## Quantum Measurements Leave Schrödinger's Cat Alive 210

Posted
by
samzenpus

from the looking-in-the-box dept.

from the looking-in-the-box dept.

First time accepted submitter Walking The Walk writes

*"Your co-workers who keep using Schrödinger's cat metaphor may need to find a new one. New Scientist reports that '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.'"*
## The BEST related animation (Score:5, Interesting)

Not my Work! - HEX

## Quantum cryptography? (Score:5, Interesting)

## Re:Equivalent of peeking without killing it ?! (Score:4, Interesting)

I always wondered why the cat didn't qualify as an observer to begin with.

## Re:Equivalent of peeking without killing it ?! (Score:5, Interesting)

No, that is not the experiment's point. That is its premise.

It is the only interpretation that I am aware of (though its precise phrasing varies). In fact, it is the only reason that anyone hopes qbits will work. Hence me not being confused.

Unlike what the original poster said, the cat is not already dead when you open the box.

Thatis the whole point of the experiment. The cat is neither alive nor dead until the point in time in which you look, at which point it has already been alive/dead all along.This principle is the one that drives the quantum computing research. The idea is that you create 512 qbits signifying a number. Since they are in their base form, they each can be either 1 or zero, which means that they are, potentially, all 2^512 possible numbers. You then pass them through a series of filters that, essentially, force them to multiply with another set of 512 qbits and form a known result. Only then do you check what each of the qbits is. You have just factored a 1024 bit integer in zero time by letting quantum mechanics test all possible combination concurrently.

Shachar

## Observing, Not Avoiding (Score:5, Interesting)

From the 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 usuch 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..."

The way I read this, they aren't claiming they prevented collapse, nor that they can predict which state it will collapse to; rather, they have (1) increased the time of the collapse of the wave function (via feedback) and (2) been able to "watch" the electron collapse to whichever state it goes to. [N.B.: I am totally open to correction. I haven't paid the $32 for a copy of the paper.]

So, no Heisenberg compensator here.

## Re:Equivalent of peeking without killing it ?! (Score:5, Interesting)

It is the only interpretation that I am aware of (though its precise phrasing varies).

That is the Copenhagen interpretation. There are several others: In the many-world interpretation, there are universe in which the cat is alive, and universes in which the cat is dead. Until you open the peek, you can interact with both. Ones you have peeked, the versions of you in the universe where the cat is dead and the versions of you in the universe where the cat is alive diverges, and cannot interact anymore (roughly). Then there is the de Borglie-Bohm [wikipedia.org] interpretation, where the cat is either dead or alive (particles have a definite, deterministic position), but until you have observed it, you can only interact with the wavefunction, which is the same for dead and alive cats (I think, but I might have horribly misunderstood it). In fact, there a quite a lot different interpretations of quantum mechanics [wikipedia.org]

In fact, it is the only reason that anyone hopes qbits will work.

Qubits works because of quantum mechanics, that is, because the equations are as they are. That have nothing to do with the interpretation, which is how we understand the equations. Interpretations are not scientific, as they make exactly the same predictions as the underlying model, but are more complex. They are not really needed, but the human mind doesn't like thinking in equations, it prefers to have something that behave like something physical, so we like having them.

## Re:Equivalent of peeking without killing it ?! (Score:5, Interesting)

Meh, it's already been done experimentally

Have a look at the

Quantum Zeno Effectwhich is both one of the scariest and most awesome pieces of experimental quantum physics around. Just in case your Google is broken, the experiment stops the random decay of unstable particles by continually measuring their state. Since the cat is just an allegory for these sorts of particles, the experiment has already been done - yes you can prevent a random (quantum) event by taking continuous measurements.You don't need this thought experiment any more - as *real* physics cruised past these mind games decades ago.

## Re:first post ! (Score:5, Interesting)

This shows a fundamental misunderstanding of the cat experiment, in that the author is assuming that by opening the box the cat gets killed. When in fact the cat can be considered both alive AND dead while the box is unopened, and if you open it it might very well be alive and not dead. Thus it would be equally accurate (or inaccurate, rather) to say

"the equivalent of taking a peek at Schrodinger's metaphorical cat without making it LIVE.I'm not a quantum physicist but If I understand Schrödinger's experiment correctly (feel free to reeducate me), the cat is both alive and dead until you open the box and 'fix' it's state. Until you observe the cat all you can say is that the closer you get to an hour (the radioctive matierial decays one atom per hour) the more likely it is that the cat is dead. So have these scientists managed to observe Schrödinger's cat in it's dual live/dead 'flux' state?

## Re:first post ! (Score:4, Interesting)

The Schrödinger's cat is just great marketing for a very straightforward statement. The cat is alive (cat = 1) in 50% of the worlds, the cat is dead (cat = 0) in 50% of the worlds, but you don't know in which world you are until you look at the cat (a.k.a. "collapsing the space of probabilities", big marketing words for a straightforward concept). Schrödinger's merely says that the cat's expected value [wikipedia.org] is: 0.5 * 1 + 0.5 * 0 = 0.5 (but this is a statistics mean, doesn't imply that the cat is both alive and death, just like families never have 1 kid and a half)

The correct way to say it, is that schrödinger's cat is a projection in 3D space of a 4D space problem, where the 4th dimension is the set of "alternate scenarios", and the cat's value at a certain position, is its expected value in that position (a "fuzzy variable").

While you can't get worthwhile info from studying a single cat system without "opening the box", you can discover a lot from a system with many cats; for examples, take a look at nonograms [wikipedia.org].

## Re:Equivalent of peeking without killing it ?! (Score:4, Interesting)

But weakly observing it can have value. It would be possible, for instance, to determine whether there is a cat in the box at the moment (perhaps by weighing the box and comparing the finding with a predetermined minimum-weight-of-cat value). This is important because a cat that is not observed in any way may or may not be in any particular place. Anyone who has ever lived with a cat knows this. People who have never been owned by a cat may be incapable of understanding this, and probably should not look for a career as a quantum mechanic.

But that explanation might be too subtle for classical physicists (who likely do not much like cats, ever since Schrödinger soured them on the cute little beasties). So for them the dilemma can be stated in a more gross fashion: how can you even know whether a cat in the box is a part of the device you are trying to build unless you at least look at whether a box is or is not present? It would seem that some degree of weak observation is indeed necessary if anything is to be done.

The underlying problem is of course that quantum mechanics sits in the intersection of physics and semantics. It is not only the case that classical physics is unable to handle what is happening at the quantum level. It is also the case that as a product of this Universe, the human brain is basically incapable of understanding quantum level events. There's something happening here, but what it is ain't exactly clear... and never will be. So sayeth the Copenhagen convention.

I don't expect anyone on Slashdot to accept this on face value. But I do have a citation: check this out. [straightdope.com] One of the more obvious implications is that if you do not have a sense of humor, then becoming a quantum mechanic is not a good career choice for you.