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2008 BBN Quantum Computing Kickoff
Quantum Materials
David P. Pappas
Jeffrey S. Kline, Minhyea LeeNational Institute of Standards & Technology,
Electronics & Electrical Engineering Laboratory, Boulder, CO
Conventional superconducting circuitMaterials perspective
tunnel barrier
insulator
wiring
substrate
Substrate Si/SiOX Thermal
Wiring Nb or Al Sputtered
Insulator SiOX CVD
Barrier AlOX Thermal
Traditional
Conventional materials are usedfor a lot of really good reasons…
• Si substrate with thermal a-SiOX on top
– Smooth, standard lithography, inexpensive
• Nb or Al wiring – sputter deposit, polycrystalline
– Low temperature, smooth, relatively high TC
• a-SiOX insulators – CVD
– Smooth (no pinholes), low T, easy
• a-AlOX tunnel barrier – thermal or plasma oxidation
– Smooth, no pinholes, low T, easy, self-limiting“CMOS compatible”
Need strong motivations for change …
Phase qubit Rabi oscillation
Martinis, et. al PRL95, 210503(2005)
(SiOX insulator on Si/SiOx substrate)
(SiN insulator on sapphire substrate)
New directions
tunnel barrier
insulator
wiring
substrate
Substrate Sapphire
(Al2O3)
Crystalline Expensive, difficult to work with, can be atomically rough
Wiring Re, Al/Ru Annealed Complicated, hard to prepare, Hi-T
Insulator SiN, a-Si,
Al2O3
Sputtered
Epitaxial
High T, adhesion, processing
Homogeneity, rough
Barrier Al2O3 Epitaxial High T, homogeneity, rough
Materials Difficulties
CMOS
Outline
• Motivations for change
– a-SiOX in substrate & insulators
– a AlOX in barriers
– Superconducting wiring materials
• Milestones & goals
• Recent progress
• Roadmap
Insert qubit pic here
Qubit LStripline (C-SiOX )
Josephson Junction(L&C)
=> Measure “Q” of simple LC resonators
Qubit has SiO2 Cap in || with J.J. & around lines
SiOX AlOx
Power dependence to parallelplate capacitor resonators
wave resonator
L
f [GHz]
Po
ut [m
W]
Pin lowering
Q of the resonator with SiO2
goes down as power decreases!
Parallel plate capacitor resonators w/SiO dielectric
C
L
=> Compare to capacitors with vacuum dielectric
=> With & without SiO2 over the capacitor
C/2 C/2
L
Dissipation is in SiO2dielectric of the capacitor!
~Pout
Interdigitated capacitor resonators
Power dependence of QLC for parallel plate capacitors
HUGE Dissipation
Q decreases with at very low power(where we run qubits)
Nphotons
QL
C
Explains small T1!
L
C
•TLS bath saturates at high E (power), decreasing loss
Schickfus and Hunklinger, 1975
Two-level systems in a-SiO2
E d
SiO2 - Bridge bond
UDAmorphous material has all barrier heights present
High E
Low ELow E
Problem - amorphous SiO2
Why short T1’s in phase Josephson qubits?
Dissipation: Idea - Nature:At low temperatures (& low powers)environment “freezes out”:
dissipation lowers
dissipation increases, by 10 – 1000!
Change the qubit design:
find better substrates
find better dielectric & minimize insulators in design
Common insulator/substrate materials
• SiOX
– 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
Minimize, optimize dielectric & substrate
Rabi oscillations > 600 ns !!
Sapphire substrate + SiN insulator:
Superconductor - Aluminum
I
Tunnel junction a- AlOx-OH-
Found improvements due to optimized materials in insulators
Tunnel barrier materials
Motivations for new tunnel barriers materialsQubit spectroscopy
• Increase the bias voltage (tilt)• Frequency of |0> => |1> transition goes down
Splittings
Increasebias
Effects of splittings• Quench Rabi Oscilations – strong coupling to qubit
Rabi oscillations
Spectroscopy
Constant splitting density in a-AlOX barriers13 um2 junction 70 um2 junction
• Smaller area – Fewer splittings, large gaps (strong coupling)
• Larger area - More splittings, smaller gap (weaker coupling)
Density ~ 1/GHz/m2
Splittings are randomly distributed
Use small junctions with
low probability of splitting for
test structures (< 1 m2)Steffen, et. al (2006)
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
What we need:
Crystalline barrier-Al2O3
Poly - Al
Poly- Al
Existing technology:
Amorphous tunnel barrier a –AlOx – OH-
No spurious resonatorsStable barrier
Amorphous Aluminum oxide barrierSpurious resonators in junctionsFluctuations in barrier
Silicon
amorphous SiO2
Low loss substrate
Design of tunnel junctions
SC bottom electrode
Top electrode
Q: Can we prepare crystalline Al2O3 on Al?
Binding energy of Al AES peak in oxide60
59
58
57
56
55
54
900800700600500400300Annealing Temp (K)
AE
S E
nerg
y of
Rea
cted
Al (
eV)
Al in sapphire Al203
Metallic aluminum
Aluminum Melts
68
10 Å AlOx on Al (300 K + anneal) 10 Å AlOx on Al (exposed at elevated temp.)
Anneal the natural oxides Oxidize at elevated temp.
A: No – need high temperature bottom wiring layer
Motivations – New wiring materials• Conventional Al, Nb:
– Surface oxides with spin polarized traps• 1/f flux noise, dephasing times, density ~ 1017/m2
• Alternative materials:
– Re: resists oxidation, high melting T, hcp lattice => Al2O3,
– Al passivated with Re or Ru => resists oxidation
Koch, Clark, di Vincenzo(PRL 2007)
e- traps Kondo traps
Faoro, Ioffe PRB (2007)
Coupled TLS
McDermott, et. al (2007)
Improvement of junctionsseen in spectroscopy of 01 transition
T = 25 mK
Amorphous barrier70 m2
Epitaxial barrier70 m2
• Density of coherent splittings reduced by ~5
in epitaxial barrier qubits
• T1 = 400 - 500 ns best for SiO2 insulator
• Splitting density– ~3-5 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/Al2O3/a-Al Qubit
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
Need to improve top barrier interface!
• 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
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
Top electrode mattersAl top electrode always gives good I/V
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
Re on top makes JJ leaky
Electrical Testing Summary & ComparisonPhase qubits
Materials
Wiring & barrier
Insulator T1
(ns)
T2*
(ns)
Splitting density
(N/GHz/mm2)
Reference
Al/AlOx/Al
1 m2 w/shunting C
min-SiNx 110 90
(160)
1 Steffen - tomography
PRL 97 050502
Al/AlOx/Al
13 m2
min-SiNx 500 150 1 Martinis Dielectric loss
PRL 95 210503
Al/AlOx/Al min-SiO2 170 * 1 Simmonds 2005
Re/Al2O3/Al epi-junction max-SiO2 150 90 0.2 PRB 74 100502
12 qubit - Re/Al2O3/Al
49 m2
max-SiO2 200-400 * 0.2 Submitted APS08
12 qubit - Re/Al2O3/Al
49m2
min-SiO2 500 140 0.2 Submitted APS08
12 qubit – Re/MgO/Al 80 50 0.4 New results
Goals1. Inter-laboratory compatibility
– Infrastructure - 6”-wafer chamber for epitaxial trilayers
• Develop 6” substrate capability
• Re/Al2O3/Al, Re/Al2O3/Re
– Supply samples to flux qubit, 6” wafer fabrication facililty
2. Extend work on epitaxial tunnel barriers to flux qubits
– Continue on barriers at chip level
• Chip level
– Develop JJ and qubit circuits compatible w/flux qubits
– study fully epitaxial systems
3. Study new materials for wiring layers
– Al/Ru capping with anneal
– Push to understand flux noise and wiring surfaces
Progress1. Chamber specifications & purchasing – RFQ out
– 6” wafer capability– Sputtering (Al, Re, Al2O3), k-cell & oxygen reactor– High temperature sample anneal– RHEED, in-situ ellipsometer
2. Sub-micron junction designs– Collaborate with MIT-LL – Will Oliver– Compatible test structures– Optical lithography with vias to junctions– Cl-etch capability at NIST online soon
Progress (continued)3. SQUID design to test 1/f flux noise
– Collaborate with UW, Madison, Rob McDermott– Fabricate SQUIDS using small junction & Cl etch
Impact on roadmapVersion 2.0 April 2, 2004, Terry Orlando www.lanl.gov
1. Scalable systems with Rabi/Ramsey oscillations
2. Ability to Initialize qubits
3. Long (relative) decoherence times, much longer than the gate-operation time
1. Calculations suggest the relaxation times ~ milliseconds.
2. Experimental measurements show at present a lower bounds
1. 1–10 μs for the Relaxation time
2. 0.1–0.5 μs for the dephasing time![2,3,7–9,11].
3. Charge, flux, and critical-current noise are probably a technological and materials processing problem![2,3,7–9,11].
4. The non resonant upper levels: in principle the effects of these levels can be compensated by a pulse sequence which allows the system to act as an effective two level system!
5. Experiments have demonstrated about a thousand gate operations prior to decoherence!.
4. Universal set of quantum gates
5. Qubit specific measurement capability
6. Interconvert stationary and flying qubits
7. Ability to faithfully transmit flying qubits