In a world’s first, researchers in France and the U.S. have performed a pioneering experiment demonstrating “hybrid” quantum networking. The approach, which unites two distinct methods of encoding information in particles of light called photons, could eventually allow for more capable and robust communications and computing.

Similar to how classical electronics can represent information as digital or analog signals, quantum systems can encode information as either discrete variables (DVs) in particles or continuous variables (CVs) in waves. Researchers have historically used one approach or the other—but not both—in any given system.

“DV and CV encoding have distinct advantages and drawbacks,” says Hugues de Riedmatten of the Institute of Photonic Sciences in Barcelona, who was not a part of the research. CV systems encode information in the varying intensity, or phasing, of light waves. They tend to be more efficient than DV approaches but are also more delicate, exhibiting stronger sensitivity to signal losses. Systems using DVs, which transmit information by the counting of photons, are harder to pair with conventional information technologies than CV techniques. They are also less error-prone and more fault-tolerant, however. Combining the two, de Riedmatten says, could offer “the best of both worlds.”

Spooky Systems

In quantum networks, information is created, stored and transferred based on the tenets of quantum mechanics. Doing so theoretically allows for levels of security and computational power that surpass anything possible with classical systems.

For instance, classical bits encode information in values of either 0 or 1. Quantum networks can instead use quantum bits, or qubits, which exploit quantum effects to embody 0 and 1 at the same time. To distribute data, such networks also often rely on another effect called quantum entanglement. Famously described by Albert Einstein as “spooky action at a distance,” entanglement is generated between particles, such as photons, after they interact closely. Einstein and others considered it “spooky” because, against all intuition, even after being separated over arbitrarily long distances, entangled particles continue to influence each other’s behavior. Any change in the state of one of the particles triggers a simultaneous change in the state of the other. Computer scientists long ago realized this effect could enable ultrasecure telecommunications, in which any attempt at eavesdropping would disrupt the entanglement, making the surveillance transparently obvious.

Systems leveraging these quantum effects can take many forms, but they generally follow either a DV or CV architecture. Now scientists at the Kastler Brossel Laboratory in Paris and the U.S. National Institute of Standards and Technology have successfully united both techniques by establishing and distributing entanglement between DV- and CV-encoded states of light within a single quantum network.

Using a complicated assembly of optical components, the team successfully produced photons in two highly entangled states. One of them arose from splitting a single photon between two different paths. The other—a so-called hybrid-entangled state—emerged from entangling a DV optical qubit with a CV qubit, which was held in a superposition of two different phases of light. “By using a special procedure called Bell-state measurement between these two separately entangled states, the entanglement was transferred or ‘teleported’ to the two systems, [which] never interacted with each other,” says Julien Laurat, a professor at Sorbonne University in Paris and senior author of the study. This transference allowed the conversion of the qubits’ quantum information from one encoding method to the other, paving the way for incorporating both DV and CV approaches into a single, scalable quantum network.

From Workbench to Workhorse

For Marco Bellini of the National Institute of Optics in Italy, who was not part of the study, what makes it novel and significant is that the researchers successfully swapped entanglement between two light beams carrying two distinct varieties of encoded quantum information. Linking disparate systems together remains a major challenge. But “this experiment has demonstrated what could become an important ingredient of future networks versatile enough to connect memories and processors based on different physical quantum platforms—and faithfully carry a broad range of quantum states, including the DV and CV ones,” he says.

Much more work remains to be done before a practical hybrid quantum network is achieved, however, Bellini adds. The current experimental method is extremely inefficient: on average, it generates hybrid entanglement just three times per minute across a distance between a CV qubit and a DV one. “While this rate is still sufficient to accumulate enough data for a proof-of-principle demonstration, it is orders of magnitude too low for any practical application,” Bellini concludes.

Further breakthroughs may be imminent. Around the world, other groups are racing to develop and demonstrate additional new quantum-networking protocols—and to close the gap between such preliminary laboratory demonstrations and practical real-world devices.

One such team, led by Bellini, is also working on using the hybrid technique to manipulate entanglement by adding and subtracting single photons to and from classical light fields. Groups in Japan, Russia, Denmark and the Czech Republic are also researching the optical hybrid approach for quantum information. Sooner or later, such hybrid-entanglement experiments should become more compact and efficient, breaking free of the workbench to become workhorses that are compatible with telecoms’ existing fiber-optic networks.