Physicists seek to quantify macroscopic quantum states

June 29, 2011 by Lisa Zyga, feature

( -- "Scientists have been interested in generating and observing macroscopic quantum superpositions in order to test quantum mechanics at the macroscopic scale," physicist Hyunseok Jeong of Seoul National University in Seoul, South Korea, told "There have been many papers in which the authors claim to have generated mesoscopic or macroscopic superpositions, often called 'Schrodinger cat states.' However, quoting A. J. Leggett in 2002, a question from the theoretical side is ‘What is the correct measure of "Schrodinger's-cattiness"?' It has been considered difficult to answer this question with a general measure, and the answer has remained to be 'very much a matter of personal taste,' quoting Leggett again. Our work now provides scientists with a theoretical tool to quantify and compare different types of quantum superpositions. This can be a step toward rigorous tests of quantum mechanics in a macroscopic limit."

Jeong and his coauthor Chang-Woo Lee, also of Seoul National University, have published their study on the quantification of macroscopic quantum superpositions in a recent issue of . Having a way to quantitatively compare different types of states in terms of their size and their degree of quantum coherence will be very useful for theoretical and experimental studies on macroscopic quantum phenomena, generation of nonclassical states, and the decoherence of quantum states within various physical systems.

As the scientists wrote in their study, quantum superposition is often considered the most crucial feature of . In quantum mechanics, particles can exist in one or more energy levels. When a particle exists in just one energy level, it’s in a well-defined energy state. But when a particle exists in two or more different energy levels at once, it’s in a superposition of energy states. The most well-known example of superposition is Schrödinger’s cat, which is locked in a box with the possibility of being poisoned. Until an observer looks inside the box, the cat is considered to be both dead and alive at the same time, according to quantum mechanics.

Physicists have observed superposition in many experiments with microscopic systems. However, the question of whether a truly macroscopic system – such as a cat – can exist in a quantum superposition is much more complicated.

For the past 10 years or so, physicists have been proposing various ways to define or measure macroscopic quantum superpositions. Many of these proposals start by considering the number of particles or the distance between component states involved in the superposition. Although this approach sounds reasonable, the proposals have run into problems – particularly, they have not been general enough to be applied to different types of states.

The biggest advantage of Lee and Jeong’s method of measuring macroscopic quantum superpositions is its generality, which enables it to be applied to many different types of states and allows for direct comparison between them. The method is based on the quantum interference of a given state in phase space, which is the space in which all possible states of a system are represented.

As the scientists explained, a macroscopic quantum superposition has two (or more) well-separated peaks and some oscillating patterns between them in phase space. The scientists showed that the frequency of these interference fringes reflects the size of the superposition, while the magnitude of the interference fringes relates to the degree of genuine superposition. So using this method, the scientists could simultaneously quantify both the size of the system and its degree of quantum coherence. The method also works for superpositions that are fully or partially decoherent, which occurs when macroscopic superpositions lose due to interactions with their environments.

Overall, the method doesn’t provide a specific threshold beyond which a superposition is “macroscopic,” but instead it provides a continuous scale to compare sizes of different superpositions. The scientists found that the method also agrees with a previous method (Dür, et al.) designed to measure a specific type of state. But with its advantage of being able to measure any state represented in phase space, the new method could be widely useful for future studies on macroscopic quantum systems.

“In [this] paper, the focus was pretty much on continuous-variable states, which mainly relates to light fields,” Jeong said. “However, due to the intrinsic generality of our measure, it should be quite straightforward to extend it to various discrete-variable systems such as atomic states. I will then apply this measure (I would call it ‘superness,’ thanks to Dr. Jonas Neergaard-Nielsen) to various ‘claimed’ macroscopic superpositions. I believe that such investigations will reveal ‘where we are’ regarding macroscopic tests of quantum theory.”

Explore further: Can we detect quantum behaviour in viruses?

More information: Chang-Woo Lee and Hyunseok Jeong. “Quantification of Macroscopic Quantum Superpositions within Phase Space.” Physical Review Letters 106, 220401 (2011). DOI: 10.1103/PhysRevLett.106.220401


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1 / 5 (1) Jun 29, 2011
this is the beginning of teleportation
not rated yet Jun 29, 2011
Teleportation. So if we know the probability of being somewhere else we can go there? (DA's Improbability Drive)
What if we could choose the desired state from the "superness"? (Stephenson's Anathem)

Superness indeed!
5 / 5 (1) Jun 30, 2011
I am sooooo disappointed by the lack of comments on this article. This is HUGE. Come on people. Where are the thoughtful insights? The spirited debate?

1 / 5 (1) Jul 01, 2011
Typo corrections to the original wording in CAPS:

"So using this method, PYLE could simultaneously quantify both the size of the system and its degree of quantum coherence. The COMMENTARY THREAD also works for superpositions that are fully or partially decoherent, which occurs when macroscopic superpositions lose quantum coherence due to interactions with PYLE." - authors ( slightly edited.)

not rated yet Jul 03, 2011
Where are the thoughtful insights?

Sorry. Busy. ;) I had this on an open tab since day one and just now got to it. This IS huge. I love this f-ing site!

Google Chrome says teleportation isn't a word. It doesn't want us to talk about it. Beware the googlebot! haha

I think I may be about to confuse superposition and entanglement:

This article makes me think about a much less ominous thing; quantum consciousness theory.

Could we be able to measure the overall level of coherence(entanglement?) of the left to the right hemispheres of our brains? Could we deduce a coherence(entanglement?) between something in the experiment to something that is far from the experiment? I'm thinking about Penrose's (I think) idea of a whole mind being entangled with another (superpositioned) mind, maybe even outside the visible Verse. Am I anywhere near relevant to this article? hahaha
1 / 5 (1) Jul 07, 2011
In reference to Mirrored Quantum Reverberations, the states of a single particle/rip would inversely be super-imposed on all other particles/rips, with the the probability waves being a culmination of these inverse states and the original state in varying magnitudes due to distance and the various other culminations that form. The original particle/rip would also contain the inverse states of all the other particles/rips. So a particle/rip in essence would be a culmination of it's original state and the inverse state of all the other particles/rips. So that original particle/rip would be a product of many states, but these many states would result in only one culmination for that particle/rip at each moment in time. (At each moment in time the original particle/rip would be a culmination of the many states that exist at that moment of time.)
© Copyright 2011 Thomas A. Sullivan
not rated yet Jul 10, 2011

That made less than zero sense. Super-imposed on other particles? That's something much more than entanglement and implies coherence over any/all particles. Where did you get that? You don't seem to have answered anything.
not rated yet Jul 12, 2011

Wow, I made less than zero sense! I have created anti-sense, and it hasnt even annihilated any sense yet.
Yes, I realize that there was not enough information to fully understand my concepts. Rip Theory predicts that there is what you called a "coherence" over all particles. Rip Theory predicts that all particles are "entangled" to a certain degree by "gravity tubes". When the perimeter (edge) of one particle (rip) moves, that movement affects all other particles. The movements are "inversed" because when an edge of a particle (rip) moves, the other edges move in an opposite manner. (This is from a theory I wrote and registered with the Library of Congress. It is yet just a theory, but a Swiss team has done an experiment that seems to confirm the basis of this theory.)
© Copyright 2011 Thomas A. Sullivan
not rated yet Jul 12, 2011

I missed you earlier comment somehow. TS's ?theory? got in the way I guess.

I'm still disappointed in the lack of response to this article. I guess it flew under the radar.

I'm thinking about Penrose's (I think) idea of a whole mind being entangled with another (superpositioned) mind, maybe even outside the visible Verse.

Neal Stephenson's Anathem meanders around this theme, sort of. Not as funny or cool as Snow Crash or as scary as Diamond Age, but the physics behind it is quite clever.
not rated yet Jul 16, 2011
While the article sounds interesting, it has no actual description of the measurement. The only citation is to a paper having an equally vague abstract, and whose content is only available by purchasing it. What gives? Why is this topic impossible to describe clearly to intelligent readers? Or is this really dull research that has been hyped-up to make it seem more impressive that it really is? Call me cynical, but I don't like what has been done here.

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