When does a quantum computer actually help with quantum biology?

Quantum computing and quantum biology sound like they were made for each other. One is a machine built on quantum mechanics. The other is life apparently using quantum mechanics to do something useful. Put them together and surely the machine cracks the biology. That is the headline you keep seeing. We wanted to know if it is true yet, so we picked a real case and measured it instead of guessing.

The case: a compass made of chemistry

Birds, sea turtles, and probably eels seem to sense Earth’s magnetic field. The leading explanation is genuinely strange. Light hits a protein in the eye and knocks an electron loose, leaving two electron spins that are quantum-correlated. Earth’s weak magnetic field nudges how those spins evolve, and the chemical outcome depends on which way the animal’s head points. A compass, built out of quantum mechanics, wired to vision. It is also, conveniently, a physics problem you can hand to a computer.

One: a laptop already wins

The version of this compass that fits on today’s quantum computers is small. Small enough that an ordinary laptop solves it exactly, in a fraction of a second. We pushed the ordinary methods as far as they go and they kept working well past where people assume they quit. Running the small version on a quantum computer proves the machine functions. It teaches you no new biology. A lot of "we did it on a quantum computer" results are really "we reproduced, on an expensive noisy machine, something a cheap reliable one already does." We wanted to be clear about which kind this was.

Two: the popular shortcut is off

When the system gets too big for the exact method, chemists reach for a famous approximation from the 1970s that treats the magnetic nuclei as a random blur. It is fast and it is everywhere. We checked it against the exact answer. It overshoots the magnetic effect by roughly a factor of two in the realistic case, and by much more when one nucleus dominates. That is a real, standalone result. Some published numbers that lean on the shortcut are probably inflated.

Three: today’s hardware is deaf to it, and we know why

We built the quantum circuit correctly. On a perfect simulator it nails the answer. Then we turned on realistic hardware noise, and the signal did not merely fade. It vanished at the second step up in size. The reason is elegant. The compass signal is a difference. You compare the chemistry with the field on versus off, and the small gap between them is the whole story. Hardware noise scrambles both cases by about the same amount, because the noise does not care about the magnetic field. Subtract, and the noise cancels and takes the real signal with it. The machine is not lying to you. It is deaf to the exact thing you are trying to hear. Error-correction tricks rescued only the simplest case, and only barely.

So when does it help?

We did the arithmetic for the full molecule, the version no ordinary computer can ever finish. It needs on the order of 29 perfect logical qubits and hundreds of millions of careful operations, which in practice means tens of thousands of physical qubits wrapped in error correction. That is not a 2026 machine. It is the kind of fault-tolerant quantum computer the roadmaps point at for roughly the end of this decade. So the honest headline is not "quantum computer solves biology." It is "quantum computers cannot do this useful biology yet, here is exactly why, and here is the year to watch."

In a field full of quantum hype, measuring the limit felt more useful than dancing around it. The wonder is real. A quantum compass in a living eye is a wild idea, and on the hardware we even watched the signal fade as we dialed the field down toward the levels of an ancient near-collapse of Earth’s magnetism. The honesty about what the machine can and cannot do yet is the part we are proud of.