Quantum computers are coming right along at an ever increasing pace. Unfortunately. though, this progress could soon see a bit of a stall. Cosmic rays emanating from the deepest corners of space could compromise the integrity of the information in these quantum systems. Now, a team at MIT has shown how vulnerable these new computers are and how we can protect them.
In a conventional computer, data is stored as as either a 1 or a 0. But thanks to our understanding of quantum physics, we have been able to make computers that use ‘qubits’ which are able to exist in a superposition of more than one state at a time. Because of this, quantum computers can perform a huge number of calculations at the same time. This makes quantum computers much more powerful than existing systems.
There is, however, a lot of work to do in regard to making a practical quantum computer. Qubits have a low coherence time. This means that they can’t remain in their superpositional states for very long. This is because they are very, very sensitive to outside interference including magnetic and electric fields, and even the ultra low-level from the universe that surrounds us all.
We are constantly being showered with cosmic rays, and when those rays hit materials close to us, they produce secondary emissions which are basically atomic-sized pieces of shrapnel. These particles can do quite a number on sensitive quantum computers.
Now, MIT researchers at Pacific Northwest National Laboratory and Lincoln Laboratory have done a new study to confirm just how much this phenomena will affect these new quantum computers. This study has helped to quantify how much of a problem cosmic rays will actually be for quantum computers.
Researchers placed disks of irradiated copper close to superconducting qubits so that the effects of the radiation can be measured. The experiments took place inside something called a ‘dilution refrigerator,’ as to minimize other interference. A dilution refrigerator does so by cooling its environment to about 200 times colder than space itself. Then, the researchers placed a second irradiated copper disk outside the dilution refrigerator in order to determine how much radiation the quantum system exposed to.
The researches were able to find that qubits will only be able to hold their states for about 4 milliseconds. This was backed up by additional experiments that placed or removed radiation shielding between the source (the copper disks) and the qubits. While the shield did help, it’s not a very practical solution because the shield they had to use was a two-ton wall of lead.
So, when quantum computers become a think, they will need to be constructed with proper shielding in mind. It is possible that the best place to put a quantum computer might be deep underground. The good thing is, we already have experience doing things like this. Check out some of the neutrino-hunting experiments that need similar levels of protection from cosmic rays.
William Oliver, one of the study’s authors said, ‘If we want to build an industry, we’d likely prefer to mitigate the effects of radiation above ground. We can think about designing qubits in a way that makes them “rad-hard,” and less sensitive to quasiparticles, or design traps for quasiparticles so that even if they’re constantly being generated by radiation, they can flow away from the qubit. So it’s definitely not game-over.’