# Quantum measurements leave Schrödinger's cat alive



## Kilgore Trout (Jun 25, 2010)

> Schrödinger's cat, the enduring icon of quantum mechanics, has been defied. By making constant but weak measurements of a quantum system, physicists have managed to probe a delicate quantum state without destroying it – the equivalent of taking a peek at Schrodinger's metaphorical cat without killing it. The result should make it easier to handle systems such as quantum computers that exploit the exotic properties of the quantum world.
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> Quantum objects have the bizarre but useful property of being able to exist in multiple states at once, a phenomenon called superposition. Physicist Erwin Schrödinger illustrated the strange implications of superposition by imagining a cat in a box whose fate depends on a radioactive atom. Because the atom's decay is governed by quantum mechanics – and so only takes a definite value when it is measured – the cat is, somehow, both dead and alive until the box is opened.
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> ...


Read more here.


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## 0 1 1 2 3 5 8 13 21 34 (Nov 22, 2009)

Here is the full article: Quantum measurements leave Schrödinger's cat alive - physics-math - 03 October 2012 - New Scientist


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## Up and Away (Mar 5, 2011)

Why are they going down this path? What could it yield?


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## Tristan427 (Dec 9, 2011)

While I am happy they figured this out, this could be bad for my avatar.


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## wuliheron (Sep 5, 2011)

This isn't a violation of Indeterminacy, but a refinement of it that supports the Ozawa formulation. Essentially more evidence that the HUP needs to be refined and expanded on and possibly the Strong Equivalency Principle as well. A modern take on the subject that goes off in a different direction from the classic interpretations.

The original interpretations where all metaphysical ones that insisted the cat must be dead, alive, or in superposition and these states were all mutually exclusive. Modern contextualist interpretations since the formulation of Bell's theorem suggest it's more useful to simply focus on the pragmatic issue of what we do see than to make metaphysical interpretations. As Niels Bohr said, "Shut up and calculate".


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## Diogenes (Jun 30, 2011)

wuliheron said:


> As Niels Bohr said, "Shut up and calculate".


That's actually a quote by David Mermin.


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## jeffbobs (Jan 27, 2012)

yeah but the real question is, what would it taste like?


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## ShadoWolf (Jun 5, 2012)

Tristan427 said:


> While I am happy they figured this out, this could be bad for my avatar.


I need your avatar :O 
But I probably won't believe it just yet...


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## Stelmaria (Sep 30, 2011)

I had no idea what they were on about when reading the new scientist article.

The actual journal article is here:
http://www.nature.com/nature/journal/v490/n7418/full/nature11505.html

Here is the actual abstract:


> Abstract
> The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this 'instantaneous' process can be stretched using weak measurements, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements. Other applications include quantum state stabilization, entanglement generation using measurement, state purification and adaptive measurements.


I think the point is that they didn't 'collapse the wavefunction', but rather _prolong_ its state, which is interesting as that is one of the key limitations preventing quantum computers etc from being useful.

I still don't fully understand, because by measuring and then correcting, it is still a probabilistic process, there is still actually no guarantee that it is in the same superposition.


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## wuliheron (Sep 5, 2011)

Snow Leopard said:


> I think the point is that they didn't 'collapse the wavefunction', but rather _prolong_ its state, which is interesting as that is one of the key limitations preventing quantum computers etc from being useful.
> 
> I still don't fully understand, because by measuring and then correcting, it is still a probabilistic process, there is still actually no guarantee that it is in the same superposition.


It means they can compensate for any disturbance the act of measuring the system causes, the so-called observer effect, and that Indeterminacy is a separate and distinct issue from measurement errors. For example, to measure the air pressure in your tires it's just easier to let a little out, but that effects the measurement. This is the first proof it is possible to compensate for measurement induced errors without collapsing the wavefunction and, therefore, Indeterminacy is a separate and distinct issue. It rules out panpsychic explanations for the collapse of the wavefunction and means the HUP needs to be refined and elevated to the status of a natural law.


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## RobynC (Jun 10, 2011)

I figure this would render the uncertainty principle invalid unless you're talking about quantum duality not being totally dual _(exhibiting both particle and wave characteristics similar to the Afshar experiment)_.

I think there's something we're not being told. That being said, read my sig


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## wuliheron (Sep 5, 2011)

RobynC said:


> I figure this would render the uncertainty principle invalid unless you're talking about quantum duality not being totally dual _(exhibiting both particle and wave characteristics similar to the Afshar experiment)_.


Actually this particular experiment supports a Bohmian mechanics interpretation rather than Afshar. Bohmian mechanics assumes there is a real particle and separate wave that pushes the particle around. Take a delicate enough measurement and you can get a better idea of exactly where the particle is and it's momentum without causing the wave to collapse. However, there are countless other experiments that contradict Bohmian mechanics and suggest that if you set up experiments well enough you can get them to support whatever basic interpretation you want.

Which is precisely why the theories keep becoming more contextual and less metaphysical as time passes because they keep steadily ruling out even the slightest hint of metaphysics being applicable. One way to think about this to consider Indeterminacy as possibly the equivalent of a shadow. In and of themselves shadows convey no energy or information and can be said to not obey any laws of physics. They're the lack of these things by definition and the question then is why do quanta display such strange shadows that appear to defy common sense. Theories like Quantum Darwinism suggest it is these shadows that we are actually measuring all the time. What appears to us to be the collapse of the wavefunction is just the shadow changing. What seems to be the charge or mass of an electron is, again, just our measurements of the shadow or reflection and not the actual quanta themselves.


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## RobynC (Jun 10, 2011)

Uh duplicate post message deleted


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## RobynC (Jun 10, 2011)

@wuliheron



> Actually this particular experiment supports a Bohmian mechanics interpretation rather than Afshar. Bohmian mechanics assumes there is a real particle and separate wave that pushes the particle around.


I thought the particle was either a wave or a particle, or somewhere in between. Basically, it oscillates from wave to particle and so on... If you have a particle being pushed around by a wave, where did the wave come from?



> Take a delicate enough measurement and you can get a better idea of exactly where the particle is and it's momentum without causing the wave to collapse.


That actually defeats Heisenberg interestingly. If you can identify speed and location at the same time, then it doesn't apply...


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## wuliheron (Sep 5, 2011)

RobynC said:


> @_wuliheron_
> I thought the particle was either a wave or a particle, or somewhere in between. Basically, it oscillates from wave to particle and so on... If you have a particle being pushed around by a wave, where did the wave come from?
> 
> That actually defeats Heisenberg interestingly. If you can identify speed and location at the same time, then it doesn't apply...


Bohmian mechanics proposes it's an actual particle, but a holographic wave throwing it around. Sort of a hybrid theory where the wave explains Indeterminacy, but the particle can support realism.

Being able to measure the position and momentum simultaneously has never been a problem. The problem is the more you know about one the less you can know about the other. In this case they've shown you can get more accurate measurements by using better measurement error correction. Indeterminacy still applies, but it's possible to now distinguish it from any noise your measurement might introduce.


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## RobynC (Jun 10, 2011)

@wuliheron
Like you have two pictures of a photon from different angles? Still -- how do you get the wave between the two mediums?


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## wuliheron (Sep 5, 2011)

RobynC said:


> @_wuliheron_
> Like you have two pictures of a photon from different angles? Still -- how do you get the wave between the two mediums?


I'm not sure exactly what you are asking. Think of waves on the ocean pushing a beach ball towards shore. If there is only one way through the rocks you'll see the ball take that route in a pretty much straight line. If there are two gaps in the rocks the waves will interfere with each other and exactly where the ball will land is anyone's guess. 

In the case of light, the photons are the balls and we can't see the waves pushing them around. With just one gap in the rocks they move in a pretty much straight line as if they were ordinary balls rolling and form a cluster on the beach. However, open two gaps in the rocks and the interference between the waves causes them to land spread out over the beach instead of clustering.


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## Stelmaria (Sep 30, 2011)

wuliheron said:


> In this case they've shown you can get *more accurate* measurements by using better measurement error correction.


I don't believe they have shown that at all (unless the improved accuracy is simply closer to the uncertainty principle limits). Can you explain?


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## wuliheron (Sep 5, 2011)

Snow Leopard said:


> I don't believe they have shown that at all (unless the improved accuracy is simply closer to the uncertainty principle limits). Can you explain?


They've shown they can get measurements without collapsing the wavefunction which has never been done before at all. That's a more accurate measurement.


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## RobynC (Jun 10, 2011)

@wuliheron

Have they perfectly predicted location and speed at the same time?


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