​A communications network or a quantum computer requires the exchange of information between nodes or between memories and processors using vectors such as photons. But how can a quantum state be maintained long enough to manipulate or store it before "decoherence" erases it?

To answer this, a team of physicists from Iramis have searched for a crystalline environment that favors the electron spin coherence of erbium ions (Er³⁺).

Erbium, an atomic impurity that can be inserted for example in a CaWO₄ crystal, simultaneously possesses:

• one electron spin degree of freedom in its ground state;
• an optical transition at the wavelength of "telecommunications" optical fibers (1.5 µm);
• quantum properties suitable for the coupling of its electronic spin to neighboring nuclear spins or to another quantum system.

However, spectroscopic measurements of the Er³⁺ ion spin transition (an inhomogeneous linewidth greater than 10 MHz) and the coherence time of its electron spin (less than 50 µs) were considered disappointing, until now.
 
The researchers chose scheelite as the host matrix for the erbium ions. This natural, undoped crystal has a very low nuclear magnetic density – since only the ¹⁸³W isotope of tungsten (14% natural abundance) has a nuclear spin – and contains traces of Er³⁺ ions (1 part per billion) in place of calcium.

The crystal is probed by an EPR (Electron Paramagnetic Resonance) spectrometer, developed by the team, at the quantum limit. The core of the device is a planar superconducting microwave resonator (7 GHz) positioned on the crystal and cooled to 10 mK. A magnetic field (70 mT) is applied in the plane of the crystal to tune the erbium spin transition frequency to the detection frequency. The spins are then detected using the "spin echo" technique: in response to two successive microwave pulses, the spins re-emit an echo pulse whose amplitude is a signature of their coherence.

 The spectral width of the spin transition can reach a minimum of 1 MHz: an improvement by a factor of ten over the current state of the art.

 As for the coherence time, it is multiplied by one thousand with respect to the known values for Er³⁺, reaching 23 ms. This is the longest coherence time ever measured for an electron spin in a non-isotopically purified crystal.