We're approaching the end of the year and there's typically a flurry of announcements about quantum computing at the moment, partly because some companies are looking to meet promised schedules. Most of these involve evolutionary improvements on previous generations of hardware. But this year we have something new: the first company to bring a new qubit technology to the market.
The technology is called a dual-rail qubit and is intended to make the most common form of errors trivially easy to detect in hardware, making error correction much more efficient. And while tech giant Amazon is experimenting with it, a startup called Quantum Circuits is the first to give the public access to dual-rail qubits via a cloud service.
While the technology itself is interesting, it also gives us an insight into how the field as a whole is thinking about making error-corrected quantum computing work.
What is a dual rail qubit?
Dual-rail qubits are variants of the hardware used in transmons, the qubits favored by companies like Google and IBM. The basic hardware unit connects a loop of superconducting wire to a small cavity through which microwave photons can resonate. This arrangement allows the presence of microwave photons in the resonator to influence the behavior of the current in the wire and vice versa. In a transmon, microwave photons are used to control the current. But there are other companies that have hardware that does the opposite: they control the state of the photons by changing the current.
Dual-rail qubits use two of these systems connected together, allowing photons to move from one resonator to the other. Using the superconducting loops it is possible to control the chance that a photon will end up in the left or right resonator. The photon's actual location will remain unknown until it is measured, allowing the system as a whole to contain a single bit of quantum information: a qubit.
