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Saturday, October 24, 2026

The Cat Qubit: A Qubit Built to Ignore One Kind of Error

Most qubits suffer two kinds of error and need a large grid of checks to fix both. A cat qubit is designed so that one kind almost never happens, leaving a simpler job.

A qubit made of microwaves in a box

An ordinary superconducting qubitThe basic unit of a quantum computer. Like a ‘bit’ in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once. is a tiny circuit with two energy levels. A cat qubitA qubit stored in a microwave resonator, using two states of the field that oscillate in opposite rhythm; the name comes from Schrödinger’s cat, because the qubit can be in both states at once. instead stores information in a superconducting resonator, a small box in which microwave photons bounce back and forth.

The trick is to remove photons in pairs. That pins the field into two stable states, which oscillate exactly out of step.1 One state is the 0, the other the 1. Despite the name, the “cat states” of many entangled atoms or ions, used in some architectures, are a different thing from cat qubits.

Two errors, one held off

Qubits suffer two kinds of error. A bit flip swaps 0 and 1. A phase flipAn error that leaves 0 and 1 alone but flips the sign between them in a superposition, which classical bits never suffer. leaves them in place but scrambles the delicate relation between them.

In a cat qubit, a bit flip means the field jumping all the way from one state to the other. Adding more photons pushes the two states farther apart, and the bit-flip rate falls exponentially.2 The cost is gentle: phase flips grow only in proportion to the photon number. The result is biased noiseNoise in which one kind of error is much rarer than the other..

In one 2025 experiment, with about two photons, bit flips took over a millisecond to appear, while phase flips arrived in about 30 microseconds.5

A line of checks, not a grid

Error correctionTechniques that combine many shaky physical qubits into fewer reliable ones, so a long calculation stays correct. normally guards against both errors, which is why the surface codeAn error-correcting code that spreads one protected qubit over a square grid of physical qubits. needs a two-dimensional grid. If bit flips are held off, only phase flips are left, and a single line of qubits with checks between neighbors can catch them.3 A distance-5 surface code uses 49 qubits in all. A distance-5 line uses five cat qubits plus a few helpers that run the checks.

This does not remove error correction; it moves part of the job into the hardware. Removing photons in pairs keeps correcting bit flips on its own, continuously, inside each qubit.

What experiments show, and the catch

A 2024 experiment pushed bit-flip times beyond 10 seconds, ten thousand times better than before, but its phase-flip times were just over 490 nanoseconds.4 And every gate must keep the bias, or bit flips sneak back in.

The 2025 experiment put five cat qubits in a line, building a logical qubitA reliable ‘qubit’ built by bundling many error-prone physical qubits together with error correction. These are the units that actually matter for useful computing..5 Its phase-flip checks ran below thresholdThe error rate below which making an error-correcting code bigger starts to help instead of hurt.: errors fell as the line grew from three to five, while bit flips stayed suppressed. Still, the best total logical error per round only improved from about 1.75% to 1.65%. Each round took about 2.8 microseconds, leaving a decoderThe classical algorithm that reads the pattern of error-correction alarms and bets on which errors most likely caused them. little time.5

As of October 2026, two companies build cat-qubit chips: AWS and Alice & Bob.57

A 2023 design study estimated that about 126,000 cat qubits could run one code-breaking calculation in 9 hours, if its hardware assumptions are met.6 When you read such a claim, ask how many photons are used, how long phase flips take, and whether gates kept the bias.

Three hand-drawn panels titled The Cat Qubit. Panel A, two states: two circled dots on opposite sides of a dashed cross, labeled 0 and 1. A curved arrow arcs from one to the other, labeled bit flip = jump across. Below: more photons: farther apart. Panel B, the trade: time before an error, on a log scale, against photons in the cat, from 1 to 4. A red line rises steeply, labeled bit flips: rarer and rarer. A blue line slopes gently down, labeled phase flips: a little more often. Panel C, one line of checks: five circled dots in a row with a small square check between each neighboring pair, labeled checks catch phase flips, and below, a line, not a grid.
Figure 1: Why cat qubits need fewer checks. A: the two states sit far apart, so a bit flip means a jump all the way across. B: adding photons makes bit flips rapidly rarer, while phase flips get only a little more frequent. Shapes are schematic. C: with bit flips held off, a single line of checks can be enough to catch the phase flips.

Sources

  1. M. Mirrahimi et al., “Dynamically protected cat-qubits: a new paradigm for universal quantum computation,” New J. Phys. 16, 045014 (2014). ↩
  2. R. Lescanne et al., “Exponential suppression of bit-flips in a qubit encoded in an oscillator,” Nat. Phys. (2020). ↩
  3. J. Guillaud and M. Mirrahimi, “Repetition Cat Qubits for Fault-Tolerant Quantum Computation,” Phys. Rev. X 9, 041053 (2019). ↩
  4. U. Réglade et al., “Quantum control of a cat-qubit with bit-flip times exceeding ten seconds,” Nature 629, 778 (2024). ↩
  5. H. Putterman et al. (AWS), “Hardware-efficient quantum error correction via concatenated bosonic qubits,” Nature 638, 927 (2025). ↩
  6. É. Gouzien et al., “Performance Analysis of a Repetition Cat Code Architecture: Computing 256-bit Elliptic Curve Logarithm in 9 Hours with 126133 Cat Qubits,” Phys. Rev. Lett. 131, 040602 (2023). ↩
  7. Alice & Bob, “Alice & Bob shares first results from France’s PROQCIMA quantum programme,” press release via EIN Presswire (October 20, 2026), https://www.einnews.com/pr_news/950767473/alice-bob-shares-first-results-from-france-s-proqcima-quantum-programme ↩