Alice & Bob Boson 4 (cat qubit chip)

Overview

Alice & Bob Boson 4 is a single qubit chip, but it is the clearest demonstration so far of the idea behind the company's technology. Most qubits are vulnerable to two kinds of error, bit flips and phase flips. A cat qubit is designed so that bit flips become exponentially rare, leaving only phase flips to worry about. On Boson 4, the average time between bit flips exceeded seven minutes, compared with milliseconds for typical transmon qubits.

Boson 4 became available to outside users in 2024, which made it the first cat qubit that researchers worldwide could test on the cloud.

History and background

Alice & Bob was founded in 2020 in Paris by Théau Peronnin and Raphaël Lescanne, building on research from the École Normale Supérieure and related groups. Its early chips, the Boson series, each tackled a design problem. One step was removing a transmon element that was a source of bit flips, and Boson 3, featured in a Nature paper in 2024, showed bit-flip times above ten seconds. Boson 4 raised that to over seven minutes.

In September 2025 the company announced a newer design called Galvanic Cat, with bit-flip times between roughly 33 and 60 minutes, which surpassed the 13-minute target that its 2030 roadmap had set for sustained bit-flip protection. The company's next chips, the Helium series, combine 12 to 16 cat qubits to build a logical qubit that operates below the error-correction threshold.

How it works

A cat qubit lives in a microwave resonator, which can hold any number of photons. The qubit's two states are two large 'coherent' states, one with the field swinging one way and one with it swinging the opposite way, named after Schrödinger's cat because a general quantum state is a superposition of both. The distance between the two states grows with the average number of photons.

To keep the qubit in this two-state subspace, the chip uses a nonlinear buffer circuit that removes photons from the resonator in pairs, driven by a microwave pump. This engineered two-photon dissipation acts like a restoring force: if noise nudges the state away, the process pushes it back, but it cannot easily convert one coherent state into the other, which would require a bit flip. The larger the average photon number, the more completely bit flips are suppressed.

The trade-off is in the other error. Photon loss from the resonator causes phase flips, and the rate grows in proportion to the average photon number. Alice & Bob's design therefore targets a balance, with a modest photon number (such as around 11 in the 2025 experiment) that gives very long bit-flip times while keeping phase flips manageable for a repetition code.

Connectivity

Single qubit; later Helium chips connect 12 to 16 cat qubits in a loop for error-correction experiments

Specifications in detail

Key features

• Bit-flip lifetime of over 7 minutes, up from about 10 seconds in the previous generation
• Bit flips are suppressed exponentially as the number of photons in the cat state grows
• Phase flips increase only linearly, so only one error type needs correcting
• Available to outside users through a cloud marketplace
• Part of a roadmap that leads to the Helium, Lithium, Beryllium, and Graphene chips

Limitations

• Boson 4 is a single qubit, so it does not show multi-qubit computation
• Phase-flip errors remain and must be corrected
• Fast, high-fidelity gates on cat qubits are harder than on conventional transmons
• The path to 100 logical qubits by 2030 is a plan, not a result

Timeline

What it is used for

• Studying bosonic error correction and repetition codes with cat qubits
• Benchmarking bit-flip and phase-flip behavior
• Developing the hardware path to a fault-tolerant machine with fewer physical qubits

How it compares

• Compared with Google Willow, Boson 4 is a single qubit but its bit-flip protection is much stronger than any transmon in Willow's grid.
• Compared with IBM Heron, Boson 4 has no multi-qubit capability, but it points to a very different route to error correction.
• Compared with Amazon's Ocelot chip, which also uses cat qubits but combines them with transmons, Alice & Bob's approach aims to remove the transmon element.

Technical notes

In a cat qubit, the two computational states are two coherent states of a microwave resonator with opposite phase. A nonlinear circuit continually exchanges pairs of photons with a cold reservoir, which pushes the resonator back into the two states whenever it drifts. The bit-flip rate falls exponentially with the average photon number, while the phase-flip rate grows linearly. That asymmetry means a simple repetition code is enough to correct the remaining errors.

Why it matters

Alice & Bob claims that using cat qubits could reduce the number of physical qubits needed for a useful machine by a large factor. Boson 4 is the strongest evidence for the bit-flip half of that claim, and the cloud access means other groups can test it.

Whether the idea scales depends on what happens next: showing multi-qubit gates on cat qubits and a below-threshold logical qubit on a Helium chip. Those are the results to watch.

Frequently asked questions

Keep exploring

• Company profile: Alice & Bob
• Related processor: Google Willow
• Related processor: IBM Heron (r1, r2, and r3)
• Related processor: Xanadu Borealis

This profile is written for educational purposes and summarizes company announcements, published papers, and vendor specification sheets. Last reviewed: September 30, 2026. Figures marked as company-reported have not all been independently verified, and benchmarks differ between vendors, so compare them with care.

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