The global race to make a practical quantum computer just took a big leap forward

Error correction

Classical charge-coupled devices, the template for the types now used in Helios, were the central component in early digital cameras. There are three main advantages to now using these in quantum computation.

Gates are the fundamental building blocks of quantum circuits, which operate on qubits. One advantage of the QCCD approach is that gates operating on certain qubits do not affect their neighbours. This reduces a phenomenon known as “crosstalk”, leading to better quality performance.

The second advantage is the ability to measure and reset qubits during a computation. This enables inevitable errors in the computation to be detected and corrected at the earliest possible time, again improving performance.

Finally, as qubits can be accessed from and returned to very different parts of the memory after processing (within limitations), distant qubits can be connected, leading to more efficient operation.

The Helios device demonstrated all the above advantages. It relies on two major advances – one in hardware and one in software. First, the four-way X junction lets the system handle several tasks at once rather than one at a time, which is much faster. That was impossible in earlier QCCD machines, which could only move data back and forth in a single line or loop.

Secondly, judicious use of the freedom offered by the two dimensions of the X junction relies on a new classical control software called Helios runtime which plans the smartest, fastest route for moving and processing the data.

Together, these represent a substantial advance in the engineering of quantum computers. An important consequence is that Helios can perform computations that cannot be performed even on the largest supercomputers using known methods within reasonable amounts of time and power consumption.

Computational prowess

It shows the ability of this device, and its successors, in surpassing the computational prowess of classical computers – albeit their algorithms and hardware are rapidly advancing too.

Having said that, Helios’ computations have just been random benchmark tests, so the practical importance of this leap forward is still limited. To perform quantum computations of practical importance from areas of science and commerce, even the most optimistic estimates suggest that quantum computers must be enormously bigger and better – of the order of a million qubits.

The UK National Quantum Strategy Mission 1 proposes to have a fault-tolerant quantum computer (FTQC) with a trillion operations. For reference, the longest computation Helios ran had about 4,000 operations operating on 98 qubits.

Building FTQCs out of the Helios device, or any other known architecture or system for that matter, remains a formidable challenge. As the numbers suggest, it would require connecting thousands of QCCD devices, each with thousands of qubits, via quantum links.

Visions exist for QCCD devices with a two-dimensional grid layout resembling city streets that can operate on thousands or more qubits. The inevitable challenge of engineering and operating such a system with an immense number of junctions and concomitant qubit-jams – akin to traffic jams – is yet to be encountered, let alone overcome. The physical transporting of qubits is also a very slow process because it uses very precise voltages, and this will only grow with larger devices and more qubits.

As of now, the Helios device is an exciting advance in the field of quantum computing. Whatever form the FTQCs of the future take, they will involve imaginatively engineered quantum hardware devices assisted by smart software.