Often called the holy grail of the tech world, a quantum computer uses the seemingly magical principles of quantum mechanics to achieve speeds well beyond today’s machines. At the moment, these counterintuitive contraptions are little more than lab experiments, but eventually, they’ll instantly handle calculations that would take years on today’s machines.
The trick is that whereas the bits of a classical computer can only hold one value at any given time, a quantum bit — or qubit — can hold multiple simultaneous values, thanks to the superposition principle of quantum mechanics.
But if we build a world of quantum computers, we’ll also need a way of transporting quantum data — the multiple values so delicately held in those qubits — from machine to machine. Led by post doctoral researcher Xiuying Chang, the Tsinghua University team seeks to provide such transportation, and though their work is still largely theoretical, they’ve taken an important step in the right direction.
“Their router isn’t practical right now,” says Ari Dyckovsky, a researcher with National Institute of Standards and Technology (NIST) who specializes in quantum entanglement, “but it adds another reason that people should keep researching in this area.”
Yes, there are already ways of moving quantum data between two places. Thanks to quantum entanglement — another mind-bending principle of quantum mechanics — you can move data between two quantum systems without a physical connection between them. And you can send quantum data across a single fiber-optic cable using individual photons.
But for a true quantum internet, you need a way of routing quantum data between disparate networks — i.e., from one fiber-optic cable to another — and at the moment, that’s not completely possible. The problem is that if you look at a qubit, it’s no longer a qubit.
In a classic computer, a transistor stores a single “bit” of information. If the transistor is “on,” for instance, it holds a “1.” If it’s “off,” it holds a “0.” But with quantum computer, information is represented by a system that can an exist in two states at the same time. Thanks to the superposition principle, such a qubit can store a “0″ and “1″ simultaneously. But if you try to read those values, the qubit “decoheres.” It turns into a classical bit capable of storing only one value. To build a viable quantum computer, researchers must work around this problem — and they must solve similar problems in building a quantum internet.
The internet is all about routing data between disparate networks. A router uses a “control signal” to route a “data signal” from network to network. The trouble with a quantum router is that if you read the control signal, you break it. But in a paper recently published to the net, Xiuying Chang and her team describe an experiment in which they build a quantum router — complete with a quantum control signal — using two entangled photons.
“This leads to more freedom to control the route of quantum data,” Luming Duan, who worked on the paper, tells Wired, “and I believe it is a useful device for future quantum internet.”
As described by Technology Review, the team begins the experiment with a photon that exists in two quantum states at the same time: both a horizontal and a vertical polarization. Then they convert this single photon into two entangled protons — which means they’re linked together even though they’re physically separate — and both of these are also in a superposition of two quantum states. One photon serves as the control signal, and it routes the other photon — the data signal.
The rub is that the method isn’t suited to large-scale quantum routing. You can’t expand it beyond the photons. “It is a nice check that coherence is maintained while converting between polarization and path entanglement, which will be an important operation for a large-scale quantum network,” says Steven Olmschenk, an assistant professor of physics and astronomy at Denison University. “But as the authors are careful to point out, the implementation that they have demonstrated cannot be scaled up, and is missing some of the key — and hard — features that will be necessary in a more general implementation.”
In other words, the experiment only transmits one qubit at a time — and the quantum internet needs a bit more bandwidth than that.
But this will come.
(by Cade Metz)