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JRA2 Ultralow temperature nanorefrigerator AALTO, CNRS, RHUL, SNS, BASEL, DELFT Objectives Thermalizing and filtering electrons in nanodevices To develop an electronic nano-refrigerator that is able to reach sub-10 mK electronic temperatures To develop an electronic microrefrigerator for cooling galvanically isolated nanosamples AALTO and CNRS will develop the nanorefrigeration by superconducting tunnel junctions SNS will build coolers based on semiconductors (quantum wires, quantum dots) BASEL will work on very low temperature thermalization and ex-chip filtering DELFT and RHUL are end users of the nano-coolers

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Deliverables and milestones Task 1 D1: Analysis of combined ex-chip and on-chip filter performance (18) DONE D2: Demonstration of sub-10 mK electronic bath temperature of a nano-electronic tunnel junction device achieved by the developed filtering strategy (30) IN PROGRESS M1: Choice of the thermalization strategy (sintered heat exchangers, 3 He cell) (12) done M2: Choice of the ex-chip filtering technique (18) done Task 2 D3: Analysis of sub-10 mK nano-cooling techniques including (i) traditional N-I-S cooler with low Tc, (ii) quantum dot cooler (24) IN PROGRESS, PARTLY DONE D4: Demonstration of sub-10 mK nanocooling with a N-I-S junction (48) PLANNED M3: Choice of the superconductor material with a lower critical temperature (24) ? M4: Precise definition of the QD cooler geometry and materials (24) ? Task 3 D5: Demonstration of 300 mK to about 50 mK cooling of a dielectric platform (36) IN PROGRESS D6: Demonstration of cooling-based improved sensitivity of a quantum detector (48) (DONE!) M5: Design of the membrane patterning and of the micro-coolers, based on heat and quasiparticles diffusion calculations (18) done M6: Delivery of the first membranes to the end users (36) done

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Task 1: Thermalizing electrons in nanorefrigerators (AALTO, CNRS, BASEL) Ex-chip filtering: Sintered heat exchangers in a 3He cell Lossy coaxes/strip lines, powder filters,... On-chip filtering: Lithographic on-chip filtering W. Pan et al., PRL 83, 3530 (1999) A. Savin et al., APL 91, 063512 2007

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Environment (i.e., photon) assisted tunneling in a NIS junction Tunneling rates are affected by the hot environment. This can be treated using the P(E) theory accounting for fluctuations at a junction. Filtering, low T Hot 4 K environment, R env R = R env Typically: R env = 100 R < 1 Junction eV PHOTON ABSORPTION and TUNNELING

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PAT has the same effect as Dynes density of states with PRL 105, 026803 (2010) Experimental data fits well with the assumption of lifetime-broadened DOS. Theory yields a 1-1 correspondence assuming weakly dissipative high T environment. Improved characteristics of quantized current plateaus

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Device Electron Temperature: with microwave filters With microwave filters: T MC = 5 mK, T E = 18 2 mK for GaAs CBT and about13 mK for metallic CBT (preliminary result for the metallic device)

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D1: Analysis of combined ex-chip and on-chip filter performance (18) DONE Capacitive and resistive on-chip filters tested, and they perform well up to f >> 50 GHz (4 K). Theoretically RC on-chip filter would be even better and this can be easily fabricated as well. Ex-chip filters: Thermocoax cable (high f filtering) Miniature silver-epoxy microwave filters (low f filtering, down to a few MHz) Thermalizing of the wires by sintered silver heat exchangers located in the 3He/4He mixture D2: Demonstration of sub-10 mK electronic bath temperature of a nano-electronic tunnel junction device achieved by the developed filtering strategy (30) IN PROGRESS Electronic temperatures of quantum dot and metallic CBT:s down to 18 (10) mK Tests of ultimate performance in progress -> Dominik Zumbuhls talk M1: Choice of the thermalization strategy (sintered heat exchangers, 3 He cell) (12) done M2: Choice of the ex-chip filtering technique (18) done

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Task 2: Microkelvin nanocooler (AALTO, CNRS, SNS) Aim is to develop sub - 10 mK electronic cooler Normal metal superconductor tunnel junctions-based optimized coolers (AALTO, CNRS, DELFT) Towards lower T: Improved quality of tunnel junctions? Thermometry at low T? Lower Tc superconductor Quasiparticle relaxation studies in sc and trapping of qp:s Quantum dot cooler (SNS)

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D3: Analysis of sub-10 mK nano-cooling techniques including (i) traditional NIS cooler with low Tc, (ii) quantum dot cooler (24) IN PROGRESS, PARTLY DONE D4: Demonstration of sub-10 mK nanocooling with a NIS junction (48) PLANNED D3 (i): AlMn-Ti NIS junction. AlMn is normal, Ti superconducting with Tc = 0.4 K (cf. Al Tc = 1.2 K in thin films). Junctions are good for thermometry but no cooling was observed.

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D3: Analysis of sub-10 mK nano-cooling techniques including (i) traditional NIS cooler with low Tc, (ii) quantum dot cooler (24) IN PROGRESS, PARTLY DONE D4: Demonstration of sub-10 mK nanocooling with a NIS junction (48) PLANNED D3 (i): Standard NIS junction cooler in small perpendicular to film magnetic field (few G). Performance improved, probably thanks to enhanced quasiparticle relaxation in Al superconductor. Theoretical analysis in progress.

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D3: Analysis of sub-10 mK nano-cooling techniques including (i) traditional NIS cooler with low Tc, (ii) quantum dot cooler (24) IN PROGRESS, PARTLY DONE D4: Demonstration of sub-10 mK nanocooling with a NIS junction (48) PLANNED D3 (ii): GaAs/AlGaAs 2DEG cooler (SNS Pisa). Thermometry by quantum dot CBTs. Quasiparticle energy relaxation in the central area either by electron-phonon coupling (high T) or through the quantum dot (low T). Optimized cooler to be developed. arXiv:1007.0172, submitted.

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Task 3: Development of a 100 mK, robust, electronically-cooled platform based on a 300 mK 3He bath (AALTO, CNRS, RHUL, DELFT) Commercial, robust SiN membranes (or custom made alumina?) as platforms (AALTO) Epitaxial large area junctions (CNRS) Optimized junctions (e-beam and mechanical masks) RHUL and DELFT use these coolers for experiments on quantum devices

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Task 3 D5: Demonstration of 300 mK to about 50 mK cooling of a dielectric platform (36) IN PROGRESS D6: Demonstration of cooling-based improved sensitivity of a quantum detector (48) (DONE!) M5: Design of the membrane patterning and of the micro-coolers, based on heat and quasiparticles diffusion calculations (18) done M6: Delivery of the first membranes to the end users (36) done Two-stage photolithography for large area cooler junctions in progress at CNRS. M5: CAD-design and experimental tests of membrane coolers done. Performance still poor (few mK cooling only)

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Task 3 D5: Demonstration of 300 mK to about 50 mK cooling of a dielectric platform (36) IN PROGRESS D6: Demonstration of cooling-based improved sensitivity of a quantum detector (48) (DONE!) M5: Design of the membrane patterning and of the micro-coolers, based on heat and quasiparticles diffusion calculations (18) done M6: Delivery of the first membranes to the end users (36) done M6, D6: Delft-Aalto: Combining a membrane cooler with a KID- detector. Improved KID performance demonstrated (N. Vercruyssen talk). RHUL-Aalto collaboration: SIS- junctions on membranes fabricated.

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Achievements by July 2010 Task 1: AALTO: On-chip filtering suppresses significantly leakage and dissipation in NIS junctions theoretical model developed and experimental demonstration performed BASEL: Sophisticated microwave ex-chip filtering demonstrates electron temperature of 18 mK Task 2: SNS: Quantum dot thermometry and thermal transport measurements performed AALTO: AlMn as a normal material tested for cooler purposes CNRS: Electron and phonon temperatures measured separately Task 3: CNRS: Epitaxial large tunnel junction process under way AALTO: Coolers on silicon nitride membranes produced, cooling power degraded in the first experiments by poor efficiency of the cold finger DELFT: Kinetic inductance detector fabricated and demonstrated (performance improvement by cooling) on a platform produced by AALTO RHUL: Tests of Josephson junctions on cooler platforms (together with AALTO) 5 or more articles on JRA2 either published, submitted or under preparation for publication

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BASEL: Filtering Strategy: Stage 1 Thermocoax cables from room T to 10 mK 1.5 meters attenuation > 100 dB for f > 4 GHz Uni Basel, Zumbhl groupJRA2, filtering & thermalizing

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Stage 2: Cryogenic Miniature Microwave Filters attenuation > 100 dB for f > 150 MHz attenuation > 100 dB for f > 20 MHz mounted @ 10 mK Uni Basel, Zumbhl groupJRA2, filtering & thermalizing

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Stage 2: Cryogenic Miniature Microwave Filters RT, LN 2 and LHe attenuation no significant degradation of performance at 4 K Uni Basel, Zumbhl groupJRA2, filtering & thermalizing

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GaAs Quantum Dot Electron Thermometer 500 nm Use GaAs quantum dot as a thermometer (Coulomb blockade) differential conductance or DC current temperature broadened regime (