Barely a week goes past without another quantum computing announcement. Recent headlines have been grabbed by Honeywell, introducing their H1 10-qubit machine, but there are many other companies competing, including IonQ, IBM and Google, with DWave already out there with a quantum simulator, and a host of start-ups vying for the limelight in this very 21st Century technology race.
These new computers promise a wide range of applications which would be impossible for classical computers, and although many benefits to humanity are promised, there is also the threat implied by discovery of RSA private keys in just a few hours – something that is impossible for classical computers.This so-called quantum advantage comes from the way in which the qubits in a quantum computer can be entangled together; these quantum links can be manipulated so that the entire ensemble is set up to represent a complex problem. New quantum software tools are being developed that can present the problem in a form that a quantum computer can understand. The next stage seems a bit like magic: the quantum computer evolves the states of its entanglement qubit ensemble – usually without human intervention – until a new overall state is found, from which the solution to the original problem can be obtained.
Of course, the practice is a far cry from this. The main problem is that qubits lose their quantum connections through interactions with the environment – rather like the cooling of a cup of coffee. These quantum computers are therefore highly error-prone, and this has led to an intense effort to find error-correcting schemes. These come at a cost, namely more qubits and in fact MANY more qubits (potentially orders of magnitude more)
Nevertheless, vast global research funding is leading to dramatic improvements to quantum computers, with national governments weighing in, in what appears to be a race to the first quantum computer that shows significantly better performance than a classical computer. And the implications for cryptography are existential: RSA and similar schemes will need to be upgraded and ultimately replaced.