The ‘observer with a hammer’ effect

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.

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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”

or worse:

“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!”

ARGGHHHHH!!!!!!!!

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.

Memetic surgery

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.

Watch my IOP talk – Building Quantum Computers – now on YouTube

You may remember a while back I mentioned that I’d put the video of my IOP talk up online. Well here it is. Thanks go to my kind colleague Dom for editing and posting these videos. Here is the first installment. I have posted links to the other 6 parts below. The talk is aimed at a general audience. It was given to a class of about 80 pupils of ages 14-18, and their teachers, although it is suitable for anyone who is interested in Physics, superconductors, superconducting processors and quantum computing. I apologise that the question and answer session (in parts 6 and 7) is a little difficult to hear, as the room was not set up to record audio in this way.
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I’ll be putting a permanent link to this talk in the Resources section at some point soon. The slides are already available there if anyone wishes to look at them in more detail. Comments and feedback appreciated… Enjoy!
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Part 2
Part 3
Part 4
Part 5
Part 6
Part 7

Herding quantum cats

Two interesting arXiv papers this week:

Adiabatic quantum computation along quasienergies

A potentially new model of Quantum Computation, which is a discretized variant of Adiabatic Quantum Computation (AQC). Is it equivalent to the standard model? Is it useful? No-one knows.

This paper also got me thinking:

Electronic structure of superposition states in flux qubits.

How do you measure the cattiness of a flux qubit? Cattiness being defined as the ability of a system to exhibit quantum properties as it approaches a classical limit in terms of mass, size, or some other measure. The name comes from the question of whether or not it is possible to put an entire ‘Schrodinger’s cat’ into a macroscopic superposition of states.

I have been wondering about this problem with regards to flux qubits for a while. You might think it is possible just to ‘count’ the number of electrons involved in the Josephson tunneling, giving around 1^10 particles. But wait, the electrons all form a macroscopic state – do you count the condensate as a single particle instead? This paper argues that the actual cat state is somewhere between these two extremes. This is good news, because although the upper bound would have been cooler in terms of Macroscopic Quantum Coherence, the superconducting flux qubit might still be the ‘cattiest thing in town’.

I’m also wondering about the cattiness of nanomechanical resonators coupled to optical or microwave cavities. This system can be put in a superposition of two mechanical states relating to the position and motion of the atoms in the bar. For example, the ground state can be thought of as the fundamental harmonic of the bar (think of it like a guitar string), with an antinode in the centre, wheras the first excited state has a node in the centre and two antinodes at 1/4 and 3/4 of the way along the bar. But here we find a similar problem to that of the flux qubit: Does the number of atoms in the bar matter?

For fun let’s calculate the number of atoms in a Niobium nanomechanical resonator:

Let’s say the mechanical bar is 20nm x 20nm x 1um.
The volume of the bar is therefore 4e-22m^3
The density of Nb is 8.57g/cm^3
The mass of the bar is therefore 3.428e-17kg
The atomic mass of Niobium is 92.906amu = 1.54e-25kg.
The number of atoms in the bar is ~2.2e8

To check that value:
The atomic radius of a Nb atom: 142.9pm = 0.1429nm
In 20nm there are 139.958 atoms,
and in 1um there are 6997.9 atoms.
Therefore in the bar there are 1.37e8 atoms

which is roughly the same as by the previous method.

So does that mean the ‘cattiness of the bar’ has an upper bound of 2e8? This would make it more catty than the flux qubit. Or do you have to assign more (or less) than one ‘quantum degree of freedom’ per atom? It’s not as simple as tunneling electrons, where the quantum state is determined by the direction of current flow around the loop. If anyone has any thoughts on this they would be appreciated. Just what exactly are the quantum degrees of freedom here?

The bar is obviously constrained by its end points, albeit not ideally. The displacement of the bar may therefore probably behave more classically near the ends, or the wavefunction may extend into the structural supporting region. This may affect the actual number of atoms in the superposition. What fraction of the length of the bar is behaving quantum mechanically?

Note that the mass of both the electron condensate in the case of the flux qubit AND that of the nanomechanical bar are both much lower than Penrose’s quantum mass limit of about 1e-8kg – so we can’t test that hypothesis in the lab yet. Note this relates to a post I wrote a while ago about electrons in a lump of superconductor – there are enough electrons in a bulk sample for the mass to be greater than the Penrose limit, but they aren’t doing any useful quantum computation, you can’t put them into a well defined superposition of states for example. We need to ENGINEER and CONTROL these cat states…

Anyhow, after that complicated Physics we are definitely in need of some cake:

mrkipling_frenchfancies

We had this type of cake yesterday (amongst others) to celebrate a colleague passing his PhD viva 🙂

In response to Ray Kurzweil’s comment on Quantum Computing and the brain

I thought I’d make a little note about this because quite a lot of people have been talking about this issue.

Ray Kurzweil addressed the Singularity Summit on Sunday and gave a brief summary of his opinions on some of the other preceding talks. He specifically answered criticism from others of our ability to ever model the brain using classical computing due to the presence of quantum effects in the brain. I don’t know of any supporters of this hypothesis other than Penrose and Stuart Hameroff, but maybe they are out there. He supported his viewpoint by saying that ‘The brain doesn’t factor large numbers’.

I agree with the statement that the brain is not necessarily ‘quantum computing’, but I disagree with this particular argument, because the brain does do lots of other things which quantum computers might ALSO be good at, such as pattern recognition, image processing and memory retrieval (database searching). So I think any argument as to why the brain isn’t quantum computing needs to be a bit more watertight (start by explaining decoherence for example) if you’re going to tackle this issue.

As a secondary effect, it perpetuates the myth that factoring is the only thing QCs will ever be used for. Which is sad, because a lot of smart people might have taken that impression away with them.

Violation of Bell’s inequality in Josephson phase qubits

Very cool…another step towards demonstrating that Josephson phase/flux systems can be treated as macroscopic coherent quantum objects.

Violation of Bell’s inequality in Josephson phase qubits

I was wondering if you could demonstrate this the other day!

From Science Daily:

The measurement of a Bell violation in a superconducting circuit was recently stated to be the next primary challenge for the superconducting qubit community, according to Martinis.

Martinis said: “This experiment has met this challenge, achieved by performing a very demanding measurement on a pair of Josephson qubits, a measurement that requires excellent control over qubit state preparation, qubit entanglement, and very high fidelity single-shot state measurements of the entangled qubits. It directly proves that quantum mechanics is the only possible description for the behavior of a macroscopic electrical circuit.”

UK Transhumanists patiently listen to me ramble about QC for 2 hours!

Anyone want to watch me embarrass myself royally as I try to explain quantum computing and AI at the UK Transhumanist meeting? Thought so! This is the first of about 10 of these videos, they should all be linked from this one via YouTube.

Warning: The talk is over 100 minutes long and contains intense scenes of metaphysical speculation, which may not be suitable for children (or the QIP community).

Here’s a nice picture from the talk:

suz_talking

It was actually quite a difficult talk to present, as the audience was extremely broad. I had some people tell me that they’d really enjoyed the Physics section, and others that they hadn’t been able to follow any of that part but that it sounded good anyway. There were over 40 people turned up (I don’t know the exact number), which is good as it means that the meetings are becoming more popular.

Presentation on Quantum Computing to UKTA

ukh+I’m giving a talk in London on the 12th September to the UKTA (UK Transhumanist Association) for a regular event known as UKH+. There will be discussion sessions before and after the talk in a lovely nearby pub (The Marlborough Arms).

The talk will be about quantum computing: What quantum computers can do, and more importantly what they CAN’T do, how to build them and how they might be useful in several areas of accelerating technologies. The talk will also address some of the interesting debate around the role of quantum mechanics in consciousness and how this may have consequences in the creation of human-level artificial intelligence.

I’m expecting some lively discussion regarding the final points!

Here is some info on the talk:
(Posted in several places so I’ll link them all)

Extrobritannia
Humanity+
Facebook event
London Futurists forum

If you would like to attend please feel free – follow the links above to find times and places 🙂