According to a proposal, photonic crystals would be used to greatly increase the sensitivities of optical magnetometers that are already regarded as ultrasensitive. The proposal applies, more specifically, to a state-of-the-art type of quantum coherent magnetometer that exploits the electromagnetically-induced-transparency (EIT) method for determining a small change in a magnetic field indirectly via measurement of the shift, induced by that change, in the hyperfine levels of resonant atoms exposed to the field.
One of the key components of a magnetometer of this type is a collection of the aforesaid resonant atoms, which have an energy spectrum that is sensitive to any variation in magnetic field. These atoms are placed in a cell, wherein they are irradiated with light from a quantum source (see figure), such that the interactions between the light and the atoms produce a beam of coherent or entangled photons suitable for use in determining the magnetic field in the cell. If the conditions under which the atoms are exposed are those of EIT, then the shift of Zeeman sublevels of the atoms caused by a change in the magnetic field results in a change in the index of refraction of the region containing the atoms. The change in the index of refraction is measured by means of a Mach-Zehnder optical interferometer. Then the change in the magnetic field can be computed from the known relationship between the magnetic field and the index of refraction.
A photonic crystal is an engineered periodic dielectric structure that can be tailored by design to exhibit one or more of a rich variety of optical properties. Notable among these properties is a range of photon energies, known as the photonic band gap (PBG), in which light cannot propagate. In an optical EIT magnetometer according to the proposal, sensitivity would be increased by using a photonic crystal to control and enhance the interaction between the resonant atoms and the optical beam. A cloud of the resonant atoms would be embedded in a photonic crystal rather than in a uniform dielectric material in a cell as in a state-of-the-art optical EIT magnetometer of prior design. The photonic crystal would be designed so that the photon frequency at the edge of the PBG would closely approximate the atomic transition frequency. In the PBG-edge region, the variation of the index of refraction with a change in the magnetic field would be orders of magnitude greater than in the absence of the photonic crystal.