Study suggests quantum computer could crack RSA-2048 encryption in three days

Alice & Bob preprint puts cat-qubit requirement at about 23,000, but the machine does not yet exist

By LineZotpaper

Published5 November 2026

Read Time3 min

Illustration: Study suggests quantum computer could crack RSA-2048 encryption in three days

A preprint study from researchers at quantum computing firm Alice & Bob estimates that a machine built with about 23,000 "cat" qubits could crack RSA-2048, the encryption standard protecting much of the internet's data, in just over three days. The team says its architecture puts it at the frontier of size and speed, though the paper has not yet been peer reviewed and no such computer exists.

Using Shor's algorithm, the team calculates that a framework of 22,593 cat qubits could factor an RSA-2048 key in 3.14 days. Cat qubits, named after Schrödinger's cat thought experiment, trap photons in a small metal box, or oscillator, and are designed to exponentially suppress bit-flip errors while only linearly increasing phase-flip errors.

RSA-2048 is the digital padlock protecting much of the internet, from banking to software updates. Cracking it requires finding the two large prime numbers whose product forms a modulus up to 617 decimal digits long, a factorization Shor's algorithm, designed by mathematician Peter Shor in 1994, could exponentially speed up.

Resource estimates for Shor's algorithm have been consistently falling, from 20 million qubits in 2019 to below one million in 2025, according to Google Quantum AI's Craig Gidney and colleagues, the researchers note. Alice & Bob's own cat-qubit estimates have dropped from 350,000 in 2023 to just under 23,000, around 40 times fewer than the million previously estimated by Gidney, though the latest estimate uses its own error rates and correction capabilities. "To our knowledge, no published architecture is smaller at this runtime, which puts us at the frontier of size and speed together," the researchers say in a blog post.

The design balances qubit count and runtime, since the qubit count dictates the size of the machine and its hardware, including the cryogenics needed to cool quantum chips. "You can trade one for the other, but push too far either way and you end up with a machine that is too big to build, or too slow to matter," the blog post explains. Linking 10 quantum computers to yield 610,530 cat qubits could crack RSA-2048 in 1.25 hours. The researchers also estimate that 29,588 cat qubits could bypass the elliptic curve encryption securing Bitcoin in 61.8 minutes, but they note Bitcoin transactions occur within a 10-minute block time, so stealing cryptocurrency in transit is not yet feasible.

Important caveats remain. No one has a 23,000-qubit quantum computer; the largest array, developed by Caltech engineers, has 6,100 qubits trapped by lasers, and is an array rather than a functional computing computer. The estimates assume cat qubits can avoid bit-flip errors for 100 years, while many proven qubits may be stable only for milliseconds. Alice & Bob recently achieved an average bit-flip time of 44 minutes by placing 11 photons in a cat-box, improving standard stability by millions of times. The study suggests that relaxing the bit-flip time to about one hour would require doubling the qubit count to around 45,000.

The researchers suggest the early 2030s could see improved hardware and lower resource requirements converge, meaning the first machines could be built in a matter of years, though the timeline of actual threats remains uncertain. The study is available as a preprint on the Cryptology ePrint Archive and has not yet been peer reviewed.

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Analysis

Why This Matters

RSA-2048 is a backbone of internet security, protecting banking, software updates and private communications; a quantum machine able to break it in days would expose that data once built.
The estimated qubit requirement has fallen from 20 million in 2019 to under 23,000, suggesting the hardware needed for a practical attack is far smaller than earlier worst-case projections.
The same approach could also threaten Bitcoin: the researchers estimate its elliptic curve encryption could be bypassed in about an hour, although the 10-minute block window still blocks in-transit theft.

Background

Quantum computers use qubits, which can hold more than a simple on-off state, and are being explored for tasks that are impractical for classical machines. Shor's algorithm, published in 1994, is the best-known quantum method for factoring large numbers, the problem underpinning RSA encryption. Cat qubits are a hardware design in which photons are trapped in an oscillator, named after Schrödinger's cat; they are engineered to make one class of errors, bit-flips, exponentially rare while tolerating more of the other, phase-flips. The new paper is a resource estimate, calculating the qubit count and runtime such a machine would need, rather than a demonstration of a working attack.

What to Watch

Whether cat-qubit stability improves beyond the 44-minute average bit-flip time Alice & Bob recently reported; shorter stability would push qubit requirements up, with a one-hour relaxation doubling the count to about 45,000.
Whether quantum hardware scales from the current largest array of 6,100 qubits toward the tens of thousands of qubits the estimate requires.
Whether the convergence of improved hardware and lower resource requirements that the researchers project for the early 2030s materialises, which would determine when the first machines could be built.

Sources

A Quantum Computer Could Crack Major Internet Encryption in Just 3 Days, Study Suggests · ScienceAlert