March 1, 2024

Alternative qubit design fixes bugs in hardware

Nord Quantique

There is general consensus that running any type 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 reliable for complex calculations, so quantum information will need to be distributed across multiple qubits, allowing for monitoring of errors and intervening when they occur.

But most ways to create these “logical qubits” needed for error correction require dozens to more than a hundred individual hardware qubits. This means we will need tens of thousands to millions of hardware qubits to perform calculations. Existing hardware only surpassed the 1,000 qubit mark last month, so the future appears to be several years away 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 qubit pairs until later this year.

Discover the bosonic qubit

The technology underlying this work is called a bosonic qubit, and it’s nothing new; an optical instrument company even has a list of products that indicates their potential for use in error correction. But although the concepts behind using it in this way were well established, demonstrations were lagging behind. Nord Quantique has now published a paper on arXiv that details a demonstration of how they actually reduce error rates.

The devices are structured like a transmon, the qubit form favored by technology heavyweights like IBM and Google. There, quantum information is stored in a loop of superconducting wire and is controlled by what is called a microwave resonator – a small piece of material where microwave photons will bounce back and forth for a while before be lost.

A bosonic qubit turns this situation upside down. In this hardware, quantum information is held in photons, while the superconducting wire and resonator control the system. Both are connected to a coaxial cavity (think of a structure that, although microscopic, looks a bit like the end of a cable connector).

Extremely simplified, quantum information is stored in the way photons in the cavity interact. The state of the photons can be monitored by the connected resonator/superconductor wire. If something appears to be wrong, the resonator/superconductor wire allows interventions to be made to restore the original state. Additional qubits are not needed. “A very simple and basic idea behind quantum error correction is redundancy,” co-founder and CTO Julien Camirand Lemyre told Ars. “One thing about resonators and oscillators in superconducting circuits is that you can put a lot of photons inside the resonators. And for us, the redundancy comes from there.”

This process does not correct all possible errors, so it does not eliminate the need for logical qubits made from multiple underlying hardware qubits. In theory, though, you can detect the two most common forms of errors that qubits are prone to (bit shifts and phase shifts).

In the arXiv preprint, the Nord Quantique team demonstrated that the system works. By using a single qubit and simply measuring whether it maintains its original state, the error correction system can reduce problems by 14%. Unfortunately, overall fidelity is also low, starting at around 85%, which is significantly lower than seen in other systems that have undergone years of development work. Some qubits have been demonstrated with greater than 99% fidelity.

Becoming Competitive

So there is no doubt that Nord Quantique is well behind several leaders in quantum computing that can perform (error-prone) calculations with dozens of qubits and have much lower error rates. Again, Nord Quantique’s work was done using a single qubit – and without performing any of the operations necessary to perform a calculation.

Lemyre told Ars that although the company is small, it benefits from being a subsidiary of Canada’s University Sherbrooke’s Institut Quantique, one of Canada’s leading quantum research centers. In addition to having access to local expertise, Nord Quantique utilizes a manufacturing facility in Sherbrooke to manufacture its hardware.

Next year, the company hopes to demonstrate that the error correction scheme can work while pairs of qubits are used to perform gate operations, the fundamental units of calculations. Another high priority is to combine this hardware-based error correction with more traditional logical qubit schemes, which would allow additional types of errors to be detected and corrected. This would involve operations with a dozen or more of these bosonic qubits at a time.

But the real challenge will be in the long term. The company relies on its hardware’s ability to handle error correction to reduce the number of qubits needed for useful calculations. But if your competitors can increase the number of qubits quickly enough while maintaining control and the necessary error rates, this may not matter. Put another way, if Nord Quantique is still in the hundreds of qubits range when other companies are in the hundreds of thousands, its technology may not be successful, even if it has some inherent advantages.

But that’s the fun part about the field as it stands: we don’t really know. Some very different technologies are already in development and show promise. And there are other sets that are still early in the development process, but are believed to have a smoother path to scaling to useful numbers of qubits. All of them will have to scale to a minimum of tens of thousands of qubits, while still enabling the ability to perform quantum manipulations that were cutting-edge science just a few decades ago.

In the background is the simple fact that we never try to scale something like this to the extent necessary. Unforeseen technical obstacles may limit progress at some point in the future.

Despite all this, there are people who support each of these technologies and who know much more about quantum mechanics than I ever will. It’s a fun time.

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