A new quantum computing study claims that a recent finding in the production, storage and retrieval of "quantum data" has brought us one step closer to the quantum internet.
Currently, quantum information is unstable over long distances and quantum bits, or qubits — the carriers of quantum information — are easily lost or fragmented during transmission.
Classical computer bits are transmitted today as pulses of light through fiber optic cables using devices called "repeaters" to amplify signals across the length of the network. To transmit qubits over longer distances the way classical computer bits are transmitted today we need similar devices that can store and retransmit quantum states across the whole network, ensuring signal fidelity no matter how far the data has to go.
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These quantum memory devices could receive, store and retransmit qubit states. The new study, conducted at Imperial College London, the University of Southampton, and the Universities of Stuttgart and Wurzburg in Germany, claims to have achieved this using standard fiber optic cables for the first time. The findings were published April 12 in the journal Scientific Advances.
All in the photon source
The researchers stored and retrieved photons — one of the potential carriers of quantum information — using a new and potentially much more efficient method.
"There are two main types of single photon sources,a process called non-linear optical frequency conversion and those based on single emitters," Sarah Thomas, professor of physics at Imperial College, London, told Live Science. "It's been demonstrated many times before that we can store photons from nonlinear optics in a quantum memory because you can engineer the source and memory to match. We used a particular single emitter called a quantum dot, which is a nanocrystal of semiconductors."
Thomas said that using nonlinear optics is less reliable — a pair of usable photons isn't produced every time, whereas a single emitter quantum dot produces them at a higher rate.
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The next challenge is that the efficiency of the interface between quantum memory devices depends on matching both the wavelength and bandwidth. Discrepancies here make storage and retrieval too inefficient, but the study finally bridged the gap.
"We did it by using a high-bandwidth, low-noise quantum memory, fabricating the photon source at a very specific wavelength to match our quantum memory," Thomas said. "We were also able to do it at a wavelength where the loss in optical fiber is the lowest, which will be key in the future for building quantum networks."
Building on past work
But this is not the only recent advance in quantum computing and the quantum internet. In February, Live Science reported on a related breakthrough at Stony Brook University.
Quantum network models are more stable at extremely low temperatures, which limits their real-world applications, but the study achieved a stable connection at room temperature, which puts it within reach of real-world use.
The Imperial study builds on that success thanks to the aligned wavelengths between transmitter and receiver.
"The Stony Brook study used photons at 795 nm [nanometers] and showed interference of two photons after storage and retrieval," Mark Saffman, chief scientist for quantum information at quantum-enabled products company Infleqtion told Live Science. "The Imperial study used a photon at 1529 nm (which is the standard telecom wavelength) and stored and retrieved it, but didn't show interference. The storage and retrieval of telecom wavelength is important for low-loss fiber transmission. Both studies advance different aspects of what's needed for a quantum network."
Michael Hasse, a cybersecurity expert (one of the areas where quantum networks will have the most impact) told Live Science that the Imperial study describes a method whereas the earlier study describes a mechanism necessary for that method to work.
"The Imperial work is about a means of establishing long-distance communication using repeaters," he said. "Quantum entanglement allows communications to be far apart in theory, but in reality it's easier when they're closer together. The Stony Brook study refers to the storage of quantum information at room temperature, which is necessary for cost-effective implementation of repeaters."
