D-Wave demonstrates entanglement with dual-rail qubits

D-Wave, long known for building quantum annealers, is now also developing gate-based quantum computers. After acquiring startup Quantum Circuits, the company adopted dual-rail qubits. In a paper published Wednesday in Nature, D-Wave reports entangling two such qubits and confirms that the dominant error remains easily detectable photon loss.

Dual-rail qubits store a single photon in a superposition across two linked resonators—left and right. The most common error is photon escape, also called an erasure error, which can be detected without requiring additional qubits for error correction. Phase flips are the next most frequent error, while bit flips are extremely rare. This error hierarchy enables a simpler and more compact error-correction code.

In the experiment, D-Wave built two dual-rail qubits connected by a tunable coupler. By activating the coupler for about 200 nanoseconds (with the whole entanglement taking 500 ns), they performed an entangling gate. The results showed photon loss at roughly 0.5% per entanglement, five times more frequent than other errors, with bit flips practically non-existent at the 10⁻⁶ level. The company highlights that this preserves the error hierarchy, which is important because operations can distort the relative frequency of error types.

There is still work to be done. The researchers observed rising error rates as the number of gates increased, possibly due to calibration drift. They also need to develop mid-circuit erasure detection and refine error-correction theory for large differences between error classes. D-Wave plans to reach 181 dual-rail qubits by 2028, with the eventual goal of hosting hundreds of logical qubits.