Alice & Bob and the École Normale Supérieure de Lyon have demonstrated a cat-qubit coupler driven by a steady DC voltage of roughly one microvolt — no microwave pump required — that exchanges photons at a two-photon rate of 3 MHz, faster than any pumped coupler the company has built to date. The team published its results as a preprint in August 2026 and announced them from Paris on October 1, 2026.

The experiment targets the core engineering problem in cat qubits: these superconducting bits store information in a microwave resonator and stay stable only if photons leak out of the resonator strictly in pairs. That pair-wise dissipation happens in a component called the coupler. In Alice & Bob's current cat qubits, the coupler runs on a carefully engineered microwave signal — an approach the company has refined over many years. The new device replaces the pump with a static voltage bias, and the data suggests the design could reach considerably higher exchange rates than 3 MHz.

How the coupler works

The device couples a high-quality "memory" resonator to a lossy "buffer" resonator through a SQUID biased with a DC voltage. A Cooper pair tunneling across the SQUID exchanges an energy of 2eV with the circuit, so it can only tunnel if the SQUID's environment can absorb or supply exactly that energy. The researchers set the bias so that 2eV matches the energy difference between two memory photons and one buffer photon. Each tunneling event converts a pair of memory photons into a single buffer photon, which quickly leaks out, producing an effective two-photon dissipation of the memory at a rate of up to 1.3 MHz in the measured configuration.

The DC-biased design delivered four results the researchers call out. First, speed: the coupler outperformed all previous pumped-drive cat-qubit couplers at a two-photon exchange rate of 3 MHz. Second, reconfigurability: changing only the bias voltage switched the same chip between exchanging photons one, two or four at a time — the two-photon process ran at a bias of about 1.1 microvolts. Third, the new mechanism suppresses a parasitic effect that plagues conventional stabilization, namely an unwanted shift of the resonator frequency with photon number that is harmful to cat codes. Fourth, using Wigner tomography to image the memory's state directly, the team observed photons dissipating in pairs.

Why four-photon processes matter

Rather than replacing the company's existing microwave-driven couplers, the DC approach adds to the toolbox. Its most consequential payoff may be access to higher-order processes such as four-photon interactions, which are very difficult to achieve with pumped methods at sufficient strength. Those processes underpin four-component cat qubits, which encode information across four coherent states and offer stronger protection against certain errors.

"This new approach could support the development of four-component cat qubits, which encode information across four coherent states. Four-to-one photon processes are very difficult to achieve with sufficient strength, but dc-biasing junctions seem to be a viable approach to that goal," said Benjamin Huard, scientific advisor at Alice & Bob and professor at ENS Lyon. The team demonstrated one-to-one and four-to-one photon swaps on the same device.

Scaling implications

The engineering argument for DC bias is thermal and spatial. Quantum processors sit inside ultra-cold dilution refrigerators with strictly limited cooling budgets, and microwave pump lines generate heat and clutter. Voltage lines are compact, easy to wire, and produce little heat inside the refrigerator — attributes that matter when the roadmap calls for thousands of couplers per machine.

Alice & Bob, founded in 2020 and headquartered in Paris and Boston, has raised €180 million and employs more than 250 people. The company claims its cat architecture can cut the hardware requirements for a useful large-scale quantum computer by up to 200 times compared with competing approaches. The new coupler does not change that figure on its own, but it removes two persistent constraints — pump hardware and photon-number-dependent frequency shifts — from the scaling path. Whether the DC-biased design holds its speed advantage at scale will become clear as Alice & Bob integrates it into larger cat-qubit lattices.