Robins May Use “Quantum Entanglement” for Migration

Summary:
The European robin, a migratory bird, is said to sense the direction of Earth’s magnetic field by using quantum entanglement, helping it navigate toward its destination. Although the mechanism has not yet been fully explained scientifically, I used this idea as a simple example for quantum computing and explored the question, “Why is quantum entanglement necessary?”

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🟢Robins may sense Earth’s magnetism using quantum entanglement

A short article in the Japanese science magazine Newton (May 2022) suggested that migratory birds might make use of quantum effects. According to the article, when a robin’s eye receives blue light, a protein in the retina called cryptochrome produces many pairs of electrons in a quantum-entangled state. These pairs come in two types:

[1] singlet, where the spins point in opposite directions, and
[2] triplet, where the spins are aligned.

It is said that by comparing the proportions of these two states, the bird can sense the angle of Earth’s magnetic field.

As illustrated below, if all pairs are in the singlet state, the bird perceives itself as facing parallel to the magnetic field and flies in that direction. If singlet and triplet appear in equal amounts, it perceives itself as perpendicular to the field and will look for another direction.

🟢Checking the idea with my own quantum-circuit simulator

The article itself was only a brief overview. Rather than stopping there, I decided to reproduce the situation using my own quantum-circuit simulator. Since the robin is thought to judge direction from the proportion of singlet states, I modeled this behavior for quantum computing and ran simulations.

To summarize the results: when the singlet proportion was 0.5, the corresponding angle was 90°, so the bird could not proceed. After “trying again,” the bird found a case where the singlet proportion became 0.93. The corresponding angle was 30°, which seems suitable for flight — so the bird could continue in that direction.

This is exactly a practical use of quantum computing. As an exercise for beginners, it is a very good learning problem.

In the circuit I created, two qubits are first flipped with X gates, then passed through a Bell circuit to generate a singlet. After that, I simply apply a rotation gate RY(θ) to the first qubit. By changing θ, the proportion of singlet states increases or decreases, representing different angles between the bird and Earth’s magnetic field.

🟢Testing on IBM’s real quantum computer

Up to this point, everything was done in my simulator. However, when I ran the same circuit on IBM’s real quantum hardware, I obtained nearly the same results, as shown below.

🟢Why is quantum entanglement necessary?

The discussion so far is already complete in one sense — but why do we need quantum entanglement at all? In fact, if we remove the Bell circuit from the circuit above, it still seems possible to change the proportion of singlet and triplet states. That is a very good question, and my answer is as follows.

In short, I believe entanglement is required in order to follow “the rules of the quantum world.” Although I did not explain it earlier, in the first diagram I wrote the words “cancellation” near the singlet and “amplification” near the triplet. These words actually matter.

First, it would be extremely difficult, considering environmental disturbances, to prepare many pairs of electrons whose spins are perfectly anti-aligned from the beginning. On the other hand, if the electron pairs are entangled, it becomes possible to maintain a stable situation in which “the total spin is zero.” This is what I meant by “cancellation.”

Next, Earth’s magnetic field is very weak. When electron pairs are in an entangled state, it becomes easier for them to transition between singlet and triplet with only a very small amount of energy. For example, when moving to the triplet state, the electrons do not respond to the magnetic field one by one. Instead, they respond as an entangled pair, so their sensitivity can be amplified. That is the meaning of the term “amplification” in the triplet diagram.

In summary, if we were only changing the proportions of singlet and triplet states in a purely logical or statistical sense, quantum entanglement would not seem necessary. However, when we take into account the actual “behavior of quantum systems inside living organisms,” the story changes. What happens inside the eye of the European robin is not just a simple chemical reaction, but rather a highly refined form of “quantum computing” shaped by nature over tens of millions of years.