coherence in superconducting materials for quantum computing david p. pappas jeffrey s. kline, fabio...

Coherence in Superconducting Materials for

Quantum ComputingDavid P. Pappas

Jeffrey S. Kline, Fabio da Silva, David WisbeyNational Institute of Standards & Technology,

Electronics & Electrical Engineering Laboratory, Boulder, CO

Collaborators

Will Oliver, Paul Welander – MIT/LL

Ray Simmonds, Kat Cicak, Josh Strong NIST, Boulder

Matthias Steffen, IBM Watson

Kevin Osborn, LPS, MD

John Martinis, Haohua Wang, UCSB

Rob McDermott, U of W

Sponsors

The quantum computing challenge

QubitPrepare

Measure

QubitPrepare

Measure

QubitPrepare

Measure

Implementations

PhotonsIon trapsNeutral atomsNMR~~~~~~~~~~~~~~

~~~~~~~~~~~~~~Spins in semiconductorsQuantum dots~~~~~~~~~~~~~~

Superconducting:~~~~~~~~~~~~~~~ChargeFluxPhase

Isolation Coupling

Decoherence:

external – radiation, heat, acoustic…internal – materials, crosstalk…

interact

interact

Superconducting qubit measurement setup

• Ante

• Dilution Refrigerator

• Low temperature, < 50 mK

• RF measurement

• Low power ~ 1 photon of energy

in cavity

• Improves coherence

• Removes quasiparticles in

superconductor

• Reduces thermal radiation

• Hurts coherence:

• Low-energy, two-level

excitations in amorphous

materials

The Josephson Junction

0

ie0

1.7 nm

• Building block of superconducting quantum bits (qubit)• Josephson relations (’62, ‘73)

Al

Al

amorphous AlOX

• Not ohmic = > I periodic in d• Voltage only when phase is changing

• System is nonlinear for high I

e2

V

)sin(II 0

IU cos

TEM photo

Types of qubits

“Charge”

“Flux”

“Phase”

You & Nori, Physics Today, November (2005)

Logic

Non-linear oscillatorExcited |1>

vsGround state |0>

IslandCharged

vs.Not charged

Current circulationLeft vs

Right

Anatomy of a conventional superconducting circuitMaterials perspective

Tunnel barrier

Wiring

Insulator

Substrate

Material Preparation method

Tunnel Barrier AlOX Thermal

Wiring Nb or Al Sputtered

Insulator SiOX CVD

Substrate Si/SiOX Thermal

Traditional

Conventional materials are usedfor a lot of really good reasons…

• Si substrate with thermal amorphous a-SiOX on top

– Smooth, standard lithography, inexpensive

• 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

• Nb or Al wiring – sputter deposit, polycrystalline

– Low temperature, smooth, relatively high TC

Need strong motivations for change …

8

Short lifetimes of quantum information in solid state superconducting qubits

• Relatively short lifetimes and operation cycles

• Need lifetime/gate operation time > 1000

0.5

0.25

00 100 200

T1 = 23 ns

Pro

b. |1

> s

tate

Meas. delay (ns)

0

1

Lifetime

“Rabi” oscillation

Outline

• Electrical model of a phase qubit• Two Level Systems (TLS) as loss mechanism

– substrate & insulators a-SiOX

– tunnel barrier a-AlOX

• Test structures for materials analysis• New directions in materials

– Improved substrates a-Si & removal– Crystalline barriers Al2O3

• Recent progress

LCR electrical model for phase qubit

=

CJ~1-100 x10-12LJ~sinf

JJ CL1

0

Inte

nsi

ty

JCVG )(T :state 1 of Lifetime 1

G(V)

• Quality factor – Energy stored/Energy lost/cycle

• Q = = w0/Dw

T1 = Q/w0

• Delectric loss tangent:

• tand = Im(e)/Re(e)

= 1/Q

Rjunction – non-linear QP tunneling - ?

Rdielectric – bound dipole relaxation ~ ?

Junction & insulators

What can we easily measure & optimize?

frequency

8GHz @ s 125~T ) tan( ,10 ~ Q :Goal 16

Loss in amorphous materials (SiOX-OH-)

• Low energy displacements of dipoles, saturate at high T, P• Lose energy through phonon creation

– tan = d 3x10-3, Q ~333, T1~40 ns

• Approaches: 1) Reduce or eliminate dielectrics

2) Optimize mtls. – e.g SiN, a-Si…

Schickfuss & Hunklinger, (1974)

E d

++++ ++++

_ _ _ _ _ _ _ _

Minimize & optimize dielectric - qubit Rabi oscillations

Rabi oscillations > 600 ns !!

Sapphire substrate + SiN insulator:

Kevin Osborn Group

SiNX pillar from high-stress film

Al film

SiNX

200 nm

Optimized SiNx for coherent quantum circuits

Qi=25,000

Qi=1,400

Loss Tangent for SiNx films

The loss tangent is sensitive to PECVD growth!

smooth etch profile from HDP CVD filmprecursor ratio: N2/SiH4 = 1.8x-ray reveals polycrystalline orderstress = 600 MPa compressiveTgrowth = 300 C

other labs:NIST,UCSB

Optimize dielectrics with simple L-C circuits

L

C

LC – parallel plate C CPW

Material Q=1/tand

Si(111) 200,000

Sapphire – Al2O3 160,000

a-Si:H 45,000

a-SiN 10,000

a-SiOX 3,300

sT HSia 2.1:1

O’Connell, APL (2008)

Predicts:

Substrate

insulator

Other approach – remove dielectricsSimmonds, Strong, Cicak et. al, NIST Boulder (2008)

• Vacuum gap capacitor with an inductor

Q

Before dielectric removedSiN, 100 nm x 8000mm2

600

After dielectric is removed 40,000

=> Flexible circuit - allows us to test the loss in a junction under identical conditions

Add a 1.5 nm, 10 mm2 a-AlOX JJ to the circuit

Q

Before dielectric removedSiN 100 nm x 8000mm2

600

After dielectric is removed 40,000

With Josephson Junctiona-AlOX, 1.5 nm x 10mm2

400

• Generally understand dielectric problem – Improve & Reduce• Significant loss in the amorphous AlOX junction

• 1.5 nm thick – very strong coupling• Focus on tunnel barriers

Tunnel barrier material characterizationQubit spectroscopy

• Increase the bias voltage (tilt)• Frequency of |0> => |1> transition goes down

Splittings

IncreaseI bias

Splittings in charge qubit - Cooper-Pair Box

Vg

Cg (Ec,EJ)

1 mm

B

Al/AlOx/Alisland

BVg

gate

islandgate

junction

junction

Z Kim et al., Physical Review B 78, 144506 (2008).

Effects of splittings

• Quench Rabi Oscilations – strong coupling to qubit• Reduces the measurement fidelity

Rabi oscillations

Spectroscopy

Origin of spectroscopy splittings

• Individual, strongly coupled TLS’s in barrier

• Distribution of excitation energies - amorphous AlOX

Density of splittings ~ 1/GHz/mm2 in 1.5 nm thick junction

(1) Reduce materials where possible

(2) Improve materials by eliminating TLS’s

13 um2 junction

• Fewer splittings, large gaps

• stronger coupling

70 um2 junction

• More splittings, small gaps

• weak coupling

1) Reduce materials where possibleSteffen, et. al PRL (2006)

• Reduce size of junctions in qubits

• increases f0 due to smaller capacitance

• Add high quality external capacitor to bring f0 down (SiN, a-Si)

• T1 ~ 170 ns (SiN) & 600 ns (a-Si:H)

• Factor of 2 shorter than expected - Still have a-AlOX in barrier

Growth of single-crystal Al2O3 (sapphire) tunnel barrier

4×10-6 Torr O2, Al 10-6 Torr O2

Epitaxial Re/Al2O3

Re

@ 850 CAl

Amorphous AlOX

@ RT

Epitaxial Al2O3

@ 800 C

Polycrystalline Al

@ RT

• Rhenium bottom electrode:• Superconducting – TC ~1 K• hcp - lattice match Al2O3• high melting T

(2) Improve JJ’s with crystalline barriers - Al2O3 & MgO

• Good - High sub-gap resistance

• First high quality junctions

made with epitaxial barrier

• Fabricate into qubit

Re(0001)

Al2O3

Al

Re

Al

I-V curve

20 mK

V(mV)

• T1 > 500 ns

– best for SiO2 insulator & large junction

– No external capacitance• Splitting density reduced

– ~3-5 times lower than amorphous barrier of same area

Qubit with 25 mm2 epitaxial Al2O3 junctionKline, et. al, Supercond. Sci. Tech. 22, 015004 (2008)

Summary & OutlookMaterials in superconducting qubits

12 Qubit Test Die Layout

Bias coil Qubit loop

DC-SQUID

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 barriera-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

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 mm2 w/shunting C

min-SiNx 110 90(160)

1 Steffen - tomographyPRL 97 050502

Al/AlOx/Al

13 mm2

min-SiNx 500 150 1 Martinis Dielectric lossPRL 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 mm2

max-SiO2 200-400 * 0.2 Submitted APS08

12 qubit - Re/Al2O3/Al

49mm2

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

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

• MgO• Diamond• ZrSiO• CaO• SiC

Þ 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?

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

SputteredEpitaxial

High T, adhesion, processingHomogeneity, rough

Barrier Al2O3 Epitaxial High T, homogeneity, rough

Materials Difficulties

CMOS

• 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

=2.1kW

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:

Þ find better substrates

Þ 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

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

Superconductor - Aluminum

I

Tunnel junction a- AlOx-OH-

Found improvements due to optimized materials in insulators

Tunnel barrier materials