Chinese scientists have transmitted the quantum version of computer memory between two entangled clouds of atoms over 50 kilometers. This achievement makes the idea of a super-fast, super-secure quantum internet a much more plausible one.
The feat, which was achieved using standard fiber optic cables, breaks through the 1.3-kilometer barrier achieved by previous quantum memory experiments. Entanglement — dubbed “spooky action at a distance” by Einstein — is a cornerstone of quantum physics.
Quantum communication relies on quantum entanglement, or what Einstein called ‘spooky action at a distance’: where two particles become inextricably linked and reliant on each other, even if they’re not in the same place.
While quantum computers can do interesting things without dedicated memory, a memory would provide a lot of flexibility in terms of the sorts of algorithms they could run and how quantum systems can interact with each other and the outside world. Building quantum memory is extremely challenging, as reading to and writing from it both have to be extremely efficient and accurate, and the memory has to do something very atypical of quantum systems: hold on to its state for an appreciable length of time.
If we solve the problems, however, quantum memory offers some rather unusual properties. The process of writing to quantum memory is very similar to the process for quantum teleportation, meaning the memory can potentially be transmitted between different computing facilities. And since the storage device is a quantum object, there’s the possibility that two qubits of memory in different locations can be entangled, essentially de-localizing the qubit’s value and spreading it between two facilities.
Individual photons have been entangled across distances exceeding 1000 kilometers, but for larger systems of particles, which hold more information, maintaining this entanglement is harder. The maximum distance between a pair of entangled quantum memories so far is just 1.3 kilometers.
The team of scientists demonstrated entanglement of two atomic ensembles in one laboratory via photon transmission through city-scale optical fibers. The atomic ensembles function as quantum memories that store quantum states. They used cavity enhancement to create atom-photon entanglement efficiently and then used quantum frequency conversion to shift the atomic wavelength to telecommunications wavelengths.
The main significance of this paper lies in extending the entangling distance in [optical] fiber between quantum memories to the city scale,” said Jian-Wei Pan, of the University of Science and Technology of China (USTC),
The team’s work is an “absolutely formidable achievement,” commented Ben Buchler, a physicist at the Australian National University.
As far as communicating that data goes, quantum technology promises to improve transmission speeds and secure the data transfers using the laws of physics themselves – provided we can get it working reliably over long distances.