Debate has long raged over whether biological functions follow the principles of quantum mechanics1,2. Light harvesting3 and olfaction4 are among the most cited systems in this discussion, but it has even been suggested that quantum mechanics may be a biasing force in evolution5. And other sensory mechanisms, such as vision6 and neuronal communications7, are quickly gaining ground. One of the main sources of opposition to these ideas is the difficulty in correlating the possible presence of quantum mechanisms in vitro and the effective biological behaviour in vivo. Now, writing in Nature Physics, Kush Paul and co-workers8 have established a direct correlation between an optical stimulus and the control of biological functions, by studying the response of a photoreceptor known as channelrhodopsin-2 (ChR2) in living brain tissue.

In living organisms, sensory information is processed by specific receptors that are capable of translating an external stimulus into an electrical potential, which then travels to a specific region of the brain to be interpreted. Opsins fall into a particular class of receptor known as photoreceptors, which are activated by external luminous stimuli. These light-gated ion channels are able to transduce a light signal into an electrical response, or an ionic current, which controls neuronal activity (Fig. 1a).

Figure 1: Light transduction by opsin.
figure 1

a, The absorption of a photon by the retinal chromophore induces a conformational change that leads to the opening of a pore, through which ions can flow to generate an ionic current. b, Stimulation with positively chirped pulses, phase shaped so that the low-frequency components are in the leading edge, invokes a larger ionic current. c, Negatively chirped pulses (with high frequencies in the leading edge) result in a smaller ionic current.

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By directly measuring the current output after stimulation with light, Paul et al. demonstrated that the amount of neuronal activity associated with ChR2 strongly depends on the coherent properties of the light pulse used as stimulus (Fig. 1b,c). To achieve this result, they applied the strategies of quantum control, a method for controlling dynamical processes by light. The basic principle is to control quantum phenomena — typically by modulating the phase and the chirp of a laser pulse9. Previous experiments had succeeded in verifying that it is possible to manipulate the chemical reaction at the base of opsin activity using tailored quantum coherent controlled pulses, but they were unable to draw a clear correlation between spectroscopic observables and opsin activity in a living cell10,11.

The results of Paul et al. are a first step in this direction and provide us with a somewhat different point of view: namely, that the question of whether nature exploits quantum mechanics should probably be replaced by whether nature is capable of responding to quantum mechanics. If the answer is yes, as now seems plausible, then the design of suitable tailored pulses may enable us to directly control cellular activities.

One could legitimately object that the photo-triggering light stimuli affecting real living systems are not phase-shaped laser pulses. This is certainly a sound argument. But producing suitably tailored quantum coherent light is becoming increasingly easier these days. And there is mounting evidence suggesting that biological complexes, such as opsins and antenna complexes, seem to be able to respond to quantum stimuli. This implies that — even if nature does not function quantum mechanically in biologically relevant conditions — it may still be possible to steer and control biological activity by manipulating quantum effects. In other words, quantum coherent control of living matter could be achieved. This is an unexpected — but very appealing — development that can be framed in the wider context of quantum technologies.