Detecting spins by their fluorescence with a microwave photon counter

Area of Science:

• Quantum physics
• Solid-state physics
• Spectroscopy

Background:

• Quantum emitters radiate energy, with both coherent and incoherent (fluorescence) components.
• Electron spins are typically detected via phase-coherent echoes, not fluorescence, due to low radiative rates and lack of detectors.
• Spontaneous microwave photon emission from spins has remained unobserved.

Purpose of the Study:

• To demonstrate the detection of electron spin fluorescence at microwave frequencies.
• To develop a method for observing the incoherent radiation from electron spins.
• To explore new possibilities for magnetic resonance spectroscopy on small spin ensembles.

Main Methods:

• Utilized superconducting quantum devices operating at millikelvin temperatures.
• Enhanced spin radiative decay rates by coupling spins to a high-quality-factor superconducting resonator.
• Developed and employed a novel single microwave photon detector (SMPD) based on a superconducting qubit.

Main Results:

• Successfully detected the fluorescence from a small ensemble of donor spins in silicon at microwave frequencies.
• Demonstrated that the SMPD can also detect spin echoes.
• Achieved standard spin characterization measurements (Rabi nutation, spectroscopy) using both echo and fluorescence detection.

Conclusions:

• The study presents the first observation of electron spin fluorescence at microwave frequencies.
• The developed SMPD and fluorescence detection method offer a new tool for magnetic resonance spectroscopy.
• This technique holds potential for characterizing small numbers of spins with high sensitivity.