[Week 1] Quantum mechanical behavior in macroscopic objects

[Jie Xiang]

We know everything, big or small, is considered a wave-matter duality under the tried and true quantum mechanics. However most textbook discussions stayed at discussing wavefunctions of the tiny electrons, within atomic potentials. What about bigger objects? Are they even proven to be a wave? Here are two Nature papers that show experiments along these lines and show why we don't talk wavefunction in macroscopic stuff: they are just too hard to measure. Read these and post your thoughts on the following questions: 

1. What are the difficulties in measuring quantum-mechanical behavior in these objects?

2. What behavior can be measured that is different than classical? 

3. What do you think of these phenomena (i.e. an object can be at two states at the same time)? Can we make cool stuff happen with this? 

4. Does nanomaterial help with these work? 

Notes: 

The newer paper, by Cleland et.al., is more advanced and it's best to skim through and not to worry about some of the technical details, instead try focusing on the physical meanings. Here's a news article that may help with the picture: 

http://www.nature.com/news/2010/100317/full/news.2010.130.html

[chandra.christensen]

I'll bite! Just a few thoughts to get the discussion started:

1. What are the difficulties in measuring quantum-mechanical behavior in these objects?

Obviously there are a ton of difficulties in performing an experiment as precise as this one, many of which were discussed in the paper. The point however which struck me the most, was the necessity to include the effect of gravity on the C60 molecules in order to determine with enough precision where they would land on the screen. The inclusion of gravity, a force which is usually somewhat at odds with the standard model of particle physics, and quantum mechanics, in an experiment measuring wave interference, really drives home the absolutely macroscopic nature of the particles in this experiment.

2.What behavior can be measured that is different than classical

In the first paper, the appearance of interference fringes in the detection patterns of the fullerenes is completely non-classical! (And fits the quantum model shockingly well, which implies the entire C60 molecule is in a single coherent quantum state before the interference! (Not sure if I'm phrasing this right))

3.Opinion? Cool stuff? 

Sooooo cool. I heard about these experiments first in regards to pushing the limits of the Copenhagen interpretation of QM. This is especially interesting with respect to today's lecture, where we discussed the inaccuracy of the "collapse of the wave function" viewpoint. So Bohr loses again!

Obviously quantum computing is a huge potential application of superposition of states. It's been shown that using wave functions to (using fuzzy language) sort've explore all possible solutions in parallel can decrease the time cost of certain algorithms from exponential to linear. Quantum algorithms, for example, completely change the game with encryption standards. Appropriately to the theme of the first lecture, I believe Feynman also had a lot to do with the birth of quantum computing!

4.Nano-Materials?
These experiments were obviously RIFE with nano materials! A good example is  the slits that the molecules were diffracted through - which also provides a good example of why precise nano-fabrication is so important, the large variation in actual slit size vs. manufacturers specification caused a large differing from predicted results, and probably quite a headache for the experimenters. 

[samontoy]

Quantum Mechanics: Macroscopic Stuff

There is no doubt that quantum mechanical description of everyday objects, turns out to be a very complex problem considering everyday object in nature is a many body system in which all physical phenomena exist all together. Take for instance, Connell, et al, paper describing the quantum ground state of a mechanical resonator, and consider all the parameters that where taken into account to construct an isolated and controlled environment to extract information regarding the lowest possible energy state of an object. A system is cooled to the lowest possible temperature because we want to eliminate thermal fluctuations in the system, as well as, other many body interactions to obtain the states with lowest possible mechanical mode. This is itself posses many difficulties considering there is no real mechanism for cooling an object to absolute zero, yet we can get fairly close. From personal experience dealing with superconductors, I know that measuring the critical temperature of many elements is a very tedious and expensive task because even with liquid He we can get to about to 1.9 K; whereas many elements have a critical temperatures less than 0.5 K. Not to mention that the most common samples measured are simply thin films with a surface area of a few square millimeters. So there is no considerable doubt that cooling any everyday object possesses many challenges.  

The true question we should ask ourselves is whether we need to measure the quantum mechanical description of everyday phenomena. It’s a fact that matter is dualistic in nature and therefore all objects behave as a particle and a wave; however, any object in nature consists of an infinitely many particles and thus the quantum nature dissipates. For instance, say we consider a particle being described as a quantum mechanical harmonic oscillator. We know that interesting observations can be noted in states consisting of a few particles, but as the number of particles increases the system begins to behave like classical harmonic oscillator. Therefore, everyday physics most phenomena can be described very accurately with classical mechanics. 

When dealing with nano scale objects, it is clear that quantum mechanical effects must be considered because particle interactions play a more important role in these dimensions.  

[chandra.christensen]

In regards to your second point, my opinion is always that the science in and of itself is fascinating enough to be more than worthwhile. It also could be asked if it is worth it to build the LHC, or go to space. The answer, in my opinion, is irrevocably yes. The practical, non curiosity based rationale, however, is spin-off technology. It might be hard to cool systems down to the appropriate temperature, and hard to make equipment precise enough to make the necessary measurements, but the technological innovations that are made in these bleeding-edge scientific pursuits are numerous, and their consequences are far-reaching. A good example is the LHC -- where the same engineers that built the accelerator and detectors are applying their prowess to build the next huge Nuclear Fusion experiment (which certainly has obvious practical applications). It's my understanding (although I can't remember where I read this) that the designs for the new ITER fusion experiment wouldn't be feasible to construct without the technological innovations made when constructing the LHC.