Physicists prove 'quantum spookiness' and start chasing Schrodinger's cat

October 26, 2015 by Peter Mosley, The Conversation
It’s proven: the universe is weird. Robert Couse-Baker/Flickr, CC BY-ND

The world of quantum mechanics is weird. Objects that are far apart can influence each other in what Albert Einstein called "spooky action at a distance", and cats can potentially be dead and alive at the same time. For decades, scientists have tried to prove that these effects are not just mathematical quirks, but real properties of the physical world.

And they are getting somewhere. Researchers have finally proven in a new study that the link between particles at a distance reflects how the universe behaves, rather than being an experimental artefact. Meanwhile, another team of researchers have set out to show that a living creature, albeit a bacterium, can be in two different quantum states at the same time – just like the cat in Schrödinger's thought experiment.

Bell's inequality test

But let's begin with the paper, published in Nature, which proves that the world is inherently spooky. All systems described by can display so-called entanglement. For example an electron, like a coin, can spin in two directions (up and down). But two electrons can be entangled so that a measurement of the spin of one electron will define the spin of the other.

According to quantum mechanics, the spin of one electron cannot be known in advance of a measurement yet will be perfectly correlated with the other, even if it is in a distant location. Einstein didn't like this because it seemed to imply that the information can be sent from one electron to the other instantaneously – breaking a rule that says nothing can travel faster than the speed of light. He instead thought that there were "hidden variables" encoded in each electron that could determine the result if only we could access them.

But in the 1960s, Northern Irish scientist John Bell came up with a method to test Einstein's theory. "Bell's inequality" is satisfied only if actions in one location cannot affect another instantly and the outcomes of measurements are well-defined beforehand – something dubbed "".

Bell showed, theoretically, that quantum entanglement would violate his inequality test but local realist theories containing Einstein's hidden variables would not. This is because the link between entangled particles is stronger than Einstein wanted to believe. So if the measured correlation between pairs of particles from an experiment was above a certain threshold, it would be incompatible with hidden variables and entanglement would win the day.

The desire to test this in the lab has driven huge experimental advances in the 51 years since Bell's paper. However, all implementations of Bell tests to date have contained loopholes that have left some wiggle room for the universe to obey local realist theories.

One of these was that the efficiency of the measurements was too low (known as the detection loophole). Although the data obtained violated Bell's inequality test, it may not be a representative sample of a complete set due because some photons in the experiment couldn't be detected. Another loophole was that the measurements were too slow (the locality loophole). If the measurement devices were able to communicate via some unknown, slower-than-light channel they could share information and influence the outcome of the impending measurement.

The new study is the first experiment to simultaneously close both of these loopholes in a test of Bell's inequality. The scientists used a laser to make two specific electrons, each within a diamond located over 1km apart, to increase their energy and emit a particle of light (a photon), which was entangled with the state of the electron. The photons were then sent through an optical fibre to be united at a third location. If they arrived at just the same time, the photons would interact with each other and become entangled – meaning their remote electron buddies would become entangled too.

The electrons' spins were then measured to test Bell's inequality. The two loopholes were closed by ensuring that the efficiency and speed of the read-out were sufficiently high. As a result, the team were able to demonstrate conclusively that the universe does not obey local realism: the outcomes of measurements cannot be known in advance, and half of an entangled state can exert on its remote partner.

Physics' famous feline

Entanglement is not the only type of unusual quantum behaviour. Another effect, known as superposition, is the ability of a particle to exist in two states (for example spin or even location) simultaneously, and is now regularly observed in laboratories around the world. For example, electrons have been known to travel through two slits at the same time – when we are not watching. The minute we observe each slit to catch this behaviour in action, the particle chooses just one.

Quantum superposition made easy.

However, we do not directly observe these effects in daily life. For example, my glass cannot be in two places at once or I would struggle to drink. But because we don't encounter such bizarre things, it would seem logical that at some scale things "switch over" from the weird world of the quantum to our familiar everyday.

But what is the scale at which this switch happens? If we had a technically perfect experiment, would we be able to observe large objects in these superposition states? This is the question posed by Schrödinger's thought experiment in which a cat is placed in a sealed box with a flask of poison and a single radioactive atom, which will undergo decay at a random time. If the atom decays, the flask is broken and the cat is poisoned; if it does not, the cat lives on. By waiting for the atom to decay, does the cat exist in both states at once as the atom does? We know that when we open the box, we must find the cat alive or dead, but is it a property of the universe or the observer that makes the cat "choose" its state?

Back to the team preparing to address this very question. Their proposal involves putting a bacterium rather than a cat in a state of superposition. Recent technical advances based on superconducting microwave resonators – devices used to detect radiation and for quantum computation – have enabled physicists to observe quantum effects in tiny flexible aluminium membranes (known as micromechanical oscillators) coupled to the circuits.

Tiny membranes count as large objects in the world of quantum physics because, even with a mass of only 50 picograms (50 trillionth of a gram), they contain hundreds of billions of atoms. However, these resonators have to be cooled to within a fraction of absolute zero (-273°C) before any quantum behaviour emerges. Otherwise thermal vibrations mask the effects.

The team plans to put a bacterium on top of such a membrane, which would then be cooled to its lowest state of energy. The membrane would then be placed into a superposition of two different states of motion: two different types of oscillations. They aim to show that the effect of the bacterium on the properties of the oscillator would be minimal, with the oscillator effectively behaving as if the bacterium were not there. In this way, the bacterium would effectively be in two states of motion at once. The researchers also plan to entangle the position of the bacterium with the spin of an electron inside it.

The proposed experiment would be impressive – but mainly for showing that quantum mechanics holds true for objects bigger than subatomic particles. But it seems unlikely to answer whether Schrödinger's cat can be alive and dead at the same time because the would remain in a constant glass-like state of cryopreservation. If this were the cat, it would exist in suspended animation rather than in a superposition of simultaneous life and death.

Explore further: Researchers find a way to close both loopholes in testing entanglement with Bell's inequality

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Oct 26, 2015
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2.3 / 5 (6) Oct 26, 2015
Susskind puts it this way (to paraphrase): take a nickle and a penny. put one in Alice's pocket, put one in Bob's pocket. Now send Alice to Timbuktu (the crater on Mars). When she gets there ask Bob what's in his pocket. Faster than the speed of light, we'll know what is in Alice's pocket!! It is really spooky, right? Oh wait, no it isn't: its OBVIOUS.
As far as "the Cat". Yeah people continue to INTENTIONALLY confuse the facts. The fact is that a quantum superposition of two states is NEITHER one NOR the other. The properties of "being alive" and "being dead" are different from the POSSIBLE quantum state of (alive,dead). It is NOT a property of the organism, it is a property of the MATHEMATICAL (PROBABILITY) DESCRIPTION. It is a different 'level' of abstraction. Nothing "spooky" about it. (Likewise it is not raining tomorrow nor is it not not raining tomorrow. Is that confusing? ask me tomorrow...but for today tomorrow is in the state (raining, not raining) Is that confusing?)
5 / 5 (2) Oct 26, 2015
wait, so they are putting a frozen bacteria on a tiny quantum vibrating massage table? Science never ceases to amaze
Oct 26, 2015
This comment has been removed by a moderator.
1 / 5 (1) Oct 26, 2015
What a waste of time. She makes no sense but she tells people what they want to here and that is the point. She is only interested on getting the result that makes her feel comfortable and that is the path to insanity.
not rated yet Oct 29, 2015
Considering that the Cat of Schrodinger's, itself, is observing the radioactive atom and keeping it in it's same state by such observation, thus the cat will come out alive, unless it gets bored and falls asleep and quits observing (unless definition of observing includes residing in memory). Thus it is the actions, or in-actions, of the cat itself that makes the difference, being a living, observing, organism. Anything else is random chance.

So, the REAL take away from this Thought Experiment is that as long as there is an observer, a state can be maintained, at least on the Quantum level, but scaling this up would present some challenge, I would think.
Enthusiastic Fool
not rated yet Nov 01, 2015
Considering that the Cat of Schrodinger's, itself, is observing the radioactive atom and keeping it in it's same state by such observation, thus the cat will come out alive, unless it gets bored and falls asleep and quits observing

Downvote me to hell for postulating without a clue here please but:
Perhaps the worlds of quantum mechanics and relativity are weirder than you think. Perhaps in the cat's frame of reference it's in an absolute state. Whether the atom decays and pops the vial killing the cat or not the cat knows. Even asleep it has sound, smell, and tactile information it can observe with.
In our frame of reference the probability wave has not collapsed on one outcome or another as no information has escaped the tentatively closed system to our frame of reference.

Further derpiness from me:
What if entanglement is actually particles acting as the smallest observers of a system and the wave collapses upward from the smallest to the largest observer.
Enthusiastic Fool
not rated yet Nov 01, 2015

It's obvious that's a terrible analogy. You don't KNOW faster than the speed of light. You cannot get an accurate measurement of what's in her pocket FTL. You can only INFER what might be in her pocket based on the information at hand. That's a huge difference. She may have gone to the vending machine or it could have fallen out when she laid down for the long trip. There may have already been another nickel in her pocket. The idea of entanglement is that you can measure at one and know with certainty the state of the other end.

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