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.

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Interesting news coverage of Teleplace QC talk

So I enjoyed giving my Teleplace talk on Quantum Computing on Satuday, and I received quite a lot of feedback about it (mostly good!).

My talk was reported on Slashdot via a Next Big Future writeup, which in turn linked to Giulio’s Teleplace blog! This level of coverage for a talk has been very interesting, I’ve never had anything linked from /. before. They unfortunately got my NAME WRONG which was most irritating. Although I’m fairly impressed now that if you Google for ‘my name spelt incorrectly + quantum computing’, it does actually ask if you meant ‘my name spelt correctly + quantum computing’ which is a small but not insignificant victory 🙂 Note: I’m not actually going to write out my name spelt incorrectly out here, as it would diminish the SNR!!

The talk also prompted this guest post written by Matt Swayne on the Quantum Bayesian Networks blog. Matt was present at the talk.

I’ve had a lot of people asking if I will post the slides online. Well here they are:

LINK TO SLIDES for QUANTUM COMPUTING: SEPARATING HOPE FROM HYPE
Teleplace seminar, S. Gildert, 04/09/10

quantum computing

Or rather, that’s a direct link to them. They are also available along with the VIDEOS of the talk and a bunch of other lectures and stuff are on the Resources page. Here are the links to the VIDEOS of the talk, and look, you have so many choices!!

  • VIDEO 1: 600×400 resolution, 1h 32 min
  • VIDEO 2: 600×400 resolution, 1h 33 min, taken from a fixed point of view
  • VIDEO 3: 600×400 resolution, 2h 33 min, including the initial chat and introductions and the very interesting last hour of discussion, recorded by Jameson Dungan
  • VIDEO 4: 600×400 resolution, 2h 18 min, including the very interesting last hour of discussion, recorded by Antoine Van de Ven
  • Here are a couple of screenshots from the talk:


    Experimental investigation of an eight-qubit unit cell in a superconducting optimization processor

    Anyone who follows this blog and wants to get a real in-depth insight into the way that D-Wave’s processors are built, and how they solve problems, should definitely read this paper:

    Phys. Rev. B. 82, 024511 (2010), R. Harris et al.

    The paper itself is quite long (15 pages) but it really gives a great description of how an 8-qubit ‘portion’ of the processor is designed, fabricated, fit to a physical (quantum mechanical) model, calibrated, and then used to solve problems.

    If you don’t have access to the Phys Rev B journal, you can read a free preprint of the article here. And if you’ve never tried reading a journal paper before, why not give it a go! (This is an experimental paper, which means there are lots of pretty pictures to look at, even if the Physics gets hard to follow). For example, a microphotograph of the 8-qubit cell:

    Quantum Computing – cool new video!

    Here’s a neat video made by my friend and colleague Dr. Dominic Walliman, which gives a great an introduction to all those budding Quantum Computer Engineers of the future 🙂

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    Not only is this a Physics-based educational and entertainment extravaganza, but the video is interspersed with some cool shots of my old lab at Birmingham, and my old dilution refrigerator – I miss you, Frosty… *sniff*

    What is quantum co-tunneling and why is it cool?

    You may have see this cool new paper on the ArXiv:

    Observation of Co-tunneling in Pairs of Coupled Flux Qubits

    (I believe there is something called a ‘paper dance’ that I am supposed to be doing)….

    Anyway, here I’ll try and write a little review article describing what this paper is all about. I’m assuming some knowledge of elementary quantum mechanics here. You can read up about the background QM needed here., and here.

    First of all, what is macroscopic resonant tunneling (MRT)?

    I’ll start by introducing energy wells. These are very common in the analysis of quantum mechanical systems. When you solve the Schrodinger equation, you put into the equation an energy landscape (also known as a ‘potential’), and out pop the wavefunctions and their associated eigenvalues (the energies that the system is allowed to have). This is usually illustrated with a square well potential, or a harmonic oscillator (parabolic) potential, like this:

    Well, the flux qubit (quantum bit), which is what we build, has an energy landscape that looks a bit like a double well. This is useful for quantum computation as you can call one of the wells ‘0’ and the other ‘1’. When you measure the system, you find that the state will be in one well or the other, and the value of your ‘bit’ will be 0 or 1. The double well potential as you might imagine also contains energy levels, and the neat thing is that these energy levels can see each other through the barrier, because the wavefunction ‘leaks’ a little bit from one well into the neighbouring one:

    One can imagine tilting the two wells with respect to one another, so the system becomes asymmetric and the energy levels in each well move with respect to one another. In flux qubit-land, we ’tilt’ the wells by applying small magnetic fields to the superconducting loops which form the qubits. Very crudely, when energy levels ‘line up’ the two wells see each other, and you can get quantum tunneling between the two states.

    This effect is known as macroscopic resonant tunneling. So how do you measure it? You start by initializing the system so that state is localised in just one well (for example, by biasing the potential very hard in one direction so that there is effectively only one well), like this:

    and then tilt the well-system back a little bit. At each tilt value, you stochastically monitor which well the state ends up in, then return it to the initialisation state and repeat lots and lots of times for different levels of tilt. As mentioned before, when the energy levels line up, you can get some tunneling and you are more likely to find the system on the other side of the barrier:

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    In this way you can build up a picture of when the system is tunneling and when it isn’t as a function of tilt. Classically, the particle would remain mostly in the state it started in, until the tilt gets so large that the particle can be thermally activated OVER the barrier. So classically the probability of the state being found on the right hand side ‘state 1’ as a function of tilt looks something like this:

    Quantum mechanically, as the energy levels ‘line up’, the particle can tunnel through the barrier – and so you get a little resonance in the probability of finding it on the other side (hence the name MRT). There are lots of energy levels in the wells, so as you tilt the system more and more, you encounter many such resonances. So the probability as a function of tilt now looks something like this:

    This is a really cool result as it demonstrates that your system is quantum mechanical. There’s just no way you can get these resonances classically, as there’s no way that particle can get through the barrier classically.

    Note: This is slightly different from macroscopic quantum tunneling, when the state tunnels out of the well-system altogether, in the same way that an alpha particle ‘tunnels’ out of the nucleus during radioactive decay and flies off into the ether. But that is a topic for another post.

    So what’s all this co-tunneling stuff?

    It’s all very nice showing that a single qubit is behaving quantum mechanically. Big deal, that’s easy. But stacking them together like qubit lego and showing that the resulting structure is quantum mechanical is harder.

    Anyway, that is what this paper is all about. Two flux qubits are locked together by magnetic coupling, and therefore the double well potential is now actually 4-dimensional. If you don’t like thinking in 4D, you can imagine two separate double-wells, which are locked together so that they mimic each other. Getting the double well potentials similar enough to be able to lock them together in the first place is also really hard with superconducting flux qubits. It’s actually easier with atoms or ions than superconducting loops, because nature gives you identical systems to start with. But flux qubits are more versatile for other reasons, so the effort that has to go into making them identical is worthwhile.

    Once they are locked together, you can again start tilting the ‘two-qubit-potential’. The spacing of the energy levels will now be different (think about a mass on the end of the spring – if you glue another mass to it, the resonant frequencies of the system will change, and the energies levels of the system along with them. We have sort of made our qubit ‘heavier’ by adding another one to it.

    But we still see the resonant peaks! Which means that two qubits locked together still behave as a nice quantum mechanical object. The peaks don’t look quite as obvious as the ones I have drawn in my cartoon above. If you want to see what they really look like check out Figure 3 of the preprint. (Note that the figure shows MRT ‘rate’ rather than ‘probability’, but the two are very closely linked)

    From the little resonant peaks that you see, you can extract Delta – which is a measure of the energy level spacing in the wells. In this particular flux-qubit system, the energy level spacing (and therefore Delta) can be tuned finely by using another superconducting loop attached to the main qubit loop. So you can make your qubit mass-on-a-spring effectively heavier or lighter by this method too. When the second tuning loop is adjusted, the resulting change in the energy level separation agrees well with theoretical predictions.

    As you add more and more qubits, it gets harder to measure Delta, as the energy levels get very close together, and the peaks start to become washed out by noise. You can use the ‘tuning’ loop to make Delta bigger, but it can only help so much, as the tuning also has a side effect: It lowers the overall ‘signal’ level of the resonant peaks that you measure.

    In summary:

    Looking at the quantum properties of coupled qubits is very important, as it helps us experimentally characterise quantum computing systems.
    Coupling qubits together makes them ‘heavier’ and their quantum energy levels become harder to measure.
    Here two coupled qubits are still behaving quantum mechanically, so this is promising. This means that in the quantum computation occurring on these chips involves at least 2-qubits interacting in a quantum mechanical way. Physicists calls these ‘2-qubit processes’. There may be processes of much higher order happening too.
    This is pretty impressive considering that these qubits are surrounded by lots of other qubits, and connected to many, many other elements in the circuitry. (Most other quantum computing devices explored so far are much more isolated from other nearby elements).

    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