Two-color cavity QED
Photons are ideal carriers of information, but they rarely exchange it with one another. So researchers turn to atoms as intermediaries. The conceptually simplest approach is to store one photon’s state in an atom, whose coherent response affects the state of another photon. Researchers can boost the interactions by confining the photon and atom together in an optical cavity—a scheme known as cavity quantum electrodynamics (cavity QED). If the mirror spacing is tuned so that the cavity resonates at the frequency of an electronic transition of the atom, the photon and atom can become strongly coupled (see Physics Today, November 2004, page 25 ). Now, Christoph Hamsen, his PhD adviser Gerhard Rempe, and their colleagues at the Max Planck Institute of Quantum Optics report a cavity QED experiment in which the photons of two light fields at different wavelengths are simultaneously put into resonance with two electronic transitions of the same atom.
To pull off the achievement, the researchers built a resonator that is tunable over centimeters with subpicometer precision. They found a cavity length, around 295 µm, that could simultaneously support two light fields at wavelengths that nearly matched two resonance lines in rubidium—one at 780 nm, the other at 795 nm. To match the resonances precisely, they used the Stark effect to subtly alter the atom’s energy levels.
The two photons, called probe and signal, are sent into the cavity using two laser beams and depicted in the energy-level diagram as initiating transitions from states |1⟩ to |3⟩ and |2⟩ to |4⟩, respectively. To link the photons, the researchers added a “control” field Ωc on the |2⟩ to |3⟩ transition in order to produce electromagnetically induced transparency (EIT) on the |1⟩ to |3⟩ probe transition. (For more on EIT, see Physics Today, November 2011, page 14 ). The transparency is then perturbed by the signal photon on the |2⟩ to |4⟩ transition. That gives rise to an optical switching effect: The presence of a signal photon in the cavity blocks the transmission of a probe photon, and vice versa. The experiment opens new possibilities for quantum nondemolition photon detection, for quantum frequency conversion of photons, and for quantum logic. (C. Hamsen et al., Nat. Phys., 2018, doi:10.1038/s41567-018-0181-1 .)
