Friday, August 5, 2016

The Schrödinger Sessions: Day 2

(Picture from here.)

The Schrödinger Sessions are a collection of lectures and demonstrations of quantum physics for science fiction writers. (See here.) They are a joint production between the Joint Quantum Institute at the University of Maryland and the National Science Foundation. The three organizers are Chad Orzel, Emily Edwards and Steve Rolston. Three of the most terrific people I've ever met.

JQI is what they call low energy quantum mechanics. This involves quantum computation, low temperatures, superconductivity-- all of those sorts of things we can do in a relatively small lab. High energy quantum mechanics and physics, those things done at the Large Hadron Collider and supernovas, aren't done at JQI. That didn't prevent us from asking about it.

I found out about it when I checked out the launchpad astronomy workshop. I went down. I did the seminar. This is my little diary. It's a week late in that I wanted a chance to clean it up before I published it.

Day 2
Most of the morning was occupied with quantum computation. There’s a lot of stuff there involving the union between quantum mechanics and information theory. Christopher Monroe boiled down quantum mechanics to two rules:

Rule #1: Quantum objects are waves and can be in superposition. This can correspond to bits used in computing as long as it is understood the bits are probabilistic descriptions and not deterministic descriptions. So if you have a bit that can be 0 or 1, a superpositioned bit would have a probability of which it is. Say 80% 1 and 20% 0.

Rule #2: Rule #1 holds as long as you don’t look.

“Looking” breaks the isolation and causes a measurement. Without understanding, the output is random. What’s important is the state so the object of the exercise is to preserve state as long as possible.

The advantage of quantum is its tremendous parallelism. For example, if you have a three bit system, there is the possibility of 8 possible states: 000, 001, 010, 011, 100, 101, 110 and 111. A normal computer can hold any one of these states at a time. A quantum computer can hold all of these simultaneously, each one having a probability.

Where this gets interesting is the determination of how to manipulate the starting conditions such that when the system is actually measured you get a meaningful answer.

We had a long talk on which sorts of problems suited a quantum computer. Some problems had only a little advantage while others had significant advantage. Factoring large numbers was one. Advanced simulation of chemical systems was another.

The more I heard of this, the more I was convinced that having a desktop quantum computer by itself was going to be of limited usefulness. I think the real utility is going to be a hybrid. A machine that solves the problem part of the way, then sets up a quantum computer to solve a portion, takes back the result and continues on. In other words, the quantum computer would act as an accelerator or support application rather than be completely useful in and of itself.

We had lunch with one of the researchers who set up the Ice Box Neutrino Detector on the South Pole. Very, very cool. Imaging a cubic kilometer of ice acting as a scientific sensor.

Then, on to superfluids and superconductors.

Superfluids are strange. For one thing, they conduct heat 500 times better than copper. You can make temperature waves that propagate through them like sound waves. They have no viscosity. If you start a superfluid rotating in the beaker it won’t stop. Forever. Until the temperature changes.

What causes superfluids (and superconductors) is the different natures between bosons and fermions. Bosons are those particles (or those things that act like such particles) who have the capacity of symmetry when quantum effects are demonstrated. Fermions are those particles who have anti-symmetry under the same circumstances. An example of this is Helium-4 vs. Helium-3. He-4 “bunches”, that is, when they get to the right point they cluster together since the SE wave functions become identical. He-3 gets to the same point and has a different solution to the wave equation. This makes them Fermions brings them under the Pauli Exclusion Principle. All because He-4 has an even number of particles in the nucleus and He-3 has an odd one.

Go figure.

Turns out the PEP is incredibly important. It's the reason that electrons occupy discrete orbitals in the atom and don't all bunch up in one place. The PEP says that no two identical fermions can occupy the same quantum state-- an electron is a fermion. Think of a hydrogen atom: one proton and two discrete electrons around it. They don't bunch together. Instead they are both available for chemical reactions-- each as a different spin. Spin, here, is defined to represent the angular momentum of the electron. This is not to say the electron is actually spinning like a top-- it turns out that if spin, the quantum state, was produced by spin, the physical act of rotation, the electron would be spinning faster than the speed of light. Instead, the electron acts like it has angular momentum and they call this quantum quality, spin.

As electrons show up in different orbitals of bigger atoms they have to occupy discrete orbitals where each electron has a unique set of quantum numbers associated with them-- of which spin is one. (Turns out there are analogs to orbitals and shells in the nucleus and its why the nucleus gets bigger as you add protons and neutrons-- neutrons are absolutely essential for this. But I digress.)

Superconduction has a similarly weird set of behaviors. MRIs use liquid helium to keep their superconductors cold—which is why we should not be using helium in birthday balloons. It's a precious natural resource. So when Mercury gets below about 4 Kelvin, its resistance goes to zero. Not .1 ohms. Not .5 ohms. Zero.

Another strange thing about superconductivity is current persistence: put a current into a super conducting ring and it stays there. Forever. Also, it excludes magnetic fields. Got to see several demonstrations of this. Imagine little levitating magnets skooching around a track without ever going higher or lower. Here's a video of it I found on youtube. Seeing it in person instead of by video gives you a funny feeling. Like the world is tilted.

A talk on how ultracold works along with a lot of liquid nitrogen demonstrations. Liquid nitrogen is fun.

Then, Raman Sundram talked with us regarding higher energy physics as opposed to the low energy physics we’d been talking about. Just a few things:
  1. Quantum mechanics, relativity and gravity don’t play well together. There are theories that handle pairs of them but nothing that manages all three.
  2. At the point of the Big Bang, all three were at their maximum. Therefore at one point in the history of the universe they were together. We just don’t know how.
  3. We normally proceed through time via space. We navigate space in all directions but not time. However, it turns out (Dirac) that being able to navigate time is also necessary. This is why anti-matter is necessary. It serves as the negative time representation. (I could draw a curve of this but I won’t.)
  4. One issue is why is there essentially no antimatter now? The current theory is the War of Annihilation: there were large amounts but normal matter and antimatter destroyed each other. There was a slight imbalance on the amounts and that’s why we are here now. (One wonders if the WOA happened, could it be the source of inflation?)
  5. The vacuum energy looks to have cause/be responsible for/related to dark energy. Dark energy can be viewed as repulsive gravity. Turns out that gravity can become repulsive if the pressure is great enough. Vacuum energy has a lot of pressure. One wonders what repulsive gravity in the vacuum energy does to the passage of time in the vacuum.
End of Day 2

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