Intro to Relativistic Quantum Information

This is the draft of a short article I wrote for New York Uni Sydney's Observatory publication. It's mostly a conversational general interest summary of the research I'm interested in. There will be simplification and glossing!

The workshop venue, Customs House, Brisbane,
with a view of the river and Story bridge.

Image from Wikimedia Commons By Kgbo
[CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

I recently went to a workshop in Brisbane, and gave a talk. This is an annual workshop about Relativistic Quantum Information (RQI), which most closely aligns with my area of research. It takes a few steps to construct what ‘RQI’ means. Relativity here usually refers to special relativity; the theory in which the speed of light in vacuum is the same for any beam of light to any observer, and so space and time contort to keep this always true. Sometimes we look at space and time that are also contorted due to gravity. Quantum mechanics describes the behaviour of microscopic objects, all the way down to fundamental particles like the electron.

Relativistic quantum mechanics is the result of combining quantum mechanics with special relativity. An example of this is the standard model of particle physics, which describes the Higgs, electron, all other known fundamental particles, and their interactions. Relativistic quantum information theory then involves applying the concept of flow of information and its consequent mathematical tools to relativistic quantum mechanics.

In reality the research field has weaker borders than that. I think at its most simple we are trying to understand the consequences of quantum mechanics and relativity. Some common research areas are:

• Quantum mechanics in black holes: How does quantum information fall into a black hole, and how does it come out again when the black hole evaporates?
• The Unruh effect: An observer accelerating in what someone else sees as empty space will see particles appear.
• Vacuum entanglement: Two experimenters can have their observations become correlated without explicitly sharing any material between them. This can work across space, or across time, and has different forms depending on where they are placed near a black hole, or in empty space.

I would say these are the largest categories of research, but there are others. I find this area so fascinating because it is so different from everyday common sense mechanics, and is investigating totally novel consequences and extensions of the combination of very solid theoretical foundations: Quantum is about the microscopic and relativity is about extreme speed, so although they are difficult to see in everyday life, they are extremely well tested and experimentally supported descriptions of our world. With tested physics so far into the surprising already, there’s a good chance the bizarre consequences are real, and even the extensions are not in the realm of the outrageous.

My talk was about a toolkit in quantum mechanics that helps us describe and understand measurements of quantum systems, and applying this to describe space. Firstly why I would want to describe space using quantum mechanics is because matter (particles) is quantum, and matter causes gravity, and gravity bends space, so probably space is quantum too. This idea is called quantum gravity and has spawned large research efforts to find a working theory of quantum gravity. These theories tend to be constructed bottom-up, so there is a huge amount of theoretical machinery involved before being able to describe normal-looking space. I wanted to start with something simple; this toolkit called quantum reference frames, apply it to space, and see what the consequences are. Maybe the insight from this simpler construction can guide us with quantum gravity. This idea is definitely in the ‘extensions’ category, and hopefully is not outrageous. Basically I said that some technical issues with applying this toolkit to space had recently been overcome. I find the insights from quantum reference frames interesting in their own right, so perhaps I can write more about all this another time.

Many thanks to Tim Ralph from The University of Queensland, and Nick Menicucci from Royal Melbourne Institute of Technology for hosting and organising the workshop. I have been attending this workshop for many years and I look forward to the next one!

Box: Black Holes and Quantum theory

A black hole with radiation in and out.

 Graphic compiled by me from:
https://commons.wikimedia.org/wiki/File:Sech_soliton.svg Kraaiennest [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
https://commons.wikimedia.org/wiki/File:Wave_packet.svg Oleg Alexandrov [Public domain]
https://en.wikipedia.org/wiki/File:Black_Hole_in_the_universe.jpg NASA [Public domain]

The problem of quantum information in black holes (often called the black hole information paradox) runs as follows:

1. The picture is of a black hole in the universe. The principles of quantum mechanics tell us that since the universe is completely described as an isolated quantum system, quantum information cannot be lost or destroyed from the universe.
2. The black hole captures some quantum material, storing its quantum information.
3. The black hole completely evaporates by emitting random ‘Hawking’ radiation (which carries no information back out) and disappears from the universe. Quantum information has now been lost from the universe, contradicting the initial principle of quantum mechanics used.

The solutions to this problem involve abandoning one of the statements in the argument. Almost all have been considered in different solutions, including the fascinating calculation that black holes don’t actually properly form. For my money, the solution is that black holes affect time as well as space, so the black hole ‘exists’, but in a time disconnected from the rest of the universe.