Two particles are called entangled if they share the same fuzzy quantum state, meaning neither of them begins with definite properties such as location or polarization. Measure the polarization of one photon, and it randomly adopts a certain value, say horizontal or vertical. The polarization of the other photon will always match that of its partner.
Entanglement binds together individual particles into an indivisible whole. A classical system is always divisible, at least in principle. But an entangled system cannot be broken down in this way. Entangled particles always act as one entity.
Entanglement of two particles means that you can know everything there is to know about a composite system and yet not know everything about the individual constituents.
To some degree, the entire universe is entangled. Imagine putting electron number 1 in atom number 1 and electron number 2 in atom number 2. After waiting a while it doesn’t make sense to say that ‘electron number 1 is still in atom number 1’. It might be in atom number 2 now because there is always the chance that the electron did a quantum hop. Remember, in quantum physics everything that can happen does happen, and electrons are free to roam the universe from one instant to the next.
Since the two entangled electrons are a part of one reality, they cannot occupy the same energy level, even though they are in two different atoms and even though they might be occupying the lowest energy state in each of the respective atoms. They “know” what energy level the other one is occupying and make sure not to occupy the same space. This is because, in reality, they are one, regardless of how far apart they are. (The difference in the two energies is very small – but it exists). This logic extends to more than two atoms – if there are 24 hydrogen atoms scattered across the universe that are coupled then there are now 24 energy states, all taking on almost, but not quite, the same values.
When an electron in one of the atoms settles into a particular state it does so in full ‘knowledge’ of the state for each of the other 23 electrons, regardless of their distance away. And so every electron in the universe knows about the state of every other electron. We need not stop there – protons and neutrons are fermions too, and so every proton knows about every other proton and every neutron knows about every other neutron. There is an intimacy between the particles that make up our universe and extend across the entire universe. (It is ephemeral in the sense that for particles that are far apart, the different energies are so close to each other as to make no discernible difference to our daily lives.) Theoretically, if some sort of signaling apparatus were to exploit this coupling, we could have instantaneous communication across the universe. But this is impossible to do. (Einstein called it ‘spooky action at a distance’ and did not like it.)
We can’t perceive all the information when it’s held among entangled particles; that information is their collective secret. Things are inexorably changed by our trying to ascertain anything about them. But once the work has been done among the entangled particles, then we can look.
Physicists now routinely produce pairs of entangled photons that share, say, one polarization state between them. (However, this is only for short periods of time. The record for laboratory-induced entanglement is 50 microseconds. Also, it is generally only done with a limited number of particles because given more, the particles are likely to get entangled with external particles – they leak too much information into the environment, causing them to behave classically.) Individual atoms have also been entangled as have macroscopic objects, such as wafers of synthetic diamond. Anton Zeilinger recently observed entanglement in light flickered between two of the Canary Islands, 144 kilometers apart. Most demonstrations of entanglement at most involve a handful of particles. Larger batches are hard to isolate from their surroundings. The particles in them are likelier to become entangled with stray particles, obscuring their original interconnections. (In scientific language, we say that too much information leaks out of the environment, causing the system to behave classically.)
However, an experiment in 2003 proved that larger systems, too, can remain entangled. Lithium fluoride salt was put in a magnetic field at very low temperatures to avoid the random motions of heart energy. The atoms in the salt aligned themselves with the field. However, the atoms responded much faster than could be explained. It is thought that entanglement was the culprit. Subsequently, copper carboxylate was shown to do the same thing at room temperature. Between 1999 and 2009, at least 8 such experiments with different substances was done.
Entanglement can now be used to encrypt messages. This is known as quantum teleportation. One can entangle a photon with a member of a second entangled pair, causing the first photon to imprint its quantum state onto the other member. Teleportation could keep signals fresh in quantum computers. Eavesdroppers can only intercept the message (the entangled photons) by garbling it.
However, in one experiment a magnetic field was applied to a piece of lithium fluoride salt (2003, Gabriel Aepple of University College London). The atoms lined up as they mutually interacted with each other. However, they did this much faster than expected and it was thought that entanglement was the culprit. Today, entanglement has been discovered at room temperature in a range of different substances.
Electrons can also be coupled to a whole atom so that when it is close to the nucleus it moves the atom to the left, whereas if it is farther away, it moves the atom to the right. Since we don’t know the state of the electron, (close or far), the atom is considered moving to the left and the right at the same time.
It is thought that life itself can show this entanglement. Migrating robins are thought to have two entangled electrons in their eyes with total zero spin. The energy of the light entering the eye separates these two electrons making them susceptible to the earth’s magnetic field. If the magnetic field is included, it affects the two electrons differently , creating an imbalance that changes the chemical reaction in the molecule which is translated into neurological impulses that create an image of the magnetic field in the bird’s brain.
In 1935, Einstein, Boris Podolsky and Nathan Rosen wrote a paper in which they took for granted, (as did everyone at the time), that non-locality (objects-forces operating with each other at a great distance) must be apparent only – it must be some kind of mathematical anomaly. Since quantum physics does predict non-locality, it must be incomplete. Bohr responded to this practically overnight, saying that, although he agreed that non-locality does not exist, we have to start viewing reality as indefinite. The world itself was non-local. In 1982, Alain Aspect proved this experimentally.
Today, the division between the quantum and classical worlds appears not to be fundamental. It is just a question of experimental ingenuity, and few physicists now think that that classical physics will never really make a comeback on any scale. If anything, the general belief is that if a deeper theory ever supersedes quantum physics, it will show the world to be even more counterintuitive than anything we have seen so far, one in which space and time do not exist and only emerge out of quantum entanglements through the process of decoherence. It may even be that gravity is not a force in its own right but the residual noise emerging from the quantum fuzziness of the other forces in the universe. This idea of “induced gravity” goes back to the 1960s and the nuclear physicist and Soviet dissident Anderi Sakharov. It would suggest that efforts to “quantize” gravity is misguided.
This means that space and time, (very fundamental in Einsteinean and classical physics), do not exist in the normal sense. Einstein’s space-time explained gravity, and that objects have never resided in more than one place at the same time are really secondary. But, according to quantum physics, particles emerge from spaceless and timeless physics of entanglement through decoherence. Gravity may not even be a force in its own right, just the residual noise emerging from the quantum fuzziness of the other forces in the universe.
Scientists are already working on some of the practical applications of entanglement. One of these is quantum computers. On the one hand, quantum devices have the danger that electrons within it can simply vanish from one place and re-appear in another because their true location is quantumly indeterminate. Currents thus leak away, and signals are degraded.
But quantum computers of the future are capable of solving problems which stump today’s machines; for example, finding prime factors of numbers with hundreds of digits or trawling through large databases.
Another idea is quantum cryptography, which already exists. It rests on the phenomenon of entanglement. If two security-conscious interlocutors, known conventionally as Alice and Bob, measure the same type of polarization in two entangled particles, the value (1 or 0) they find is always the same. But if Alice measures one type of polarization and Bob another, quantum mechanics dictates that the results will only match part of the time.
This can be used to detect Eve, the eavesdropper. All Alice and Bob need do is for each to pick a type of polarization for successive photons at random and then compare which type they used. Since they are not comparing values, but only how they were measured, this can be done over an unsecured channel. They keep only those where they measured the same thing. If they then compare a subset of these and get a perfect match, they can use the remaining values to encrypt their future chats. Had Eve intercepted one of the entangled photons, she would have had to send another unentangled one. As a result, Alice’s and Bob’s readings would not tally. They would throw away the key and ask for another.
Ever since Artur Ekert harnessed this phenomenon to distribute cryptographic keys in 1991, the field has burgeoned. Dr. Ekert used pairs of light particles created by firing a laser at a material called a non-linear crystal.
In 2007 the Swiss authorities used a secure fibre-optic quantum network constructed along similar principles to Dr. Ekert’s to transmit voting data during a general election. The same year a team led by Anton Zeilinger, from the University of Vienna, successfully dispatched an entangled photon 144km through the air from the Canary island of Tenerife to another, La Palma. Though in 2010 two teams showed that some early versions of theoretically impregnable quantum-cryptographic systems were not foolproof and could be hacked by attacking the weakest links, this only spurred the makers of these systems to eliminate the loopholes.
Should they succeed, there would be another bizarre consequence of entanglement that would make such systems desirable. In theory, Alice and Bob can get a secure key even if the photons were sent by Eve herself, because Eve does not know which polarization they will measure. This means that you can buy a quantum encoder from an adversary and use it without fear of being snooped on. Such “device-independent” cryptography is currently all the rage and Dr. Ekert is exploring how to test if a given device, irrespective of provenance, generates secure keys.
Following the success of the Tenerife-La Palma linkup, Dr. Zeilinger, too, has grander ambitions. He is working together with the European Space Agency and the Chinese Academy of Sciences to take entanglement ballistic. The hope is to put the quantum bird in orbit by 2016 and to use it to beam secure keys down to seven ground stations sprinkled across Europe.
There are at least two rival attempts to do the same.
Alex Ling, Dr. Ekert’s colleague, is planning to test the space-worthiness of their design by subjecting it to various trials, including the vibrations of a simulated launch, and, they hope, lofting it high into the atmosphere on a weather balloon to ensure it can operate remotely on the edge of space. If all goes according to plan, the experiment will launch in 2013 on a small satellite designed by Dr. Ling’s collaborators at Singapore’s Nanyang Technological University.
Satellites, of course, even in low-earth orbit, are several hundred kilometers away. But this is less of a problem than might appear at first blush. When a beam of photon travels parallel to the earth’s surface, as in the Canaries, it must pass through the densest part of the atmosphere, increasing the likelihood of bumping into a molecule of air, which destroys the entanglement. But point a beam up to a satellite, (as in Dr. Jennewein’s project), or down from one (as in Dr. Zeilinger’s), and most of the distance is filled with a vacuum. (Dr. Ling’s device will test entanglement in orbit, without exchanging entangled photons with ground stations.)
Besides offering the world’s paranoid a way to foil eavesdroppers, the quantum-equipped satellites will serve a more rarefied purpose. Comparing one of a pair of entangled photons moving through space at around 25,000kph with another on the ground will let physicists probe what happens when quantum entanglement meets the theory of relativity, which kicks in at such speeds, upending ordinary notions of space and time.