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. 1 st 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. 2 nd 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 3 rd 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 Project Milestones

3. Improvement of junctions seen in spectroscopy of 0  1 transition T = 25 mK Splittings decohere qubits during measurements Amorphous barrier 70  m 2 Epitaxial barrier 70  m 2 Density of coherent splittings reduced by ~5 in epitaxial barrier qubits Need test bed for rapid materials screening

4. R eff C eff LJ()LJ() L Josephson Junction non-linear LC-Oscillator with Ray Simmonds, NIST Boulder Q internal =  r R eff C eff Flux Bias Coil  w in  w out => Simpler alternative to full qubit Only one junction Relaxed conditions on I C Coherent oscillators in junction will be pumped  r  2 L eff (  ) C eff 1 =

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

6. Re/epi-Al 2 O 3 /Al ~15 splittings/GHz 7.0 7.8 f  w (GHz) Flux Bias Al/a-AlO x /Al few splittings observed JJ non-linear resonator: 13  m 2 in JJ area

7. JJ resonator no T 1, T 2 & still don’t have 100% yield die Observation –Tunnel junction I C 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  m 2, 25  m 2, 49  m 2 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 Materials test bed considerations

8. 12 Qubit Test Circuit Common qubit microwave line Common flux bias line S1S1 S7S7 S6S6 S5S5 S2S2 S3S3 S4S4 S8S8 S9S9 S 10 S 11 S 12 12  m 2 25  m 2 49  m 2

9. 12 Qubit Test Circuit Common qubit microwave line Common flux bias line S7S7 S6S6 S5S5 S2S2 S3S3 S4S4 S8S8 S9S9 S 10 S 11 S 12 12  m 2 25  m 2 49  m 2 S1S1 S1S1

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

11. 12 qubit results –two 49  m 2 devices worked –Visibility ~ 75% –T1 ~ 200,400 ns –Splittings comparable to 13  m 2 amorphous device –one 49  m 2 devices –Visibility ~ 80% –T1 ~ 500 ns –T2 = 140 ns –Splittings comparable to 13  m 2 amorphous device Si-O 2 dielectric min Si-O 2 dielectric

12. T 1 = 400 ns  good for SiO 2 dielectric Splitting density –~3 times lower than amorphous barrier of same area Future plan: –advanced wiring dielectrics – SiN, a-Si – 1  s T 1 ? –Use to test wiring layer Min-SiO 2 Epitaxial Re Qubit

13. Electrical Testing Summary & Comparison MaterialsWiring DielectricT1T1 Reference e-beam junction w/Shunting capacitor min-SiN x 450PRL 97 050502 Al/AlO x /Almin-SiN x 500PRL 95 210503 Al/AlO x /Almin-SiO 2 170Simmonds 2005 Re/Al 2 O 3 /Al epi-junctionmax-SiO 2 150PRB 74 100502 12 qubit - Re/Al 2 O 3 /Almax-SiO 2 200-400Present work 12 qubit - Re/Al 2 O 3 /Almin-SiO 2 500Present work 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 ~1.5 nm epi-Re interface non-epi Al interface Oxygen Re Al a-AlO x

15. 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

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

17. 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 Al top electrode always gives good I/V

18. Look at Magnesium oxide as tunnel barrier a MgO aVaV 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 –T C = 5.4 K –Smooth surface morphology Compatibility with crystalline MgO –MgO(001)-FCC is lattice matched to V(001)R45  -BCC –mismatch ~ 1%

19. V/MgO/Al fabrication 1.Sputter deposit V -800C, 2 nm/min, Ar 2.MgO growth – reactive evaporation in O 2 3.Evaporate Al substrate MgO V Al

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

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

22. –Oxidation of vanadium during trilayer growth –Reduces T C and the gap at the interface –Adversely affects I/V’s –How does this affect qubit?? Yes - vanadium base electrode oxidizes! Vanadium base electrode: as grown After exposure to oxygen

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 –Al 2 O 3 Try to reduce splittings by using atomic oxygen Install new UHV system for three/six inch wafers

25. Road Map to Epitaxial Qubits 2007 Re JJ IVs Completed, submitted, or Published In progress Future rogram Epi growth on Re Re qubit w/low perf. dielectrics Growth on six inch wafer Atomic oxygen experiment Al 2 O 3 Epitaxial Qubit MgO Epitaxial Qubit Epi growth on V Re qubit w/high perf. dielectrics JJ Oscillator study 12 qubit design V JJ IVs Textured growth on Re Re JJ IVs Re qubit w/high perf. dielectrics Growth on six inch wafer Epi Growth on NbN NbN JJ IVs NbN qubit w/high perf. dielectrics