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

Outline • 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?

Superconductivity: conventional & unconventional Superconducting - Cooper - order parameter pairs Y = Y 0 e i j - Conventional Unconventional pairing mechanism: electron-phonon interaction pairing mechanism ? total spin S=0 total spin S=0 orbital angular momentum L=2 orbital angular momentum L=0 - - k y k y nodes - sign change + + + + k x k x + + - d  -wave s-wave 2 y 2 x

Discovery of superconductors: critical temperature vs. time 160 Hole doped cuprates HgBa 2 Ca 2 Cu 4 O 8 120 Temperature (Kelvin) Bi 2 Sr 2 Ca 2 Cu 3 O 10 YBa 2 Cu 3 O 7 80 liquid Nitrogen, 77 K MgB 2 (La/Sr)CuO 4 Electron doped cuprates La 2-x Ce x CuO 4 40 Nb 3 Ge Nb NbN Nd 2-x Ce x CuO 4 Pr 2-x Ce x CuO 4 Hg Pb SrTiO 3 (1911) liquid Helium, 4.2 K 0 1960 1980 2000 1910 Sr 2 RuO 4 Year Conventional superconductors

Josephson junction: dc effect B J = J c sin ( j 2 j 1 ) B. D. Josephson, Cambridge (1962) U 0 I I c

 -junction: s-wave & d-wave superconductors I = – I c sin ( j 2 j 1 ) =I c sin ( j 2 j 1 +) I + + - + I D.Wollman, Van Harlingen, W.Lee, D.M.Ginsberg, A.J.Leggett, Phys.Rev.Lett. 71, 2134 (1993)

 -junction & Qubits Quantum computation : a qubit with 2 persistent current states L. B. Ioffe et al., Nature 398, 679 (1999)

 -junctions: d-wave superconductors only! hole-doped YBa 2 Cu 3 O 7 R.R.Schulz, B. Chesca, et al., Appl. Phys. Lett. 76 , 912 (2000)

 -junctions: d-wave superconductors only! electro-doped La 2-x Ce x CuO 4 U U I I  LaCeCuO - design  -design 0.1 30 0 Critical Current ( A)  B residual  voltage criterion, 2 V 0.0 LaCeCuO 0-design 0-design 30 0 0.1 30 0 B residual 0.0 1 mm 1 mm I I U U -0.5 0.0 0.5  Magnetic Field ( T) B. Chesca et al., Phys. Rev .Lett. 90 , 057004 (2003)

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)

Josephson junction: ac effect Superconductor Barrier B. D. Josephson, MW/THz Cambridge (1962) Superconductor U supercurrent oscillates locally 0 natural MW/THz generator I I c

22 x 20 asymmetrical Josephson junction array 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. 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 of 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;

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)

I c (I ctrl ) at 77K: highly asymmetric B. Chesca, D. John, M. Kemp, J. Brown, and C. Mellor, Appl. Phys. Lett. 103 , 092601 (2013)

Magnetic sensors: SQUIDs & SQIFs Why superconducting magnetic sensors? the best getting less expensive: 77K SQUID-arrays better than single-SQUID 4.2 K

SQUID arrays SQUID 770 SQUID array flux coherent & non-interacting SQUID array [ ] [ ] Noise Array = N 1/2 Noise SQUID 1 Noise Noise = V V Array N 1/2 V Array = N V SQUID SQUID

SQUID arrays @ 77K better than SQUIDs @ 4.2 K -172  A 484 SSA, 40 K 770 SSA, 83 K 10 -15 V, mV 1/2 ) 770 SSA  (    Hz -30 T=83K -212  A max(  V)=6.8 mV -15 0 15 1/2  1/N B,  T HTS SQUID 1/2 @ 77 K S nano-HTS SQUID @ 4.2 K 1 LTS SQUID @ 4.2 K 0 200 400 600 800 Number of SQUIDs 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 !