Development of superconducting electronics and its applications at VTT

Development of superconducting electronics and its applications at VTT Panu Helistö VTT Technical Research Centre of Finland PO Box 1000, 02044 Espoo, Finland panu.helisto@vtt.fi www.vtt.fi/palvelut/cluster1/topic1_7/sensors_quantumsensors.jsp 29.11.2006 Panu Helistö: Superconducting electronics at VTT 1

Outline 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 29.11.2006 Panu Helistö: Superconducting electronics at VTT 2

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 bolometric/calorimetric radiation detectors utilizing strong electrothermal feedback in the transition 4. But - is there any near-future commercial potential? 5. Cryocooling needed - messy and impractical? 29.11.2006 Panu Helistö: Superconducting electronics at VTT 3

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 => 4 K not a problem! 29.11.2006 Panu Helistö: Superconducting electronics at VTT 4

Tunnel junction devices 29.11.2006 Panu Helistö: Superconducting electronics at VTT 5

JOSEPHSON (SIS) JUNCTION 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 S -I -S SC 1 SC 2 I Exploited in Josephson voltage standards, SQUIDs, Single Flux Quantum electronics etc -V/2 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 29.11.2006 Panu Helistö: Superconducting electronics at VTT 6

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 Picture source: ESA 29.11.2006 Panu Helistö: Superconducting electronics at VTT 7

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 Feedback Resistor 4.2 K 300 K Preamplifier SQUID + gradiometric input coil Signal Coil SQUID Bias Voltage FET Feedback Coil Noise cancellation readout Gate Voltage GND 29.11.2006 Panu Helistö: Superconducting electronics at VTT 8

SQUID application: Brain diagnostics MagnetoEncephaloGraphy (MEG) Main customer: Elekta Neuromag Oy 303 SQUID channels at 4 K map the cortex neural activity (only SQUIDs have sufficient resolution) 10 000 VTT SQUIDs in daily use around the world Example of a successful industrial superconducting sensor application Spinoff company Aivon Oy starts in 2007 29.11.2006 Panu Helistö: Superconducting electronics at VTT 9

Quantum voltage standard ~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 Nb 2 O 5 Application: metrology 20 μm Junction bound Nb 2 O 5 Tunnel barrier Al 2 O 3 Niobium II Insulator II Trilayer 1 μm Insulator I Niobium I 29.11.2006 Panu Helistö: Superconducting electronics at VTT 10

Micrographs of different JJ array realizations Notch filter (a) Shunt resistor (b) Inductive strips 20 μm (c) Contact hole Shunt resistor 20 μm 0.1 mm Josephson junction 29.11.2006 Panu Helistö: Superconducting electronics at VTT 11

Quantum voltage standard Practical challenges: Large, very expensive component Requires an expensive mmwave generator + phase lock => VTT is searching improved realizations of JJ arrays 29.11.2006 Panu Helistö: Superconducting electronics at VTT 12

VTT RSFQubit process Nb4 Ins3 Ins2 Ins1 Res Substrate Nb 2 O 5 Nb3 Al/AlO x Nb2 Nb1 Cooling fin Res 4 Nb layer process developed for complicated, mk compatible JJ circuits Applications: superconductíng quantum bit manipulation and integration to superconducting readout and control circuitry 29.11.2006 Panu Helistö: Superconducting electronics at VTT 13

Towards quantum computing with superconducting qubits 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). 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 J. Hassel, H. Seppä, P. Helistö, J. Kunert, L. Fritzsch and H.G. Meyer, Appl. Phys. Lett. (2006) 29.11.2006 Panu Helistö: Superconducting electronics at VTT 14

Semiconductor-superconductor junction refrigerators Cooling and temperature measurement at mk Al-Si junction SOI mesa A 20 μm I V Al I th B V th T e (mk) 400 300 200 S-Sm cooler junction Thermometer BOX, SiO 2 n++ SOI film Al Si subst Optical micrograph of a Si-Al refrigerator on Silicon-on-insulator (SOI) substrate and schematic cross-section along AB. [1] 100-0.4 0.0 0.4 V (mv) Experimental electron temperature vs. bias voltage at different substrate temperatures.[1] Efficient cooling is mediated by weak electron-phonon coupling. [2] [1] A. M. Savin, M. Prunnila, P. P. Kivinen,et al., Appl.Phys. Lett. 79, 1471 (2001). [2] M. Prunnila et al., Phys. Rev. Lett. 95, 206602 (2005). Mika Prunnila & Jouni Ahopelto, VTT Micro and Nanoelectronics 29.11.2006 Panu Helistö: Superconducting electronics at VTT 15

Superconduction transition detectors 29.11.2006 Panu Helistö: Superconducting electronics at VTT 16

Nb bolometers for THz detection Nb microwire (35 x 1 x 0.1 um 3 ) at the center of a wide-band log-spiral antenna, biased electrically at superconducting transition THz power heats the bolometer and increases its resistance 29.11.2006 Panu Helistö: Superconducting electronics at VTT 17

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 Transition during natural warmup 3 mm 1200 1000 Al NbN 10 μm Resistance [Ω] 800 600 400 200 Si 0 6 8 10 12 14 16 Lakeshore temperature [K] 29.11.2006 Panu Helistö: Superconducting electronics at VTT 18

The room temperature readout - noise elimination by ETF pat. pending 3T c 2T c T c T 0 0 10 9 I 8 7 Johnson 0 1 2 3 4 5 V/V 6 0 Normal 5 Phonon 4 3 Supercond 2 Readout l/ 4 l/2 3l/4 l 1 0.8 1.0 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 V [mv] NEP [fw/sqrthz] At high voltages, Johnson noise > phonon noise = bad, very high amplifier noise = bad At low voltages, phonon noise > Johnson noise = fine, high current responsivity, very high amplifier noise = bad At IV curve bottom, phonon noise ~ Johnson noise = good, negligible amplifier noise = very good! 29.11.2006 Panu Helistö: Superconducting electronics at VTT 19

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) i n (pa/rthz) 10 1 (b) 100 10 NEP P n (fw/rthz) I [μa] 7.0 6.5 6.0 5.5 R l = 2029 Ohm G l/4 = 2.9 nw/k ΔT = 5 K (a) 100 1000 10000 100000 f(hz) 5.0 2 4 6 8 10 V [mv] 29.11.2006 Panu Helistö: Superconducting electronics at VTT 20

Pulse tube cryocooler being rewired for THz imaging Cryoflex cables will be used 29.11.2006 Panu Helistö: Superconducting electronics at VTT 21

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 Skin at body temperature 29.11.2006 Panu Helistö: Superconducting electronics at VTT 22

Under construction: video-rate passive THz imager Cryocooler 128-pixel detector array Rotating mirror 10-30 m Target: rapidly growing security market, also medical and pharma applications demonstrated => potential for a real mass product(?) (Detector-readout concept resembles the proven MEG brain scanner concept) 29.11.2006 Panu Helistö: Superconducting electronics at VTT 23

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 29.11.2006 Panu Helistö: Superconducting electronics at VTT 24

XEUS prototype instrument 29.11.2006 Panu Helistö: Superconducting electronics at VTT 25

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 29.11.2006 Panu Helistö: Superconducting electronics at VTT 26

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 29.11.2006 Panu Helistö: Superconducting electronics at VTT 27