Harnessing the weird properties of entangled photons

October 26, 1999

After battling the strangeness of time and space, it must have taken a lot to freak out Albert Einstein. Yet there was one aspect of quantum physics that brought him out in a cold sweat: entanglement.

Take two photons, for instance, tie them together with an unbreakable quantum bridge and their link becomes so close it is almost telepathic. Einstein described this apparently supernatural behaviour as "spooky". It seems, for instance, that performing an experiment on one photon of an entangled pair instantaneously affects its partner, whether they're in the same lab, or at different ends of the Universe. No surprise, then, that researchers in Austria have recently used this bizarre trick in quantum teleportation experiments (New Scientist, 14 March 1998, p 26).

But relax, says Malvin Teich of Boston University's department of electrical and computer engineering. "This spooky behaviour has been verified time and time again," he says, and simply comes from the fact that every pair of entangled photons are born from the same photon mother.

It turns out that these twins can be pretty handy. So handy, in fact, that a growing band of researchers-including Teich and colleagues Bahaa Saleh and Alexander Sergienko-are dispensing with normal light altogether and turning instead to Einstein's tormentors. They have found that they can use their perfectly synchronised behaviour to reveal exactly what happens when the brain and the peripheral nervous system go wrong. With a beam of entangled photons and a microscope, they can light up the finest details inside delicate nerve cells, without frying them to death.

And by harnessing the weirder properties of entangled photons, Teich and his colleagues believe they can move into previously inaccessible territory that by rights they shouldn't even be able to see: the intimate quantum secrets of a single atom.

Surprisingly for such bizarre beasts, entangled photons are extremely simple to create. All you need is a laser beam and a "nonlinear" crystal-one made from a material such as beta barium borate, whose optical properties depend on its orientation. Fire the laser light into the crystal and if the alignment is just right, each photon in the laser beam splits in half, and two beams emerge instead of one.

Like any quantum particle, each photon can also be described as a wave. Mathematically, this is represented by a "wave function"-a kind of fuzzy cloud which defines the probability of finding a particle at any particular point in space and time. For a photon, the wave function describes properties such as its colour and direction of travel.

But, says Teich, a photon loses its individuality when it forms one half of an entangled pair. At the instant of their creation-like twins emerging from the womb-entangled photons have a good number of shared characteristics. Their polarisation, direction of travel and colour are all linked: not necessarily identical, but definitely related. In the quantum world, this entangled pair is described by a 2-particle wave function that is inseparable. "In fact," says Teich, "it's a single entity that just happens to be made up of two photons."

Until recently, this abstract idea has been little more than an interesting conundrum and a useful window into the strangeness of the quantum world-a physicist's plaything that could eventually lead to unbreakable codes and unimaginably powerful quantum computers (New Scientist, 22 August 1998, p 26). Instead, says Teich, why not put them to work right now? Their roles may be less glamorous, but the results should be no less amazing. And what better to start with than the ultimate optical microscope?

One of the simplest ways to look deep inside delicate living cells is with a technique called fluorescence microscopy. Add fluorescent dye to your sample and, depending on the dye molecules' size and charge, they will be taken up by different parts of the cells. Now focus a spot of light onto them and the dye glows brightly wherever it is present, showing up previously invisible detail. Scan the focused spot around and you can measure fluorescence anywhere in the sample. And with image-processing software, you can turn the fluorescence intensity map into a three-dimensional image of a cell.

Tightly focused
Unfortunately, you won't only see fluorescence from the spot where the light is focused-dye in the tissue above and below this point will also glow and blur the image. So researchers reduce this "noise" by replacing light bulbs with narrow laser beams that can be tightly focused into minute spots. They also insert tiny pinholes at certain points in the microscope which block out most of the unwanted fluorescence.

However, solving one problem produces another: the pinholes also cut out a lot of the real signal and the image becomes faint. The only solution is to crank up the power of the laser beam. But dyes used to stain cells are broken down by bright light. Without the fluorescence there's no image, but under intense light, the dyes can also react with nearby molecules of water or protein, churning out energetic free radicals that chemically attack and kill the cells. Delicate samples simply don't stand a chance.

Physicists thought they had found the answer with two-photon microscopy. This technique relies on dye molecules that only fluoresce when they absorb two photons simultaneously. Fluorescence only comes from areas where the laser beam is brightest-at its focus, for instance-since it is only here that a dye molecule has a good chance of absorbing two photons at the same instant. Dye anywhere else in the sample can't fluoresce since it can't absorb light.

But even this isn't perfect, says Teich. Researchers still need to pour loads of light onto their samples, since the chances of two photons arriving at a single dye molecule just nanometres across, within billionths of a microsecond of each other, are pretty slim. "They need to jack the intensity way high," says Teich. And that's bad news for fragile cells.

Teich and Saleh have a solution: a ready-made beam of photons, arriving perfectly two by two, wherever and whenever you want them. They call the idea entangled photon microscopy: instead of using normal laser light-and waiting for the chance arrival of two photons-an entangled light source provides photons that only ever come in pairs (see Diagram).

Entangled photons give an added advantage: they come down different beam paths. The time and place where the photons in these two paths meet can be accurately controlled by fine-tuning the position of the microscope's mirrors. The result is high-resolution 3D imaging, with easily directed, low-intensity light.

Where better to unleash this technique than on some of the most intricate and delicate tissues around: the neurons of the brain. Together with Sergienko, biology professor Kristen Harris and a research grant of $960 000 from the David and Lucile Packard Foundation, Saleh and Teich plan to use entangled photons to shed light on the workings of dendritic spines. These are tiny nerve junctions-synapses-that pass signals from one nerve cell to another.

In humans, malformation during the development of these spines inhibits brain function, but no one knows how to ensure healthy growth. Harris is hoping that watching living, growing spines could provide some clues. "We know these spines are crucial to cognitive processes such as learning and memory," says Harris, "but we know little about how they work."

The spines are too small to be seen through today's light microscopes and other techniques such as electron microscopy only allow researchers to see into dead synapses and cells. The ability to look at healthy, living tissue in fine detail makes entangled photons seem attractive. But Teich is cautious about making too many early claims for them: he and Saleh have written up the theoretical idea and taken out a patent, but the group has yet to build a prototype microscope. And they are still working out exactly what problems quantum mechanics may throw in their path. "I am reasonably confident it's going to work," says Teich, "but there are quantum issues that come into play."

By this, Teich means that the photon pairs may not always be completely entangled when they are formed. When a photon of laser light splits into two, the photons of each pair it creates can pop into existence at slightly different times and in slightly different places. The gaps-in space and time-between them are tiny, but can be big enough so that their wave functions are not entirely inseparable. The extent of this "partial entanglement" changes the chances of their being absorbed, says Teich. "We need to know: will every pair of photons give rise to an absorption in the microscope, or only some portion of them due to the partially entangled nature of the light source?"

Teich and his team don't yet know the answers-they are still working on a full quantum mechanical description of the way that the dye absorbs entangled photons. And they are half-expecting some surprises along the way: a close examination of quantum mechanics, they have discovered, almost always throws up some bizarre new behaviour.

Take another one of their calculations, for example. A relatively simple investigation of the absorption of entangled photons by an atom has shown that these photons can use a quantum sleight of hand to reveal all of the atom's innermost secrets.

Electrons in an atom are restricted to certain discrete regions called orbitals, and the energy of each electron depends on the orbital it occupies. Suppose you wanted to discover the energy levels occupied by electrons in an atom of hydrogen-a common kind of problem for physicists. The best way to do this is to make the electrons jump up to higher energy levels by tickling them with a little light energy. As they drop back down to their original levels, they give out their energy as light at certain frequencies, and these frequencies reveal the relative positions of their energy levels.

But to ensure that every electron makes the jump, you have to give each one precisely the right amount of energy. And the only way to do this is to bombard them with light at a whole range of energies. That way you can be pretty sure you'll get it spot on. But only a few photons are absorbed-the rest play no part.

Enter the ultimate inquisitors: entangled photons. Use these instead of normal light and Teich calculates that you only need photons at a single energy to do the same job. As they interact with the hydrogen atom, they seem to grab information about all of its electron states in one go. Nothing seems safe from their prying wave function. The initial indications of this possibility shocked the researchers. "We were astonished," says Teich. "We had no idea that this would emerge from the math."

It's no magic trick, of course-it's just the way the quantum world works. Entangled photons have a wave function which includes information about the phase difference between them. This behaves like a blank sheet of paper: the atom can use it to "write down" an estimate of the probability that each of its electron's energy states will be occupied, based on the energy and phase of the entangled photons. In a kind of "quantum interference", the entire description of the electrons in the atom becomes folded into the photons' wave function.

To extract the information, you simply vary the degree of photon entanglement, for example by changing the electric field applied to your nonlinear crystal, or the delay time between the photons. If, at the same time, you measure how much light the sample absorbs-which relies on excited electrons-you'll quickly pry out the energy levels of every electron in the atom.

"Because of the magical relationship between the two photons, you can extract that information," says Teich. Measurements made with four or five different values of the electric field-producing pairs of photons all with the same energy, but with slightly varying phase relationships-should be enough to glean all the information about the electrons that Teich needs to lay the atom bare. Eventually, he believes, this technique may lead to an "automatic" spectroscopy machine that could reveal all kinds of hidden detail within an atom, without spending the time and effort to sweep through all possible energy permutations.

Examples like this may be the first signs of a coming quantum revolution, when entities with strange properties will take over the tasks that have, until now, been the preserve of the predictable world of classical physics. And the advance guard, led by Sergienko, is already implementing a new regime: quantum calibration.

Catch the light
Sergienko, who along with Teich and Saleh co-directs Boston University's Quantum Imaging Lab, has found a way to use entangled photons to determine a quantity that is usually very difficult to get a handle on: exactly how much light a material absorbs. This quantity-called the absolute quantum efficiency-is a measure of how many of the photons that strike a material are absorbed. It is used by engineers to calibrate light-sensitive materials.

This is an important task: these materials are used to make the sensors that pop up in everything from exposure meters in cameras and scanners at supermarket checkouts to the tiny photosensors in CD players. But to accurately measure the efficiency of a new light detector is almost impossible unless you can compare its performance with that of a sensor that has already been calibrated. However, these sensor "standards" are housed in only a few specialised labs around the world-places such as the National Institute of Standards and Technology (NIST) at Gaithersburg in Maryland. So portable sensors must be calibrated against these standards, and others calibrated against them. The sensor that calibrates a CD player's photodetector on the factory floor may be five, or even fifty times removed from the original, and its accuracy can be way off the mark.

So just how do you find out whether your new light sensor will spot every photon that hits it? With a source of entangled photons, this task becomes comparatively simple. The key, says Sergienko, is the unique correlation in time and space between a pair of entangled photons. Create such pairs with a nonlinear crystal and you generate two beams, with one photon of the pair in each. Now aim one beam onto your new material or sensor and the other at a detector capable of picking out single photons (see Diagram).

This detector acts like a signpost, telling you whenever a pair of entangled photons arrives. Watch and wait for this signpost to announce the arrival of an entangled photon and the moment it does, says Sergienko, you know that the photon's twin has just arrived at your test sensor.

So did the sensor see the photon? "If nothing happens, you know it has missed the photon," he says. Repeat this simple experiment a few hundred times-looking at the signal from the test sensor only when you know a photon has just hit it-and you'll learn exactly how many photons have arrived, and how many of these photons have been absorbed and detected. In other words, you'll measure its absolute quantum efficiency.

Best of all, this calibration process doesn't rely on the sensitivity of your signpost detector: its sole purpose is to tell you the precise moment that you must watch what your new sensor is doing. "You only look when you're absolutely sure a photon is arriving," Sergienko explains.

He is currently working with NIST to develop a prototype testing facility. If his scheme succeeds, it could make the NIST level of accuracy available to almost any lab or factory with access to a similar facility. "Quantum mechanics really is a democratic standard," says Sergienko. "We can define an absolute standard in every country around the world."

Sergienko suspects that entangled photons could also be used to probe biological systems that respond to light. What is the quantum efficiency of the human eye, for example? And how, exactly, do the arrays of chlorophyll molecules-the "antenna"-used by plants to collect sunlight work?

"The general motivation is very simple," says Sergienko. "The quantum system is more flexible than the classical one." And with persistence and creativity, Sergienko believes, there is an enormous amount that researchers can learn using entanglement. "So far it's been good enough to keep us busy for the next five to ten years. But the excitement of this is that the surprises just keep coming: you do one thing, touching different sides of the same process, it gives you more ideas."
Michael Brooks is a science writer based in Lewes, East Sussex
Email: mebrooks@compuserve.com

New Scientist issue date: 30th October 99

Further reading: "Entangled-photon virtual state spectroscopy", by Bahaa Saleh and others, Physical Review Letters, vol 80, p 3483 (1998)
"Entanglement-induced two-photon transparency", by Hong-Bing Fei and others, Physical Review Letters, vol 78, p 1679 (1997)
For more information see: http://photon.bu.edu/sergienko/home.html and http://photon.bu.edu/teich/


New Scientist

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