A lab in China is reporting that it has constructed the first memory device that uses single photons to store quantum data. This is a significant breakthrough that takes us further down the path towards a quantum Internet, and potentially quantum computing as well.
As it currently stands, we already make extensive use of photons – the bulk of the Internet and telecommunications backbone consists of photons traveling down fibre optic cables. Rather than single photons, though, these signals consist of carrier light waves of millions of photons, with the wave being modulated by binary data. These pulses are never stored, either; when they reach a router, they’re converted into electrical signals, and then stored in RAM before being converted back into light.
Now, however, Dong-Sheng Ding and fellow researchers at the University of Science and Technology of China have announced that they have generated a single photon, stored it in a “cigar-shaped atomic cloud of rubidium atoms” for 400 nanoseconds, and then released the photon.
The single photon is created using a process called spontaneous four-wave mixing, and the rubidium cloud stores the photon due to electromagnetically induced transparency (EIT).
EIT causes a phenomenon called “slow light,” which is used here to “store” the photon for 400ns (more than long enough to count as computer memory).
The generation and storage would be a big achievement in itself, but there’s more: The rubidium trap also preserves the orbital angular momentum (OAM) of the photon – the image below shows the generation of a single photon (a), and the storage of a single photon with OAM (b). As we’ve noted before, electromagnetic waves (including photons) can have both spin and orbital angular momentum. Spin angular momentum (SAM), which is equivalent to the Earth spinning on its own axis, produces polarisation – and then there’s OAM, which is equivalent to the Earth rotating around the Sun.
Generally, in wireless and wired communications, signals only use SAM and are therefore flat – but by introducing OAM, a signal becomes a 3D helix. You can encode a lot more data into a carrier wave – perhaps an infinite amount – if you play with both the SAM and OAM. By preserving the OAM of the single photon, the Chinese researchers could be onto something very big indeed.
Moving forward, a photonic quantum memory is absolutely vital if we ever want to build a quantum Internet out of quantum routers. Even if we pull back from lofty quantum applications, if we could introduce OAM to the world’s fibre optic networks, the Internet would suddenly get a whole lot faster.
Research paper: arXiv:1305.2675 – “Single-Photon-Level Quantum Image Memory Based on Cold Atomic Ensembles”