Contributor Comms Physics
Story of the Invention of a Cold-Electron Bolometer
Leonid KUZMIN
Professor, Chalmers University of Technology
We demonstrate the photon-noise-limited operation of Cold-Electron Bolometers (CEBs) due to highly efficient self-cooling of the absorber. This study suggests t
hat CEBs are potential candidates for advanced radio astronomy projects avoiding dilution refrigerators. In particular, balloon telescopes could not afford a heavy dilution refrigerator. For space missions, the problem is even more severe for dilution refrigerator due to the absence of gravity. This problem is one of the main obstacles for the COrE space mission that is not accepted by the European Space Agency from 2007 due to cryogenics.
We have developed CEB with on-chip cooling that could replace a dilution refrigerator in principle for all frequency channels. This progress is fully based on the invention of a Cold-Electron Bolometer. However, the story of the invention was not a smooth process with progress, fails and, finally, with the development of the very effective ultimate CEB.
Fig. 1. Falls and Ups in development of a Cold-Electron Bolometer. 1. Hot-electron Bolometer with SIN tunnel junction as the thermometer and Andreev mirrors for RF coupling and thermal isolation. M. Nahum and J. M. Martinis (1993). 2. Hot-electron Bolometer with SIN tunnel junction as thermometer and other SIN junctions for RF coupling and thermal isolation. L. Kuzmin (1998). 3. Six-legged Bolometer with SIN tunnel junctions as the thermometer, other SIN tunnel junctions as electron cooler, and Andreev mirrors for RF coupling, and thermal isolation. L. Kuzmin, I. Devyatov, & D. Golubev (1998). 4. Cold-Electron Bolometer (CEB) with SIN tunnel junctions as the thermometer, electron cooler, RF capacitive coupling and thermal isolation. L. Kuzmin (2002).
Historical development of the CEB is shown in Fig. 1. The story was started from the elegant suggestion of M. Nahum and J. M. Martinis to use Andreev reflection for RF coupling from one side and thermal protection from another side (step 1). They used three-legged bolometer. However, this bolometer has limited thermal protection only up to 70 GHz, determined by the superconducting gap of Aluminum. Besides that, the readout by SIN tunnel junction from the middle of absorber disturbed the RF electrodynamics strongly.
The next step was the replacement of Andreev contact by SIN tunnel junction for better thermal protection (step 2). However, RF scheme became even more disturbed by two SIN tunnel junctions for response measuring.
Step 3 was an attempt to include electron cooling in operation of the bolometer by connecting two more large cooling junctions to the absorber. This step was the apotheosis of the absurdity: this six-legged bolometer was too complicated. It never should work and it did not. Besides complexity, these two pairs of junctions, for cooling and for measuring, fight each other for signal and considerably decreased responsivity. However, sometimes the top of absurdity is good because it stimulated intensive thinking for a better decision. As a result, the optimal decision in Step 4 has been found. Instead of "six-legged cuttlefish" it was invented the two-legged Cold-Electron Bolometer (CEB) with only one pair of SIN tunnel junctions! These junctions combined all four functions: RF capacitive coupling, thermal isolation, thermometer, and electron cooler (Fig. 2).
Fig. 2. a) Cold-Electron Bolometer with two SIN tunnel junctions, b) Energy diagram of operation of SIN tunnel junctions with combination of four functions: RF capacitive coupling, thermal isolation, thermometer, and electron cooler.
Sometimes the question arises, is this not a cheating nature: such a combination of four functions in one element simultaneously? Answer: in a sense, it seems that yes. However, it works because we used the conversion of energy to different forms. Electromagnetic energy penetrates through the capacitance of tunnel junction. Then this energy converts in absorber in thermal energy with thermal protection by the high potential barrier of the tunnel junction. After that, the hottest electrons tunnel through the barrier in the form of hot quasiparticles providing measuring of incoming power and simultaneously cooling the absorber.
All first three concepts did not survive and stop their existence. Only the fourth concept of CEB has found wide applications and is in the stage of further intensive development.
Finally, a Cold Electron Bolometer that operates at an electron temperature that is less than the phonon temperature was presented for the first time. The electron temperature in a CEB array can go from 310 to 120 mK. Due to effective electron self-cooling of the absorber, photon-noise-limited operation of a 2D array of CEBs has been achieved at 310 mK.
This study suggests that CEBs with internal self-cooling are potential candidates for advanced radio astronomy projects that must avoid dilution refrigerators. Therefore, this can solve the main problem of the COrE space mission that was not accepted by the European Space Agency from 2007 due to necessity to find a compromise between sensitivity, cryogenics and cost. We can develop arrays of CEBs practically for any frequency range achieving ultimate sensitivity at 300 mK without dilution refrigerator.