2007 quantum computing qc quantum algorithms qa program review
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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

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

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2007 quantum computing qc quantum algorithms qa program review

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


Project milestones

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


Improvement of junctions seen in spectroscopy of 0 1 transition

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


2007 quantum computing qc quantum algorithms qa program review

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


Josephson junction non linear lc oscillator die layout

Josephson Junction non-linear LC-Oscillatordie layout

w in

w out

JJ

JJ

JJ

Flux Bias Coil


Jj non linear resonator 13 m 2 in jj area

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


Materials test bed considerations

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


12 qubit test circuit

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


12 qubit test circuit1

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


12 qubit die layout

Qubit loop

Bias coil

DC-SQUID

12 Qubit Die Layout


12 qubit results

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


Min sio 2 epitaxial re qubit

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


Electrical testing summary comparison

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


Need to develop better tunnel junctions and better electrodes

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


Source of residual tlfs al al 2 o 3 interface

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


Top electrode matters

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


Al top electrode always gives good i v

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


Look at magnesium oxide as tunnel barrier

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


V mgo al fabrication

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


Mgo tunnel barrier on v @ rt is epitaxial

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


V mgo al josephson junction iv curve

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


Yes vanadium base electrode oxidizes

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??


V mgo conclusions

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


2008 milestones

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


Road map to epitaxial qubits 2007

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


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