1 / 69

Quantum Information Storage in Solid State Systems

David P. Pappas National Institute of Standards & Technology Boulder, CO M. Vissers, Jeff Kline. Quantum Information Storage in Solid State Systems. Hydrogen atom energy levels. n=2. n=1. -3.4 eV. 2. Quantum leap. 0.1 meter. -13.6 eV. 1. D E = 10 electron-volts

gilead
Download Presentation

Quantum Information Storage in Solid State Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. David P. Pappas National Institute of Standards & Technology Boulder, CO M. Vissers, Jeff Kline Quantum Information Storage in Solid State Systems

  2. Hydrogen atom energy levels n=2 n=1 -3.4 eV 2 Quantum leap 0.1 meter -13.6 eV 1 DE = 10 electron-volts 1 eV =1.602 x 10-19 Joules Quantum Leap - DE ~ 1 x 10-18 Joules DE = mgh = (60 kg) x (9.8 m/s2)x(0.1 m) = 60 kg m2/s2 DE ~ 60 Joules Something... “took a Quantum Leap!” Misnomer? 3 Equivalent of man jumping 0.000 000 000 000 000 001 meters high < millionth of an atom

  3. => Quantum Hydrogen • electron acts like a wave • not measured • electron acts like particle • when measured • at some position “Quantum Leap” ≡ fundamental change in paradigm Classical Hydrogen - + • electron charged particle • accelerating e- loses energy • spirals in • atom collapses

  4. Quantum mechanics is Nature’s calculator…very very fast Caffeine Aspirin Ethanol • Example – Self assembly of atoms into molecules • Difficult for classical computer to simulate the simplest molecule • Can we turn situation around? • make artificial atoms • redefine rules=> system solves problem 5 “qubit” molecule NMR R. Marx, A. F. Fahmy, J. M. Myers, W. Bermel, S. J. GlaserPhys. Rev. A 62, 012310-1-8 (2000) BOC-(13C2-15N-2D -glycine)-fluoride

  5. Quantum Information • Quantum Computing • Factor large numbers • Simulate physical systems • Search databases • Quantum Cryptography • Send messages securely • Alternative to public key • Detect eavesdroppers • What is different about Quantum Information? • How is it useful? • Can we control it? • Software • Hardware

  6. Classical Cryptography Bob Alice Pick 2 private keys, 3 & 5 Send public key 15=3 x 5 Use private keys to decode Receive public key = 15 Encode data with public key Send encrypted message back Send public key =15 • Public key Send message back Eve Eavesdropper needs to figure out 15=3x5 to decode : 10941738641570527421809707322040357612003732945449205990913842131476349984288934784717997257891267332497625752899781833797076537244027146743531593354333897= 102639592829741105772054196573991675900716567808038066803341933521790711307779 x 106603488380168454820927220360012878679207958575989291522270608237193062808643 • 7 months to reduce this number into it’s two prime factors • Need faster factoring - Quantum Computing • Need code that can’t be eavesdropped - Quantum Cryptography

  7. Progress in Quantum Information • Quantum Computing • Software • Quantum error correction – Steane, Shor PRL (1996) • Fault tolerant algorithms – Knill, Nature (2005) • Hardware • NMR quantum computers have reached 5 bits • Grovers algorithm - Vandersypen APL (2000) • Shor’s alorithm - Nature (2001) • Ion traps – demonstrated factoring • Semi-classical QFT – Wineland, Science (2005) • Quantum error correction – Nature (2004) • Six atom Schroedinger Cat State – Nature (2005) • Linear optics gate with photons – Franson PRA (2001) • Solid state qubits demonstrate 2 coupled qubits • Quantum cryptography • Demonstrated, tested • Practical demos - Zeilinger Optics Expr. (2004) • 1.45 km through Vienna sewer system • Scaling up to commercial products City Hall Bank

  8. Quantum Computing software – Digital Logic “0” • 2 states • One classical bit “1” • Quantum bit (qubit) Measure 50% if a=b or 50% • 6 states

  9. Quantum computing more possible states & intrinsically parallel Output f(S1) f(S2) S1 =00…000 S2 =00…001 S3 =00…010 … Classical computation: n bits Classical Gate f(x) n bits => 2n possible states 2n computations Spans all input states Quantum computation: n qubits => possible states Quantum gate f(x) Output state measure 1 computation operates on all states

  10. Quantum Interference Speed up of unsorted data search • Classical algorithm: • N =8 Items (states): • On average, need to search N/2 (=4) items to find • : O(N) • Quantum Algorithm: • Compares all coefficients at the same time

  11. Grover’s Search Algorithm a b c d e f g h Weights: F( ) =>change sign if matches “diffusion” operator - reflect around average a b c d e f g h

  12. Quantum problem solving is fast • Grover’s algorithms conducts search in O(N1/2) vs. O(N) • Quantum systems “recognize” solutions • classical database doesn’t gain for simple search • N operations to load! • Resources - NlogN amplitudes for algorithm • Cryptography key search algorithms can benefit greatly • Shor’s algorithm factors numbers • Quantum Fourier Transform: • Exponential speedup of FT • Used to find factors of numbers • Crack public key encryptions • Quantum algorithms take fundamentally different mindset!

  13. Quantum Computing Hardware • Initialize • Store • Interact • Measure Coupling Systems: Superconductors Phase, charge, flux ~~~~~~~~~~ Semiconductor spin Quantum dot ~~~~~~~~~ NMR Neutral atoms Ions Photons Lifetimes

  14. NIST - Quantum computing with real atoms • 9Be+ ions - MEMs RF traps • Addressed with focused lasers • Shuttle ions around • Store • interact • Long lifetimes • Challenge is scaling & interaction • initialization • operation • measurement

  15. Making artificial atoms I 1) Use Superconductors • Zero DC resistance - PDG-conductor at AC • Complete diamagnetism • Macroscopic quantum effects 2) Make LC circuit 3) Add non-linearity to define energy levels Thin, non-linear junction & capacitor Intensity Quality factor: L C Lifetime: frequency

  16. What temperature & frequency? • Freeze out quasiparticles (unpaired e-) in the superconductor • Excitation energy >> Thermal • f = 2 – 10 GHz range • Need high Q for long lifetimes ~ 106 gives ms time constants • Need enabling technology for low T operation • 0.05 to 0.1 K!

  17. Closed cycle plug & play refrigerators T< 0.1 K • Adiabatic demagnetization refrigerattor (ADR) • High Precision Devices – Louisville, CO • Automated magcycl & T PID • Demonstrated T < 100 mK > 400 hr • < 1 day turnaround • Hold time > 12 hr @ 0.1 K w /2 coax • Closed cycle dilution refrigerators (DR) • Multiple vendors – Oxford, BlueFors , Cryomagnetics, HPD, Quantum Design … • 20 mK base • Continuous > 400 μW power - ~ 100 coax • ~ 10 kW power for He compressor & electronics

  18. NIST – Quantum computing with superconductors • Al/AlOX junctions on Si wafers • optical lithography • ~ 1 mm features • standard processing • Al wiring w/SiO2 dielectric • Addressed with RF pulses, DC bias currents • Couple qubits with capacitors • Challenge • Improve lifetimes • Reduce decoherence • Operate at 20 mK • Expected to reduce thermal • decoherence sources

  19. Solid State Quantum Iinformation • Challenges • Need many qubits – 10 ~ 100 (logical) • Need long quantum coherence times ~ ms • Need high measurement fidelity ~ 99% • Superconducting Josephson junction “phase” qubits • Operation • Spectroscopy • Materials • Al ring w/weak link “Josephson Junction” • SiOx substrate, insulator • Al0x tunnel barrier • Identification of decoherence • Improvements

  20. How do you talk to a phase qubit? • Artificial atom - use radiation • Apply bias to adjust frequency • “Listen” with SQUID magnetometer qubit junction SQUID Meas. RF in ~ 5-10 GHz current bias in

  21. Al Mesa Tunnel Barrier Al/a-AlOx/Al a-SiOx Fabrication – trilayer process (Al) Al – superconductor, TC ~ 1 K a-SiO2: amorphous SiO2 - substrate & insulation Chemical vapor deposition a – AlOx: amorphous Al2O3 + OH- - tunnel barrier Self passivated oxide ~ 1.5 nm thick Si

  22. 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

  23. LJ ~1/cosd CJ I = Electrical circuit model of Josephson junction • Qubit oscillates like an L-C circuit • Adjust potential to get two state system – “0” & “1” • Want very low damping of oscillator • High Q gives long lifetimes

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

  25. T1’s in phase qubits – historically been short 0.5 13 m2 junctions: T1~ 25 ns T1 = 23 ns Prob. |1> state 70 m2 junctions: T1~ 40 to 100 ns 0.25 0 200 100 0 Meas. delay (ns) • Loss mostly external to junction

  26. Coherence of first phase quantum bit • Single operation time can be ~ 1 ns • Information in system decayed rapidly ~ 100 ns • Need to preserve information > 104 x operation time to be useful • => Increase coherence times & measurement fidelity!

  27. Materials development – can we improve coherence? Al a-SiOx • Absorption in dielectric & junction (a-SiO2) & (a-AlO2) • Gets worse at low temperature (two – level fluctuators) Si • Focus on: • Insulators: along traces & around junction • in substrate • crossovers • Tunnel barrier - between superconductors

  28. Test actual L-C resonators using thin SiO2 Q decreases at low temperature! ~T RSiO2=2.1kW

  29. 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

  30. 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

  31. Common insulator/substrate materials • SiO2 • 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

  32. Minimize, optimize dielectric & substrate Phys. Rev. Lett. 95, 210503 (2005) Maximum Dielectric SiO2 Rabi oscillations > 600 ns !! Minimum Dielectric SiO2 Minimum Dielectric SiNx Sapphire substrate + SiN insulator: Minimum Dielectric SiNx (UCSB, high T) Time (ns)

  33. Can couple 2 qubits => Do logical operations in ~ 100 ns – density matrix J. Martinis

  34. Found improvements due to optimized materials in insulators => Can we improve the tunnel barrier? Superconductor - Aluminum I Tunnel junction a- AlOx-OH-

  35. Qubit spectroscopy • Increase the bias voltage (tilt) • Frequency of |0> => |1> transition goes down Increase bias Resonances

  36. Effects of resonances • Quench Rabi Oscilations – strong coupling to qubit Spectroscopy Rabi oscillations

  37. Number & splitting of Resonators 70 um2 junction 13 um2 junction • Smaller area – Fewer resonators, larger splitting (strong coupling) • Larger area - More resonators, smaller splitting (weaker coupling) •  Resonances are randomly distributed

  38. 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

  39. 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

  40. 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

  41. Chose bottom superconducting electrode to stabilize crystalline Al2O3 tunnel barrier Elements with high melting temperature

  42. Elements with TC > 1K

  43. Elements that lattice match sapphire (Al203)

  44. Elements that form weaker bond with oxygen than Al

  45. Elements that are not radioactive • Use Re for Al2O3 barrier

  46. Load Lock UHV surface science &growth system Al2O3 growth: Al thermal deposition under O2 exposure on top of base epitaxial Re. LEED, RHEED, Auger Re Sputtering STM/AFM O2 Al Oxygen

  47. 100 nm Re Base layer @ 850 C on sapphire 0.5 x 0.5 mm • 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

  48. Al2O3 Re(0001) AFM/RHEED of single crystal Al2O3 on Re(0001) Epi Re Grow Al2O3 @ RT + Anneal @ 800 ◦C 4x10-6 Torr O2 3m

  49. Source of residual two-level fluctuators: Al-Al2O3 interface? Oxygen Profile across the tunnel barrier Oxygen is white Long tail Sharp Al2O3 Al Re 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 ???

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

More Related