Unveiling dark states via two-dimensional magnetic pulse spectroscopy
The study and manipulation of low dipole moment quantum states has been historically difficult due to their inaccessibility by conventional spectroscopic techniques. Controlling the spin in such states requires unfeasibly strong magnetic fields to overcome the typical decoherence rates. However, the advent of terahertz technology and its application to magnetic pulses opens up a new scenario. In this article, we employ an electron-hole pair model to demonstrate that it is possible to control the precession of the spins and to modify the transition rates to different spin states. Enhancing transitions from a bright state to a dark state with different spin means that the latter can be revealed by ordinary spectroscopy. We propose a modification of the standard two-dimensional spectroscopic scheme in which a three laser pulse sequence is encased in a circularly polarised magnetic pulse, whose role is to induce ultrafast coherent transitions between a bright and a dark spin state making the latter susceptible to spectroscopic investigation.