Scientists have succeeded for the first time in entangling two separate qubits by connecting them via a cable, in a breakthrough that will likely accelerate the creation of quantum networks – which, by combining the capabilities of several quantum devices, could boost the potential of the technology even in its current limited state.
The researchers, from the University of Chicago's Cleland Lab, created two quantum nodes, themselves containing three superconducting qubits each. Using a one-meter-long superconducting cable to connect the nodes, the scientists then chose one qubit in each node and entangled them together by sending so-called "entangled quantum states" through the cable.
Taking the form of microwave photons, these entangled quantum states are extremely fragile, which makes the process particularly challenging; but the researchers nevertheless managed to transfer the entanglement from one node to the other, linking the qubits into a special quantum state that is still both fascinating and confounding to quantum scientists.
Qubits, or quantum bits, are the basic unit of quantum information, and their properties can be exploited to create next-generation quantum technologies; one of those properties is entanglement. Entanglement happens when two qubits are made to interact in a certain way, and they become inexplicably linked. Once entangled, they start sharing the same properties, no matter how distant they are from each other.
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This means that by looking at one half of an entangled pair, scientists can know the properties of the other particle, even if they are thousands of kilometers away. Using entanglement, scientists could create webs of linked qubits, which could in turn help make quantum computing more powerful, as well as lay the groundwork for future quantum communication networks.
"Developing methods that allow us to transfer entangled states will be essential to scaling quantum computing," said Andrew Cleland, professor at the University of Chicago, who led the research.
For entanglement to be useful, it has to be established in the first place – something that is easier said than done. Within the Cleland Lab scientists' two-node experimental set-up, entanglement was transferred from node to cable to node in only a few tens of nanoseconds. With a nanosecond representing just one billionth of a second, the achievement was widely hailed as a successful one.
Quantum scientists around the world are actively working on different ways to establish entanglement between two qubits, but the most common procedure so far has consisted of creating a pair of entangled particles, and then distributing them between two points.
For example, once they are entangled, qubits can travel through networks of optical fiber. Last year, in fact, another group of researchers from the University of Chicago used an existing underground network of optical fiber to support entangled photons travelling across a 52-mile network in the city's suburbs.
Another method consists of using satellites as a source of entangled photons, which allows the particles to travel over much longer distances. China is leading in this space: in 2017, the country's satellite Micius successfully delivered entangled particles to ground stations up to 1,200 kilometers away.
Transferring entanglement from one qubit to another one located in another quantum node, however, is an unprecedented experiment. It doesn't stop here: once the Cleland Lab researchers used the cable to entangle two qubits in each of the two nodes, they then managed to extend this entanglement to the other qubits in each node.
In other words, Cleland and his team "amplified" the entanglement of qubits, until all six qubits in the two nodes were entangled in a single globally entangled state. The next challenge? To expand the system to three nodes, to build three-way entanglement.
By building up this small-scale network of entangled particles, the scientists are getting closer to establishing a quantum network that could have big implications for quantum computing. Entanglement could effectively be used to create quantum clusters, made up of linked qubits located in different quantum devices.
Much like supercomputers today carry out parallel calculations on many CPUs connected to one another, it is widely expected that in the future, quantum computing will be enabled by many different modules of such entangled qubits, all connected to each other to run a computation. "These modules will need to send complex quantum states to each other, and this is a big step towards that," said Cleland.
The quantum computers currently developed by tech giants the likes of IBM and Google can only support less than 100 qubits – nowhere near enough for the technology to start having a real-world impact. The companies are confident that quantum computers will scale up sooner rather than later; but a quantum network could, in principle, start showing results before a fully-fledged quantum computer sees the light of day.
In effect, by linking together quantum devices that, as they stand, have limited capabilities, scientists expect that they could create a quantum supercomputer more powerful than a quantum device operating on its own.
In addition to advancing quantum computing, a network of interlinked qubits could also enable new applications in the realm of quantum communications. The US and Chinese governments, as well as the EU, have all shown a marked interest in developing a quantum internet in recent years, which will rely on entanglement to exchange quantum information between quantum devices. One of the key applications of such a quantum network would be quantum key distribution – an un-hackable cryptography protocol that, once more, relies on inter-linked quantum particles.