The proposed experiment would use a beam-splitter to create path-entangled photons sent to a single-photon detector and a human observer. Before detection, a displacement in phase space—accomplished with coherent light from a laser, incident on an unbalanced beam-splitter—would amplify the entanglement signal, potentially allowing detection by the unaided human eye. [Image: Valentina Caprara Vivoli]
The strange phenomenon of quantum entanglement has by now been established many times in the lab, using computers and sophisticated single-photon detectors. A team of scientists led by Nicolaus Sangouard of the University of Basel, Switzerland, now proposes an experiment that, the researchers suggest, could in principle allow entangled photons to be detected directly by the experimentalist’s eye (Optica, doi: 10.1364/OPTICA.3.000473). While the proposed experiment faces some practical hurdles, the scientists believe that their feasibility study offers a path toward “realizing the first experiment where entanglement is observed with the eye.”
Finding an amplifier
The human eye, while tuned by eons of evolution to be superb at picking out predators on the African savannah, has some shortcomings as a quantum-entanglement detector. Sensitivity constitutes a particular problem: Previous experiments have shown that, on average, coherent states of several hundred photons need to be incident on the eye for a glimmer of light to be perceived. And the number of photons from such a collection that actually reach a photoreceptor on the retina is much smaller—perhaps only seven, if the eye is modeled as a threshold detector with a transmission efficiency of 8 percent.
Any hope of naked-eye detection of entanglement, therefore, requires some way to amplify the signal from the entangled photon, without destroying the delicate entangled state itself. Sangouard, along with colleagues Valentina Caprara Vivoli of the University of Geneva, Switzerland, and Pavel Sekatski of the University of Innsbruck, Austria, suggest that using a stream of coherent light to systematically add the same displacement in phase space to each of the two spatially separated, entangled quantum states could act as just such a “magnifying glass,” allowing the human eye to turn into an entanglement detector.
Phase-space displacement
The team’s proposed experiment begins with the creation of a pair of photons through the process of spontaneous parametric down-conversion. One of the photons is detected directly, acting as a herald for the existence of the other photon. The heralded twin photon, meanwhile, passes through a beam-splitter that creates a pair of entangled quantum states (specifically, entanglement between two optical modes sharing a delocalized single photon) that are directed over different paths—one to a photon-counting detector, and the other to a human experimentalist.
Before the entangled optical modes reach the detector or the human eye, however, they are passed through an unbalanced beam-splitter on which a laser showers photons in a coherent state—an apparatus that creates the desired phase-space displacement in the entangled modes at both the human and nonhuman ends of the path. The displacement has two effects. First, it serves as a state-independent photon amplifier, boosting the signal from the entangled quantum state to the hundred-photon threshold required for detection by the human eye, without breaking the entanglement. And, second, because the amplitude and phase of the displacement are statistically related to the probability of a detection event in both the eye and the single-photon detector, the setup, over many repeated measurements, allows entanglement to be inferred.
The big hurdles: Time and patience
Sangouard and his colleagues acknowledge that their intriguing thought experiment has a rather significant real-world pitfall: “According to an initial estimate,” Sangouard says, “several hundreds of thousands of runs would be necessary until we have enough data to determine if we've actually detected entangled photons.” That would translate into hundreds of hours of lab time for the (very unfortunate) human subject doing the detecting. In a more practical vein, though, the team notes that the proposal offers some insight on how other kinds of threshold detectors might be upgraded, using the same kind of coherent amplification, to the point at which they become useful in quantum-optics experiments.
Whatever the practical limitations of this first-cut experimental proposal, the researchers see a lot of appeal in the general problem of human detection of these exotic phenomena. “It is safe to say that probing the human vision with quantum light is a terra incognita,” they conclude in their paper. “This makes it an attractive challenge on its own.”