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."

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There's a strong consensus that tackling most useful problems with a quantum computer will require that the computer be capable of error correction. There is absolutely no consensus, however, about what technology will allow us to get there. A large number of companies, including major players like Microsoft, Intel, Amazon, and IBM, have all committed to different technologies to get there, while a collection of startups are exploring an even wider range of potential solutions.

We probably won't have a clearer picture of what's likely to work for a few years. But there's going to be lots of interesting research and development work between now and then, some of which may ultimately represent key milestones in the development of quantum computing. To give you a sense of that work, we're going to look at three papers that were published within the last couple of weeks, each of which tackles a different aspect of quantum computing technology.

Hot stuff

Error correction will require connecting multiple hardware qubits to act as a single unit termed a logical qubit. This spreads a single bit of quantum information across multiple hardware qubits, making it more robust. Additional qubits are used to monitor the behavior of the ones holding the data and perform corrections as needed. Some error correction schemes require over a hundred hardware qubits for each logical qubit, meaning we'd need tens of thousands of hardware qubits before we could do anything practical.

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iMessage is getting a major makeover that makes it among the two messaging apps most prepared to withstand the coming advent of quantum computing, largely at parity with Signal or arguably incrementally more hardened.

On Wednesday, Apple said messages sent through iMessage will now be protected by two forms of end-to-end encryption (E2EE), whereas before, it had only one. The encryption being added, known as PQ3, is an implementation of a new algorithm called Kyber that, unlike the algorithms iMessage has used until now, can’t be broken with quantum computing. Apple isn’t replacing the older quantum-vulnerable algorithm with PQ3—it's augmenting it. That means, for the encryption to be broken, an attacker will have to crack both.

Making E2EE future safe

The iMessage changes come five months after the Signal Foundation, maker of the Signal Protocol that encrypts messages sent by more than a billion people, updated the open standard so that it, too, is ready for post-quantum computing (PQC). Just like Apple, Signal added Kyber to X3DH, the algorithm it was using previously. Together, they’re known as PQXDH.

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There's a general consensus that performing any sort of complex algorithm on quantum hardware will have to wait for the arrival of error-corrected qubits. Individual qubits are too error-prone to be trusted for complex calculations, so quantum information will need to be distributed across multiple qubits, allowing monitoring for errors and intervention when they occur.

But most ways of making these "logical qubits" needed for error correction require anywhere from dozens to over a hundred individual hardware qubits. This means we'll need anywhere from tens of thousands to millions of hardware qubits to do calculations. Existing hardware has only cleared the 1,000-qubit mark within the last month, so that future appears to be several years off at best.

But on Thursday, a company called Nord Quantique announced that it had demonstrated error correction using a single qubit with a distinct hardware design. While this has the potential to greatly reduce the number of hardware qubits needed for useful error correction, the demonstration involved a single qubit—the company doesn't even expect to demonstrate operations on pairs of qubits until later this year.

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 family portrait of IBM's quantum processors, with the two new arrivals (Heron and Condor) at right. (credit: IBM)

On Monday, IBM announced that it has produced the two quantum systems that its roadmap had slated for release in 2023. One of these is based on a chip named Condor, which is the largest transmon-based quantum processor yet released, with 1,121 functioning qubits. The second is based on a combination of three Heron chips, each of which has 133 qubits. Smaller chips like Heron and its successor, Flamingo, will play a critical role in IBM's quantum roadmap—which also got a major update today.

Based on the update, IBM will have error-corrected qubits working by the end of the decade, enabled by improvements to individual qubits made over several iterations of the Flamingo chip. While these systems probably won't place things like existing encryption schemes at risk, they should be able to reliably execute quantum algorithms that are far more complex than anything we can do today.

We talked with IBM's Jay Gambetta about everything the company is announcing today, including existing processors, future roadmaps, what the machines might be used for over the next few years, and the software that makes it all possible. But to understand what the company is doing, we have to back up a bit to look at where the field as a whole is moving.

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.