Materials Science in Quantum Computing

Materials scientist view of qubit

• Materials

– SiOx sub substrate

– Superconductor (Al,Nb)

– SiOx dielectric

– Al0x tunnel barrier

The quantum computing challenge

superconductors

Superconducting Josephson junction phase qubit principle

TunnelJunction~1.5 nm

Top superconductor

Bottom superconductor

Itop= 0

bot= 0ei

Cooper pairwavefunction

Josephsonrelations

e2V

)sin(II 0

• I depends on

• Voltage only when

phase is

changing

Quantum behavior - potential that phase qubit lives in• Increase bias => cubic potential lifts degeneracy

231201 EEE • Use the |0> and |1> states for information

1 of lifetimeT1

Insert qubit pic here

Qubit LStripline (C-SiO2 )

Josephson Junction(L&C)

=> Measure “Q” of LC resonators

Qubit has SiO2 Cap in || with J.J.

SiO2 AlOx

Power dependence to parallelplate capacitor resonators

wave resonator

L

f [GHz]

Po

ut [m

W]

Pin lowering

Q of the resonator goes downas power decreases!

Parallel plate capacitor resonators w/SiO dielectric

C

L

Room-temperature deposited SiO2 over the capacitor

C/2 C/2

L

Data with and without SiO2 on Cap

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

10-1

100

101

102

103

104

105

102

103

104

105

Q

number of photons

|E| [V/m]

1/lo

ss ta

ng

ent

• Spin (TLS) bath: saturates at high power, decreasing loss

high power

SiOx

(amorphous)

Schickfus and Hunklinger, 1977

1 10 102 104

• Loss “saturates” from each TLS• Pabs ~ number saturated ~ d*E• Solve Bloch eqn’s for bath : 2

21real

imag

ETT

d

Theory: Why 1/E Dependence?

Uncompensated spins in SiOx

E d

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:

single crystal sapphire substrates

SiN dielectric & minimize dielectric in design

SiN/sapphire: Significant improvement in T1, T2*

0 Time (s) 2 0 Time (s) 1

P(1

)

0.4

P(1

)

• T1 increased to nearly 600 ns

• T2* nearly 300 ns

• Still need to deal with low fidelity => junctions

• Do spectroscopy on qubits

11

1

Qubit spectroscopy• Increase the bias voltage (tilt)• Frequency of |0> => |1> transition decreases

Resonances

Increasebias

• Resonance density increases with junction size

Microscopic two-state fluctuators in junction

Amorphous AlO tunnel barrier

• Continuum of

metastable vacancies

• Changes on thermal cycling

• Origin?

• uncompensated spins in barrier

• O atoms tunneling between sites

Resonators must be 2 level, coherent with qubit!

qubit - |0> or |1>

res. - |g> or |e>

|0e>

|0g> 0

|1e>

|1g>

On resonance

E=0

Anti-crossing, splitting S

|1e>

|1g>E

Off resonance

Qubit-resonator coupled interaction

• Off resonance - |1> decays

•On resonance, put qubit in |1>

• Wait some time, int/2

• Qubit goes into |0>

=>Wait int

Qubit goes back to |1>

with enhanced amplitude!

States oscillate

|1g> <=> |0e>

Resonator has longer

coherence time than qubit

off

off

on

on

What we need:

Crystalline barrier-Al2O3 ?

Interfaces: Smooth Stable No dangling bonds

Poly - Al

Poly- Al

Existing technology:

Amorphous tunnel barrier a -AlOx

Rough interfaces Unstable at room temp. Dangling bonds

No spurious resonatorsStable barrier

Amorphous Aluminum oxide barrierSpurious resonators in junctionsFluctuations in barrier

Silicon

amorphous SiO2

dangling bonds at interface

Low loss substrate/dielectric : SiN

Re-design 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

Chose bottom superconducting electrode to stabilize crystalline Al2O3 or MgO tunnel barrier

Elements with high melting temperature

Elements with TC > 1K

Elements that lattice match sapphire (Al203)

Elements that form weaker bond with oxygen than Al

Elements that are not radioactive

LEED, RHEED, AESRe

Sputtering

LoadLock

STM

Ex-situ AFM

Al

Oxy

gen

O2

Al2O3 growth: Al thermal deposition under O2 exposureon top of base Epi Re.

UHV system: Pbase< 5x10-10 Torr

• 1.5 nm RMS roughness

• 1-2 atomic layer steps

• Screw dislocations on mesas

• Stranski-Krastanov growth

– Initial wetting of substrate

• Formation of 3-d islands

– Islands fill in gradually

0.5 x 0.5 m100 nm Re Base layer @ 850 C on sapphire

Epi Re Grow Al2O3 @ RT+ Anneal @ 800 ◦C 4x10-6 Torr O2

3m

Single crystal Al2O3 on Re(0001)

Re(0001)

Al2O3

Fabricate test junctions with epi-Al2O3 barrier

• First high quality junctions

made with epitaxial

barrier!!Re(0001)

Al2O3

Al

1/R vs. Areaat 300 K

V(mV)

I-V curve at 20 mKRe

Al

Conclusions• Amorphous dielectrics can have HUGE

loss due to two level system (spin bath)

– Problem with phase qubit:

Loss in dielectric

– Fix by using SiN dielectric

• Tunnel junctions have coherent two state systems that are detrimental to the fidelity of the measurements

• Materials optimization is critical to long

coherence times

• Status

– Testing qubits w/epi-barriers

– Eliminating/improving dielectrics

around qubit

1