Optical-domain spectral super-resolution enabled by a quantum memory

Super-resolution methods of optical imaging hold a solid place as an application in biological and chemical sciences, but many new developments allow the beating of diffraction limit in better and more subtle ways by fully exploiting spatial information already present in the optical field. By analogy, full spectral information of the optical field leads to a super-resolution spectroscopy, in which we can detect frequency separation of two emitters with precision surpassing the Fourier limit. We employ an optical quantum memory with embedded time-frequency processing capabilities to implement a time-inversion interferometer for input light, thus projecting the optical field in the symmetric–antisymmetric mode basis. This is accomplished by engineering a frequency-dependent dispersion combined with time-dependent temporal phase modulation that allows us to split, rotate and interfere the signal pulses in the chronocyclic space. Analysis based on quantum metrology shows the advantage of our technique over both conventional spectroscopy as well as heterodyne measurements. Moreover, our work not only establishes a new super-resolution spectroscopy method but also provides an exceptionally good spectral resolution, not seen even in Fourier spectrometers.