Over the last 100 years, scientists have realized, first in rats, that neurons in mammalian brains were capable of producing photons, or "biophotons." The photons appear, though faintly, within the visible spectrum, running from near-infrared through violet, or between 200 and 1,300 nanometers. The question is why?
The team wanted to know whether or not there existed an infrastructure over which light could travel from one place to another in the brain across the distances required, focusing on myelinated axons. Axons are the fibers that carry a neuron’s electrical signal outward; myelinated axons are covered in myelin, a fatty substance that electrically insulates the axon.
They modeled such axons, doing computations on how light would behave as the fibers bent, lost or gained thickness in their biophoton-absorbing myelin coating, or how they’d behave when crossing each other. The team concluded that light conduction across myelinated axons is feasible.
The axons could pass between 46% and 96% of the light they receive over a distance of 2 millimeters, the average length of a human brain’s axons, the percentage depending on bending, sheath thickness, etc. They also worked out that, though rat brains can pass just one biophoton per neuron a minute, human brains, with many more neurons, could convey more than a billion biophotons per second. All together, the researchers conclude, “This mechanism appears to be sufficient to facilitate transmission of a large number of bits of information, or even allow the creation of a large amount of quantum entanglement.” So there's what could act as an entire network for light-based communication in place. But we don’t know what, if anything, it’s doing. The researchers proposed a set of in vitro and in vivo experiments for others to perform that could confirm their findings.
Meanwhile, did they say “entanglement?” Given the presence here of photons, the possibility has to cross one’s mind, since they go hand in hand, as it were, with entanglement. In the paper, the scientists are intrigued in particular with the interactions between photons and nuclear spins — the way nuclei turn causes different chemical effects — and how that affects things like magnetoreception in animals.
Given that there’s some distance between the biophotons and nuclear spins, the scientists wonder if there’s entanglement involved, saying, “for individual quantum communication links to form a larger quantum network with an associated entanglement process involving many distant spins, the nuclear spins interfacing with different axons must interact coherently. This, most likely, requires close enough contact between the interacting spins. The involvement of synaptic junctions between individual axons may provide such a proximity mechanism.” And since some people think entanglement could be behind whatever process it is that produces consciousness, well, where is this going to lead?
The core element of our quantum repeater is a cube of glass. We put two independent photons in, and as long as we can detect two photons coming out the other sides we know that we can perform entanglement swapping.
Nature Communications today published research by a team comprising Scottish and South African researchers, demonstrating entanglement swapping and teleportation of orbital angular momentum ‘patterns’ of light. This is a crucial step towards realizing a quantum repeater for high-dimensional entangled states.
Quantum communication over is integral to security and has been demonstrated in and fibre with two-dimensional states, recently over distances exceeding 1200 km between satellites. But using only two states reduces the information capacity of the photons, so the link is secure but slow. To make it secure and fast requires a higher-dimensional alphabet, for example, using patterns of light, of which there are an infinite number. One such pattern set is the (OAM) of light. Increased bit rates can be achieved by using OAM as the carrier of information. However, such photon states decay when transmitted over long distances, for example, due to mode coupling in fibre or turbulence in free space, thus requiring a way to amplify the signal. Unfortunately such “amplification” is not allowed in the quantum world, but it is possible to create an analogy, called a quantum repeater, akin to optical fibre repeaters in classical optical networks.
An integral part of a is the ability to entangle two photons that have never interacted – a process referred to as “entanglement swapping”. This is accomplished by interfering two photons from independent entangled pairs, resulting in the remaining two photons becoming entangled. This allows the establishment of entanglement between two distant points without requiring one photon to travel the entire distance, thus reducing the effects of decay and loss. It also means that you don’t have to have a line of sight between the two places.
Alphabet of OAM modes. OAM modes are sometimes called twisted light as the light appears as a ring with a vortex in the middle. The light can be twisted once, twice, three times and so on to create a high-dimensional alphabet. Credit: Wits University
An outcome of this is that the information of one can be transferred to the other, a process called teleportation. Like in the science fiction series, Star Trek, where people are “beamed” from one place to another, information is “teleported” from one place to another. If two photons are entangled and you change a value on one of them, then other one automatically changes too. This happens even though the two photons are never connected and, in fact, are in two completely different places.
In this latest work, the team performed the first experimental demonstration of entanglement swapping and teleportation for orbital angular momentum (OAM) states of light. They showed that quantum correlations could be established between previously independent photons, and that this could be used to send information across a virtual link. Importantly, the scheme is scalable to higher dimensions, paving the way for long-distance with high .
Schematic of the experiment. Four photons are created, one pair from each entanglement source (BBO). One from each pair (B and C) are brought together on a beam splitter. When all four photons are measured in together one finds that photons A and D, which previously where independent, are now entangled. Credit: Wits University
Background
Present communication systems are very fast, but not fundamentally secure. To make them secure researchers use the laws of Nature for the encoding by exploiting the quirky properties of the quantum world. One such property is entanglement. When two particles are entangled they are connected in a spooky sense: a measurement on one immediately changes the state of the other no matter how far apart they are. Entanglement is one of the core resources needed to realise a quantum network.
Yet a secure communication link over long distance is very challenging: Quantum links using patterns of light languish at short distances precisely because there is no way to protect the link against noise without detecting the photons, yet once they are detected their usefulness is destroyed. To overcome this one can have a repeating station at intermediate distances – this allows one to share information across a much longer without the need for the information to physically flow over that link. The core ingredient is to get independent photons to become entangled. While this has been demonstrated previously with two-dimensional states, in this work the team showed the first demonstration with OAM and in high-dimensional spaces.
During almost four years of observing the cosmos, the Herschel Space Observatory traced out the presence of water. With its unprecedented sensitivity and spectral resolution at key wavelengths, Herschel revealed this crucial molecule in star-forming molecular clouds, detected it for the first time in the seeds of future stars and planets, and identified the delivery of water from interplanetary debris to planets in our solar system.
Out to much grander scales, beyond our solar system and the Galactic confines of the Milky Way, Herschel has detected water in many other galaxies. As already highlighted by some of its predecessors, the findings corroborate the crucial role of this all-important molecule in the processes that lead to the birth of stars throughout the cosmos.
Herschel’s infrared view of part of the Taurus Molecular Cloud, about 450 light-years from Earth and is the nearest large region of star formation, within which the bright, cold pre-stellar cloud L1544 can be seen at the lower left at top of the page, I surrounded by many other clouds of gas and dust of varying density.
Water is essential to life as we know it on Earth. It covers over 70 percent of our planet's surface and is present in trace amounts in the atmosphere. While it may seem abundant, especially if we're looking at the blue-hued stretch of a lake, sea or ocean, water is only a minor component of the total mass of Earth. In fact, it is not at all clear whether the water that is currently present on our blue planet was there around the time of its formation, 4.6 billion years ago, or it is was delivered by later impacts of smaller celestial objects.
According to one of the leading theories to explain how the solar system came into being, Earth and the inner planets were extremely hot and dry for the first several hundred million years after their formation. In this scenario, water was delivered to these planets only later by violent impacts of small bodies such as meteorites, asteroids, and/or comets – the remaining debris of the protoplanetary disc out of which the planets and their moons took shape.
There are various avenues to investigate the origin of this crucial molecule on our planet, either following the clues in our cosmic neighborhood – the solar system – or looking into the stellar nurseries where analogues of our sun and planets are being born.
ESA's Herschel Space Observatory, an extraordinary mission that was launched in 2009 and that observed the sky at far-infrared and sub-millimetre wavelengths for almost four years, took a comprehensive approach, tracing water from stars and planets in the forming across our Milky Way galaxy to planets and minor solar system bodies in our own neck of the woods.
Water was first detected in star-forming molecular clouds in the late 1960s. At the time, it was the sixth interstellar molecule to be identified, compared to the nearly 200 that are known to date. Ever since its discovery, astronomers suspected that water would be present in a variety of cosmic environments. After all, it is made up of the two most abundant reactive elements that exist – hydrogen, which dates back to the Big Seed, and oxygen, produced in the furnaces of stars throughout the history of the Universe.
The mosaic below combines several observations of the Taurus Molecular Cloud performed by ESA's Herschel Space Observatory. Located about 450 light-years from us, in the constellation Taurus, the Bull, this vast complex of interstellar clouds is where a myriad of stars are being born, and is the closest large region of star formation.
In fact, water has been observed in celestial objects as diverse as planets, moons, stars, star-forming clouds, and even beyond our Milky Way, in the stellar cradles of other galaxies. However, due to the water vapour present in the Earth's atmosphere, studying this molecule with astronomical observations is anything but trivial.
Over the decades, astronomers have used a wide range of facilities to study water in the cosmos, from ground-based observatories in the dry climate of mountain-tops and airborne telescopes to experiments on stratospheric balloons and space observatories and even on the Space Shuttle. Far from the moist environment of our planet, a space telescope is of course the ideal tool to investigate cosmic water.
The first satellite dedicated to this topic, ESA's Infrared Space Observatory (ISO), was launched in 1995 and operated until 1998, shortly followed by NASA's Submillimeter Wave Astronomy Satellite (SWAS) and Spitzer Space Telescope, and by the Swedish-led, international Odin satellite.
Stepping into this long-established tradition, Herschel pushed the quest of cosmic water to new heights with a phenomenal piece of hardware, the Heterodyne Instrument for the Far Infrared (HIFI) – one of the three instruments on board.
To reveal the presence of a molecule in a cosmic source, astronomers look for a set of very distinctive fingerprints, or lines, in the source's spectrum, which are caused by rotation or vibration transitions in the structure of the molecule.
These lines are observed within a stretch of the electromagnetic spectrum, covering infrared to microwave wavelengths, depending on the type of molecule and its temperature. In the case of water, some of the most interesting lines – the ones that correspond to the lowest energetic configuration of water vapour, in other words its ground or 'cold' state – are found in the far-infrared and sub-millimetre ranges, which are inaccessible from the ground.
Specially designed for the hunt for water and other molecules, Herschel's HIFI instrument had an unprecedented spectral resolution that could target about 40 different water lines, each coming from a different transition of the water molecule and thus sensitive to a different temperature.
In particular, unlike its predecessors, Herschel was sensitive to two different transitions of the ground state of water that correspond to the two 'spin' forms of the molecule, called ortho and para, in which the spins of the hydrogen nuclei have different orientations. This key feature allowed astronomers to determine the temperatures under which the water formed by comparing the relative amounts of ortho and para water.
Two of the observatory's Key Programs– Water in Star-forming regions with Herschel and Water and Related Chemistry in the solar system – dedicated several hundred hours to the quest for cosmic water.
Exploiting the outstanding data collected by HIFI, along with observations performed with Herschel's two other instruments, the Photodetector Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE), astronomers have been able to greatly expand our understanding of the role of water in the Universe.
While water vapor in star-forming regions had been known for quite a while, Herschel discovered it, for the first time, in a pre-stellar core – a cold lump of dense material that will later turn into a star. The pre-stellar core, called Lynds 1544, is located in the Taurus molecular cloud, a vast region of gas and dust that is incubating the seeds of future stars and planets.
With the Herschel data, astronomers could estimate also the amount of water vapor in Lynds 1544 – the equivalent of over 2000 times the water content of Earth's oceans. Spectrum of water vapor shown below. The water vapor derives from icy dust grains, hinting at a reservoir of over a thousand times more water in the form of ice. If any planets are to emerge around the star taking shape from this core, it is likely that some of the water detected by Herschel will find its way to the planets as well.
En route to becoming stars, pre-stellar cores keep accreting matter from their parent cloud until they separate from it, turning into a protostar, an independent object that is collapsing under its own gravity. Normally, a rotating disc of gas and dust – a protoplanetary disc – takes shape around protostars, providing the material for the formation of future planets. Finally, when nuclear reactions ignite in the core of the protostar, counteracting the collapse, a fully-fledged star is born.
Herschel has spotted water in objects spanning all stages of star formation, including in a large number of low-mass protostars found in many nearby star-forming regions.
For the first time, astronomers using Herschel have detected cold water vapour in a protoplanetary disc. While previous studies had revealed either hot water vapour in the inner part of similar discs, or water ice in their outskirts, Herschel's observations targeting the disc around the nearby young star TW Hydrae were the first to identify cold water vapour, with temperatures lower than 100 K, in such an object.
The cold vapour appears to be located in a thin layer at intermediate depths in the disc, where the evaporation of gas and the freeze-out of ice find a balance. The data indicate a small amount of cold vapour, equivalent to about 0.5 per cent of the water in Earth's oceans, but point to a much larger reservoir of water ice – several thousand Earth oceans – in the disc.
This was the first evidence that large amounts of water ice can be stored in the precursor of a planetary system like our own, thus contributing more evidence to tackling the puzzle of the origin of water on Earth and other planets.
Besides proving that water is an important constituent of stars and planets since their early formation, Herschel also followed its trail all the way to our local neighbourhood, the solar system.
To compare water found in different celestial bodies, astronomers analyse the relative abundance of molecules with a slightly different composition. Most notably, they look at the D/H ratio, comparing 'ordinary' water, composed of two hydrogen (H) and one oxygen (O) atoms, and semi-heavy water, where one of the hydrogen atoms appears as deuterium (D), an isotopical form with an extra neutron.
Before Herschel, this measurement had been performed on a handful of comets, all of them thought to originate in the Oort cloud at the outskirts of our solar system, and all of them revealing higher proportions of deuterium to 'normal' hydrogen than that found in Earth's oceans. These results seemed to suggest that comets – icy leftovers of our ancient protoplanetary disc – could not have been the source of our planet's water, while a specific class of meteorites, called Cl carbonaceous chondrites, possessed the 'right' D/H ratio and thus seemed to be the main culprit.
In 2011, Herschel's observations of water in Comet 103P/Hartley 2 reopened this fascinating debate. This measurement was the first of its kind performed for a Jupiter-Family comet – a class of comets with orbits governed by Jupiter's gravity and with much shorter period with respect to their Oort-cloud counterparts – and revealed, for the first time, water with a deuterium to hydrogen proportion similar to that found on our planet.
Herschel contributed two more observations to the debate, finding a Jupiter-Family comet (45P/Honda-Mrkos- Pajdušáková) with Earth-like water, and an Oort-cloud comet (2009P1) with a different blend from that of our planet's water.
The plot thickened when ESA's Rosetta mission reached Comet 67P/Churyumov–Gerasimenko in 2014 and sampled the water content in its atmosphere. Rosetta's comet is also a Jupiter-Family one but, unlike the two observed by Herschel, it does not contain Earth-like water; on the contrary, it turned out to have the highest D/H ratio ever measured for a comet.
While Rosetta revealed that not all Jupiter-Family comets contain water that is similar to that of our planet's oceans, Herschel's earlier detections had importantly pointed out that comets with the right composition do exist and some might indeed have contributed to Earth's water budget.
In fact, current models indicate that a broad and diverse range of minor bodies contributed to the critical role of bringing water to our planet.
Elsewhere in the solar system, Herschel has gone as far as confirming that at least one comet has contributed to enriching a different planet – Jupiter – with water. By investigating the distribution of water vapour in the stratosphere of the giant planet, astronomers found evidence that almost all of it was delivered by the famous impact of Comet Shoemaker-Levy 9 in 1994.
Following water throughout the solar system, Herschel has found this molecule in many more places, from the dwarf planet Ceres, the largest body in the asteroid belt, to a giant torus of water vapour surrounding Saturn, which appears to be supplied by the planet's small moon Enceladus.
As revealed by the NASA/ESA/ASI Cassini mission, Enceladus exhibits plumes of water drawing from the underground ocean lurking under its icy crust.
Farther away from the sun, Herschel revealed highly reflecting surfaces on several Trans-Neptunian Objects (TNOs), indicating that water ice might be present even on these ancient, remote objects. While TNOs date back to the early formation of our solar system, astronomers suspect that their bright icy coating may be more recent – a speculative but not unfeasible hypothesis given the availability of water on outer planets like Uranus and Neptune, and on their major moons. Such a recent coating might also suggest that the surface of these long-thought 'dead' objects can in fact be alive, as highlighted also by the in-situ observations performed in 2015 by NASA's New Horizon probe of another TNO, the dwarf planet Pluto.
Given its chemical composition, water unsurprisingly is ubiquitous in the Universe, and, after Herschel, there is no longer any doubt that cosmic water trails go a long way, from planets to stars, and even to the vastness of interstellar space.
However, Herschel has only begun scratching the surface of the proverbial iceberg, having spotted water in individual cosmic sources that are, in many cases, one of a kind. These exciting discoveries call for future surveys to follow up on Herschel's observations, collecting larger samples of each type of sources to scrutinise water and other molecules and delve into the physical mechanisms underlying their formation and delivery across the cosmos.
