1. Ultralow temperature nanorefrigerator
Cooling Gn
Electron
system
Electrical
environment
Lattice

2. NIS junction as a refrigerator
Cooling power of a NIS junction:
Optimum cooling power is reached at VC  2D/e:
Optimum cooling power per junction, when superconducting reservoirs are not overheated, TS << TC
Temperature TN on the island is determined by the balance of heat fluxes, e.g.:
Electron-phonon heat flux:
(dominates at high temperatures, negligible at low temperatures)
Efficiency (coefficient of performance) of a NIS junction refrigerator:

3. Experimental status
Nahum, Eiles, Martinis 1994 Demonstration of NIS cooling
Leivo, Pekola, Averin 1996, Kuzmin 2003, Rajauria et al Cooling electrons 300 mK -> 100 mK by SINIS
Manninen et al Cooling by SIS’IS see also Chi and Clarke 1979 and Blamire et al. 1991, Tirelli, Giazotto et al. 2008
Manninen et al. 1997, Luukanen et al Lattice (membrane) refrigeration by SINIS
Savin et al S – Schottky – Semic – Schottky – S cooling
Clark et al. 2005, Miller et al x-ray detector refrigerated by SINIS
Refrigeration of a membrane with separate thermometer
Refrigeration of a ”bulk” object
A. Clark et al., Appl. Phys. Lett. 86, (2005).
A. Luukanen et al., J. Low Temp. Phys. 120, 281 (2000).
For a review, see Rev. Mod. Phys. 78, 217 (2006).

4. NIST 2007-08 Done Robust, wafer-scale solid-state refrigerators
1st cooling of bulk material
1st integrated NIS-cooled detectors
mm bolometer (Goddard)
X-ray microcalorimeter
Spectra above Tc
Future (someone else)
Improve cooling
300  100 mK
Cooling platform for general payloads
Attach your own detector chip

5. Specifications, objectives
Now:
Temperature reduction (electrons): 300 mK -> 50 mK
Temperature reduction (lattice): 200 mK -> 100 mK
Cooling power: 30 pW at 100 mK by one junction pair
Objectives (NanoFridge, EPSRC, Microkelvin):
Electron cooling from 300 mK -> 10 mK
Cooled platform for nanosamples: 300 mK -> 50 mK, cooling power 10 nW at 100 mK by an array of junctions
Cooler from 1.5 K down to 300 mK using higher Tc superconductor
Experiments in progress at TKK:
Thermodynamic cycles with electrons: utilizing Coulomb blockade, heat pump with P = kBT f (proposal 2007)
Refrigeration at the quantum limit (Meschke et al., Nature 2006, Timofeev et al. 2009, unpublished)
Brownian refrigerator, Maxwell’s demon (proposal 2007)
Cooling mechanical modes in suspended structures, i.e., nanomechanics combined with electronic refrigeration (Preliminary experiment, Muhonen et al. and Koppinen et al. 2009)

6. JRA2 Ultralow temperature nanorefrigerator TKK, 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

7. Roles of the participants
TKK and CNRS will develop the nanorefrigeration by superconducting tunnel junctions
SNS will build coolers based on semiconducting electron gas
BASEL will work mainly on very low temperature thermalization and filtering DELFT and RHUL are mainly end users of the nano-coolers

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

9. Task 2: Microkelvin nanocooler (TKK, CNRS, SNS)
Aim is to develop sub - 10 mK electronic cooler
Normal metal – superconductor tunnel junctions-based optimized coolers (TKK, CNRS, DELFT)
10 mK to 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)

10. Thermometry at low T
SNS Josephson junction

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

12. Deliverables
Task 1
D1: Analysis of combined ex-chip and on-chip filter performance (18)
D2: Demonstration of sub-10 mK electronic bath temperature of a nano-electronic tunnel junction device achieved by the developed filtering strategy (30)
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)
D4: Demonstration of sub-10 mK nanocooling with a N-I-S junction (48)
Task 3
D5: Demonstration of 300 mK to about 50 mK cooling of a dielectric platform (36)
D6: Demonstration of cooling-based improved sensitivity of a quantum detector (48)