Transcription
A group of physicists just found a way to overcome one of the biggest limitations of quantum physics. So big, it's a theorem. The no cloning theorem. This means that contrary to what we thought, quantum information can be copied. That's quite something. Have physicists just been wrong for half a century? Will this finally make quantum computers work? Let's have a look.
When I can't sleep at night, and that's happened a lot recently, I wonder whether Captain Kirk dies when he goes through the teleporter. One way to think about what the teleporter does is to convert Kirk into pure information, send this information elsewhere, and reassemble him. The other way is that the teleporter just reads the information, destroys the original, and then rebuilds the copy elsewhere. Except that quantum physics has what's called the no cloning theorem that says you can't copy a quantum state without destroying the original. And since everything is ultimately quantum information, including you and I, doesn't this mean that there is only ever one real kirk? And as a corollary, you can't back up yourself onto a computer, which is unfortunate because I'd like a restore point before I read the comment section.
In case that didn't already give you a headache, a group of physicists just reported they found a way around the no cloning theorem. It's probably the most basic theorem of quantum physics, but it's one of the reasons why it's so hard to make quantum computers work. It says that you can't duplicate quantum states. And this means that the most obvious way to prevent errors. Just make several copies and do the same calculation on all of them doesn't work on a quantum computer. You have to do something more difficult.
The no cloning theorem was first proved in the early 1980s. So it's somewhat of a late comer in the history of quantum physics. It's fairly easy to understand, really. You only need to know that in quantum physics we describe everything by a wave function usually denoted s. But if we have multiple wave functions that might be fi or sai or some other weird Greek symbols. If you want to know something like what's the probability that a particle with wave function s actually behaves like some other wave function phi, then you take the square of the product of these wave functions. Okay, that sounds a little mysterious. What does it mean that a wave function behaves like some other wave function? Well, you might ask, for example, if I have a particle with a wave function that's smeared out all over the place, what's the probability that it behaves as if it was only over here? You do this by taking the product of the wave functions and then taking the absolute square. But in particular, the probability that s behaves like itself is one. So the absolute square of any wave function is one. If you wanted to clone a wave function, you need a sort of cloning apparatus into which you shove a wave function and an empty slot that I'll call zero. And out comes the wave function. And the previously empty slot is now the same wave function. So you have duplicated it. For such a cloning machine to be possible in quantum physics, this operation must preserve probabilities. But you see this immediately creates a problem because suppose you shove a second wave function into the cloning machine. Fi and zero goes in, and out comes five. Now this operation must preserve all probabilities, in particular that of the size zero to appear like 5 0, that must still be the same after the cloning, but 0 0 is just one. So this means that 5i is equal to 5i squared, and this just is not the case for most states. This means the copy machine can't exist.
The authors of the new paper now say that there is a clever workaround for this. They show that, contrary to what we thought all along, that one can make perfect copies of an unknown quantum state of, say, a quantum bit, a cubit, one just has to make sure that one can only ever read out one of the copies. And this isn't just maths. They actually showed that this works on an IBM quantum computer with about 150 cubits. They showed that, indeed, it works despite the hardware noise. So let me be clear. It's not that they found a mistake in the no cloning theorem. Rather, they demonstrated both mathematically and experimentally that it isn't as restrictive as we thought it is. I give this paper a zero out of 10 on the meter. They ticked all the boxes. Good maths, good experiment, good interpretation. Zabina approves.
What does this mean? First, it means we have to rethink what we thought we knew about quantum information. Second, it might have practical uses. For one thing, it might lead to better quantum computing algorithms. So, maybe we'll get some use out of them sooner than we expected. But it might also come in handy for future quantum internet.
There is a deeper lesson in this that I also learned from my tax advisor. If you follow the rules precisely enough, you can do the thing you were told you can't do.
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