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Repeater Boosts Long-Range Quantum Entanglement

Quantum repeater

Researchers in Austria and France have shown how a quantum repeater made from two ions of calcium can improve the distribution of entanglement by dividing a single fiber link into two less-lossy halves. [Image: Universit├Ąt Innsbruck/H. Ritsch]

Physicists in Austria and France have demonstrated a key component for future quantum networks—a quantum repeater node capable of joining a pair of separately entangled links over long distances (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.130.213601). Using a memory made from two calcium ions trapped inside an optical cavity, the researchers showed that two 25-km-long fiber optic links spliced together would outperform a single 50-km stretch of fiber. They reckon that modest improvements could see the technology spanning distances of up to 800 km.

Quantum memory

The ability to send quantum information across long-distance networks could potentially enable a range of applications not possible with classical technology, such as unhackable cryptography, distributed quantum computing and large-scale quantum sensing. However, quantum bits (qubits) cannot be copied like classical information can, which prevents amplification used to overcome loss in optical fibers. This, in turn, limits the length of fiber along which data can be sent at useful rates.

Quantum repeaters would overcome this problem through an iterative process. Particles would first be entangled over segments of fiber each perhaps a few tens of kilometers long—beyond which loss becomes excessive. Neighboring segments would then be combined by “swapping” the individual entanglements for a joint one, thanks to a quantum measurement made at the segments’ junction via a repeater. This double-length segment could itself then be joined to another newly created link and so on, enabling quite long distances to be spanned with a fairly limited number of swaps.

Scientists are currently developing memory qubits made from a range of quantum objects, including electrons trapped in diamond crystal defects, ensembles of atoms, single atoms and single ions.

There is a catch, however. Fiber loss and other noise mean that there is only a finite chance that any given attempt at entanglement is successful. Entangled states must therefore be saved for some minimum amount of time—long enough that all the other segments involved become entangled, allowing a single entangled link to be created. That calls for quantum memory.

Scientists are currently developing memory qubits made from a range of quantum objects, including electrons trapped in diamond crystal defects, ensembles of atoms, single atoms and single ions. All have their strengths and weaknesses, with single ions yielding clean entangled states and emitting photons that easily convert to telecom wavelengths—an essential feature for minimizing loss. But extracting photons from ions is a tricky business, in part because ions are strongly affected by charge noise (which makes them hard to integrate into small optical cavities).

Quantum repeater node

In the latest work, Ben Lanyon of the University of Innsbruck, Austria, and colleagues at Innsbruck, the Austrian Academy of Sciences and Paris-Saclay University, France, have demonstrated the basis for a working quantum repeater node based on two ions of calcium-40. They were able to boost the ions’ photon collection efficiency by using electric fields to hold them at specific positions within a low-loss optical cavity and employing lasers with carefully chosen frequencies to manipulate the ions’ quantum states.

Lanyon and colleagues executed their protocol 44,720 times over the course of 33 minutes, making more than 2 million attempts to entangle the end nodes and succeeding just over 2,000 times—a success rate of about 1 in 1,000.

Operating the repeater involves exciting the ions with a blue laser and collecting the photon that each ion emits when it drops to a lower energy state. Those photons, entangled with their respective ions, are converted to the telecom wavelength (1550 nm) and then sent in opposite directions to measuring nodes at the ends of two sections of 25-km-long (spooled) optical fiber. Only when messages return from the nodes, signalling that both photons have been detected, is the final stage of the repeater protocol implemented—a Bell-state measurement on the ions, which swaps the entanglement from the ion-photon pairs to the two photons.

Lanyon and colleagues executed their protocol 44,720 times over the course of 33 minutes, making more than 2 million attempts to entangle the end nodes and succeeding just over 2,000 times—a success rate of about 1 in 1,000. By measuring the photon states at the end nodes, they were able to establish how close on average the quantum links were to ideal entangled states—registering a “fidelity” of 0.72, well above the 0.5 that proves the existence of entanglement.

To establish whether setting up the repeater node was worth all the bother, the researchers also measured the performance of a single 50-km-long link by joining the spools of fiber and sending both ion-entangled photons to a single end node. The outcome was positive—the rate of successful entanglements using the single, longer link was just below 7 Hz. With the repeater dividing up the channel into two shorter, less-lossy halves, the success rate was instead a little over 9 Hz.

“Not an absolute vindication”

Lanyon says that the result is not an absolute vindication of repeaters, since some future technology could always enable quicker point-to-point transmission. But he points out that repeaters themselves introduce loss into the system, arguing that the demonstration of a net advantage despite this loss is “highly nontrivial.”

“The indistinguishability of our photons certainly needs to be improved, and it’s probably the most challenging part of building such a quantum repeater,” Lanyon says.

The researchers also worked out how much their repeater’s performance would likely have to improve to hook up multiple such devices to span 800 km—the kind of distance needed for continent-wide quantum networks. They concluded that the fidelities relevant to a single repeater would only need small improvements, while the fidelity of links between photonic qubits from different repeaters would require a more substantial boost. Lanyon explains that this would involve making the photons more identical to one another, which, he adds, could be achieved by shrinking the cavity such that ions couple to it more effectively. “The indistinguishability of our photons certainly needs to be improved, and it’s probably the most challenging part of building such a quantum repeater,” he says.

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