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
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