2007 Quantum Computing (QC) & Quantum Algorithms (QA) Program Review

1. 2007 Quantum Computing (QC) & Quantum Algorithms (QA) Program Review Quantum Materials Jeffrey S. Kline, Seongshik Oh*, David P. Pappas National Institute of Standards & Technology, Electronics & Electrical Engineering Laboratory, Boulder, CO *present address Rutgers University, Piscataway, NJ

2. Project Milestones • 1st Year: Al2O3-based epitaxial materials • Re/Al2O3/Re Josephson junctions • Obtained leaky IV curve due to pinholes in tunnel barrier • Oxygen segregation study • Obtained oxygen profile which indicates undesirable diffusion of oxygen from the barrier into the aluminum top electrode • Fabricate devices with new circuit design and better wiring dielectrics • Completed new design and made low temperature measurements at UCSB. Integration with better wiring dielectrics in progress. • 2nd Year: MgO-based epitaxial materials • V/MgO/V trilayer growth • STM and Auger characterization complete. • Fabricate test junctions • Junctions are leaky due to pinholes in MgO • Measure V/MgO/V qubits • Not possible with leaky barrier • NbN/AlN-based Josephson junctions • Attempted to grow NbN but cannot obtain high quality films due to incompatible apparatus. Will try with new MBE system. • Three inch wafer MgO growth • MgO-based non-epitaxial materials • Fabricate Re/MgO/Re Josephson junction oscillators • Not possible with leaky MgO barrier • Fabricate Re/MgO/Al qubit • 3rd Year: Commission MBE Chamber • Set up vacuum chamber • Move-in complete, not under vacuum yet • Set up characterization tools • Grow samples on three inch wafers

3. Improvement of junctionsseen in spectroscopy of 01 transition Epitaxial barrier 70 m2 Amorphous barrier 70 m2 T = 25 mK Splittings decohere qubits during measurements • Density of coherent splittings reduced by ~5 in epitaxial barrier qubits • Need test bed for rapid materials screening

4. 1 wr(f)2 = Leff(f) Ceff Josephson Junction non-linear LC-Oscillatorwith Ray Simmonds, NIST Boulder • => Simpler alternative to full qubit • Only one junction • Relaxed conditions on IC • Coherent oscillators in junction will be pumped Flux Bias Coil w out w in L Ceff Reff LJ(f) Qinternal = wrReffCeff

5. Josephson Junction non-linear LC-Oscillatordie layout w in w out JJ JJ JJ Flux Bias Coil

6. JJ non-linear resonator: 13 m2 in JJ area Al/a-AlOx/Al Re/epi-Al2O3/Al 7.8 fw(GHz) Flux Bias 7.0 few splittings observed ~15 splittings/GHz

7. Materials test bed considerations • JJ resonator no T1, T2 & still don’t have 100% yield die • Observation • Tunnel junction IC is exponentially depends on thickness • Oxide deposition time =410±5 seconds with R doubling every 5 s • Need 4 junctions to work simultaneously on qubit • (75% probability)4 => 25% yield • 3 different qubit junction areas • 12 m2, 25 m2, 49 m2 • 4 devices of each • All 12 qubits share common flux bias and microwave lines • Advantage – simplify circuit and bonding • Drawback – only measures one qubit at a time

8. 12 Qubit Test Circuit Common qubit microwave line Common flux bias line S2 S1 S4 S3 • 12 m2 • 25 m2 • 49 m2 S6 S5 S8 S7 S11 S10 S9 S12

9. S1 S1 12 Qubit Test Circuit Common qubit microwave line Common flux bias line S2 S4 S3 • 12 m2 • 25 m2 • 49 m2 S6 S5 S8 S7 S11 S10 S9 S12

10. Qubit loop Bias coil DC-SQUID 12 Qubit Die Layout

11. 12 qubit results Si-O2 dielectric min Si-O2 dielectric • two 49 mm2 devices worked • Visibility ~ 75% • T1 ~ 200,400 ns • Splittings comparable to 13 mm2 amorphous device • one 49 mm2 devices • Visibility ~ 80% • T1 ~ 500 ns • T2 = 140 ns • Splittings comparable to 13 mm2 amorphous device

12. Min-SiO2 Epitaxial Re Qubit • T1 = 400 ns good for SiO2 dielectric • Splitting density • ~3 times lower than amorphous barrier of same area • Future plan: • advanced wiring dielectrics – SiN, a-Si – 1 ms T1? • Use to test wiring layer

13. Electrical Testing Summary & Comparison • 12 – qubit design has become standard UCSB test platform • We need to: • Test wiring layers for loss • Find materials with better interfaces

14. Need to develop better tunnel junctions and better electrodes! • 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

15. 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

16. Top electrode matters V/c-MgO/Re substrate Re/c-AlO/Re Re on top makes JJ leaky Re top electrode Tunnel barrier Bottom electrode => Pinholes in tunnel barrier

17. Al 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

18. Look at Magnesium oxide as tunnel barrier • MgO • Room temperature crystalline growth possible • Compare to Al2O3 which requires high temp (~800C) anneal • Cubic lattice • Compare to Al2O3: hexagonal • Lattice matches to Vanadium • Desirable electrode properties • TC = 5.4 K • Smooth surface morphology • Compatibility with crystalline MgO • MgO(001)-FCC is lattice matched to V(001)R45-BCC • mismatch ~ 1% aV aMgO

19. Al MgO V MgO substrate V/MgO/Al fabrication • Sputter deposit V -800C, 2 nm/min, Ar • MgO growth – reactive evaporation in O2 • Evaporate Al

20. MgO tunnel barrier on V @ RT is epitaxial • MgO • RT growth • Thickness ~2 nm • Single atomic steps • Wide terraces JK104.1.R1 JK127.1.m3_p1 STM: 800x800 nm2

21. V/MgO/Al Josephson junction IV curve T = 50 mK • Vanadium energy gap () reduced from 0.8 meV (bulk V) to 0.10 meV • Unintentional oxidation of vanadium base electrode? observed gap expected (bulk) gap

22. Yes - vanadium base electrode oxidizes! • Vanadium base electrode: as grown • After exposure to oxygen • Oxidation of vanadium during trilayer growth • Reduces TC and the gap at the interface • Adversely affects I/V’s • How does this affect qubit??

23. V/MgO Conclusions • V base electrode is oxidized • We have tried • V/MgO/V: leaky • V/MgO/Re: leaky • V-VN/MgO/Al: reduced gap • V-Mg/MgO/Al: reduced gap • Mg proximity layer • V/MgO/Al: reduced gap • Need to test V/MgO/Al qubits

24. 2008 Milestones • High performance dielectrics • Hydrogenated amorphous silicon • Tunnel barriers • MgO • Rhenium base electrode • AlN • Al2O3 • Try to reduce splittings by using atomic oxygen • Install new UHV system for three/six inch wafers

25. Road Map to Epitaxial Qubits 2007 Al2O3 Epitaxial Qubit MgO Epitaxial Qubit Epi growth on Re Epi growth on V Re JJ IVs V JJ IVs Textured growth on Re Re qubit w/low perf. dielectrics JJ Oscillator study Re JJ IVs 12 qubit design Re qubit w/high perf. dielectrics Re qubit w/high perf. dielectrics Growth on six inch wafer Epi Growth on NbN Atomic oxygen experiment NbN qubit w/high perf. dielectrics NbN JJ IVs Growth on six inch wafer Completed, submitted, or Published In progress Future rogram