What is Quantum Physics? Disentangling Quantum Entanglement
Particles in different places at the same time? The world of quanta seems to contradict familiar everyday principles. Nevertheless, the basic ideas of the quantum world are fairly easy to understand.
We humans are medium-sized – at least from a physical point of view. We are significantly smaller than stars and galaxies, but significantly larger than molecules and atoms. Our everyday world is characterized by medium-sized things, such as tomatoes, footballs or family homes – things that obey the laws of classical physics.
At any given moment, a tomato is in a very specific place and has a very specific speed. If the tomato has been in the fridge and is now on the table, then it must have passed all the points on a continuous line connecting the fridge and the table at some point in time. These are rules that we take for granted. We do not question them. We have learnt from childhood that this is the way things work in our familiar world of medium-sized things.
But this does not have to be the case. When we look at molecules, atoms and elementary particles, we see that these basic rules no longer seem to apply in the world of small particles. A quantum particle can be in several places at once. If an electron used to be located to the left of the atomic nucleus and is now to the right of the atomic nucleus, then it has not necessarily passed through all the points in between.
Our human intuition cannot cope with this – after all, it has been trained on classical medium-sized everyday objects, not on electrons, atoms or molecules. This is precisely why the world of quantum physics seems so strange, unfamiliar and crazy to us humans. But that should not bother us. The only thing that matters is that we can explore the laws of the quantum world, describe them with mathematical equations and use them to solve problems.
The Superposition principle: In Different States at the Same Time
One of the most fundamental laws of quantum theory is the so-called superposition principle. It tells us that if a quantum object can be in different states, then it can also be in a combination of those states.
If an electron can move to the left or to the right, then the laws of quantum theory also allow it to move both to the left and to the right at the same time. If an atom can rotate clockwise or anticlockwise, then the laws of quantum theory also allow it to rotate both clockwise and anticlockwise at the same time. If a molecule can either break apart or remain whole when hit by a flash of light, then it is also possible for it to have both broken apart and remained whole afterwards. Such a combination of states is called a “superposition state”.
This does not mean that we simply do not know the state of our quantum particle. It is not the case that the atom spins clockwise 50% of the time and anticlockwise 50% of the time and that we simply lack information about which of these possibilities has actually occurred. No, the particle is actually in a combination of different states at the same time. The question “which of these is the real one?” has no answer. The universe simply does not contain that information. You could say that the atom itself does not know in which direction it is spinning.
But what if you send this atom into a measuring device that can determine the atom’s direction of rotation? Now something strange happens: the superposition state is destroyed. A device that can measure the two states “clockwise” and “anticlockwise” forces our particle to choose one of the two possibilities. At the moment of measurement, the superposition state “both anticlockwise and clockwise” becomes either the state “anticlockwise” or the state “clockwise”.
Which will be chosen? That depends on pure chance. Even if we know everything there is to know about the particle, we simply cannot predict the result of the measurement. Quantum physics can only tell us the probabilities with which the possible outcomes will occur. But which of them will actually become the measured reality cannot, in principle, be predicted.
A New Kind of Randomness
From a philosophical point of view, this is a radical idea: until the advent of quantum physics, it was assumed that everything in the universe happened for a reason. The state of the universe at any given moment must be a logical and unambiguous consequence of the state of the universe at the previous moment, and of the laws of nature!
But quantum physics breaks with this principle: when a quantum measurement turns a superposition of states into a particular result, it does so purely arbitrarily. Another result would have been just as physically possible, but it did not happen. The result of the quantum measurement is (at least according to the current interpretation of quantum physics) random in the most fundamental sense – it is an effect without a cause.
Only as Long as no one is Watching
Measurement plays a crucial role in quantum physics: quantum particles can be in different states at the same time, but only as long as they are not being measured. When measured, this superposition state is inevitably destroyed. The strange quantum-physical “both at the same time” becomes a very ordinary “either-or”.
This may sound unsatisfactory at first: quantum theory predicts a strange new kind of superposition state, but when we actually want to see it, it is no longer there? If so, how do we even know that it ever existed? Doesn’t this sound strange, like a magician assuring us that he can pull a rabbit out of a hat – but only when no one is watching?
Not quite. Quantum theory is still testable. Even if the superposition states cannot be observed directly, quantum theory provides statements that can be verified in experiments. Using the equations of quantum physics, it is possible to calculate exactly how these strange superposition states evolve over time, how they interact with other things and what the effects are. These effects can be observed – and the observations are in perfect agreement with the mathematical predictions. But only if we assume that these superposition states, which seem so unusual to us humans, exist while the quantum system is unobserved.
Quantum Entanglement
Having accepted these peculiar quantum states, we can now think about more complicated situations involving multiple particles. For example, let us imagine that a particle decays into two smaller particles that fly off in different directions and rotate. If the original particle had no intrinsic angular momentum of its own, then (according to the law of conservation of angular momentum) the direction of rotation of the two flying particles must be different: if one particle rotates clockwise, then the other particle must rotate anticlockwise – or vice versa. The two particles are therefore linked: The state of one particle (clockwise or anticlockwise) can tell us about the state of the other.
But we already know that the particle may not have a fixed state. It can be in a state where it is spinning both anticlockwise and clockwise – and so can the second particle. What happens if we measure the state of one particle and get the result “clockwise”? Then we have determined the quantum state of that particle – we have forced it to change from its superposition state to a clearly defined measured state.
But since the state of the other particle has been logically linked to the state of the first particle – in this case the two particles are said to be “entangled” – the state of the other particle is now also determined. It must now necessarily be in the “anticlockwise” state. For this particle, too, the superposition state has turned into a well-defined, measured state – but no measurement has been made on this particle!
This is the really confusing thing about quantum entanglement: we measure one particle, thereby changing its state – and at the same time we also change the state of another particle, without having interacted with it. The other particle may be in a completely different place during the measurement. Imagine a pair of quantum-entangled particles travelling through the universe for millions of years, located in different galaxies. Nevertheless, a measurement of one particle would automatically determine the state of the other.
Confusing, But Useful
If you find this confusing, you are in good company: Even Albert Einstein refused to believe it. Nature could not possibly be that strange! Perhaps, Einstein thought, quantum theory was just the precursor to a deeper, more comprehensive theory in which such craziness as quantum entanglement would disappear.
But numerous experiments – including those of the Austrian quantum physicist Anton Zeilinger – have shown that the seemingly crazy predictions of quantum theory are in fact true. Nature is indeed stranger than Albert Einstein ever imagined.
And that is cause for celebration: these quantum physical curiosities have revolutionized our world over the last hundred years. From lasers to computer chips, from photovoltaics to magnetic resonance imaging, much of today’s technology would be inconceivable without quantum physics. The first century of quantum physics has been an incredible success story – and there is no doubt that it is far from over.
The Double Slit experiment
What exactly is the difference between a particle and a wave? This question can be explored using the famous double-slit experiment. Imagine shooting small particles at a plate with a slit – for example small paintballs flying through the slit and hitingt a wall behind it. Depending on whether the slit is on the left or right, a spot will appear on the wall behind, slightly shifted to the left or right. If two slits are open, two spots will appear. They may overlap in the middle.
What happens if we do the same experiment with waves? We could, for example, divide a pool of water into two parts using a board with two slits. Now we create waves that move towards the board, propagate through both slits at the same time and then form a wave pattern on the other side – similar to the wave pattern in a pond that is created when two stones are thrown into it. Such a pattern – quite unlike the simple spots we saw in the previous experiment – is characteristic of waves.
Schrödinger's Cat
Quantum theory says that particles can be in a “superposition” – a combination of different states. For example, a radioactive atomic nucleus can either remain whole or decay. So, according to quantum physics, it can also be in a state where it is both whole and decayed at the same time – at least until we force the atomic nucleus to choose one or the other by measuring it. Is this just a bizarre detail of particle physics that has no consequences for our world of big things?
It's not that simple, as Erwin Schrödinger explained in his famous thought experiment about Schrödinger's cat. Suppose we take a radioactive atom. We put a detector next to it that registers when the atom decays. When that happens, a mechanism drops a bottle of poison, and the cat sitting next to it dies. We now put all this in a sealed box and wait.
We can open the box at any time – then we can see whether the atom has decayed or not, and whether the cat is still alive. But what if we don't open the box? Is the atom, as long as we do not observe it, in a combination of “decayed” and “not decayed”? And is the bottle of poison in a combination of “intact” and “shattered”? And is the cat in a combination of “alive” and “dead”? Have we just transferred the quantum superposition state of a particle to a cat?
No. The process of observation by a human being is irrelevant in this case. The cat’s state is not determined when the box is opened. The decisive step comes much earlier – when an object of the quantum world (our atom) first comes into contact with an object of the world of larger things (in this case: the measuring device). This first contact is a measurement, regardless of human observation. A decision is made at that very moment. Large objects such as bottles of poison or cats cannot remain in a state of quantum physical superposition for any significant length of time – they are much too large for that.
It is still an intensely researched question as to how quantum effects can be demonstrated even in large (by quantum physics standards) objects – for example, in large molecules.