2008 NIST/Sandia Kickoff Materials For Superconducting Qubits David P. Pappas Jeffrey S. Kline Minhyea Lee Fabio da Silva Will Kindel National Institute.

2008 NIST/Sandia Kickoff

Materials For Superconducting Qubits

David P. Pappas

Jeffrey S. Kline

Minhyea Lee

Fabio da Silva

Will Kindel

National Institute of Standards & Technology,

Electronics & Electrical Engineering Laboratory, Boulder, CO

Experimental Implementation of Quantum Computers

•Trapped Ions •Neutral Atoms

•NMR•Semiconductor Spin•Quantum Dot•Superconducting SET (Charge Qubit)

•Superconducting Flux Qubit

•Superconducting Phase Qubit (this work)

1nm

100nm

10μm

1μm

1Å

Coh

eren

ce

easi

er

Cou

plin

g ea

sierNEC, Yale, Saclay

MIT, Delft, Berkeley

NIST, UCSB, Maryland

Materials view of a phase qubit

• Loop of superconducting with weak link

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

2 RSiO2Ceff 27ns

~T

RSiO2

=2.1k

Temperature Dependence of QQ also decreases at low temperature!

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)

12 Qubit Test Die Layout

Bias coil Qubit loop

DC-SQUID

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

•“Medium” K dielectrics?• Si• SiN• Al2O3

• MgO• Diamond• ZrSiO• CaO• SiC

Need to use thicker insulators

• “low” K dielectrics? • doped SiOx (F, C• Porous SiOx• Spin-on polymers (HSQ)

Probably not

new

Other potential new insulators – from VLSI world?