The dawn of the quantum internet has begun: A group of scientists successfully built a fully functional prototype of such a network, which uses diamonds as a foundation and calls on paradoxical quantum concepts that were once restricted to the pages of a textbook.
Described by a paper published April 15 in Science, the mechanism would most feasibly serve as a means to enhance quantum computing. Like the normal internet, it could connect servers to expand space — offering something like the cloud — and increase computing power. However, in contrast to the normal internet, it obeys the laws of quantum physics, namely superposition and entanglement.
"We are the first to make entanglement across three nodes, essentially, in this ready-to-be-used fashion," Matteo Pompili, a Ph.D. student at Delft University of Technology in the Netherlands and lead author of the study, told The Academic Times. "The big advantage that we did is that once we establish entanglement, we basically get a flat signal that tells us, 'Now entanglement is done; you can use it for something else,' which is necessary if you want to use [the system] for later."
But for entanglement to be established, superposition is required.
Superposition can be thought of by considering a standard playing card. Suppose the card is face down. Before it's turned over, in the observer's mind, it could be hearts, spades or another suit. In that moment, for the observer, the possibility of each different suit exists at the same time; the card is in superposition.
Modifying the case to include two cards face down offers insight into entanglement. If the observer is told that if card A is a spade then card B is not a spade, these cards are interdependent. Each card's superposition — before it's turned over — is entangled in the other one. Even if the observer checks only card A, the spade-bearing possibility of card B is affected, and so, its superposition is interrupted.
These examples appear extremely abstract, and perhaps even subjective, as they depend on the mind of the observer. But that's because cards aren't quantum particles. In the quantum domain, these things really do happen.
Except instead of suits, they revolve around spin. A quantum particle, namely an electron, has either a spin up or spin down, but both spins exist simultaneously. That is, until one tries to check, or turns the card over. Applied to quantum computing, superposition determines the state of a bit, which stores computer language.
Normal bits have a state of either 0 or 1, and when several bits are put together, a sequence forms. That sequence is kind of like a letter in computer language. Intuitively, if a bit is made out of quantum material, it can be in superposition, both 0 and 1 at the same time — until someone checks. These are called qubits, and when spread across systems via the quantum internet, they're multiplied in capacity.
"A 200-qubit quantum computer is not just twice as powerful as [if there were] two 100 qubit-quantum computers — it's exponentially powerful," Pompili said.
That means the baffling implications — the reason why these qubits matter — of quantum computers can happen quicker. For instance, quantum computers can test unimaginable amounts of combinations at once, due to the nature of superposition. That's speculated to be helpful in making medication, because every potential molecule wouldn't need to be separately tested.
"There is a lot of interest in simulating complex molecules or scaling up complex proteins … for example, in drug design," Pompili explained. "By connecting two computers, you can distribute your computation."
The researchers behind the study found a way to transfer these qubits between nodes, which can be thought of as the prototype's placeholders for quantum computers. The nodes are named Alice, Bob and Charlie. Each one includes a diamond, because in diamonds, there are defects.
"An electron [gets trapped] around this defect," Pompili said. "We can address this single electron or the spin of this electron via lasers and via microwave pulses … basically, with this, we can access this spin of this quantum system and we can use this as a qubit."
A qubit of Alice links with a qubit of Bob via a connection, making the two entangled. But to communicate whatever data those qubits hold, an issue is that — like in the card analogy — once the system is touched, it's interrupted and the superposition of information is sort of lost.
To get around this, the team found a way to "entanglement swap" between nodes. That's where Charlie comes in.
"We can move the entanglement from the communication qubit of Bob to an additional qubit that we have on the same diamond in Bob," Pompili said. "This is given by a surrounding nuclear spin that basically is affected by the spin of the electron of this defect."
After the second qubit of Bob takes the original qubit's data, it leaves the original qubit ready to entangle again, even though it retained its original information. That will happen with the qubit of Charlie, effectively communicating quantum information from Alice to Charlie, with Bob as the mediator.
A timely application of the system is with quantum cryptography, to increase privacy online. If two users are sharing photos or sensitive information, but the qubits in both devices are entangled, any potential hacker would disrupt the entanglement — turn the card over — and both parties would know, instantly.
But perhaps most important, Pompili explains, is the exciting thought that if quantum internet becomes available to the public, the possibilities are boundless.
"Once you make a technology available to people that are not in the field, you start to have out-of-the-box thinking, like what happened with the internet," he said. "The nicest apps don't come from the people that have designed the internet, but come from the people that can use the internet to do things … have different backgrounds, and so can can design things that we have no idea about."
The study, "Realization of a multinode quantum network of remote solid-state qubits," published April 15 in Science, was authored by M. Pompili, S. L. N. Hermans, S. Baier, H. K. C. Beukers, P. C. Humphreys, R. N. Schouten, R. F. L. Vermeulen, M. J. Tiggelman, L. dos Santos Martins, B. Dirkse, S. Wehner and R. Hanson, Delft University of Technology.