Why many worlds?
Quantum mechanics is often presented as being “our most successful theory ever”. Despite 100 years of stringent experimental tests it has never been proved wrong. It has been confirmed to an accuracy of 1 part in 1012 and has even now been tested in space-based experiments. It underlies much of modern technology, including pretty much the whole information and computing industry. And it has predictive power in an extreme range of scenarios, from the smallest constituents of our universe to a fraction of a second after the Big Bang. For these reasons, it seems reasonable to assume that quantum mechanics is correct, and does not need to be modified, at least not yet.
We can then ask the question: If quantum mechanics is correct, then what does it tell us about the universe? But before answering this, we should probe whether this is a reasonable question in the first place. Continue reading
This blog post follows on from a previous post, “What does quantum mechanics tell us about reality”, in which I tried to give a balanced and non-technical overview of some of the interpretations of quantum mechanics. In this post I will take a different approach: this will be a slightly biased, critical, and more technical follow-up. I recommend reading the earlier post first, but those already familiar with the interpretations of quantum mechanics should be able to dive straight into this.
In the Schrödinger’s cat thought experiment, a cat is placed in a box with a device that contains a radioactive atom and a vial of poison. If the atom decays, then the device is designed to release the poison, thus killing the cat. It is now well known that such an atom can be put into a state in which it has decayed, and not decayed simultaneously – this is known as a superposition state. Now, if this system is studied using the central equation in quantum mechanics, the Schrödinger equation, then the following result will be found: if the atom is in a superposition state, then this will lead to the cat being in a superposition state. The cat will be dead and alive simultaneously! Now suppose you open the box – what will you find? The Schrödinger equation again predicts that, if the cat was in a superposition of being dead and alive, then when you open the box you will also enter into a superposition. You will be in a superposition of either seeing the dead cat, whilst simultaneously seeing the alive cat.
How much coal do I need to put into my steam engine to get to Glasgow? How efficient can I make my power station? What’s the best way to cool down a cup of tea? The fundamental answers to these questions lies within the field of thermodynamics.
On the other side of the spectrum, scientists are asking seemingly disparate questions within the field of quantum information. How much entanglement do I need to pass on this message? How can I stop my quantum computer from losing its quantum-ness? What’s the best way I can make a quantum superposition?
Now it may seem that there can be no possible link between the answers to these questions. That a steam engine has nothing to do with quantum entanglement, well that is mostly true. However, all of questions are concerned with the same problem, the extractability and conservation of a given resource. On the thermodynamic scale, engineers want to know how heat and work behave. On the quantum scale, theorists and experimentalists alike are interested in defining and conserving the quantum-ness of a given system.
In order to bridge a gap between these fields, Continue reading
In this blog post I will introduce a core philosophical issue at the heart of quantum mechanics, and some of the most popular resolutions to this problem. This will be a non-technical introduction – I will try to avoid jargon and unnecessary details. But for those who like the gritty details, a second blog post will soon follow in which I will give a more technical and thorough (but also slightly biased) discussion of the same issues.
I first came across the Entanglement picture about 11 years ago. Nearing the end of my year-long visit to the Centre for Quantum Computation in Cambridge (UK), I was returning to Salerno (Italy) to start working on my PhD thesis. As my dissertation was entitled Entanglement of Gaussian states, I decided to include some pictures related to (quantum) entanglement at the beginning of each part. The first few hits for “entanglement” found at the time on Google Images eventually made the cut (check the final version at https://arxiv.org/abs/quant-ph/0702069) However, one of the images in particular captured my imagination – and stole my heart – so much that I chose it for the best spot: Part II, showcasing the bulk of my original results on bipartite entanglement. The image was a relatively low-res photo of a painting entitled, quite aptly, Entanglement, and taken from the website of its creator, American artist Pamela (Pam) Ott: http://www.hottr6.com/ott/.
A paper of ours featured on the cover of J. Phys. A (2014)
I have recently been appointed to the the Editorial Board of Journal of Physics A: Mathematical and Theoretical (in short, J. Phys. A). J. Phys. A is a highly respected journal with a long history of seminal contributions to mathematical and theoretical physics, belonging to the non-profit Institute of Physics (IOP) Publishing family. I have enjoyed publishing in J. Phys. A over the years (including two Topical Reviews) and always experienced a very constructive peer review process; my students love it as well. We had our latest (the first for me) Editorial Board meeting a month ago in Edinburgh and it was a really pleasant and interesting experience, also because I got to spend the week-end there with my family and the weather was surprisingly nice 🙂
The publishers of the IOP blog JPhys+ interviewed me recently about my career, current research and what it is I find so appealing about the topics I study. The full text of the Q&A interview with Phil Brown, originally appeared here, is copied below.
If you have ever asked a quantum researcher about their subject, you will have realised that our first response is to tell you that the quantum world is weird, spooky, and counter-intuitive. It cannot be explained easily because we only just about understand it ourselves! After a bit more conversation, you’ll find us telling you that all these weird features are actually very useful: we talk about a potential for new technologies that can harness the quantum, from quantum computers to cryptography. The idea is to treat these quantum features as useful resources that help us to do things in an improved way. To do this, we need to carefully construct a framework that allows us to rigorously characterise these resources. Such a framework is called a quantum resource theory.
In this post, I will give a short explanation of resource theories. We begin by outlining the concept generally, but then focus on two particular examples of resources. The first example considers the resource of money in a fictional bank account, while the second example moves properly into the quantum world with the familiar resource of quantum entanglement.