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The Strange World of Quantum Entanglement
Brian Clegg received a physics degree from Cambridge University and is the author of numerous books and articles on the history of science. His most recent book is The God Effect : Quantum Entanglement, Science’s Strangest Phenomenon.
- What you write in your book about entanglement is so startling, it’s hard to believe. Let’s start with a definition. What is quantum entanglement?
Entanglement is a strange feature of quantum physics, the science of the very small. It’s possible to link together two quantum particles — photons of light or atoms, for example — in a special way that makes them effectively two parts of the same entity. You can then separate them as far as you like, and a change in one is instantly reflected in the other. This odd, faster than light link, is a fundamental aspect of quantum science. Erwin Schrödinger, who came up with the name “entanglement” called it “the characteristic trait of quantum mechanics.” Entanglement is fascinating in its own right, but what makes it really special are dramatic practical applications that have become apparent in the last few years.
- Is it possible that entangled particles are not actually in immediate communication, but are simply programmed to behave in the same way? Much like twins separated at birth who live eerily similar lives — assume the same professions, marry similar spouses, etc.
- This is an obvious possibility. John Bell, who devised a lot of the theory for testing the existence of entanglement, covered it in a paper called “Bertlmann’s Socks and the Nature of Reality.” Reinhold Bertlmann, a colleague of Bell’s, always wore socks of different colors. Bell pointed out that, if you saw one of Bertlmann’s feet coming around the corner of a building and it had a pink sock on, you would instantly know the other sock wasn’t pink, even though you had never seen it. The color difference was programmed in when Bertlmann put his socks on. But the quantum world is very different. If you take some property of a particle, the equivalent of color, say the spin of an electron, it doesn’t have the value pre-programmed. It has a range of probabilities as to what the answer might be, but until you actually measure it, there is no fixed value. What happens with a pair of entangled electrons is you measure the spin of one. Until that moment, neither of them had a spin with a fixed value. But the instant you take the measurement on one, the other immediately fixes its spin (say to the opposite value). These “quantum socks” were every possible color until you looked at one. Only then did it become pink, and the other instantly took on another color.
- You write that Einstein among other scientists could not accept quantum entanglement. It seems to throw out the whole notion of cause and effect. How confident are physicists that quantum entanglement exists and what are the implications for science and the scientific method?
- Einstein had problems with the whole of quantum physics — which is ironic, as it was based on his Nobel Prize winning paper on the photoelectric effect. What he didn’t like was the way quantum particles don’t have fixed values for their properties until they are observed — he couldn’t relate to a universe where probability ruled. That’s why he famously said that God doesn’t play dice. I think an even better quote, less well known, was when he wrote: “I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will, not only its moment to jump off, but also its direction. In that case, I would rather be a cobbler, or even an employee in a gaming house, than a physicist.” Einstein believed that underneath these probabilities were fixed, hidden realities we just couldn’t see. That was why he dreamed up the idea of entanglement in 1935. It was to show that either quantum theory was incomplete, because it said there was no hidden information, or it was possible to instantly influence something at a distance. As that seemed incredible, he thought it showed that quantum theory was wrong.
- It did take a long time to prove that entanglement truly existed. It wasn’t until the 1980s that it was clearly demonstrated. But it has been shown without doubt that this is the case. Entanglement exists, and is being used in very practical ways.
- Entanglement doesn’t throw away the concept of cause and effect. But it does underline the fact that quantum particles really do only have a range of probabilities on the values of their properties rather than fixed values. And while it seems to contradict Einstein’s special relativity, which says nothing can travel faster than light, it’s more likely that entanglement challenges our ideas of what distance and time really mean. Similarly, entanglement is no challenge to the scientific method. We need to use a different kind of math, but this is still the same science.
- Where do you see the first practical applications of entanglement?
The first thing most people think of, including a report produced for the Department of Defense shortly after entanglement was proved real, is being able to use it to communicate faster than light. The link of entanglement works instantaneously at any distance. So it would be amazing if it could be used to send a signal. In fact this isn’t possible. Although there is a real connection between two entangled particles, we don’t know what the information is that it’s going to send. If I measure the spin of an entangled electron, yes it communicates the value somehow to its twin — but I can’t use it. I had no idea what the spin was going to be. This is just as well, as faster than light messages travel backwards in time. If I could send a message instantly it would be received in the past, and that really would disrupt cause and effect. However, there are still real and amazing applications of entanglement. It can be used to produce unbreakable encryption. If you send each half of a set of entangled pairs to either end of a communications link, then the randomly generated but linked properties can be used as a key to encrypt information. If anyone intercepts the information it will break the entanglement, and the communication can be stopped before the eavesdropper picks up any data.
- Then there are quantum computers. These are conceptual machines that can crack problems that would take an ordinary computer longer than the lifetime of the universe to solve. We already know how to program a quantum computer to do some amazing things. For instance, if I have an unsorted database with a million entries, I will typically have to try out 500,000 of these before hitting on the right one. (Try looking for a specific number, rather than a person, in the paper version of the New York telephone directory.) But using a quantum computer it only takes 1,000 attempts. Unfortunately, though, Quantum computers are almost impossible to make.
- Instead of storing information in bits on silicon chips, each of which can hold 0 or 1, a quantum computer uses quantum particles like photons or atoms as the information stores. Each particle can store infinitely long numbers, but if you look at the particle, it changes the value. Entanglement means you can’t interact with these quantum bits (qubits for short) without frying your quantum memory. There are several technologies being tried to build the first, basic quantum computers, but they all rely on entanglement to get information into and around the system.
- Most dramatic of all is quantum teleportation.
- And for those Trekkies out there, tell us about the possibility of teleportation.
- It’s more than a possibility, it has been done, but only on a very small scale. What a Star Trek transporter is supposed to do is make an exact copy of an object or a person somewhere else. There’s a fundamental problem here. Because looking at a quantum particle changes it. You can’t scan a particle, see what it looks like and make an exact copy. So it might seem that teleportation is impossible. Entanglement lets you get around this restriction. By interacting the particle with one half of an entangled pair, and then putting the other half of the pair through a special process, a bit like a logic gate in a computer, it’s possible to make an identical particle at a remote location. We can only do this because the entanglement transfers the quantum information without us ever knowing what it was. In the process, the original particle loses its properties. Teleportation isn’t copying, it effectively destroys the original.
- This doesn’t mean you’ll be able to rush out and buy a transporter at Radio Shack next week. This process has been done with large molecules, similar in size to a bacterium, so it’s possible that we could teleport something living. But it won’t work with something as big as a person. You would have to scan every single molecule in the body and reassemble at the other end, which doesn’t look like it’s every going to be practical.
- Maybe this isn’t so bad, though. Remember, the original is destroyed (something Star Trek glosses over). Okay, you get an identical copy, but would you be prepared to be vaporized if you knew an exact, indistinguishable copy was going to be created the other side of the world? I’m not ecstatic about flying, but by comparison it sounds a safe option.
- Could entanglement prove to be the “Holy Grail” for merging scientific and mystical, religious thought?
- There have certainly been people who have tried to draw this kind of conclusion, but I think they are mistaken. Entanglement is a wholly physical process. I called my book The God Effect because it has been suggested that entanglement is the working mechanism of the Higgs boson, a very special particle that gives everything its mass, and has been called the God Particle, because it’s so fundamental. But that’s just a label. It’s also true that Nobel Prize winning physicist Brian Josephson has suggested that entanglement could explain telepathy (much to the irritation of paranormal debunker James Randi), but Josephson was saying if telepathy exists, then here’s a physical mechanism that could explain it — he wasn’t indulging in mystical navel-gazing.
- What entanglement (and quantum theory in general) does do is remind us is that the real world is much stranger than we imagine. That’s because the way things are in the world of the very small is totally different to large scale objects like desks and pens. We can’t rely on experience and common sense to guide us on how things are going to work at this level. And that can make some of the effects of quantum physics seem mystical. In the end, this is something similar to science fiction writer Arthur C. Clarke’s observation that “any sufficiently advanced technology is indistinguishable from magic.”
Tags: Einstein, Erwin Schrödinger, John Bell, Physics, quantum computers, quantum entanglement, quantum physics