# Synopsis: A Traffic Light for Photons

#### Photon routing in cavity QED: Beyond the fundamental limit of photon blockade

Serge Rosenblum, Scott Parkins, and Barak Dayan

Published September 29, 2011

Efforts to implement quantum information processing with photonic qubits have focused on techniques for storing and gating individual photons. One possibility is to borrow a page from the work of researchers looking at control of electrons. In a phenomenon called Coulomb blockade, an electron tunnels through a barrier into a small capacitance device; the resulting voltage increase prevents another electron from tunneling in. For quantum information processing, the goal is to have a similar method for photon control and routing. Writing in Physical Review A, Serge Rosenblum at the Weizmann Institute of Science, Israel, and co-workers theoretically analyze the complications of doing so and suggest a way forward.

Recent demonstrations of photon blockade have relied on a two-level system such as an atom or a quantum dot coupled to a small resonant cavity: entry of one photon alters the cavity properties enough to prevent a second photon from entering. Rosenblum et al. find, however, that photon blockade, or in fact any effect based on a two-level system, has inherent limitations for photon routing. The reason is that a two-photon pulse short enough to transport both photons within the memory time of the system would have a bandwidth that is too large to establish a strong enough interaction. Instead, the authors propose and extensively analyze the use of a three-level system, which avoids these problems. Such a system combined with a modified cavity arrangement could provide a robust, efficient, and controllable method for controlling the traffic of photons. – David Voss