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Quantum superposition, big enough to see! Answered

In today's Nature online, there's a report of an experimental group who have put a naked-eye visible oscillator (an aluminum-nitride paddle about 30 microns wide) into a quantum superposition of moving and not moving simultaneously.  They cooled the paddle to a few tens of millikelvins, and connected it to a superconducting circuit that could drive the paddle to osciilate. 

After confirming that the paddle was in its quantum ground state (no vibrational energy), they put the superconducting circuit into a superposition of on- and off states, and directly measured that the paddle was in the corresponding superposition of vibrating and not vibrating.

In the words of J.B.S. Haldane, "Life is not only queerer than we imagine, it is queerer than we can imagine."

Update #1:  Science News also has a report on this result, with some slightly different details.  The paper itself is a pay-per-view Nature article.

Update #2:  Science magazine now also has a report on this result, with a better and more detailed explanation of the superconducting phase circuit.



8 years ago

Agree with lemonie, this is tantalizing.  How did they "put the quantum circuit into a superposition of 'push' and 'don't push'"?  This may be too hard to explain to the lay person, I admit.  But if it's really "an object large enough to be visible to the naked eye" WHY ISN'T THERE A PICTURE OF IT IN THE QUANTUM STATE???

Hi again, Rachel.  Adrian Cho at Science Magazine has written the best news summary so far on this result.  He has a very nice paragraph explaining the superconducting phase circuit they used:
Cleland and Martinis rely not on a SET, but on a widget called a "phase qubit," a strip of superconductor with a non-superconducting patch in it that acts a bit like a sandbar in a stream of free-flowing electrons.

The details aside, the phase qubit is itself a highly controllable quantum-mechanical system with a ground state and one higher-energy state. Researchers can ease the qubit from one state to the other—or even put it into both states at once—by applying microwaves of a specific frequency.

Without going into engineering details, that's probably the best description you'll get.

Awesome, thanks!  That is indeed the article I was hoping to read.

To answer your VERY LOUD QUESTION ;->

The picture you see there is an electron micrograph of the etched chip, showing the oscillator (paddle) structure.  When the whole apparatus is assembled, that chip is part of a structure, all of which is immersed in a liquid helium cryostat (to get to 4 K) which surrounds a dilution refrigerator (to get to millikelvins).  Those things are stainless steel tanks; kind of hard to see through :-/

OK fine the actual (non-superposed) vibration is probably not visible to the naked eye anyway, even if there were a camera inside the tank (there are obviously some kind of sensors in there to detect the Wigner density or whatever they did measure).  But maybe super super slo-mo?  I don't know enough about cameras to know if there are any that can detect changes in the megahertz range.  I'd be happy even to look at a tiny blur, though :)

I may just have to buy a subscription to Nature.  All the articles there look fascinating, if utterly obscure to my under-educated self.

It's not hard to explain in principle, but I don't want to explain the wrong thing.  The actual research article is behind a pay-per-view barrier on the Nature website, and there's no preprint out on arXiv :-( 

If you look up Wikipedia articles on "quantum bits" or "qubits", you should find some general information on how experimenters generate superposition states.

I'm a bit confused. I'm guessing the object that they manipulated into the quantum superposition is the aluminum-nitride paddle, which is the slightly bent rectangle in the picture? But when watching it, would you see it moving, or not moving? After reading the Science News article, it almost sounds like they dont know, especially this sentence: "If the researchers could make a resonator with longer-lasting vibrations, scientists might be able to test superposition on the macroscopic scale."

Yes, the picture shows the resonator, which is 30 microns wide (big enough to see).  That sentence in the SN article does seem to contradict Nature's own news report, which suggests that they did observe the paddle in a superposition state (presumably by doing weak measurements to map out the Wigner density, but I'm just guessing).

I have no idea what the paddle would "look like" in a superposition.  It's natural frequency is in the megahertz range, so I'm not sure you'd actually "see" anything moving; think about a POV display.  

Thanks. Of course, now : "megahertz range, so I'm not sure you'd actually "see" anything moving; think about a POV display. " Doesn't that mean that it is actually moving? Or am I just not understanding it?

Well, I was interpreting your and Rachel's questions as, "If I were looking at this thing myself, what would I see?"  My point is that I am not sure whether the superposition state would actually have any naked-eye visible difference from either the ground or the vibrational state.  That conclusion is based on the frequency, which is too fast for the eye to resolve.

If you're asking how the experimenters deduced that (a) they could distinguish the ground from the vibrational state, and (b) that they had put the paddle into a superposition of those two states, then you'd need to read the article.

It's hard to grasp how it's possible for something to be moving and not moving at the same time - it feels terribly illogical. Is this just my brain used to thinking about physics classically? I thought that when you got right down to it, the cat was either alive or dead, we just don't know which. That is, its state is undetermined, but that doesn't mean that uncertainly translates to a new mised state. Is that wrong?

The cat's state of being is determined when you open the box, before that "I don't know" is equivalent to the superimposition of the two states - like going to the bookies and betting on it - the result hasn't been declared yet, it's either at the same time.
With this, I'd like to know whether the box has been opened or not, and I've got some further reading from K on this.


"Weak measurement" is a method that allows you to determine the state (including superpositions vs. eigenstates) without opening the box.  It's technically a limiting case -- you open the box "as little as possible," and in the appropriate limit get the result for not opening it at all. 

I won't claim to be able to describe the actual technology involved -- that's not my field -- but the concept at least makes sense to me.

Yes I can grasp that, but I need to probe it a bit more. At present I'm looking at the superimposition of "I understand this" and "I don't understand this". I'm sure you'll appreciate that.


What is superposition?  It is not a "mixed state."  Mathematically, a mixed state doesn't have a phase relationship between the underlying components.  Consider two bags of marbles, one filled with red, and one filled with black.  If you pour them together, you get a bag with a mix of red marbles and black marbles.  That's a mixed state.  If you pull marbles at random from the bag, some will be red and some will be black, but each will be just one color or the other.

On the other hand, suppose you run the marbles through a machine that mushes them together in pairs, so you end up with marbles that are red on one side and black on the other.  Every marble you pull out will be both red and black at the same time, depending on which side you look at.  That's analogous to a superposition.

Superposition is an inherent feature of waves.  Just consider a guitar string.  If you carefully set it vibrating, you can get exactly half a wave (no nodes) on the string -- the ends are pinned, and the middle moves back and forth.  Or you can get two half-waves -- the middle doesn't move, and the 1/4 and 3/4 points move back and forth in opposition.  Or three half-waves, four, five, and so on.  These are called "pure states."

Now pluck the string at some arbitrary point along its length.  It'll vibrate in some complicated fashion.  But you can decompose that vibration into a sum of the pure states I described above, each with some amplitude (loudness).  This is a superposition state.

In quantum mechanics, everything has a wavelike aspect, which means you have pure states, and can form superpositions of them.

It feels illogical because your brain (and mine, and everyone's) is only trained to think about physics classically.  However, this sort of experiment (as discussed in the article) really cuts to point of that seeming illogic.

There are so many things in the universe that are completely outside our everyday (and evolutionary) experience.  Taking "intuition" as a basis would rule out atoms, X-rays, galaxies, relativity, along with such nice things as lasers, transistors, and medical imaging :-)

"When you get right down to it" is a dangerous argument -- it leads, in fact, to exactly Schrodinger's "paradox."  Our intuition tells us the cat must be either alive or dead, that the paddle must be either at rest or vibrating.  But what about an electron?  Or a photon?  We know that those "little things" can be in superposition states (I'll make a separate post about that!), and we can do simple experiments to demonstrate it. 

When you get right down to it, the cat is built completely out of little things that we know obey the laws of quantum mechanics.  Those equations don't have anything in them which specifies a "maximum size."  So why shouldn't the cat obey the same laws of physics?  What is it that makes our intuition, our everyday experience, contradict the reality of quantum behaviour?  Is it size?  Interaction with a stochastic (random) thermal environment?  Some magical cut-off?

Experiments like the one I posted about attack those questions head on.  If the quantum-classical transition is driven by size, then as we build bigger and bigger systems like this, eventually we should find one we can't play quantum tricks with.  If it's driven by the environment, then with good enough isolation we should be able to play this game with a virus, or a cell, or a large sapphire crystal.

OK I get the idea, but can you explain what they did.
I wonder about disturbing things with measurements, but there's insufficient information to work out what they did in detail.


Hi, Lemonie.  Sorry for the lack of detail.  As you can tell, this is a news report in Nature, not the actual paper.  The paper itself is behind Nature's pay-per-view barrier.  I just searched arXiv for the primary author (A.N. Cleland), but there is no preprint version of this result :-(

There is a Science News article about this result, which has some different details (including the resonator structure).

If you look up general techniques like "weak measurement", you should get more detail on how to map out a quantum superposition state.

Oh that Science News article has a lot more info.  Forget all that I said about internal cameras above, obviously they'd be doing this in absolute darkness. *sigh*