Paul Kwiat asks his volunteers to sit inside a small, dark room. As their eyes adjust to the lack of light, each volunteer props his or her head on a chin rest—as you would at an optometrist’s—and gazes with one eye at a dim red cross. On either side of the cross is an optical fiber, positioned to pipe a single photon of light at either the left or the right side of a volunteer’s eye.
Even as he verifies the human eye’s ability to detect single photons, Kwiat, an experimental quantum physicist at the University of Illinois at Urbana–Champaign, and his colleagues are setting their sights higher: to use human vision to probe the very foundations of quantum mechanics, according to a paper they submitted to the preprint server arXiv on June 21.
Rather than simply sending single photons toward a volunteer’s eye through either the left or the right fiber, the idea is to send photons in a quantum superposition of effectively traversing both fibers at once. Will humans see any difference? According to standard quantum mechanics, they will not—but such a test has never been done. If Kwiat’s team produces conclusive results showing otherwise, it would question our current understanding of the quantum world, opening the door to alternative theories that argue for a dramatically different view of nature in which reality exists regardless of observations or observers, cutting against the grain of how quantum mechanics is interpreted today. “It could possibly be evidence that something’s going on beyond standard quantum mechanics,” says Rebecca Holmes, Kwiat’s former student who designed the equipment, and who is now a researcher at the Los Alamos National Laboratory.
The effort to determine whether humans can directly detect single photons has a storied history. In 1941 researchers from Columbia University reported in Science the human eye can see a flash from as few as five photons landing on the retina. More than three decades later Barbara Sakitt, a biophysicist then at the University of California, Berkeley, performed experiments suggesting that the eye could see a single photon. But these experiments were far from conclusive. “The problem with all these experiments is that they were just trying to use ‘classical’ light sources” that do not reliably emit single photons, Holmes says. That is, there was no guarantee each of these early trials involved just one photon.
Then, in 2012, came firm evidence that individual photoreceptors, or rod cells, can detect single photons—at least in the eyes of a frog. Leonid Krivitsky of the Agency for Science, Technology and Research in Singapore and his colleagues extracted rod cells from adult frogs’ eyes and performed laboratory tests showing the cells reacted to single photons. Now, “there’s absolutely no doubt that individual photoreceptors respond to single photons,” Kwiat says. That is not the same as saying those rod cells do the same in a living frog—or, for that matter, a human being. So Kwiat, along with Illinois colleague physicist Anthony Leggett and others, began envisioning tests of human vision using single-photon sources. Soon Kwiat’s group, which now included Holmes, was actually experimenting. But “we got beat on that,” Holmes says.
In 2016 a team led by biophysicist Alipasha Vaziri, then at the University of Vienna, reported using single-photon sources to show “humans can detect a single-photon incident on their eye with a probability significantly above chance.”
Kwiat’s team, somewhat skeptical of the result, wants to improve the statistics by doing a much larger number of trials with many more subjects. Their key concern is the low efficiency of the eye as a photon detector. Any incident photon has to get past the cornea, the clear outer layer of the eye, which reflects some of the light. The photon then enters a lens that, together with the cornea, focuses the light on the retina at the back of the eye. But between the lens and the retina is a clear, gel-like substance that gives the eye its shape—and this too can absorb or scatter the photon. Effectively, less than 10 percent of the photons that hit the cornea make it to the rod cells in the retina, which result in nerve signals that travel into the brain, causing perception. So getting statistically significant results that rise above chance is a daunting challenge. “We are hoping in the next six months to have a definitive answer,” Kwiat says.
That has not stopped them from dreaming up new experiments. In the standard setup a half-silvered mirror steers a photon to either the left or the right fiber. The photon then lands on one side or the other of a volunteer’s retina, and the subject has to indicate which by using a keyboard. But it is trivial (using quantum optics) to put the photon in a superposition of going through both fibers, and onto both sides of the eye, at once. What occurs next depends on what one believes happens to the photon.
Physicists describe a photon’s quantum state using a mathematical abstraction called the wave function. Before the superposed photon hits the eye its wave function is spread out, and the photon has an equal probability of being seen on the left or the right. The photon’s interaction with the visual system acts as a measurement that is thought to “collapse” the wave function, and the photon randomly ends up on one side or the other, like a tossed coin coming up “tails” or “heads.” Would humans see a difference in the photon counts on the left versus the right when perceiving superposed photons as compared with photons in classical states? “If you trust quantum mechanics, then there should be no difference,” Kwiat says. But if their experiment finds an irrefutable, statistically significant difference, it would signal something amiss with quantum physics. “That would be a big. That would be a quite earth-shattering result,” he adds.
Such a result would point toward a possible resolution of the central concern of quantum mechanics: the so-called measurement problem. There’s nothing in the theory that specifies how measurements can collapse the wave function, if indeed wave functions do collapse. How big should the measuring apparatus be? In the case of the eye, would an individual rod cell do? Or does one need the entire retina? What about the cornea? Might a conscious observer need to be in the mix?
Some alternative theories solve this potential problem by invoking collapse independently of observers and measurement devices. Consider, for instance, the “GRW” collapse model (named after theorists Giancarlo Ghirardi, Alberto Rimini and Tullio Weber). The GRW model and its many variants posit wave functions collapse spontaneously; the more massive the object in superposition, the faster its collapse. One consequence of this would be that individual particles could remain in superposition for interminably long times whereas macroscopic objects could not. So, the infamous Schrödinger’s cat, in GRW, can never be in a superposition of being dead and alive. Rather it is always either dead or alive, and we only discover its state when we look. Such theories are said to be “observer-independent” models of reality
If a collapse theory such as GRW is the correct description of nature, it would upend almost a century of thought that has tried to argue observation and measurement are central to the making of reality. Crucially, when the superposed photon lands on an eye, GRW would predict ever-so-slightly different photon counts for the left and the right sides of the eye than does standard quantum mechanics. This is because differently sized systems in the various stages of the photon’s processing—such as two light-sensitive proteins in two rod cells versus two assemblies of rod cells and associated nerves in the retina—would exhibit different spontaneous collapse rates after interacting with a photon. Although both Kwiat and Holmes stress it is highly unlikely they will see a difference in their experiments, they acknowledge that any observed deviation would hint at GRW-like theories.
Michael Hall, a theoretical quantum physicist at the Australian National University who was not part of the study, agrees GRW would predict a very small deviation in the photon counts, but says such deviations would be too tiny to be detected by the proposed experiment. Nevertheless, he thinks any aberration in the photon counts would deserve attention. “It would be quite serious. I find that unlikely but possible,” he says. “That would be amazingly interesting.”
Kwiat also wonders about the subjective perception of quantum states versus classical states. “Is there any perceptual difference on the part of the person when they directly observe a quantum event?” he asks. “The answer is ‘probably not,’ but we really don’t know. You can’t know the answer to that unless either you have a complete physical model down to the quantum mechanical level of what’s going on in the human visual system—which we don’t have—or you do the experiment.”
Robert Prevedel, a member of Vaziri’s 2016 team who is now at the European Molecular Biology Laboratory in Germany, is more interested in teasing out exactly where collapse actually occurs in the chain of events. Does it happen at the beginning, when a photon strikes a rod cell? Or in the middle, with generation and transmission of neural signals? Or does it happen at the end, when the signals register in conscious perception? He suggests firing superposed photons at extracted retinas and recording from different levels of visual processing (say, from rod cells or from the different types of photo cells that make up the retina) to see how long the superposition lasts.
Prevedel thinks first absorption by a rod should destroy the photon’s superposition. But “if we can see quantum [superposition] in any of the subsequent levels inside the different cell layers in the retina, or any downstream neuronal circuits even, that would be really a breakthrough,” he says. “This would be an amazing finding.”
There is, of course, an elephant in the room: human consciousness. Could conscious perception ultimately cause the collapse of the quantum state, making the photon show up on one or the other side? Prevedel doubts consciousness has anything whatsoever to do with measurement and collapse.
“Consciousness…arises in our brain as the combined effect of millions, if not billions, of cells and neurons. If there is a role of consciousness in the detection of quantum superposition, it’d involve a really macroscopic object on the level of the entire brain, i.e. a huge ensemble of atoms and electrons that make up the biological cells,” Prevedel says. “From all that we know, this kind of macroscopic object would not be able to sustain quantum [superposition].”