High-Tc Josephson junctions for quantum computation Boris Chesca - - PowerPoint PPT Presentation

Workshop on Theory and Practice of Adiabatic Quantum Computers and Quantum Simulations 22-26 August 2016, ICTP Trieste

High-Tc Josephson junctions for quantum computation

Boris Chesca Department of Physics, Loughborough University

 Daniel John, postdoc (PhD @ Loughborough) Matthew Kemp, Jeffrey Brown, final year project students Department of Physics, Loughborough University  Christopher Mellor School of Physics and Astronomy, Nottingham University

• Introduction

Superconductivity: conventional unconventional s-wave d-wave Josephson junctions

• Qubits & (d-wave) high-Tc Josephson junctions
• Devices with hundreds/thousends of high-Tc junctions

Flux-flow MW/THz generators Transistors Magnetic sensors (SQUIDs, SQIFs) Quantum computers?

Outline

+

ky + kx + + +

nodes sign change

2 2 y

x

d

 -wave

Conventional

pairing mechanism: electron-phonon interaction

Unconventional

pairing mechanism ? total spin S=0

• rbital angular momentum L=2

Y = Y0eij

Cooper pairs

• s-wave

total spin S=0

• rbital angular momentum L=0
• Superconducting
• rder parameter

Superconductivity: conventional & unconventional

ky + +

• kx

Discovery of superconductors: critical temperature vs. time

Conventional superconductors

liquid Nitrogen, 77 K liquid Helium, 4.2 K

160 40 80 120 1910 1960 1980 2000 Year Temperature (Kelvin)

Hg Pb Nb NbN (1911) MgB2

HgBa2Ca2Cu4O8 Bi2Sr2Ca2Cu3O10 YBa2Cu3O7 (La/Sr)CuO4 SrTiO3 Nd2-xCexCuO4 La2-xCexCuO4 Pr2-xCexCuO4 Sr2RuO4

Nb3Ge

Hole doped cuprates Electron doped cuprates

Josephson junction: dc effect

I Ic U B

• B. D. Josephson,

Cambridge (1962)

J = Jc sin (j2 j1)

-junction: s-wave & d-wave superconductors

D.Wollman, Van Harlingen, W.Lee, D.M.Ginsberg, A.J.Leggett, Phys.Rev.Lett. 71, 2134 (1993)

I = – Ic sin (j2 j1) =Ic sin (j2 j1+)

I

+ +

I

+

-junction & Qubits

• L. B. Ioffe et al., Nature 398, 679 (1999)

Quantum computation: a qubit with 2 persistent current states

R.R.Schulz, B. Chesca, et al., Appl. Phys. Lett. 76, 912 (2000)

hole-doped YBa2Cu3O7 -junctions: d-wave superconductors only!

I U I U 1 mm I U I U 1 mm

300 300 300

• B. Chesca et al., Phys. Rev .Lett. 90, 057004 (2003)
• 0.5

0.0 0.5 Magnetic Field ( T) Critical Current ( A)

 

LaCeCuO - design



voltage criterion, 2 V



Bresidual LaCeCuO 0-design Bresidual 0.0 0.1 0.0 0.1

0-design -design

electro-doped La2-xCexCuO4 -junctions: d-wave superconductors only!

Devices with hundreds/thousends of high-Tc junctions Flux-flow MW/THz generators Transistors Magnetic sensors (SQUIDs, SQIFs) Quantum computers ?

Flux-flow MW/THz generators why superconducting generators?

natural frequency is tunable (voltage, B field)

Superconductor Barrier Superconductor

MW/THz I Ic U supercurrent oscillates locally natural MW/THz generator

• B. D. Josephson,

Cambridge (1962)

Josephson junction: ac effect

A vortex corresponds to a soliton propagating along the chain. Each pendulum hangs almost straight down for much of the time, but when the soliton passes by, the pendulum overturns rapidly and oscillates for the period between passing solitons. These oscillations are the analogue

• f the EM radiation excited by the vortex. A resonance occurs if the pendulum oscillates precisely

an integer number of times (m) between successive passages of the soliton;

JJ array = chain of N identical pendulums driven by a constant torque

each pendulum is damped & free to move transverse to the axis of the chain coupled to its nearest neighbours by torsional springs has an identical behaviour except for a constant shift in time.

22 x 20 asymmetrical Josephson junction array

Flux-flow @ 77 K: MW is 0.1 W @ (1.5-25) GHz

• B. Chesca, D. John, and C. Mellor, Supercond. Sci. Technol. 27, 085015 (2014)

Transistors why superconducting transistors?

high switching speed low power dissipation low noise

Flux-flow resonances: ideal for high-gain transistors

• B. Chesca, et al, Appl. Phys. Lett. 103, 092601 (2013)

Ic(Ictrl) at 77K: highly asymmetric

• B. Chesca, D. John, M. Kemp, J. Brown, and C. Mellor, Appl. Phys. Lett. 103, 092601 (2013)

Why superconducting magnetic sensors?

the best getting less expensive: 77K SQUID-arrays better than single-SQUID 4.2 K

Magnetic sensors: SQUIDs & SQIFs

SQUID arrays

SQUID 770 SQUID array

Noise Noise V V =

flux coherent & non-interacting SQUID array

NoiseArray = N1/2 NoiseSQUID VArray = N VSQUID

[ [ ] ]

SQUID Array N1/2

1

200 400 600 800 1 10

• 15

15

• 30
• 15
• 212 A

V, mV B, T

770 SSA

• 172 A

max(V)=6.8 mV T=83K

Number of SQUIDs S

1/2

 (Hz

1/2)

HTS SQUID @ 77 K nano-HTS SQUID @ 4.2 K LTS SQUID @ 4.2 K 484 SSA, 40 K 770 SSA, 83 K

1/N

1/2

SQUID arrays @ 77K better than SQUIDs @ 4.2 K

• B. Chesca, J. Daniel, C. Mellor, Appl. Phys. Lett. 107, 162602 (2015)

2D 20000 SQUID arrays design

• E. E. Mitchell et al,, Supercond. Sci. Technol. 29, 06LT01 (2016)

Quantum Computers? why superconducting Quantum Computers?

D-wave produced 2 (Google and NASA)

1000 qubit processor with 128K low-Tc Josephson junctions

Conclusions

High-Tc junctions: very significant progress simple and reliable fabrication: bicrystal, step-edge high performance devices with hundreds/thousands junctions quantum computing with high-Tc junctions worth a try !