A new quantum computer sets a high watermark for accuracy – are we on the verge of a big breakthrough?

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Transformative technologies like quantum computing rarely arrive in a single leap. Helios is not shown.<br>Anna Szilagyi / EPA Images

https://theconversation.com/a-new-quantum-computer-sets-a-high-watermark-for-accuracy-are-we-on-the-verge-of-a-big-breakthrough-285753

https://theconversation.com/a-new-quantum-computer-sets-a-high-watermark-for-accuracy-are-we-on-the-verge-of-a-big-breakthrough-285753

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In a laboratory in Broomfield, Colorado, 98 atoms are suspended in mid-air, held in place by electric fields and cooled to temperatures close to absolute zero.

Each atom is far smaller than anything the naked eye could ever see, yet each carries information in a form that has no counterpart in classical physics.

Together, they form Helios, a new quantum computer built by the British-American company Quantinuum. Quantum computers use the power of quantum mechanics, the rules that govern how physics operates at atomic and sub-atomic scales. Those that use Helios’ model of suspended atoms are known as trapped-ion.

A paper published in Nature describes it as a 98-qubit processor with very high accuracy and performance that pushes beyond what can easily be simulated on classical machines. That sounds impressive, but the important question is not simply whether this is a<br>bigger quantum computer (the previous biggest, System Model H2, had 56 qubits). It is whether it is a better one.

Quantum computers are not just faster versions of ordinary computers. The qubits (quantum bits) that they use to process information can exist in quantum states that do not behave like the ones and zeroes of conventional digital technology.

This allows some calculations to be arranged in ways that may eventually outperform even the largest supercomputers. The possible applications are fascinating: new materials, better optimisation methods, improved chemistry simulations and new approaches to cryptography.

An introduction to the Helios quantum computer (Quantinuum).

The difficulty is that qubits are extremely fragile. They are disturbed by temperature variations, imperfect control, unwanted interactions with the environment and, in some systems, even the act of moving information around the device.

For this reason, the race in quantum computing is not only about having more qubits. It is about having more good qubits, controlled accurately enough to perform long and meaningful calculations.

Why it matters

This is why Helios’ result matters. Quantum computing has been promising to change the world for decades, but many announcements still tend to focus on the number of qubits.

This is like judging a race by the number of runners at the starting line. What matters is how many reach the finish, and in what condition. Helios takes both sides of that challenge seriously. Not only is the 98 qubits relatively large; it also reports very low error rates at this scale.

Errors are more common with quantum computers than with classical ones, so error correction is a big challenge in this area.

The Nature paper gives an average error rate for single-qubit gates of about 2.5 in 100,000 for Helios. A quantum gate is the building block of a circuit in quantum computers. For two-qubit gates in Helios, which are harder and more important for useful computation, the average error rate is about 7.9 in 10,000. This is similar to the best demonstrations of around 5 in 10,000 errors.

Quantum operations are cumulative. A small error in one step may not matter much, but a useful quantum algorithm may require thousands, millions or more operations. Lower error rates mean that more complex calculations become possible before the quantum information falls apart.

Helios’ other notable feature is all-to-all connectivity. In many quantum computers, qubits can interact only with their nearest neighbours, rather like people who can speak only to those sitting next to them. If two distant qubits need to interact, the information must be moved through a chain of intermediate steps. Each extra step adds time and error.

In Helios, any qubit can in principle interact with any other. This is especially valuable for algorithms where the required pattern of interactions does not fit neatly onto a fixed grid.

Quantum railway

The hardware behind this is also interesting. Trapped-ion quantum computers such as Helios use charged atoms as qubits. These ions are held using electric fields and manipulated with laser pulses.

The approach is known for high accuracy, but scaling it up while preserving that accuracy is technically difficult. Helios uses barium ions in what is called a quantum charge-coupled device, or QCCD, architecture. A useful way to picture it is as a tiny quantum railway.

Ions can be stored...