/Ru capping with anneal –Push to understand flux noise and wiring surfaces

38. “Medium” K dielectrics? Si SiN Al 2 O 3 MgO Diamond ZrSiO CaO SiC  Need to use thicker insulators “low” K dielectrics? doped SiOx (F, C Porous SiOx Spin-on polymers (HSQ) Probably not new Other potential new insulators – from VLSI world?

39. New directions tunnel barrier insulator wiring substrate SubstrateSapphire (Al 2 O 3 ) CrystallineExpensive, difficult to work with, can be atomically rough WiringRe, Al/RuAnnealedComplicated, hard to prepare, Hi-T InsulatorSiN, a-Si, Al 2 O 3 Sputtered Epitaxial High T, adhesion, processing Homogeneity, rough BarrierAl 2 O 3 EpitaxialHigh T, homogeneity, rough Materials Difficulties CMOS

40. TLS bath saturates at high E (power), decreasing loss Schickfus and Hunklinger, 1975 Two-level systems in a-SiO2 E d SiO 2 - Bridge bond Amorphous material has all barrier heights present High E Low E

41. ~T R SiO2 =2.1k  Temperature Dependence of Q Q also decreases at low temperature!

42. Problem - amorphous SiO 2 Why short T 1 ’s in phase Josephson qubits? Dissipation: Idea - Nature: At low temperatures (& low powers) environment “freezes out”: dissipation lowers dissipation increases, by 10 – 1000! Change the qubit design:  find better substrates  find better dielectric & minimize insulators in design

43. Common insulator/substrate materials SiO X –Bridge bond, unstable Amorphous films have uncompensated O -, H, OH - Si 3 N 4 –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 Al 2 O 3 –Amorphous – high loss, similar to a-SiO2, has H, OH - in film –Single crystal (sapphire) - Very low loss system

44. Insert qubit pic here Qubit L Stripline (C-SiO X ) Josephson Junction (L&C) => Measure “Q” of simple LC resonators Qubit has SiO 2 Cap in || with J.J. & around lines SiO X AlO x

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