Light in an optical fiber is a stream of photons, and quantum communication requires single photons that can be sent one at a time. But making them emerge quickly while remaining indistinguishable from one another is a tough challenge. In the new work, physicists took a semiconductor quantum dot, a tiny crystal that acts as a trap for electrons, and placed it in a microresonator. The resonator amplifies the emission and helps channel the photons in the desired direction.
By driving the dot with laser pulses, the authors achieved single-photon emission at a clock rate of 2.5 GHz: that is, 2.5 billion times per second. Moreover, the photons are nearly indistinguishable: the interference visibility exceeded 85%, and the probability of emitting two photons at once instead of one was below 4%. This is close to the theoretical limit for such a system.
This photon source operates in the telecom C-band (around 1550 nm), where losses in optical fiber are minimal. That means it can be integrated into existing communication lines. High speed and photon indistinguishability are needed for quantum protocols where information is encoded in the states of individual light particles. The result brings us closer to quantum networks that can transmit data over long distances without compromising security.
A rate of 2.5 GHz means the gap between photons is just 0.4 nanoseconds. In that time, light travels only 12 centimeters.