Materials for csq
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Materials for CSQ. PM: Karl Roenigk, IARPA PI: David P. Pappas, NIST Staff: Danielle Braje, Robert Erickson, Fabio da Silva, Jeff Kline IC Postdoc - David Wisbey Collaboration : CU Denver: H. Fardi, M. Huber Colorado School of Mines – Brian Gorman, Mike Kaufman.

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Materials for CSQ

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Materials for csq

Materials for CSQ

PM: Karl Roenigk, IARPA

PI: David P. Pappas, NIST

Staff: Danielle Braje, Robert Erickson,Fabio da Silva, Jeff Kline

IC Postdoc - David Wisbey

Collaboration :

CU Denver: H. Fardi, M. Huber

Colorado School of Mines – Brian Gorman, Mike Kaufman


Programmatics matrix management

ProgrammaticsMatrix management


Outline

Outline

  • Ongoing theoretical work:

    • Analyze junction response

      • DC and AC

    • Simulate absorption of materials

  • Potential work:

    • Atomistic calculations of interface structure

      • Need cubic spinel sturctures as templates

      • Steve Helberg – NRL

    • Q reference material ILC?

      • Q is a function of temperature and power

      • Q is high at high T & P, low at low T&P

      • Qubits operate at low T & P

      • RM is critical to define milestones of program


Technical objectives table

Technical Objectives Table


Work flowchart

Work flowchart


Materials for csq

Example: sapphire/Al(111)/Al2O3/Al(111)

T=1.8K

Trilayer flowchart

Substrate

Need ADR

For TLS

analysis

RF?

RHEED?

AFM?

Bottom Electrode

Need ADR

TC Al <1.8 K

RHEED?

AFM/STM?

RF?

Tunnel Barrier

RHEED?

AFM?

Top Electrode

CIPT shows shorted junction

CIPT

TEM, AFM

Thy

Process junction

CAFM

Thy

Insulator, wiring

RF

IV

T1,T2

Thy


Epitaxial junction problem

Epitaxial Junction Problem

S.C.

Oxide

S.C.

  • Tunneling barrier interface defects:

    • random roughness (O diffusion within Al overlayers),

    • terraces (epitaxially produced)

    • pinholes

S.C.

Oxide

S.C.

Oxide

S.C.

Al

Oxygen Diffusion

Terraces

Pinholes


Example al 111 epi al 2 o 3 al 111 structure on sapphire

Example:Al(111)/epi-Al2O3/Al(111) structure on sapphire

TEM

Brian Gorman

CSM

FIB Pt

Oxide / Al / Re

Epitaxial Al

Sapphire Substrate


Hrtem of oxide layer

HRTEM of Oxide Layer

0.37nm

Top Al(111)

Oxide Al2O3

0.23nm

<111> Al

<111>Al

<0001> Al2O3

Epi Al(111)


Approach

Approach

  • Development of quick and efficient procedure for detecting junction interface defects

  • Electrical IV measurements (DC)

    • Barrier tunneling model

      • account for subharmonic energy-gap structure

      • assess interface defects, particularly pinholes

  • Electric RF absorption (AC)

    • Engineered Fe+3 magnetic impurities

    • Probes of terracing & thickness deviations


Materials for csq

x

S

I

S

d2

d1

Barrier Tunneling Model

  • Applicable to S-I-S geometry [Arnold]

  • Tunneling current formalized using non-equilibrium single-particle Green functions [Keldysh]

  • Non-transfer Hamiltonian—transmission probability T2 included to all orders [Feuchtwang]

(first principles equivalent)


Materials for csq

2 - 

e

2 - 3

CP



 1 =  - 

e

CP

2 = 0

-

h

-

(evanescent)

S

I

S

Multiple Andreev Reflection (eV =  - )

Barrier Tunneling Model

  • Subharmonic gap structure (barrier contribution)

    • MAR: Multiple Andreev Reflection [KBT]

    • MPT: Multi-Particle Tunneling [Shrieffer and Wilkins]

  • MAR accounts for voltages below threshold /e, which are not addressed by MPT


Materials for csq

e

2 +3

2 + 

 1 =  + 



e

CP



2 = 0

h

-

-

(evanescent)

S

I

S

Multiple Andreev Reflection (eV =  + )

Barrier Tunneling Model

  • Voltages above threshold /e


Materials for csq

Barrier Model and Pinholes

  • Example applied to Nb/AlOx/Nb tunnel junctions [Kleinsasser]

  • Subgap current attributable to multiple Andreev reflection

  • Extended to account for pinholes via parameters:

    • Pinhole transmission probability T2 (near unity, by definition)

    • Ratio of pinhole conductance to that of barrier

4% of current due to pinholes (T2 ~ 1)


Materials for csq

z

Al

z(x,y)

Al-O

z(x,y)=d

Fe3+

x

Al

Fe3+ Probes of Roughness

  • Fe3+ impurities can be used as probes of junction interface roughness when microwaves are applied

,V0

  • By Faraday’s law, roughness induces a driving magnetic induction that couples to Fe3+ impurities

  • For example, Fe3+ power absorption depends on the variance of this induction (ħ0 ~ 12 GHz)


How fe 3 impurities couple to junction phase qubit

How Fe3+ Impurities Couple to Junction Phase Qubit

z

d<<R

d

JJ

Fe3+

R

  • Phase of Cooper-pair wave function is shifted by Fe3+ impurities in single-crystal sapphire junction*

  • This introduces time-independent interaction terms in the washboard potential of the Hamiltonian

  • Provides mechanism for decoherence and 1/f noise

* R.P. Erickson and D.P. Pappas, “Model of magnetic impurities within the Josephson junction of a phase qubit”, Submitted to PRB Rapid Comm.

16


Materials for csq

Progress

  • Barrier tunneling model

    • Correspondence with G. Arnold; source code provided

    • Initial implementation to be completed June 1, 2009

    • Next step: application to NIST I-V measurements

    • Then extend model to include terrace-induced channels

  • Fe3+ probes of roughness

    • Initial theory development completed April 1, 2009

    • Next step: application to NIST measurements

    • Then extend model to self-consistency within London gauge


Nature of the faults in al 111 on sapphire

Nature of the faults in Al(111) on sapphire

Faults in epi Al initiate at substrate, are transferred vertically through the Al to the oxide layer, near where the growth abnormalities seem to form in most cases


Dark field imaging of faults in al 111 on sapphire

Dark Field Imaging of Faults in Al(111) on sapphire

Left: 2-beam bright field image using the 006 reflection shown in the inset SADP

Right: CDF image using the same 006 reflection as in the left image

Note that the top layer of Al are not illuminated using this reflection, indicating that the bright areas in the CDF image are slightly misoriented in-plane


Slightly tilt sample

Slightly tilt sample:

Left: 2-beam bright field image using the 006 reflection shown in the inset SADP

Right: CDF image using the 006 reflection as in the left image,

Note the top layer of Al are not illuminated, indicating that the bright areas in the CDF image are slightly misoriented in-plane with respect to the previous 006 CDF image


Basal plane sapphire atomic placement

Basal Plane Sapphire Atomic Placement

Al(111)

Sapphire

Atomic positions of the Al (pink) and O (gray) for sapphire oriented down the basal plane.

Note the rotation of the oxygen atoms in the c-direction of the crystal


Potential solution

Potential solution

  • Change substrate to cubic spinel

    • e.g. MgAl2O4 (111)

    • Lattice matched between Al & Al2O3

    • No staggered O atom sub-lattices

    • High T material

  • Suggest simulating superconductor-spinel interface

  • Potential NRL contribution


Lcr electrical model for phase qubit

LCR electrical model for phase qubit

LJ~sinf

CJ~1-100 x10-12

Rjunction – non-linear QP tunneling

Rdielectric – bound dipole relaxation Junction & insulators

=

G(V)

Intensity

What can be quantified?

  • Quality factor – Energy stored/Energy lost/cycle

    • Q = = w0/Dw

    • T1 = Q/w0

  • Delectric loss tangent = 1/Q

    • tand = Im(e)/Re(e)

frequency


Test dielectrics with simple lc cpw circuits

Test dielectrics with simple LC & CPW circuits

O’Connell, APL (2008)

LC – parallel plate C

CPW

C

L


Cpw simulations model field around center conductor

CPW simulations Model field around center conductor

Electric Field around CPW

  • Absorption in dielectric reduces Q

  • Primary absorption due to two-level fluctuators

  • Active at low T & Pwr


Materials for csq

Saturation of TLSs at low T & P

PPMS

ADR

|e

|e

wRF

|g

|g

10% effect @ 1.8 K

80% effect @ 0.1 K


Materials for csq

Quality factor can appear higher due to poor T and Pwr control

Increasing T

Increasing P

1/Q =


Cryogenic measurement standard

Cryogenic Measurement Standard

Measurements of all labs are not created equal

Q appears higher for high Temperature & Power

  • Objectives:

    • Standardize inter-laboratory results

    • Set the bar for superconducting coherent measurements

  • Approach

    • Design test samples, which are relevant to the field

      • High Q CPWs for single frequencies

        • T & Pwr dependence

      • RF resonator combs for full transfer function

    • Fabricate AND measure samples at NIST

    • Conduct Inter-laboratory Comparison (ILC)

    • Generate SRM for community

  • Methods

    • State-of-the-art superconducting circuit test facility

    • Perfect Quality Factor measurements

    • Traceable to NIST standards (frequency and voltage)

  • Vision

    • Give researchers SMA box with calibration standard





Q =  

“To provide for the dissemination of an internationally consistent, accurate, reproducible, and measurable cryogenic measurement standard”


Summary

Summary

  • Ongoing theoretical work:

    • Analyze junction response

      • DC and AC

    • Simulate absorption of materials

  • Potential work:

    • Atomistic calculations of interface structure

      • Need cubic spinel sturctures as templates

      • Steve Helberg – NRL

    • NIST Q-factor SRM

      • Q is a function of temperature and power

      • Q is high at high T & P, low at low T&P

      • Qubits operate at low T & P

      • RM is critical to define milestones of program


Hpd adr delivered cooled

HPD ADR delivered & cooled

  • Agilent 20 GHz VNA ordered

  • Wiring for

    • 32 test junctions (4x25 pin)

    • 1 resonator (2 SMA)

  • Demonstrated T < 50 mK

  • Will enable in-house:

    • Sub-gap structure in epitaxial tunnel junctions

    • Process controll

    • Q-measurements at Low T, P to measure TLS’s


Opportunities issues

Opportunities & Issues

  • NIST Leverage

    • B1E Cleanroom B1E being installed

      • Can get significant space & leverage

      • Deposition systems, low noise space

      • Chlorine etch coming on line

    • Quantum information high priority

Action Items

  • New techniques

    • Ellipsometry

    • Fe impurities at barriers to evaluate roughness

    • ADR – Lower temperature & TLS evaluation

  • Flip chip

    • Design SQUID & qubits

  • Stay on track with GANTT chart


Capres cipt nist 12 tip probe

Capres CIPT – NIST12-tip probe

Re(10 nm)

Al(10)

Barrier

Re or Al(150)

Top surface must be conductive

(Au, RuO, Re)


Materials for csq

Percent of total energy in dielectric

(50 micron trench depth )


Materials for csq

Model:

Ideal CPW – lossless

Prelim Data 1.8 K


Rf substrate evaluation through q

RF Substrate Evaluation Through Q

T = 1.8 K

  • Q ~ 105 for Si, sapphire

  • Q decreases at lower RF power (~ 103 photons)

    • Influence of two-level systems

    • Next step: go to low temp & power with Al, Re in ADR


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