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