Remote sensing—a category broad enough to include both personal medical monitors and space weather forecasting—is poised for a quantum upgrade, much like computing and cryptography before it. A new type of quantum sensor that promises both higher sensitivity and greater security has been proposed and tested in proof-of-concept form. What remains to be seen is how broadly it will be adopted, and whether such quantum enhancements might ultimately make for better medical and space weather tech.
“Our scheme is hybridizing two different quantum technologies,” says Jacob Dunningham, professor of physics at the University of Sussex in the United Kingdom. “It’s combining quantum communications with quantum sensing. So it’s a way of being able to measure something and get the data back in a way that no eavesdropper can hack into or spoof.”
Dunningham and PhD student Sean Moore—now a postdoc at the LIP6 computer science lab in Paris—proposed what they are calling their secure quantum remote sensing (SQRS) system on 14 January in the journal Physical Review A.
The researchers’ simplest SQRS model uses individual photons as the workhorse qubit of the system, although unlike qubits used in, say, quantum computing, none of the qubits here need to be entangled. Their SQRS model also assumes some classical communications on an open channel, between sender and receiver of the qubits. And with these ingredients, the researchers suggest, one could perform high-precision remote measurements whose results are available neither to the person doing the actual measurement nor to any potential eavesdropper who might hack into the communications channels.
Alice and Bob and SQRS
Say that Alice wants a measurement performed remotely. To make this measurement via SQRS, she would need to send individual photons to Bob, who’s located where Alice wants the measurement performed. Bob then performs the measurement, encoding his results onto the phase of the single photons that Alice has sent as part of the process. Bob then messages his encoded measurement results back to Alice via the classical communication channel. Because the method ensures Bob doesn’t know the original states of the photons Alice sent, he can’t extract any meaningful information out of the phase data he sends back to Alice. He may have performed the measurement, but he doesn’t have access to the measurement’s result. Only Alice has that.
Plus, any eavesdropper, Eve, could intercept Alice’s individual photons and classical messages from Bob back to Alice, and she wouldn’t be able to wring meaning from it either. This is because, in part, Bob’s measurement also introduces quantum randomness into the process in ways that Eve cannot plausibly recreate—and Bob could not observe without disturbing the system.
According to Moore, the proposed SQRS protocol addresses the sort of remote measurement situation where Bob is what the researchers call an “honest and curious” observer. “Honest and curious is a certain perspective used in quantum cryptography where we assume that some party does what they’re told, [such as not actively trying to leak data]” Moore says. “But we don’t necessarily want them to gain any information.”
Last month, a team of researchers at Guangxi University in Guangxi, China reported they confirmed the SQRS protocol works, at least at a proof-of-principle level. (The group’s findings, however, have to date only been published on the ArXiv online preprint server and have not yet been peer reviewed.)
According to Wei Kejin, associate professor at Guangxi’s school of physical science and engineering, the group was able to use a weak light source—not even a single-photon generator, but rather a simpler light source that, over time, deals out individual photons only statistically on average.
Such relatively accessible, entanglement-free light sources, Kejin says, “are generally easier to implement, making them more suitable for real-world applications.”
The Guangxi group reports 6 percent of their SQRS system’s remote measurements were erroneous. However, Kejin says that a 6 percent error rate in the setup is less significant than it may at first appear. This is because the statistics improve in the SQRS system’s favor with more photons generated. “Error correction and privacy amplification techniques can be employed to distill a secure key,” Kejin says. “Thus, the technology remains viable for real-world applications, particularly in secure communications where high precision and reliability are paramount.”
Next Steps for SQRS—and Its Applications
According to Jaewoo Joo, senior lecturer in the school of mathematics and physics at the University of Portsmouth in the U.K., who’s unaffiliated with the research, one practical SQRS application could involve high-precision, quantum radar. The enhanced quantum-level accuracy of the radar measurements would be one attraction, Joo says, but also no adversary or interloper could hack into the radar’s observations, he adds. Or, Joo says, medical monitors at a patient’s home or at a remote clinic could be used by doctors centrally located in a hospital, for instance, and the data sent back to the hospital would be secure and free from tampering or hacking.
To realize the kinds of scenarios Joo describes would very likely involve whole networks of SQRS systems, not just the most basic SQRS setup, with one Alice and one Bob. Dunningham and Moore describe that simple, foundational model of SQRS in a paper published two years ago. It was the basic, foundational SQRS setup, in fact, that the Guanxi group has been working to experimentally test.
The more complex, networked SQRS system that’s likely to be needed is what’s described in January’s Physical Review A paper. The networked SQRS system involves Alice along with multiple “Bobs”—each of which operates their own individual sensor, on which each Bob performs similar kinds of measurements as in the basic SQRS protocol. The key difference between basic SQRS and networked SQRS is in the latter system, some of the qubits in the system do need to be entangled.
Introducing networks of sensors and entangled qubits, Dunningham and Moore find, can further enhance the accuracy and security of the system.
Dunningham says quantum effects would also amplify the accuracy of the overall system, with a boost that’s proportional to the square root of the number of sensors in the network. “So if you had 100 sensors, you get a factor of 10 improvement,” he says. “And those sort of factors are huge in metrology. People get excited about a few percent. So the advantages are potentially very big.”
Envisioning a networked SQRS system, for instance, Dunningham describes enhanced atomic clocks in orbit providing ultra-high-precision timekeeping with high-security quantum protections ensuring no hacking or spoofing.
“You can get a big, precision-measurement advantage as well as maintaining the security,” he says.
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