Enlarge / Quantinuum's H2 "racetrack" quantum processor. (credit: Quantinuum)

Today's quantum computing hardware is severely limited in what it can do by errors that are difficult to avoid. There can be problems with everything from setting the initial state of a qubit to reading its output, and qubits will occasionally lose their state while doing nothing. Some of the quantum processors in existence today can't use all of their individual qubits for a single calculation without errors becoming inevitable.

The solution is to combine multiple hardware qubits to form what's termed a logical qubit. This allows a single bit of quantum information to be distributed among multiple hardware qubits, reducing the impact of individual errors. Additional qubits can be used as sensors to detect errors and allow interventions to correct them. Recently, there have been a number of demonstrations that logical qubits work in principle .

On Wednesday, Microsoft and Quantinuum announced that logical qubits work in more than principle. "We've been able to demonstrate what's called active syndrome extraction, or sometimes it's also called repeated error correction," Microsoft's Krysta Svore told Ars. "And we've been able to do this such that it is better than the underlying physical error rate. So it actually works."

Enlarge / The current generation of hardware, which will see rapid iteration over the next several years. (credit: QuEra)

On Tuesday, the quantum computing startup Quera laid out a road map that will bring error correction to quantum computing in only two years and enable useful computations using it by 2026, years ahead of when IBM plans to offer the equivalent . Normally, this sort of thing should be dismissed as hype. Except the company is Quera, which is a spinoff of the Harvard Universeity lab that demonstrated the ability to identify and manage errors using hardware that's similar in design to what Quera is building.

Also notable: Quera uses the same type of qubit that a rival startup, Atom Computing, has already scaled up to over 1,000 qubits . So, while the announcement should be viewed cautiously—several companies have promised rapid scaling and then failed to deliver—there are some reasons it should be viewed seriously as well.

It’s a trap!

Current qubits, regardless of their design, are prone to errors during measurements, operations, or even when simply sitting there. While it's possible to improve these error rates so that simple calculations can be done, most people in the field are skeptical it will ever be possible to drop these rates enough to do the elaborate calculations that would fulfill the promise of quantum computing. The consensus seems to be that, outside of a few edge cases, useful computation will require error-corrected qubits.

Enlarge / Some of the optical hardware needed to get QuEra's machine to work. (credit: QuEra)

There's widespread agreement that most useful quantum computing will have to wait for the development of error-corrected qubits. Error correction involves distributing a bit of quantum information—termed a logical qubit—among a small collection of hardware qubits. The disagreements mostly focus on how best to implement it and how long it will take.

A key step toward that future is described in a paper released in Nature today. A large team of researchers, primarily based at Harvard University, have now demonstrated the ability to perform multiple operations on as many as 48 logical qubits. The work shows that the system, based on hardware developed by the company QuEra, can correctly identify the occurrence of errors, and this can significantly improve the results of calculations.

Yuval Boger, QuEra's chief marketing officer, told Ars: "We feel it is a very significant milestone on the path to where we all want to be, which is large-scale, fault-tolerant quantum computers.

Enlarge / The qubits of the new hardware: an array of individual atoms. (credit: Atom Computing)

Today, a startup called Atom Computing announced that it has been doing internal testing of a 1,180 qubit quantum computer and will be making it available to customers next year. The system represents a major step forward for the company, which had only built one prior system based on neutral atom qubits—a system that operated using only 100 qubits.

The error rate for individual qubit operations is high enough that it won't be possible to run an algorithm that relies on the full qubit count without it failing due to an error. But it does back up the company's claims that its technology can scale rapidly and provides a testbed for work on quantum error correction. And, for smaller algorithms, the company says it'll simply run multiple instances in parallel to boost the chance of returning the right answer.

Computing with atoms

Atom Computing, as its name implies, has chosen neutral atoms as its qubit of choice (there are other companies that are working with ions). These systems rely on a set of lasers that create a series of locations that are energetically favorable for atoms. Left on their own, atoms will tend to fall into these locations and stay there until a stray gas atom bumps into them and knocks them out.

Enlarge / IBM's Eagle processor has reached Rev3, which means lower noise qubits. (credit: IBM )

Today's quantum processors are error-prone. While the probabilities are small—less than 1 percent in many cases—each operation we perform on each qubit, including basic things like reading its state, has a significant error rate. If we try an operation that needs a lot of qubits, or a lot of operations on a smaller number of qubits, then errors become inevitable.

Long term, the plan is to solve that using error-corrected qubits . But these will require multiple high-quality qubits for every bit of information, meaning we'll need thousands of qubits that are better than anything we can currently make. Given that we probably won't reach that point until the next decade at the earliest, it raises the question of whether quantum computers can do anything interesting in the meantime.

In a publication in today's Nature, IBM researchers make a strong case for the answer to that being yes. Using a technique termed "error mitigation," they managed to overcome the problems with today's qubits and produce an accurate result despite the noise in the system. And they did so in a way that clearly outperformed similar calculations on classical computers.

Enlarge / The quantum network is a bit bulkier than Ethernet. (credit: ETH Zurich / Daniel Winkler )

A new experiment uses superconducting qubits to demonstrate that quantum mechanics violates what's called local realism by allowing two objects to behave as a single quantum system no matter how large the separation between them. The experiment wasn't the first to show that local realism isn't how the Universe works—it's not even the first to do so with qubits.

But it's the first to separate the qubits by enough distance to ensure that light isn't fast enough to travel between them while measurements are made. And it did so by cooling a 30-meter-long aluminum wire to just a few microKelvin. Because the qubits are so easy to control, the experiment provides a new precision to these sorts of measurements. And the hardware setup may be essential for future quantum computing efforts.

Getting real about realism

Albert Einstein was famously uneasy with some of the consequences of quantum entanglement. If quantum mechanics were right, then a pair of entangled objects would behave as a single quantum system no matter how far apart the objects were. Altering the state of one of them should instantly alter the state of the second, with the change seemingly occurring faster than light could possibly travel between the two objects. This, Einstein argued, almost certainly had to be wrong.

Enlarge / Two generations of Google's Sycamore processor. (credit: Google Quantum AI)

Today, Google announced a demonstration of quantum error correction on its next generation of quantum processors, Sycamore. The iteration on Sycamore isn't dramatic—it's the same number of qubits, just with better performance. And getting quantum error correction isn't really the news—they'd managed to get it to work a couple of years ago.

Instead, the signs of progress are a bit more subtle. In earlier generations of processors, qubits were error-prone enough that adding more of them to an error-correction scheme caused problems that were larger than the gain in corrections. In this new iteration, adding more qubits and getting the error rate to go down is possible.

We can fix that

The functional unit of a quantum processor is a qubit, which is anything—an atom, an electron, a hunk of superconducting electronics—that can be used to store and manipulate a quantum state. The more qubits you have, the more capable the machine is. By the time you have access to several hundred, it's thought that you can perform calculations that would be difficult to impossible to do on traditional computer hardware.

Enlarge (credit: QuEra)

Quantum computing has entered a bit of an awkward period. There have been clear demonstrations that we can successfully run quantum algorithms, but the qubit counts and error rates of existing hardware mean that we can't solve any commercially useful problems at the moment. So, while many companies are interested in quantum computing and have developed software for existing hardware (and have paid for access to that hardware), the efforts have been focused on preparation. They want the expertise and capability needed to develop useful software once the computers are ready to run it.

For the moment, that leaves them waiting for hardware companies to produce sufficiently robust machines—machines that don't currently have a clear delivery date. It could be years; it could be decades. Beyond learning how to develop quantum computing software, there's nothing obvious to do with the hardware in the meantime.

But a company called QuEra may have found a way to do something that's not as obvious. The technology it is developing could ultimately provide a route to quantum computing. But until then, it's possible to solve a class of mathematical problems on the same hardware, and any improvements to that hardware will benefit both types of computation. And in a new paper, the company's researchers have expanded the types of computations that can be run on their machine.

Enlarge (credit: Aurich Lawson / Getty Images)

Inspired by the functioning of pulsed lasers, scientists from France and Japan have developed an acoustic counterpart that enables the precise and controlled transmission of single electrons between quantum nodes.

Riding the waves

The spin of an electron can serve as a basis for creating qubits—the basic unit of information of quantum computing. In order to process or store that information, the information in qubits may have to be transported between quantum nodes in a network.

One option is transporting the electrons themselves, something that can now be done by having them ride sound waves. “More than 10 years ago, we demonstrated it for the first time,” said lead researcher Christopher Bauerle of the Grenoble-based Institute Néel .