A French quantum hardware team tamed a specific error mode in 'cat qubits'

A joint Alice & Bob and École Normale Supérieure de Lyon (ENS Lyon) experiment shows a specific way to bleed off the photons that usually break multi-photon cat qubits. Cat qubits store information in an oscillator state named for Schrödinger's cat; keeping them intact is one of the harder jobs in quantum error-correction hardware.

The teams used a DC-voltage-biased SQUID, a small superconducting circuit, to link a high-Q memory resonator to a lossy buffer. The energy gained or lost by tunneling Cooper pairs, the paired electrons that carry current without resistance in a superconductor, matches the conversion of memory photons into a single buffer photon. The swap works at one, two, and up to four-to-one photon ratios. In the September 30 announcement, Alice & Bob reports a two-photon exchange rate of about 3 MHz and an effective two-photon dissipation rate up to 1.3 MHz, both company-reported numbers from a single preprint and trade-publication summary.

The architecture is incremental, not finished. The paper says unwanted Kerr and detuning terms only cancel at first order, leaving residual effects from spurious modes. Bias-voltage noise and radio-frequency filtering are flagged as practical constraints, and the swap itself cannot coherently move a quantum state between memory and buffer. It can only engineer dissipation. The work is one step in a long fault-tolerance relay.