Figuring out how to route information between two different types of quantum nodes has been a significant technical barrier to the quantum net.
Companies like Google and IBM are currently locked in a race to be the first to achieve quantum supremacy – that is, make a quantum computing chip that can outperform the most powerful classical computers – but ,once these devices come online, the next major challenge will be networking it effectively.
So-called "quantum internet" already exist in its infancy. Only a handful of governments, financial and research institutions have had small internal networks for routing information between experimental quantum computers for years. Earlier this year, China successfully routed quantum information through a satellite in low Earth orbit. Quantum internet will offer a number of advantages over its classical predecessor, such as perfectly secure data exchange and far more efficient data processing.
With currently existing quantum networks, information is being routed between the same types of quantum nodes, which transmit, store and process qubits as they are routed through the network. Researchers at the Institute of Photonic Sciences (ICFO) in Spain recently announced that the team manage to transfer information between two different types of quantum nodes in their laboratory, overcoming a significant hurdle on the route to a quantum internet.
Normal computers store information in binary bits - either as a 1 or a 0 – but quantum computers traffic in qubits, which store information as a 1, a 0, or a superposition of both at the same time. This information is generally encoded in a particle of light – called a photon – and in the case of the ICFO experiment, the qubit was encoded in the photon using a technique called time-bin encoding.
It requires a photon to pass through an interferometer which is a device used for superimposing light waves, from which it's guided onto either a short or long path of an optical fibre. When it emerges on the other side, the photon is encoded with a superposition of the time it takes to traverse the long and short paths. According to the IFCO researchers, this type of encoding makes the qubit particularly robust to decoherence, or the destruction of the information contained in the qubit.
Photons are the informational medium par excellence in quantum networks, quantum nodes can be achieved using a variety of different materials, each of which has its own strength. For instance, according to the IFCO researchers, a node made from a cloud of laser-cooled rubidium atoms is an ideal medium for generating qubits and encoding them in photons. Nodes made from a crystal infused with small amounts of praseodymium ions, on the other hand, is better for sorting qubits for "long" periods of time (in the quantum realm, this would be measured in microseconds).
Quantum networks would benefit by being able to use different types of nodes depending on the network's application, but figuring out how to send a photon from one type of node to another has proven difficult for researchers.
"It's like having nodes speaking in two different languages," Nicolas Maring, a research fellow at ICFO, said in a statement. "In order for them to communicate, it is necessary to convert the single photon's properties so it can efficiently transfer all the information between these different nodes."
In their experiment, the IFCO research team used laser-cooled rubidium atoms to generate a qubit encoded in a single photon with a very short wavelength – 780 nanometers. The device then converted this photon into wavelengths used to route the non-quantum communication through optical fibre – 1552 nanometers – and the photon was sent through the optical fibre to an adjacent lab. At this other lab, a device developed by the scientist was used to convert the photon to a 606-nanometer wavelength so that it could be received by a crystal infused with small amounts of praseodymium.
The qubit was able to be sustainable for 2.5 microseconds in the crystal. Which is not a lot, but it was long enough to retrieve the qubit with hardly any information being lost. As IFCO researcher, Hugues de Riedmatten, told an online source, "this type of crystal could possibly be used to store qubits for as long as a minute in the future."
According to the team, the next step is to create larger quantum networks that consist of more than two different nodes, as well as distributing entangled photons between different nodes.
"Finding quantum nodes that can have all the required capabilities (storage, processing, etc) is extremely challenging," Riedmatten said. "The ability to combine different systems, which could have different capabilities could solve the problem, would be a big step towards a quantum internet."