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Quantum Compass of Migratory Birds
devices to detect weak magnetic felds and to use the GMF to navigate. Te anisotropic hyperfne cou
pling (HPC) between the electron spins and the surrounding nuclear spins can play a crucial role in
avian magnetoreception. Te HPC can afect not only the product yields but also the entanglement of
the electron spin states. By involving more nuclear spins one can greatly enhance the quantum entangle
ment (Sadiek et al., 2008). Additionally, mimicking this anisotropic magnetic environment can be very
useful for creating detectors of weak magnetic felds. By studying the role of the intensity of the mag
netic feld in avian navigation, for instance, Zhang et al. (2015) found that birds could be able to detect
the change of the GMF intensity and the approximate direction of parallels instead of sensing the exact
direction. Te plausible mechanism in which birds can utilize the signal has been investigated.
While the strong magnetic feld efects have been investigated and understood in depth, Kerpal et al.
(2019) made progress to obtain a more complete picture of the typically much less pronounced sensitiv
ity to weak felds. Experimentally, only one study on a model system has succeeded in providing proof
for an isotropic Earth strength efect, while an orientation dependence of the magnetic feld efect was
only observed for felds >3 mT (Maeda et al., 2008). Te orientation dependence in this high feld region
is caused by anisotropic HPCs in the radical pair, the anisotropic dipolar coupling being negligible
compared to HPCs or indeed their anisotropies (D = 0.06 mT for the center-to-center distance of 3.6 nm
in this pair) (Di Valentin et al., 2005). Previously only founded in theoretical simulations, it is specu
lated that these anisotropic HPCs in a radical pair with restricted motion may result in an orientation-
dependent magnetic feld response even in extremely weak magnetic felds including the GMF (Ritz
et al., 2000).
Using a custom-designed transient absorption spectrometer, Kerpal et al. (2019) verifed this hypoth
esis by testing if a quantum compass can function in felds as weak as the GMF. Tis is not only cru
cial regarding the discussion of the magnitude of any expected efects, but, importantly, the quantum
dynamics in high- and low-feld regimes are dominated by diferent processes (Lewis et al., 2018). Te
previously demonstrated existence of a chemical compass response of certain radical pair-based reac
tions in high felds (Maeda et al., 2008) is, therefore, a necessary condition but by no means sufcient
to explain the avian compass sense within the quantum system’s low feld regime (Kerpal et al., 2019).
As shown in Figure 4.3, chemical structure, photocycle, and time dependence of the magnetic feld
efect (MFE) of carotenoid-porphyrin-fullerene (CPF) moieties are presented by Kerpal et al. (2019).
Figure 4.3a shows the structure of the investigated model chemical compass, a molecular triad consist
ing of covalently linked carotenoid (C), porphyrin (P), and fullerene (F) moieties. Its photophysical
behavior and response to high felds, in the absence and presence of resonant radiofrequency felds,
have been studied previously (Kodis et al., 2004; Maeda et al., 2008, 2011, 2015). As shown in Figure 4.3b,
photoexcitation of the porphyrin at 532 nm is followed by rapid intramolecular electron transfer, frst
generating a primary radical pair C−P+−F•− of picosecond lifetime, before subsequent electron transfer
leads to the formation of the secondary radical pair C•+−P−F•−, which lives for up to roughly a micro
second. Previous work, in similar solvent and temperature conditions to those employed here, demon
strated that this secondary radical pair is formed predominantly in the singlet state, S[C•+ ± P−F•−], with
just 7% of radical pairs being created in the triplet state (Maeda et al., 2011). While each radical pair is
born in a spin-correlated state (either singlet or triplet), the magnetic feld characteristics of the radical
pair ensemble are complex.
Te measurements were carried out at 120 K, where the solvent, 2-methyltetrahydrofuran (MTHF),
forms an optically transparent glass. Recombination of C•+ ± P−F•− is possible from either the singlet or
triplet states and occurs with rate constants kS and kT, respectively. Te rates are strongly dependent on
the solvent properties, notably its dielectric constant. Under similar conditions, kS has been shown to be
some three orders of magnitude faster than kT, and consequently, a signifcant change in the recombina
tion kinetics is observed upon application of a magnetic feld (Maeda et al., 2011).
Most experimental investigations of magnetic feld efects have relied on optical methods in which
either the concentration of the radicals themselves or of one of their recombination products is deter
mined as a function of feld. Here nanosecond transient absorption spectroscopy was used to obtain