Here is another short essay about quantum mechanics-related stuff. It’s a very high level essay, so any practising quantum physicists probably shouldn’t read it 😉 It is more aimed at a general audience (and news reporters!) and talks about the ‘spooky’ and ‘weird’ properties of superposition and decoherence that people seem to like to tie in with consciousness, cats, and ‘the observer effect’. It doesn’t really go into entanglement directly, I think that should be an issue for a separate post! It is also a fun introduction to some issues when trying to perform experimental quantum computing and quantum physics in general.
I’ve also put this essay in the Resources section as a permanent link.
The not-so spooky after all ‘observer-with-a-hammer’ effect
S. Gildert November 2010
I’m so sick of people using phrases like this:
“Looking at, nay, even thinking about a quantum computer will destroy its delicate computation. Even scientists do not understand this strange and counter-intuitive property of quantum mechanics”
“The act of a conscious observer making a measurement on a quantum computer whilst it is performing a calculation causes the wavefunction to collapse. The spooky nature of these devices means that they just don’t work when we are looking at them!”
These kind of phrases spread like viral memes because they are easy to remember and they pique people’s curiosity. People like the idea of anthropomorphizing inanimate systems. It makes them seem unusual and special. This misunderstanding, the idea that a quantum system somehow ‘cares’ or is emotionally sensitive to what a human is doing, is actually what causes this meme to perpetuate.
So I’m going to put a new meme out there into the-internet-ether-blogosphere-tubes. Maybe someone will pick up on this analogy and it will become totally viral. It probably won’t, because it seems pretty dull in comparison to spooky ethereal all-seeing quantum systems, but if it flicks a light switch in the mind of but a single reader, if on contemplating my words someone’s conceptual picture of quantum mechanics as a mystical, ever elusive resource is reduced even by the tiniest amount, then my work here will be done.
Let’s start by cutting the yukky tumorous part from this meme and dissecting it on our operating table:
“Looking at a quantum system changes it.”
Now I don’t necessarily disagree with this statement, but I think you need to define what you mean by ‘looking’….
Usually when physicists ‘look’ at things, they are trying to measure something to extract information from it. To measure something, you need to interact with it in some way or other. In fact, everything in the world interacts with many other things around it (that’s why Physics is interesting!). Everything one could ever wish to measure is actually sitting in a little bath of other things that are constantly interacting with it. Usually, we can ignore this and concentrate on the one thing we care about. But sometimes this interacting-background property can cause unwanted problems.
Measuring small things
Brownian motion can give us a nice example of a nasty background interaction. Imagine that a scientist wanted to investigate the repulsion (or attraction) of some tiny magnetic particles in a solution that had just precipitated out of an awesomely cool chemical reaction. (I don’t know why you’d want to do this, but scientists have some weird ideas). So she starts to take measurements of the positions of the little magnetic particles over time, and finds that they are not obeying the laws of magnetism. How dare they! What could be wrong with the experiment? So our good scientist takes the solution in her beaker and you start to adjust various parameters to try and figure out what is going on. It turns out that when she cools the solution, the particles start to behave more in line with what is expected. She figures that the Brownian motion – all the other molecules jostling and wiggling around near the magnetic particles – are actually kicking the experiment around, ruining the results. But by lowering the temperature, it is possible to stop the environment in which the particles sit from disturbing them as much.
In this example, the scientist was able to measure the positions of the particles with something like a ruler or a laser or some other cool technique, and it was fairly easy, even though the environment had become irritatingly convolved with our experiment. Once she had got around how to stop the interaction with the environment, then our experiment worked well.
Quantum systems are small, and small things are delicate. But quantum systems are so small that the environment, the ‘background-interaction’ around them, is no longer something that they, or we, can ignore. It pushes them around. In order to have a chance at engineering quantum systems, researchers have to isolate them carefully from the environment (or at least the bits of the environment that kick them around). Scientists spend a lot of time trying to stop the environment from interacting with their qubits. For example, superconducting processors need to be operated at very cold temperatures, in extremely low magnetic field environments. But I won’t digress into the experimental details. The main idea is that no matter how you build your quantum computer, you will have to solve this problem in some way or other. And even after all this careful engineering, the darn things still interact with the environment to some degree.
It gets worse
But with quantum systems, there is an extra problem. The problem is not just the environment. To illustrate this problem, I’ll propose another little story of the striving scientists.
Imagine that our scientists have developed a technique to measure the diameter of bird eggs using a robotic arm. The arm has a hand that grasps the eggs, measures them, and then displays the diameter on a neat built-in display. (Alternatively, you can Bluetooth the results to your iPhone, so the scientists tell me). Anyway, this robotic arm is so ridiculously precise that it can measure the diameter of eggs more accurately than any pair or vernier calipers, any laser-interferometer array or any other cool way of measuring eggs that has ever existed. The National Standards laboratories are intrigued.
However, there is a slight problem. Every time the robot tries to measure an egg, it breaks the darn thing. There is no way to get around this. The scientific breakthrough relating to the accuracy of the new machine comes from the fact that the robot squeezes the egg slightly. Try and change the way that the measurement is performed, and you just can’t get good results anymore. It seems that we just cannot avoid breaking the eggs. The interaction of the robot with the egg is ruining our experiment.
Of course, a robot-egg measuring system like this sounds ridiculous, but this is exactly the problem that we have with quantum systems. The measuring apparatus is huge compared to the quantum system, and it interacts with it, just like the pesky environment does. It pushes and squeezes our quantum system. The result is that anything huge that we use to try to perform a delicate measurement will break it. And worse still, we can’t just try to ‘turn it off completely’ like we could with the environment surrounding the particles in the solution. By the very nature of what we are trying to do, we need the measurement apparatus to interact with the qubits, otherwise how can we measure them? What a pain. We end up measuring a kind of qubit-environment-combination mess, just like trying to measure the diameter of a broken egg whose contents are running all over our robotic measurement apparatus.
I can’t stress enough how comparatively big and clumsy quantum measurement apparatus is. Whilst scientists are trying to build better measurement techniques that don’t have such a bad effect on quantum systems, ultimately you just can’t get around this problem, because the large-scale things that we care about are just not compatible with the small-scale of the quantum world.
This doesn’t mean that quantum computers aren’t useful. It just means that the information we can extract from such systems is not neat, clean and unique to the thing we were trying to measure. We have to ‘reconstruct’ information from the inevitable conglomerate that we get out of a measurement. In some cases, this is enough to help us do useful computations.
Hammering the message home
Nowhere here does one need to invoke any spookiness, consciousness, roles of the observer, or animal cruelty involving cats and boxes. In fact, the so-called ‘observer’ effect could perhaps be more appropriately termed the ‘observer-with-a-hammer’ effect. We take for granted that we can measure large classical systems, like the 0 or 1 binary states of transistors, without affecting them too much. But measuring a quantum system is like trying the determine the voltage states of a single transistor by taking a hammer to the motherboard and counting the number of electrons that ended up sticking to the end of it. It kind of upsets the computation that you were in the middle of. It’s not the observer that’s the problem here, it’s the hammer.
So, the perhaps-not-so-viral phraseology for one to take away from my relentless ranting is thus:
“When you try and measure a delicate quantum system with clumsy apparatus, you actually end up with a messy combination of both!”
Alternatively, you could say ‘you can’t make a quantum measurement without breaking a few eggs’ – But if that terrible pun sticks then I will forever be embarrassed.