Quantum Computer With 22,593 Cat Qubits Could Break RSA-2048 in 3.14 Days

A new preprint estimates that a quantum computer built from 22,593 so-called cat qubits could break RSA-2048 encryption, a widely used standard for online security, in 3.14 days. ScienceAlert reported on the work, which comes from a team led by researchers at the quantum computing company Alice & Bob, and which has not yet been peer reviewed.

The result is a calculation on paper, not a demonstration. No one has a quantum computer of that size, and the study's authors acknowledge that major engineering hurdles remain.

What RSA-2048 protects and how it could be cracked

RSA encryption helps protect activities such as social media use, software updates and banking transactions, according to ScienceAlert. The RSA-2048 version works by multiplying two large prime numbers into a modulus up to 617 decimal digits long. Anyone can see that number, but breaking the lock requires finding the two original primes, a step called factorization.

In 1994, mathematician Peter Shor designed a quantum algorithm that can speed up that factorization exponentially. The new study is a resource estimate for running Shor's algorithm: how many qubits and how much time a machine would need.

Why cat qubits change the estimate

Cat qubits are an alternative to the transmon qubits used in many quantum computers. Named after the Schrödinger's cat thought experiment, a cat qubit is made of photons trapped in a tiny metal box, or oscillator. According to ScienceAlert, this design exponentially suppresses one main type of quantum error, bit-flips, while increasing the other type, phase-flips, only linearly.

That built-in protection reduces the hardware needed for error correction. Alice & Bob's own estimates have fallen from 350,000 qubits in 2023 to just under 23,000 now. The figure is about 40 times lower than the million qubits that Google Quantum AI's Craig Gidney and colleagues previously estimated, though the new number rests on Alice & Bob's own assumptions about error rates and error correction. Estimates from Gidney's group had themselves dropped from 20 million qubits in 2019 to below one million in 2025.

The numbers in the study

• 22,593 cat qubits: enough to factor an RSA-2048 key in 3.14 days.
• 610,530 cat qubits: linking 10 machines could do the job in 1.25 hours.
• 29,588 cat qubits: could break the elliptic-curve locks that secure Bitcoin ownership in 61.8 minutes.
• 10 minutes: Bitcoin's block time, a window the attack would need to beat to steal coins in transit.

The researchers say that, to their knowledge, no published design is smaller at this runtime. They describe a trade-off in which a machine can be too big to build or too slow to matter.

Why it matters for everyday security

For ordinary internet users, the study does not signal an immediate threat. ScienceAlert noted that the largest current array, built by Caltech engineers, holds 6,100 qubits trapped by lasers, and that it is an array rather than a functional computing machine. Challenges in temperature control, scalability and stability, including linking many chips, still have to be overcome.

The estimate also rests on a demanding assumption: that the cat qubits can avoid bit-flip errors for 100 years. Many proven qubits may stay stable for only milliseconds. Alice & Bob physicists recently reached an average bit-flip time of 44 minutes by putting 11 photons into a cat qubit. According to the study, relaxing the requirement to about one hour would roughly double the qubit count to around 45,000.

The result adds to evidence that the resources needed to break current encryption keep shrinking. Some observers, ScienceAlert reported, think RSA-2048 could be deprecated around the start of the next decade.

What happens next

The paper is posted on the Cryptology ePrint Archive and still awaits peer review. In their press release, the researchers say the early 2030s could bring together better hardware and lower resource requirements, so the first machines might be built within years. The timeline for any real-world threat remains uncertain.

Around Prague, based on reporting by ScienceAlert