Narrow-Linewidth and High-Generation-Rate Flying Qubits

Flying qubits are single photons, and quantum information is essentially the wave function in quantum mechanics. Since photons possess ultimate speed, hardly interact with the environment, and never collide with each other, they are the ideal carriers of quantum information. Due to single photons’ nature of randomness in time, biphotons, i.e. pairs of single photons, are more practical in quantum processes. As one photon triggers or starts an operation of a quantum process or device, the other will be employed in the operation and thus is called the heralded single photon. Here, we produced the biphotons or heralded single photons with a linewidth of 960 kHz and at a generation rate of 3.7×105 pairs/s from a hot atomic medium. The high generation rate together with narrow linewidth results in a spectral brightness of 3.8×105 pairs/s/MHz, which is the best result to date of all kinds of biphoton sources. A biphoton source of a larger generation rate can produce carriers faster to enable a higher bandwidth of quantum information transmission. A quantum operation using biphotons of a narrower linewidth can achieve a better success probability. Thus, the spectral brightness or the generation rate per linewidth is the measure of success rate in quantum communication. The spectral brightness achieved in this work is close to the ultimate limit, an important milestone in quantum communication.