Macroscopic quantum superposition of spin ensembles with ultra-long coherence times via superradiant masing

TL;DR

This paper demonstrates a method to create macroscopic quantum superpositions of large spin ensembles in solids, achieving coherence times of about 10^8 seconds using superradiant masing, with potential applications in quantum technology and magnetometry.

Contribution

It introduces a superradiant masing scheme that significantly extends the coherence time of macroscopic spin superpositions in solids, surpassing previous limitations.

Findings

Achieved coherence times of ~10^8 seconds for spin ensembles.

Demonstrated superradiant masing as a mechanism for long-lived quantum states.

Potential for ultrasensitive magnetometry with ~10 fT/(Hz)^{1/2} sensitivity.

Abstract

Macroscopic quantum phenomena such as lasers, Bose-Einstein condensates, superfluids, and superconductors are of great importance in foundations and applications of quantum mechanics. In particular, quantum superposition of a large number of spins in solids is highly desirable for both quantum information processing and ultrasensitive magnetometry. Spin ensembles in solids, however, have rather short collective coherence time (typically less than microseconds). Here we demonstrate that under realistic conditions it is possible to maintain macroscopic quantum superposition of a large spin ensemble (such as about ~10^{14} nitrogen-vacancy center electron spins in diamond) with an extremely long coherence time ~10^8 sec under readily accessible conditions. The scheme, following the mechanism of superradiant lasers, is based on superradiant masing due to coherent coupling between collective spin excitations (magnons) and microwave cavity photons. The coherence time of the macroscopic quantum superposition is the sum of the magnon life time and the cavity lifetime, further elongated by the total number of coherent magnons and photons, which have macroscopic values when masing occurs. The macroscopic quantum coherence of spin ensembles can be exploited for magnetometry with sensitivity ~10 fT/(Hz)^{1/2}. The long-living collective states of spin ensembles in solids will provide a new platform for studying macroscopic quantum phenomena and are useful for quantum technologies.