Quantum Physics

Keeping secrets is hard. Kids know it. Celebrities know it. National security experts know it, too.

And it’s about to get even harder.

There’s always someone who wants to get at the juicy details we’d rather keep hidden. Yet at every moment, untold volumes of private information are zipping along internet cables and optical fibers. That information’s privacy relies on encryption, a way to mathematically scramble data to prevent any snoops from deciphering it — even with the help of powerful computers.

But the mathematical basis of these techniques is under threat from a foe that has, until recently, seemed hypothetical: quantum computers.

In the 1990s, scientists realized that these computers could exploit the weird physics of the minuscule realm of atoms and electrons to perform certain types of calculations out of reach for standard computers. That means that once the quantum machines are powerful enough, they could crack the mathematical padlocks on encrypted data, laying bare the world’s secrets.

Today’s quantum computers are far too puny to defeat current security measures. But with more powerful quantum machines being regularly rolled out by the likes of IBM and Google, scientists, governments and others are beginning to take action. Experts are spreading the word that it’s time to prepare for a milestone some are calling Y2Q. That’s the year that quantum computers will gain the ability to crack the encoding schemes that keep electronic communications secure.

“If that encryption is ever broken,” says mathematician Michele Mosca, “it would be a systemic catastrophe.”

Y2Q is coming. What does it mean?

Encryption pervades digital life — safeguarding emails, financial and medical data, online shopping transactions and more. Encryption is also woven into a plethora of physical devices that transmit information, from cars to robot vacuums to baby monitors. Encryption even secures infrastructure such as power grids. The tools Y2Q threatens are everywhere. “The stakes are just astronomically high,” says Mosca, of the University of Waterloo in Canada, who is also CEO of the cybersecurity company evolutionQ.

The name Y2Q alludes to the infamous Y2K bug, which threatened to create computer havoc in the year 2000 because software typically used only two digits to mark the year (SN: 1/2/99, p. 4). Y2Q is a similarly systemic issue, but in many ways, it’s not a fair comparison. The fix for Y2Q is much more complex than changing how dates are represented, and computers are now even more inextricably entwined into society than two decades ago. Plus, no one knows when Y2Q will arrive.

Confronted with the Y2Q threat, cryptography — the study and the practice of techniques used to encode information — is facing an overhaul. Scientists and mathematicians are now working urgently to prepare for that unknown date by devising new ways of encrypting data that won’t be susceptible to quantum decoding. An effort headed by the U.S. National Institute of Standards and Technology, or NIST, aims to release new standards for such post-quantum cryptography algorithms next year.

Meanwhile, a longer-term effort takes a can’t-beat-’em-join-’em approach: using quantum technology to build a more secure, quantum internet. Scientists around the world are building networks that shuttle quantum information back and forth between cities, chasing the dream of communication that theoretically could be immune to hacking.

How public-key cryptography works

If you want to share a secret message with someone, you can encrypt it, garbling the information in such a way that it’s possible to decode it later.

Schoolkids might do this with a simple cipher: For example, replace the letter A with the number 1, B with 2 and so on. Anyone who knows this secret key used to encrypt the message can later decode the message and read it — whether it’s the intended recipient or another sneaky classmate.

It’s a simplified example of what’s called symmetric-key cryptography: The same key is used to encode and decode a message. In a more serious communication, the key would be much more complex — essentially impossible for anyone to guess. But in both cases, the same secret key is used to encode and decode.

This strategy was used in cryptography for millennia, says computer scientist Peter Schwabe of the Max Planck Institute for Security and Privacy in Bochum, Germany. “It was either used in a military context or it was used between lovers that were not supposed to love each other.”

But in the globally connected modern world, symmetric-key cryptography has a problem. How do you get the secret key to someone on the other side of the planet, someone you’ve never met, without anyone else getting their hands on it?

To solve this quandary, in the 1970s cryptographers devised public-key cryptography, which uses special mathematical tricks to solve the symmetric-key conundrum. It uses two different, mathematically related keys. A public key is used to encrypt messages, and a mathematically related private key decodes them. Say Alice wants to send a message to Bob. She looks up his public key and uses it to scramble her communication. Only Bob, with his private key, can decode it. To any snoops that intercept the message, it’s meaningless.

Public-key techniques are also used to create digital signatures. These signatures verify that someone online really is who they say they are, so you know you’re really downloading that new app from Apple, not some nefarious impersonator. Only the owner of a private key can sign the message, but anyone can use the public key to verify its authenticity.

The public-key cryptography that permeates the internet is directly vulnerable to full-scale quantum computers. What’s more, symmetric-key cryptography often relies on public-key cryptography to share the secret key needed to communicate. That puts the majority of internet security under threat.

Why quantum computers will threaten public-key cryptography

If public-key encryption keeps your data hidden away under the floorboards, then to read that information, you need to build a way in. You have to be able to access the data with your private key. “There’s got to be a secret door somewhere in there, where if I knock the right way, it opens up,” Mosca says.

Constructing such a trapdoor demands special mathematical tactics, based on operations that are easy to perform in one direction but hard in the opposite direction. Multiplying two prime numbers together is quick work for a computer, even if the numbers are very large. But it’s much more time-consuming for a computer to calculate the primes from their product. For large enough numbers, it’s impossible to do in a practical amount of time with a standard computer.

The challenge of finding the prime factors of a large number is behind one of the main types of public-key encryption used today, known as RSA. A hacker using a classical computer wouldn’t be able to deduce the private key from the public key. Another math problem, known as the discrete logarithm problem, is a similar one-way street.

These two mathematical problems underlie nearly all of the public-key cryptography in use today. But a sufficiently powerful quantum computer would blow their trapdoors wide open. “All of those public-key algorithms are vulnerable to an attack that can only be carried out by a quantum computer,” says mathematician Angela Robinson of NIST, in Gaithersburg, Md. “Our whole digital world is relying on quantum-vulnerable algorithms.”

This vulnerability came to light in 1994, when mathematician Peter Shor, now at MIT, came up with an algorithm that would allow quantum computers to solve both of these math problems. In quantum machines, the bits, called qubits, can take on values of 0 and 1 simultaneously, a state known as a superposition. And qubits can be linked with one another through the quantum connection called entanglement, enabling new tactics like Shor’s (SN: 7/8/17 & 7/22/17, p. 34).

“Back then, that was an interesting theoretical paper. Quantum computers were a distant dream,” says mathematician Dustin Moody of NIST, “but it wasn’t a practical threat.” Since then, there’s been a quantum computing boom (SN: 7/8/17 & 7/22/17, p. 28).

The machines are being built using qubits made from various materials — from individual atoms to flecks of silicon to superconductors (which conduct electricity without resistance) — but all calculate according to quantum rules. IBM’s superconducting quantum computer Osprey, for example, has 433 qubits. That’s up from the five qubits of the computer IBM unveiled in 2016. The company plans to roll out one with more than a thousand qubits this year.

That’s still far from the Y2Q threshold: To break RSA encryption, a quantum computer would need 20 million qubits, researchers reported in 2021 in Quantum.

Mosca estimates that in the next 15 years, there’s about a 50 percent chance of a quantum computer powerful enough to break standard public-key encryption. That may seem like a long time, but experts estimate that previous major cryptography overhauls have taken around 15 years. “This is not a Tuesday patch,” Mosca says.

The threat is even more pressing because the data we send today could be vulnerable to quantum computers that don’t exist yet. Hackers could harvest encrypted information now, and later decode it once a powerful quantum computer becomes available, Mosca says. “It’s just bad news if we don’t get ahead of this.”

New algorithms could safeguard our security

Getting ahead of the problem is the aim of Moody, Robinson and others who are part of NIST’s effort to select and standardize post-quantum encryption and digital signatures. Such techniques would have to thwart hackers using quantum machines, while still protecting from classical hacks.

After NIST put out a call for post-quantum algorithms in 2016, the team received dozens of proposed schemes. The researchers sorted through the candidates, weighing considerations including the level of security provided and the computational resources needed for each. Finally, in July 2022, NIST announced four schemes that had risen to the top. Once the final standards for those algorithms are ready in 2024, organizations can begin making the post-quantum leap. Meanwhile, NIST continues to consider additional candidates.

In parallel with NIST’s efforts, others are endorsing the post-quantum endeavor. In May 2022, the White House put out a memo setting 2035 as the goal for U.S. government agencies to go post-quantum. In November, Google announced it is already using post-quantum cryptography in internal communications.

Several of the algorithms selected by NIST share a mathematical basis — a technique called lattice-based cryptography. It relies on a problem involving describing a lattice, or a grid of points, using a set of arrows, or vectors.

In math, a lattice is described by a set of vectors used to produce it. Consider Manhattan. Even if you’d never seen a map of the city, you could roughly reproduce its grid using two arrows, one the length and direction of an avenue block and the other matching a street block. Discounting the city’s quirks, such as variations in block lengths, you’d just place arrows end-to-end until you’ve mapped out the whole grid.

But there are more complicated sets of vectors that can reproduce the city’s grid. Picture two arrows starting, for example, at Washington Square Park in lower Manhattan, with one pointing to Times Square in Midtown and the other to a neighboring landmark, the Empire State Building. Properly chosen, two such vectors could also be used — with more difficulty — to map out the city’s grid.

A math problem called the shortest vector problem asks: Given a set of long vectors that generate a lattice, what is the shortest vector that can be used as part of a set to produce the grid? If all you knew about the city was the location of those three landmarks, it’d be quite a task to back out the shortest vector corresponding to the city’s blocks.

Now, picture doing that not for a 2-D map, but in hundreds of dimensions. That’s a problem thought to be so difficult that no computer, quantum or classical, could do it in a reasonable amount of time.

The difficulty of that problem is what underlies the strength of several post-quantum cryptography algorithms. In lattice-based cryptography, a short vector is used to create the private key, and the long vectors produce the public key.

Other post-quantum schemes NIST considered are based on different math problems. To choose among the options, NIST mathematicians’ chief consideration was the strength of each algorithm’s security. But none of these algorithms are definitively proved to be secure against quantum computers, or even classical ones. One algorithm originally considered by NIST, called SIKE, was later broken. It took just 10 minutes to crack on a standard computer, researchers reported in April in Advances in Cryptology – EUROCRYPT 2023.

Although it might seem like a failure, the SIKE breakdown can be considered progress. The faith in the security of cryptographic algorithms comes from a trial by fire. “The more [that] smart people try to break something and fail, the more confidence we can get that it’s actually hard to break it,” Schwabe says. Some algorithms must perish in the process.

A quantum internet could bolster security

Quantum physics taketh away, but also, it gives. A different quantum technique can allow communication with mathematically proved security. That means a future quantum internet could, theoretically at least, be fully safe from both quantum and classical hacks.

By transmitting photons — particles of light — and measuring their properties upon arrival, it’s possible to generate a shared private key that is verifiably safe from eavesdroppers.

This quantum key distribution, or QKD, relies on a principle of quantum physics called the no-cloning theorem. Essentially, it’s impossible to copy quantum information. Any attempt to do so will alter the original information, revealing that someone was snooping. “Someone who was trying to learn that information would basically leave a fingerprint behind,” says quantum engineer Nolan Bitner of Argonne National Laboratory in Lemont, Ill.

This quirk of quantum physics allows two people to share a secret key and, by comparing notes, determine whether the key has been intercepted along the way. If those comparisons don’t match as expected, someone was eavesdropping. The communicators discard their key and start over. If there is no sign of foul play, they can safely use their shared secret key to encrypt their communication and send it over the standard internet, certain of its security. It’s a quantum solution to the quandary of how two parties can share secret keys without ever meeting. There’s no need for a mathematical trapdoor that might be vulnerable to an undiscovered tactic.

But QKD can’t be done over normal channels. It requires quantum networks, in which photons are created, sent zipping along optical fibers and are manipulated at the other end.

Such networks already snake through select cities in the world. One threads through Chicago suburbs from the University of Chicago to Argonne lab and Fermilab in Batavia, for a total of 200 kilometers. In China, an extensive network connects cities along a more than 2,000-kilometer backbone that wends from Beijing to Shanghai, along with two quantum satellites that beam photons through the air. A quantum network crisscrosses South Korea, and another links several U.K. cities. There are networks in Tokyo and the Netherlands — the list goes on, with more to come.

A quantum network in China extends more than 2,000 kilometers from Beijing to Shanghai and includes a quantum satellite that beams photons to ground stations in Xinglong and Nanshan. Other quantum networks are being built and tested around the world.Y.-A. CHEN ET AL/NATURE 2021, ADAPTED BY C. CHANG

A quantum network in China extends more than 2,000 kilometers from Beijing to Shanghai and includes a quantum satellite that beams photons to ground stations in Xinglong and Nanshan. Other quantum networks are being built and tested around the world.Y.-A. CHEN ET AL/NATURE 2021, ADAPTED BY C. CHANG

Many of these networks are test-beds used by researchers to study the technology outside of a lab. But some are getting real-world use. Banks use China’s network, and South Korea’s links government agencies. Companies such as ID Quantique, based in Switzerland, offer commercial QKD devices.

QKD’s security is mathematically proven, but quantum networks can fall short of that guarantee in practice. The difficulty of creating, transmitting, detecting and storing quantum particles can open loopholes. Devices and networks must be painstakingly designed and tested to ensure a hacker can’t game the system.

And one missing component in particular is holding quantum networks back. “The number one device is quantum memory,” says quantum physicist Xiongfeng Ma of Tsinghua University in Beijing. When sending quantum information over long distances through fibers, particles can easily get lost along the way. For distances greater than about 100 kilometers, that makes quantum communication impractical without the use of way stations that amplify the signal. Such way stations temporarily convert data into classical, rather than quantum, information. That classical step means hackers could target these “trusted nodes” undetected, marring QKD’s pristine security. And it limits what quantum maneuvers the networks can do.

It’s not possible to create pairs of particles that are entangled over long distances in a network like this. But special stations sprinkled throughout the network, called quantum repeaters, could solve the problem by storing information in a quantum memory. To create far-flung entangled particles, scientists could first entangle sets of particles over short distances, storing them in quantum memories at each quantum repeater. Performing certain operations on the entangled particles could leapfrog that entanglement to other particles farther apart. By repeating this process, particles could be entangled across extended distances.

But, thanks in part to quantum particles’ tendency to be easily perturbed by outside influences, scientists have yet to develop a practical quantum repeater. “When that does appear, it’s likely to catalyze global quantum networks,” says David Awschalom, a physicist at the University of Chicago. Not only will such technologies allow longer distances and better security for QKD, but they will also enable more complicated tasks, like entangling distant quantum computers to allow them to work together.

A European effort called the Quantum Internet Alliance aims to build a network with quantum repeaters by the end of 2029, creating a backbone stretching over 500 kilometers, in addition to two metropolitan-scale networks. The effort is “super challenging,” says physicist and computer scientist Stephanie Wehner of Delft University of Technology in the Netherlands. “We are on a moon shot mission.” Eventually, scientists envision a global quantum internet.

Awschalom imagines the networks becoming accessible to all. “Wouldn’t it be great to be able to go to a public library and be able to get onto a quantum network?”

A link between a ground station (red and green lasers shown in this time-lapse image) and the quantum satellite Micius shows the potential for long-distance secure communications. The satellite beams photons to the ground station, in Xinglong, China.JIN LIWANG/XINHUA/ALAMY LIVE NEWS

What does the future of cryptography look like?

QKD and post-quantum cryptography are complementary. “In order to overcome the threat of the quantum computers we need both,” says physicist Nicolas Gisin of the University of Geneva and cofounder of ID Quantique. When people are exchanging information that doesn’t require the utmost security — say, using a mobile phone to post cat memes on Reddit — post-quantum cryptography will be more practical, as it doesn’t demand a to-and-fro of individual quantum particles. But “there are really situations where we want to make sure that the security is going to last … for several decades, and post-quantum cryptography cannot guarantee that,” Gisin says.

Eventually, quantum techniques could allow for even more advanced types of security, such as blind quantum computing. In that scheme, a user could compute something on a remote quantum computer without anyone being able to determine what they’re computing. A technique called covert quantum communication would allow users to communicate securely while hiding that they were exchanging messages at all. And device-independent QKD would ensure security even if the devices used to communicate are potentially flawed (SN: 8/27/22, p. 10).

The appeal of such extreme secrecy, of course, depends upon whether you’re the secret-keeper or the snoop. In the United States, government agencies like the FBI, CIA and the National Security Agency have argued that encryption makes it difficult to eavesdrop on criminals or terrorists. The agencies have a history of advocating for back doors that would let them in on encrypted communications — or building in secret back doors.

But quantum techniques, done properly, can prevent anyone from intercepting secrets, even powerful government agencies.

“It’s interesting to think about a world where, in principle, one might imagine perfect security,” Awschalom says. “Is that a good thing or is that a bad thing?” More

Anyons, anyone?

Scientists have created strange new particle-like objects called non-abelian anyons. These long-sought quasiparticles can be “braided,” meaning that they can be moved around one another and retain a memory of that swapping, similar to how a braided ponytail keeps a record of the order in which strands cross over each other.

Two independent teams — one led by researchers at Google, the other by researchers at the quantum computing company Quantinuum — have reported creating and braiding versions of these anyons using quantum computers. The Google and Quantinuum results, respectively reported May 11 in Nature and May 9 at arXiv.org, could help scientists construct quantum computers that are resistant to the errors that currently bedevil the machines.

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Non-abelian anyons defy common intuition about what happens to objects that swap locations. Picture the street game with cups and balls, where a performer swaps identical cups back and forth. If you weren’t watching closely, you’d never know if two cups had been moved around one another and back to their original positions. In the quantum world, that’s not always the case.

“It’s predicted that there is this crazy particle where, if you swap them around each other while you have your eyes closed, you can actually tell after the fact,” says physicist Trond Andersen of Google Quantum AI in Santa Barbara, Calif. “This goes against our common sense, and it seems crazy.”

Particles in our regular 3-D world can’t do this magic trick. But when particles are confined to just two dimensions, the rules change. While scientists don’t have a 2-D universe in which to explore particles, they can manipulate materials or quantum computers to exhibit behavior like that of particles that live in two dimensions, creating objects known as quasiparticles.

All fundamental subatomic particles fall into two classes, based on how identical particles of each type behave when swapped. They are either fermions, a class that includes electrons and other particles that make up matter, or bosons, which include particles of light known as photons.

But in two dimensions, there’s another option: anyons. For bosons or fermions, swapping identical particles back and forth or moving them around one another can’t have a directly measurable effect. For anyons, it can.

In the 1990s, scientists realized that a specific version of an anyon, called a non-abelian anyon, could be used to build quantum computers that might safeguard fragile quantum information, which is easily knocked out of whack by minute disturbances.

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“For fundamental reasons these anyons have been very exciting, and for practical reasons people hope they might be useful,” says theoretical physicist Maissam Barkeshli of the University of Maryland in College Park, who was not involved with either study.

Google’s team created the anyons using a superconducting quantum computer, where the quantum bits, or qubits, are made of material that conducts electricity without resistance. Quantinuum’s study, which has yet to be peer-reviewed, is based on a quantum computer whose qubits are composed of trapped, electrically charged atoms of ytterbium and barium. In both cases, scientists manipulated the qubits to create the anyons and move them around, demonstrating a measurable change after the anyons were braided.

Scientists have previously created and braided a less exotic type of anyon, called an abelian anyon, within a 2-D layer of a solid material (SN: 7/9/20). And many physicists are similarly questing after a solid material that might host the non-abelian type.

But the new studies create non-abelian states within qubits inside a quantum computer, which is fundamentally different, Barkeshli says. “You’re kind of synthetically creating the state for a fleeting moment.” That means it doesn’t have all the properties that anyons within a solid material would have, he says.

In both cases, much more work must be done before the anyons could create powerful, error-resistant quantum computers. Google’s study, in particular, produces an anyon that’s akin to a fish out of water. It’s a non-abelian within a more commonplace abelian framework. That means those anyons may not be as powerful for quantum computing, Barkeshli says.

It’s not all about practical usefulness. Demonstrating that non-abelian anyons really exist is fundamentally important, says Quantinuum’s Henrik Dreyer, a physicist in Munich. It “confirms that the rules of quantum mechanics apply in the way that we thought they would apply.” More

In keeping with the grand tradition of tubby cats, a newly created quantum “cat” is particularly massive — at least for the quantum realm.

Scientists put a jiggling piece of sapphire crystal in what’s known as a “cat state,” in which an object exists in two different states simultaneously. It’s a situation reminiscent of physicists’ favorite imaginary feline, Schrödinger’s cat, known for being alive and dead at the same time.

The new sapphire cat is a relatively hefty 16 micrograms, physicists report in the April 21 Science. That’s close to half the mass of an eyelash, and more than 100 trillion times the mass of cat states previously created with molecules. “We’ve reached a new regime where quantum mechanics apparently does work,” says physicist Yiwen Chu of ETH Zurich.

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In a quantum parable dreamt up in the 1930s by physicist Erwin Schrödinger, a cat is trapped in a box and, due to quantum effects, winds up alive and dead at the same time (SN: 5/26/16). This paradoxical scenario doesn’t happen in the real world. While quantum particles are capable of existing in two distinct states simultaneously — what’s called a superposition — those effects wash out for cat-sized stuff.

Quantum effects are typically confined to atoms, molecules and the like. The everyday world visible to human eyes doesn’t exhibit quantum properties. Scientists can coax certain tiny objects to display quantum features (SN: 4/25/18). But scientists don’t fully understand the border between the quantum and nonquantum realms.

“We really have only just begun to understand that intermediate regime,” says Benjamin Sussman of the University of Ottawa, who was not involved with the new study. “It’s of really profound interest to see how these quantum systems scale and how they behave.”

Cat states are a special variety of quantum behavior that come close to re-creating Schrödinger’s idea. They are superpositions of two states that are distinct according to the classical physics that describes the everyday world— like an alive or dead cat — rather than two states that exist only in the quantum domain, such as the energy levels of an atom.

In the new experiment, the researchers jiggled a portion of a sapphire crystal in such a way that its atoms moved in two directions at once. That’s a distinction that “captures the spirit” of Schrödinger’s cat, Chu says.

The jiggling was confined within a sliver of the crystal consisting of 100 million billion atoms. That’s large enough that, if extracted from the rest of the crystal, it would be visible to the naked eye, Chu says.

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Still, the oscillations of the atoms were tiny, about a millionth of a billionth of a millimeter — not exactly the scale of everyday objects. Other demonstrations of cat states have demonstrated much larger spatial separation, despite being made up of fewer atoms.

In future work, Sussman says he’d like to see the researchers scale up not only the mass, but also the size of the oscillations. “That’s going to be really hard but will be really interesting.” More

To shrink error rates in quantum computers, sometimes more is better. More qubits, that is.

The quantum bits, or qubits, that make up a quantum computer are prone to mistakes that could render a calculation useless if not corrected. To reduce that error rate, scientists aim to build a computer that can correct its own errors. Such a machine would combine the powers of multiple fallible qubits into one improved qubit, called a “logical qubit,” that can be used to make calculations (SN: 6/22/20).  

Scientists now have demonstrated a key milestone in quantum error correction. Scaling up the number of qubits in a logical qubit can make it less error-prone, researchers at Google report February 22 in Nature.

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Future quantum computers could solve problems impossible for even the most powerful traditional computers (SN: 6/29/17). To build those mighty quantum machines, researchers agree that they’ll need to use error correction to dramatically shrink error rates. While scientists have previously demonstrated that they can detect and correct simple errors in small-scale quantum computers, error correction is still in its early stages (SN: 10/4/21).

The new advance doesn’t mean researchers are ready to build a fully error-corrected quantum computer, “however, it does demonstrate that it is indeed possible, that error correction fundamentally works,” physicist Julian Kelly of Google Quantum AI said in a news briefing February 21.

Quantum computers like Google’s require a dilution refrigerator (pictured) that can cool the quantum processor (which is installed at the bottom of the refrigerator) to frigid temperatures.Google Quantum AI

Logical qubits store information redundantly in multiple physical qubits. That redundancy allows a quantum computer to check if any mistakes have cropped up and fix them on the fly. Ideally, the larger the logical qubit, the smaller the error rate should be. But if the original qubits are too faulty, adding in more of them will cause more problems than it solves.

Using Google’s Sycamore quantum chip, the researchers studied two different sizes of logical qubits, one consisting of 17 qubits and the other of 49 qubits. After making steady improvements to the performance of the original physical qubits that make up the device, the researchers tallied up the errors that still slipped through. The larger logical qubit had a lower error rate, about 2.9 percent per round of error correction, compared to the smaller logical qubit’s rate of about 3.0 percent, the researchers found.

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That small improvement suggests scientists are finally tiptoeing into the regime where error correction can begin to squelch errors by scaling up. “It’s a major goal to achieve,” says physicist Andreas Wallraff of ETH Zurich, who was not involved with the research.

However, the result is only on the cusp of showing that error correction improves as scientists scale up. A computer simulation of the quantum computer’s performance suggests that, if the logical qubit’s size were increased even more, its error rate would actually get worse. Additional improvement to the original faulty qubits will be needed to enable scientists to really capitalize on the benefits of error correction.

Still, milestones in quantum computation are so difficult to achieve that they’re treated like pole jumping, Wallraff says. You just aim to barely clear the bar. More

Quantum tech is going green.

A new take on highly sensitive magnetic field sensors ditches the power-hungry lasers that previous devices have relied on to make their measurements and replaces them with sunlight. Lasers can gobble 100 watts or so of power — like keeping a bright lightbulb burning. The innovation potentially untethers quantum sensors from that energy need. The result is an environmentally friendly prototype on the forefront of technology, researchers report in an upcoming issue of Physical Review X Energy.

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The big twist is in how the device uses sunlight. It doesn’t use solar cells to convert light into electricity. Instead, the sunlight does the job of the laser’s light, says Jiangfeng Du, a physicist at the University of Science and Technology of China in Hefei.   

Quantum magnetometers often include a powerful green laser to measure magnetic fields. The laser shines on a diamond that contains atomic defects (SN: 2/26/08). The defects result when nitrogen atoms replace some of the carbon atoms that pure diamonds are made of. The green laser causes the nitrogen defects to fluoresce, emitting red light with an intensity that depends on the strength of the surrounding magnetic fields.

The new quantum sensor needs green light too. There’s plenty of that in sunlight, as seen in the green wavelengths reflected from tree leaves and grass. To collect enough of it to run their magnetometer, Du and colleagues replaced the laser with a lens 15 centimeters across to gather sunlight. They then filtered the light to remove all colors but green and focused it on a diamond with nitrogen atom defects. The result is red fluorescence that reveals magnetic field strengths just as laser-equipped magnetometers do.

Green-colored light shining on the diamond-based sensor in a quantum device can be used to measure magnetic fields. In this prototype, a lens (top) collects sunlight, which is filtered to leave only green wavelengths of light. That green light provides an environmentally friendly alternative to the light created by power-hungry lasers that conventional quantum devices rely on.Yunbin Zhu/University of Science and Technology of China

Changing energy from one type to another, as happens when solar cells collect light and produce electricity, is an inherently inefficient process (SN: 7/26/17). The researchers claim that avoiding the conversion of sunlight to electricity to run lasers makes their approach three times more efficient than would be possible with solar cells powering lasers.

“I’ve never seen any other reports that connect solar research to quantum technologies,” says Yen-Hung Lin, a physicist at the University of Oxford who was not involved with the study. “It might well ignite a spark of interest in this unexplored direction, and we could see more interdisciplinary research in the field of energy.”

Quantum devices sensitive to other things, like electric fields or pressure, could also benefit from the sunlight-driven approach, the researchers say. In particular, space-based quantum technology might use the intense sunlight available outside Earth’s atmosphere to provide light tailored for quantum sensors. The remaining light, in wavelengths that the quantum sensors don’t use, could be relegated to solar cells that power electronics to process the quantum signals.

The sunlight-driven magnetometer is just a first step in the melding of quantum and environmentally sustainable technology. “In the current state, this device is primarily for developmental purposes,” Du says. “We expect that the devices will be used for practical purposes. But there [is] lots of work to be done.” More

Stealthy communication just got more secure, thanks to quantum entanglement.

Quantum physics provides a way to share secret information that’s mathematically proven to be safe from the prying eyes of spies. But until now, demonstrations of the technique, called quantum key distribution, rested on an assumption: The devices used to create and measure quantum particles have to be known to be flawless. Hidden defects could allow a stealthy snoop to penetrate the security unnoticed.

Now, three teams of researchers have demonstrated the ability to perform secure quantum communication without prior confirmation that the devices are foolproof. Called device-independent quantum key distribution, the method is based on quantum entanglement, a mysterious relationship between particles that links their properties even when separated over long distances.

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In everyday communication, such as the transmission of credit card numbers over the internet, a secret code, or key, is used to garble the information, so that it can be read only by someone else with the key. But there’s a quandary: How can a distant sender and receiver share that key with one another while ensuring that no one else has intercepted it along the way?

Quantum physics provides a way to share keys by transmitting a series of quantum particles, such as particles of light called photons, and performing measurements on them. By comparing notes, the users can be sure that no one else has intercepted the key. Those secret keys, once established, can then be used to encrypt the sensitive intel (SN: 12/13/17). By comparison, standard internet security rests on a relatively shaky foundation of math problems that are difficult for today’s computers to solve, which could be vulnerable to new technology, namely quantum computers (SN: 6/29/17).

But quantum communication typically has a catch. “There cannot be any glitch that is unforeseen,” says quantum physicist Valerio Scarani of the National University of Singapore. For example, he says, imagine that your device is supposed to emit one photon but unknown to you, it emits two photons. Any such flaws would mean that the mathematical proof of security no longer holds up. A hacker could sniff out your secret key, even though the transmission seems secure.

Device-independent quantum key distribution can rule out such flaws. The method builds off of a quantum technique known as a Bell test, which involves measurements of entangled particles. Such tests can prove that quantum mechanics really does have “spooky” properties, namely nonlocality, the idea that measurements of one particle can be correlated with those of a distant particle. In 2015, researchers performed the first “loophole-free” Bell tests, which certified beyond a doubt that quantum physics’ counterintuitive nature is real (SN: 12/15/15).

“The Bell test basically acts as a guarantee,” says Jean-Daniel Bancal of CEA Saclay in France. A faulty device would fail the test, so “we can infer that the device is working properly.”

In their study, Bancal and colleagues used entangled, electrically charged strontium atoms separated by about two meters. Measurements of those ions certified that their devices were behaving properly, and the researchers generated a secret key, the team reports in the July 28 Nature.

Typically, quantum communication is meant for long-distance dispatches. (To share a secret with someone two meters away, it would be easier to simply walk across the room.) So Scarani and colleagues studied entangled rubidium atoms 400 meters apart. The setup had what it took to produce a secret key, the researchers report in the same issue of Nature. But the team didn’t follow the process all the way through: The extra distance meant that producing a key would have taken months.

In the third study, published in the July 29 Physical Review Letters, researchers wrangled entangled photons rather than atoms or ions. Physicist Wen-Zhao Liu of the University of Science and Technology of China in Hefei and colleagues also demonstrated the capability to generate keys, at distances up to 220 meters. This is particularly challenging to do with photons, Liu says, because photons are often lost in the process of transmission and detection.

Loophole-free Bell tests are already no easy feat, and these techniques are even more challenging, says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. “The requirements for this experiment are so absurdly high that it’s just an impressive achievement to be able to demonstrate some of these capabilities,” says Shalm, who wrote a perspective in the same issue of Nature.

That means that the technique won’t see practical use anytime soon, says physicist Nicolas Gisin of the University of Geneva, who was not involved with the research.

Still, device-independent quantum key distribution is “a totally fascinating idea,” Gisin says. Bell tests were designed to answer a philosophical question about the nature of reality — whether quantum physics really is as weird as it seems. “To see that this now becomes a tool that enables something else,” he says, “this is the beauty.” More

Machine learning can get a boost from quantum physics.

On certain types of machine learning tasks, quantum computers have an exponential advantage over standard computation, scientists report in the June 10 Science. The researchers proved that, according to quantum math, the advantage applies when using machine learning to understand quantum systems. And the team showed that the advantage holds up in real-world tests.

“People are very excited about the potential of using quantum technology to improve our learning ability,” says theoretical physicist and computer scientist Hsin-Yuan Huang of Caltech. But it wasn’t entirely clear if machine learning could benefit from quantum physics in practice.

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In certain machine learning tasks, scientists attempt to glean information about a quantum system — say a molecule or a group of particles — by performing repeated experiments, and analyzing data from those experiments to learn about the system.

Huang and colleagues studied several such tasks. In one, scientists aim to discern properties of the quantum system, such as the position and momentum of particles within. Quantum data from multiple experiments could be input into a quantum computer’s memory, and the computer would process the data jointly to learn the quantum system’s characteristics.

The researchers proved theoretically that doing the same characterization with standard, or classical, techniques would require exponentially more experiments in order to learn the same information. Unlike a classical computer, a quantum computer can exploit entanglement — ethereal quantum linkages — to better analyze the results of multiple experiments.

But the new work goes beyond just the theoretical. “It’s crucial to understand if this is realistic, if this is something we could see in the lab or if this is just theoretical,” says Dorit Aharonov of Hebrew University in Jerusalem, who was not involved with the research.

So the researchers tested machine learning tasks with Google’s quantum computer, Sycamore (SN: 10/23/19). Rather than measuring a real quantum system, the team used simulated quantum data, and analyzed it using either quantum or classical techniques.

Quantum machine learning won out there, too, even though Google’s quantum computer is noisy, meaning errors can slip into calculations. Eventually, scientists plan to build quantum computers that can correct their own errors (SN: 6/22/20). But for now, even without that error correction, quantum machine learning prevailed. More