Fast photon source lights up quantum technologies

Researchers at the University of Sheffield in the UK have built a nanoscale chip that can emit rapid pulses of single, mostly indistinguishable photons. The research team, led by Feng Liu, exploited the physics behind the Purcell effect to design the system, helping them to reduce losses and achieve increased photon production rates.

Physicists have been keen to develop on-chip sources of single photons with indistinguishable quantum states for several applications, such as secure data transmission and photonic quantum technologies. However, previous designs have suffered from high losses of single photons, mainly due to imperfect geometries in the chips. Currently, the most advanced technologies can only efficiently create pulses containing no more than 3 to 5 photons.

To solve the issue, Liu’s team made use of the Purcell effect, which describes how the spontaneous emission rates of quantum systems can be enhanced by their surrounding environments. In their design, the necessary conditions are created by incorporating a quantum dot – just a few atoms of a semiconducting material – into the resonant cavity of a larger photonic crystal. When a rapid laser pulse is fired at the dot, one of its electrons becomes excited and then releases a single photon as it relaxes back into its ground state. The photons created in this process resonate inside the cavity, before being emitted in rapid succession.

Liu and colleagues coupled the quantum dot to a waveguide that funnelled the emitted photons away from the cavity, ensuring they did not interfere with the laser pulse. The technique enabled the cavity to produce one photon every 22.7 picoseconds – around 50 times faster than would be achievable without the Purcell effect. This may not be the fastest photon production rate yet developed but, unlike previous systems, more than 90% of the photons remained indistinguishable from each other on sufficiently long timescales for 20 photons to be emitted.

Using the insights gathered by Liu and colleagues, chips containing rapid single-photon sources could soon be used in a variety of applications. Since a single photon cannot be interfered with without alerting its sender, such chips would be highly desirable for government or security organizations wishing to transmit large amounts of data confidentially. They would also be advantageous in photonic quantum technologies, with applications including boson sampling, and improving the sensitivity of interferometers.

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