2007 Quantum Computing (QC)

& Quantum Algorithms (QA)

Program Review

Quantum Materials

Jeffrey S. Kline, Seongshik Oh*, David P. PappasNational Institute of Standards & Technology,

Electronics & Electrical Engineering Laboratory, Boulder, CO

*present address Rutgers University, Piscataway, NJ

• 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

Project Milestones

Improvement of junctionsseen in spectroscopy of 01 transition

T = 25 mKSplittings decohere qubits during measurements

Amorphous barrier70 m2

Epitaxial barrier70 m2

• Density of coherent splittings reduced by ~5

in epitaxial barrier qubits• Need test bed for rapid materials screening

Reff

Ceff

LJ()

L

Josephson Junction non-linear LC-Oscillatorwith Ray Simmonds, NIST Boulder

Qinternal = rReffCeff

Flux Bias Coil

w in w out

=> Simpler alternative to full qubit• Only one junction• Relaxed conditions on IC

• Coherent oscillators in junction will be pumped

r2

Leff() Ceff

1=

Josephson Junction non-linear LC-Oscillatordie layout

JJJJ

w in w out

JJ

Flux Bias Coil

Re/epi-Al2O3/Al

7.52

7.54

7.56

7.58

7.6

7.62

7.64

7.66

7.68

7.7

Flux (0)

Freq

uenc

y (GH

z)

Power Out vs Frequency and Flux

~15 splittings/GHz

7.0

7.8

f w (

GH

z)

Flux Bias

Al/a-AlOx/Al

few splittings observed

JJ non-linear resonator: 13 m2 in JJ area

• 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

Materials test bed considerations

12 Qubit Test CircuitCommon qubit microwave line Common flux bias line

S1

S7S6S5

S2 S3 S4

S8

S9 S10 S11 S12

12 m2

25 m2

49 m2

12 Qubit Test CircuitCommon qubit microwave line

Common flux bias line

S7S6S5

S2 S3 S4

S8

S9 S10 S11 S12

12 m2

25 m2

49 m2

S1

S1

12 Qubit Die Layout

Bias coil Qubit loop

DC-SQUID

12 qubit results

– two 49 m2 devices worked– Visibility ~ 75%– T1 ~ 200,400 ns– Splittings comparable to 13

m2 amorphous device

– one 49 m2 devices– Visibility ~ 80%– T1 ~ 500 ns– T2 = 140 ns– Splittings comparable to

13 m2 amorphous device

Si-O2 dielectric min Si-O2 dielectric

• 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 s T1?– Use to test wiring layer

Min-SiO2 Epitaxial Re Qubit

Electrical Testing Summary & Comparison

Materials Wiring Dielectric T1 Reference

e-beam junction w/Shunting capacitor

min-SiNx 450 PRL 97 050502

Al/AlOx/Al min-SiNx 500 PRL 95 210503

Al/AlOx/Al min-SiO2 170 Simmonds 2005

Re/Al2O3/Al epi-junction max-SiO2 150 PRB 74 100502

12 qubit - Re/Al2O3/Al max-SiO2 200-400 Present work

12 qubit - Re/Al2O3/Al min-SiO2 500 Present work

• 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!

• Interfacial effect• ~1 in 5 oxygens at Al interface• Agrees with reduced splitting density

~1.5 nm

epi-Re interface

non-epi Al interfaceOxygen

Re

Al

a-AlOx

Source of Residual TLFs: Al-Al2O3 interface?

Electron Energy Loss Spectroscopy (EELS) from TEM shows1. Sharp interface between Al2O3 and Re2. Noticeable oxygen diffusion into Al from Al2O3

1. Indicates presence of a-AlOx at interface2. Will “heal” pinholes

Distance (μm)

Oxy

gen

cont

ent

Al2O3White is oxygen

Re on top makes JJ leaky

0

10

20

30

0 500 1000

V/c-MgO/Re

V (uV)

V/c-MgO/Re

0

10

20

30

0 200 400 600

Re/c-Al2O3/Re/Al

V (uV)

Re/c-AlO/Re

substrate

Re top electrodeTunnel barrierBottom electrode

=> Pinholes in tunnel barrier

Top electrode matters

0

5

10

15

20

25

0 100 200 300 400 500

Al/a-AlOx/Al

V (uV)

Al/a-AlO/Al

0

4

8

12

0 200 400 600

Re/c-Al2O3/Al

V (uV)

Re/c-AlO/Al

0

5

10

15

20

0 200 400 600

Re/c-MgO/Al

V (uV)

Re/c-MgO/Al

a: Amorphousc: Crystalline

Supports conclusion that Al top electrode “heals” pinholes

substrate

Al top electrodeTunnel barrierBottom electrode

Al top electrode always gives good I/V

Look at Magnesium oxide as tunnel barrier

aMgO

aV

• 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%

V/MgO/Al fabrication

1. Sputter deposit V -800C, 2 nm/min, Ar

2. MgO growth – reactive evaporation in O2

3. Evaporate Al

substrate MgOV MgOAl

MgO tunnel barrier on V @ RT is epitaxial

• MgO– RT growth– Thickness ~2 nm– Single atomic steps– Wide terraces

STM: 800x800 nm2JK127.1.m3_p1

JK104.1.R1

• 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

– Oxidation of vanadium during trilayer growth– Reduces TC 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

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

• 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

Re JJ IVs

Completed, submitted, or PublishedIn progress

Future rogram

Epi growth on Re

Re qubit w/low perf.dielectrics

Growth on six inch wafer

Atomic oxygen experiment

Al2O3 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 IVsNbN qubit w/high perf.

dielectrics