The butterfly, the fruit fly and the quantum robin (pt. 3)

The final part.

Electronic bonds between atoms are formed when a pair of electrons are shared. They are entangled, and almost always in a singlet. They can remain entangled even when separated into different atoms (and are thus called free radicals). There is now a possibility that one of the electrons has flipped spin - so now the radicals are in a superposition of being in a singlet and triplet state. However, the probabilities of existing in the corresponding states are not necessarily equal, and can be influenced by the angle of the magnetic field. As radical pairs are unstable, they tend to quickly recombine, forming the products of the chemical reaction. And here is where we can have the earth’s magnetic field, weak as it may be, influence chemical reactions. 

Schulten claimed that this mechanism could be behind birds’ magnetoreception - but where could this reaction be taking place? If their magnetoreception did not function when they were blindfolded, the eyes were an optimal candidate. Leask proposed this, and Wiltschko tested it. He transported pigeons away from their home, while ensuring they were under pitch-black conditions. They had trouble finding their way home, confirming that their magnetoreception did require access to light. 

Yet… cryptochrome, as mentioned earlier, has the ability to form free radicals when it interacts with light. A paper was published in 2000. The proposed process is as follows: 

1) A photon (wavelength corresponding to blue) is absorbed by FAD - a light-sensitive pigment molecule. 

2) An electron is ejected.

3) The remaining space is occupied by a donated electron from an entangled pair, from tryptophan (an amino acid in cryptochrome). 

4) The donated electron has the ability to remain entangled.

5) The entangled electrons form a superposition of singlet/triplet states. 

6) Because it is incredibly sensitive to the magnetic field, the angle of the magnetic field influences the products of the chemical reaction.

7) The results are somehow translated and interpreted by the bird’s brain.

In 2007, a group led by Mouritsen confirmed that cryptochrome produced free radicals when exposed to blue light. Further evidence: in 2004, researchers found three types of cryptochrome in robins’ eyes. Now, how do we gather evidence for our findings?

In this proposed chemical compass, the oscillations would have an incredibly high frequency - millions per second (superposition between singlet and triplet, which have different frequencies). Thus, if we expose them to oscillations with an incredibly high frequency, it would disrupt this ability. Ritz and Wiltschko, working together, designed and conducted an experiment, and found that a high-frequency field, oscillating at 1.3 million cycles per second, was able to disrupt the birds’ ability to orient themselves. Their findings also determined that the entangled electrons must survive for at least a millionth of a second in order to experience these oscillations. 

Note that a later study, upon removing the background electromagnetic noise, found a wider range of frequencies which disrupted their ability. This suggests that we are missing a piece - Vedral has proposed that the entangled pairs must last tens of microseconds - longer than earlier researchers thought. 

The research outlined above has focused on the robin, but cryptochrome is present in other organisms that have magnetoreception, including chickens and fruit flies - and even cockroaches.