Development of superconducting electronics and its applications at VTT

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Development of superconducting electronics and its

applications at VTT

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• Why superconducting electronics? • Tunnel junction devices and sensors

• One junction: STJ detectors • Two junctions: SQUIDs

• ~10000 junctions: Quantum voltage standards

• 100-10000 junctions: Circuits for manipulation of quantum bits • 1-4 junctions: Electron cooling with SINIS-junctions

• Superconducting transition detectors

• Nb and NbN microbolometers for THz • Room temperature readout

• Towards video-rate imaging

• X-ray transition-edge calorimeters and their readout • Conclusions

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Why superconducting electronics and devices

1. Benefit of low operation temperature

• low thermal noise - improved performance 2. Special properties of tunnel junctions

• unparalleled resolution as sensor

• generation of absolute quantum voltage

• fast, low noise analog and digital electronics • applications in future quantum computing(?) • cryocooling at low temperatures

3. Steep superconducting transition

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electro-Cryocooling

• Cooling allows for substantial increase in performance

• Necessary for superconductivity • But - is it industrially feasible? • Liquid helium handling - only for

physicists?

• Commercial turn-key 4 K cryocoolers available

• Reliable - such coolers have run for 10-15 years with no problems in tropical conditions for mining applications

• Price: 20-35 k€

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JOSEPHSON (SIS) JUNCTION

SC 1 SC 2 V/2 I -V/2

• Two superconductors (S) separated by a thin (2 nm) insulating barrier (I)

• Two operation modes: 1) supercurrent tunneling

• Dc and ac Josephson effect

• Exploited in Josephson voltage standards, SQUIDs, Single Flux Quantum electronics etc

2) quasiparticle tunneling

• 'Normal' current due to thermal or photon excitation of quasiparticles

• Properties of the superconducting gap essential

• STJ detectors, SIS mixers, electron coolers etc

S

-I

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Superconducting tunnel junction (STJ) detector

• Single epitaxial SIS junction biased near the gap voltage • Photodetection mode: photon

hitting the junction breaks Cooper pairs and generates quasiparticle bunch (compare semicond. det.) • Applications: IR - X-ray detection,

mass spectrometry, astrophysics

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SQUID : superconducting quantum interference device

• Two Josephson (SIS) junctions in a superconducting loop • Output current depends on the magnetic flux in the loop • Best available magnetic field sensor (~ 1 fT/sqHz)

• Magnetic applications: medical, mining, military etc

• Current amplifier for low-impedance high-resolution detectors

SQUID + gradiometric input coil Noise cancellation readout SQUID FET Signal Coil Feedback Coil Bias Voltage Gate Voltage 300 K 4.2 K Feedback Resistor Preamplifier GND

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SQUID application: Brain diagnostics

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MagnetoEncephaloGraphy (MEG)

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Main customer: Elekta Neuromag Oy

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303 SQUID channels at 4 K map the cortex neural activity (only SQUIDs have sufficient resolution)

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10 000 VTT SQUIDs in daily use around the world

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Example of a successful industrial superconducting sensor application

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Quantum voltage standard

Niobium I Insulator I Trilayer Niobium II Insulator II Junction bound Nb₂O₅ Tunnel barrier Al₂O₃ 20 μm 1 μ m

• ~10 000 series-connected Josephson junctions generate absolute voltage • Dc voltage uncertainty: ~1 nV @ 10 V

• VTT trilayer technology: Nb/Al/AlOx/Nb junctions bounded by Nb₂O₅ • Application: metrology

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Micrographs of different JJ array realizations

0.1 mm

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Inductive strips _{Contact hole}

Josephson junction

Notch filter Shunt resistor _{Shunt resistor}

(b)

(c)

20 μm

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Quantum voltage standard

Practical challenges: • Large, very expensive

component

• Requires an expensive mm-wave generator + phase lock => VTT is searching improved

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VTT RSFQubit process

Substrate Ins1 Ins2 Ins3 Nb3 Nb4 Res Nb2 Nb1 Nb2O5 Al/AlOx Cooling fin Res

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Towards quantum computing with superconducting qubits

• Main problem: decoherence due to thermal noise and fluctuators in junctions and their neighborhood

• Results 2006

• high quality JJ junctions => reduced JJ noise

• e-ph coupling 'cooling fins' to achieve record low base temperature of shunt resistors

• RC shunts to isolate qubit from RSFQ dissipations

Maria Gabriella Castellano, Leif Grönberg, Pasquale Carelli, Fabio Chiarello, Carlo Cosmelli, Roberto Leoni, Stefano Poletto, Guido Torrioli, Juha Hassel, Panu Helistö, Superconducting Science and Technology 19, 860864 (2006).

J. Hassel, H. Seppä, P. Helistö, J. Kunert, L. Fritzsch and H.G. Meyer, Appl. Phys. Lett. (2006)

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Thermometer S-Sm cooler junction BOX, SiO2 Si subst Al n++ SOI film 20 μm I_th V V_th I SOI mesa Al-Si junction Al A B -0.4 0.0 0.4 100 200 300 400 T e (mK) V (mV)

Semiconductor-superconductor junction refrigerators

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NbN bolometer arrays

• NbN - higher normal state resistance

=> lower thermal conductance than Nb (lower noise), easier matching to antenna • Tc can be tailored to optimize performance 0 - 13 K

• 8-pixel subarrays developed => goal 128-pixel linear array

3 mm

10 μ

m

Al

Si

NbN

6 8 10 12 14 16 0 200 400 600 800 1000 1200 Resistance [ Ω ] Lakeshore temperature [K]

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The room temperature readout - noise elimination by ETF

• At high voltages, Johnson noise > phonon noise = bad, very high amplifier noise = bad 0 2T_c T_c T₀ 3T_c l l/2 Normal Supercond 0 1 2 3 4 5 V/V₀ I l/4 3l/4 1 0.8 1.0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 2 3 4 5 6 7 8 9 10 V [mV] N E P [fW /s qr tH z] Phonon Johnson Readout pat. pending

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Electrical noise of a NbN microbolometer + RT readout

• Smooth I-V curve - good quality bolometer

• Electrical NEP ~ 9 fW/sqHz (lowest measured at 4 K)

100 1000 10000 100000 1 10 i n (p A /rt H z ) f(Hz) 10 100 P n (fW /rt H z ) (b) 5.0 5.5 6.0 6.5 7.0 R_l = 2029 Ohm G_l/4 = 2.9 nW/K Δ_{T = 5 K} I [ μ A] (a) NEP (fW/rtHz) V [mV] 2 4 6 8 10

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Passive THz imaging with superconducting bolometers

Passive imaging at THz - ultimate resolution needed VTT-Millilab-NIST collaboration: • 1-pixel Nb bolometer 0.1 - 1 THz • RT readout, mechanical scanning • State-of-the-art resolution

Background (reflected by concealed objects) at room temperature

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Under construction: video-rate passive THz imager

Cryocooler 128-pixel detector array Rotating mirror 10-30 m

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XEUS - planned ESA X-ray telescope mission

Goals

• Formation of clusters of

galaxies in the early universe (dark matter, dark energy,…) • Role of supermassive black

holes in galaxy formation • Gravity theory at high fields

(what happens to physics at the event horizon?)

• Matter under extreme

conditions (neutron star - a huge strange nucleus)

Probe

• Spectroscopic imaging of X-and (red-shifted) gamma rays

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VTT SQUID readout for TES calorimeters

• mK SQUID design for XEUS

• Frequency division multiplexing (FDM) readout

• Terrestrial application: high resolution X-ray fluoresence materials characterisation

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Conclusions

• Supeconducting electronics is perhaps the only solution to high-end scientific goals such as the quantum computer or experimental

astrophysics and cosmology

• But superconducting electronics has also established or emerging industrial applications with reasonable-to-large potential

• VTT has a broad experience in designing and fabricating superconducting electronics and in developing applications based on them