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Cooling electron spins with microwave photons

A team from the CEA-Iramis has shown that it is possible to bring an ensemble of electron spins to a temperature lower than that of the supporting crystal, thanks to their being coupled to an electromagnetic field within a resonant microwave cavity. This patented discovery greatly improves the detection of EPR (Electron Paramagnetic Resonance) and NMR (Nuclear Magnetic Resonance) signals.

Published on 15 June 2020

In EPR (or NMR), the electron (or nuclear) spins are aligned (or polarized) by an external magnetic field and then excited by a microwave electromagnetic field pulse (or radiofrequency, for nuclear spins). Their relaxation generates a signal carrying the desired information. Because of the weak polarization, this signal is drowned in a strong "noise".

To enhance the useful signal, the polarization can be strengthened by increasing the intensity of the magnetic field or by reducing the temperature of the sample. Even better, it is possible to cool the spins alone, which are then said to be "hyperpolarized". Thus, the "dynamic nuclear polarization" method consists in cooling the nuclear spins by coupling them to an ensemble of electron spins, which are more easily polarized. However, this is not applicable to electron spins – except in a few rare cases where the spins can be cooled optically.

Physicists at the Iramis (SPEC) have proposed a method for hyperpolarizing electron spins derived from their recent work (the first demonstration of the "Purcell effect"). For this, the sample to be analyzed is inserted in a microwave cavity. The external magnetic field is adjusted so that the difference between the energy levels of the electron spins corresponds exactly to the resonance frequency of the cavity. Due to the coupling with the electromagnetic field, radiative relaxation of the spins (i.e. emission of a microwave photon) is much more likely than non-radiative de-excitation by phonon emission (vibration quantum), which is a sign of the Purcell effect.

The ensemble of electron spins contained in the sample – atoms of bismuth implanted in a silicon crystal – are coupled to a superconducting niobium resonator with a frequency of 7.4 GHz in the Purcell regime, cooled to a temperature of 850 mK. By connecting the input of the resonator to a resistor cooled to 20 mK, the researchers have demonstrated that they can reduce the electromagnetic field in the cavity, which reduces the population of spins in the higher energy level and thus the effective temperature of the spins. The measured EPR signal (the spin echo measurement) is then multiplied by 2.3, which is consistent with a spin temperature reduced to 350 mK.

This method should also be applicable at higher sample temperatures, especially that of liquid helium (4.2K). Moreover, it should be possible in principle to achieve a near-perfect polarization by combining the radiative cooling of the spins experimentally tested in this study with the "active" cooling of the microwave field, which was recently demonstrated using Josephson junctions.

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