Quantum computing progress: higher temperatures, better error correction

There is a strong consensus that tackling the most useful problems with a quantum computer will require the computer to be able to correct errors. However, there is absolutely no consensus on what technology will allow us to achieve this. A large number of companies, including major players such as Microsoft, Intel, Amazon and IBM, have committed to different technologies to get there, while a host of startups are exploring an even broader range of potential solutions.

We probably won’t have a clearer idea of ​​what will probably work for a few years. But there will be a lot 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 taste of that work, we’ll look at three papers that were published in recent weeks, each addressing a different aspect of quantum computing technology.

hot things

Bug fixes will require connecting multiple hardware qubits to act as a single unit called a logical qubit. This distributes a single bit of quantum information across multiple hardware qubits, making it more robust. Additional qubits are used to monitor the behavior of those holding the data and make necessary corrections. Some error correction schemes require more than a hundred hardware qubits for each logical qubit, meaning we would need tens of thousands of hardware qubits before we could do anything practical.

Several companies have looked at that problem and decided that we already know how to create hardware at that scale: just look at any silicon chip. So if we could etch useful qubits using the same processes we use to make today’s processors, then scaling wouldn’t be an issue. Typically, this has meant fabricating quantum dots on the surface of silicon chips and using them to store individual electrons that can contain a qubit in their spin. The rest of the chip contains more traditional circuitry that performs startup, control, and readout of the qubit.

This creates a notable problem. Like many other qubit technologies, quantum dots must be kept below 1 Kelvin to prevent the environment from interfering with the qubit. And, as anyone who’s ever owned an x86-based laptop knows, all the other circuitry on the silicon generates heat. Therefore, there is the very real possibility that trying to control qubits will increase the temperature to the point that the qubits cannot maintain their state.

That may not be the problem we thought, according to work published Wednesday in Nature. A large international team including people from the startup Diraq has shown that a silicon quantum dot processor can operate well at a relatively warm 1 Kelvin, compared to the usual milliKelvin at which these processors typically operate.

The work was performed on a two-qubit prototype made with materials specifically chosen to improve noise tolerance; The experimental procedure was also optimized to limit errors. The team then performed normal operations starting at 0.1 K and gradually increased temperatures up to 1.5 K, checking performance as they did so. They found that one of the main sources of errors, state preparation and measurement (SPAM), did not change dramatically in this temperature range: “SPAM around 1 K is comparable to that at mikelvin temperatures and remains viable at least up to 1 .4K”.

The error rates they saw depended on the state in which they were preparing. One state in particular (both spin-ups) had a loyalty of more than 99 percent, while the rest were less restricted, somewhere above 95 percent. The states had a lifetime of more than a millisecond, which is considered long-lived in the quantum world.

All of which is pretty good and suggests that the chips can tolerate reasonable operating temperatures, meaning that the on-chip control circuits can be used without causing problems. The error rates of hardware qubits are still well above what would be necessary for error correction to work. However, the researchers suggest that they have identified error processes that can potentially be compensated. They hope that the ability to do industrial-scale manufacturing will ultimately lead to hardware that works.