There is no local realism within the quantum realm!

What is reality? Is there a mathematically rigorous way to define it? Following the German philosopher Immanuel Kant, we can at least start by vaguely identifying two different kinds of reality: anything that we come across a posteriori as a result of an observation, so-called phenomenon, and anything-in-itself a priori with respect to our observation, so-called noumenon. In the following we will be interested in the latter form of reality and ask the question: is it there even if we do not observe it? In other (Einstein’s) words: is the moon there when no one looks? Of course many other questions immediately follow, such as: how can we ever prove that there is no reality-in-itself if we are by definition not allowed to observe it? Can we resort to the phenomenon to prove the nonexistence of the noumenon?

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A somewhat coherent post on a robust idea

[This is a guest post by our long-time friend and collaborator Marco Piani, cross-posted on his blog Quantum Rules]

The word “coherence” has different meaning for different people. Most people may think of the notion of being logical and consistent, be it in speaking or in acting. Actually, we all hope to deal with people — especially politicians(!) — who exhibit coherence between what they say and what they do. And we all hope that the next major blockbuster movie is coherent, with no major plot holes that make you grind your teeth in your seat, unable to fully enjoy your popcorn.

Nonetheless, to a physicist, coherence is also a notion associated with wave behaviour. More precisely, it is associated with the possibility of seeing the effects of superposition, which is the coherent(!) combination of different physical possibilities. For example, the superposition of sounds waves is what allows people to listen to music in the background, while pleasantly chatting.

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Posted in Interference, Quantum research, Superposition | 4 Comments

Tangram puzzles and the cost of calculating entanglement

I haven’t got many memories of my time as a kid. They mostly survive through exhilarating accounts narrated by my parents to other relatives and friends over and over again… but I do not remember living most of them (not sure why). Yet one of the distinctive memories I have is that of me sitting in the one and only barber shop in my home village, sometime during my primary school years, and discovering a great way of killing time during the long waits: Tangram. Yes, Tangram, the traditional dissection puzzle invented ages ago in China and imported in the western world during the 19th century, a sample of which eventually ended up in that barber shop.


Tangram set (source: Wikipedia)

I clearly remember myself playing with the seven wooden pieces, and the cheer of satisfaction when I finally figured out a way to reassemble them into the designed large square. More recently, with the advent of smartphones, plenty of Tangram puzzle apps became available for download, all based on the same principle: reassembling a number of small polygonal pieces in a puzzle, to reconstruct a target, more complex shape. Sometimes, there is only one way to achieve the goal, given the set pieces; some other times, there are multiple solutions to a puzzle.

Today I am going to discuss a variation of the Tangram puzzles, with an important twist: price. Imagine that each of the set pieces, to be used for assembling the final target shape, comes at a cost (that we will indicate with the currency symbol ¥). For any given target shape, the goal of the game is then to find the solution which minimizes the overall cost. This may not be necessarily the one with the least amount of used pieces, but must be the solution (or solutions) involving the overall cheapest possible combination of the pieces, in order to reconstruct the target.

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Posted in Entanglement, Geometry, Quantum research | 1 Comment

On the Motive Power of Intuition

I’m sure that most of you were taught thermodynamics in high school. You’ll probably remember listening to things like “…thermodynamics is the branch of physics that studies heat and work…”, or “…thermodynamics is concerned with large objects, like heat engines…”. For most people thermodynamics was indeed a pleasant subject to study. Its math was simple. At least, simple compared to those horrible vectors and scary integrals from electromagnetism or mechanics. And the theory was fairly easy too: You just had to memorise a couple of formulas and those good old Three Laws.
(Or were there four of them?)
There’s no doubt that some may have liked thermodynamics better than others, but we all understood it.

In my case, I later started a physics degree and had to take yet another “Basic Thermodynamics” course. While electromagnetism and mechanics had become considerably more scary in the first year of the degree, thermodynamics still looked pretty much the same.
(Nothing to worry about. Piece of cake)
However, some things got me thinking. I started to ask myself about basic concepts that had never troubled me before. For instance, the ‘state variable’ T stands for ‘temperature’ and appears all over the place in thermodynamics but, did I really understand what temperature was?

“You must be kidding. Everyone knows what temperature is!”, you may say.

“Really? Then tell me what is it”, I would reply defiantly.

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Posted in Miscellanea, Quantum research, Thermodynamics | Tagged | Leave a comment

What dancing, Nuclear Magnetic Resonance and Quantum Information have to do with each other?

When we hear the words “Nuclear Magnetic Resonance” (NMR) what first comes to our minds is probably those huge and noise machines in hospitals. However, much more than that, NMR is a powerful technique that helps us to understand how the microscopic world arranges itself, and its applications extend to many different areas in chemistry, physics, biology… In this post, I will briefly explain the principles behind this technique and how it is applied to quantum information studies.

In the same way that any physical body has “mass” as an intrinsic property and this masscompass interacts with Earth’s gravitational force, some nuclei (like hydrogen, nitrogen…) have also an intrinsic “nuclear spin”. This spin allows those particles to feel and respond when a magnetic field is present. Like a compass that will orient itself according to Earth’s magnetic field, the nuclear spins will follow the local magnetic field around its position.

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Posted in Dancing quanta, Quantum research | 1 Comment

So, what are you researching? Connections between quantum entanglement and metrology

“So, what are you researching?”

The dreaded question I am asked on countless occasion. From friends, family, acquaintances, this is a very natural question to ask of somebody doing a PhD – research is both their passion and their purpose. So why do I find it so tough to articulate what I do?

Well, let me introduce myself first! I’m Tom, I have been doing my PhD in Nottingham for just over two years now and, like the rest of the group, specialise in quantum information theory. My research has focussed on quantum correlations beyond entanglement, quantum coherence, and quantification of multipartite entanglement. If you’re feeling a bit nonplussed after that last sentence, I don’t blame you! How can such abstract concepts be done justice with a simple sentence or two without explanation? This is exactly the task I face when I try to explain what I am researching to my inquisitive interrogator. As much I love what I am doing, it seems that some of the wonder is lost in my words. Whilst most of my work lies firmly within the realms of “blue skies research”, I find my explanations drawn to the more tangible applications in quantum information science like the quantum computer, quantum teleportation and cryptography. The result is an unsatisfactory jumble of hand waving explanations and brief discussions of Star Trek teleporters!

Now, using the power of this blog, I have the opportunity to give a satisfactory explanation once and for all! In my series of posts I will give accessible descriptions of some of the topics researched in our group, linking them in with some of the realistic applications of quantum information science that are gradually being introduced into the modern world. So, without further ado, on with our first topic!

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Posted in Entanglement, Quantum foundations, Quantum research, Superposition | 6 Comments

Quanta rei, or not?

“Τα πάντα ρει” says famous Greek philosopher Heraclitus; everything flows. With this expression, Heraclitus probably wanted to express the world view that, at the most basic level, everything is in constant motion. What is the most basic level of nature according to modern physics? Of course, the quanta! If Heraclitus’ view that everything flows is correct, then ..quanta rei as well; or, to take advantage of my Greek origins, ..τα κβάντα ρει, right?

A few weeks ago, Gerardo, the Italian captain of our Quantum Correlations group in Nottingham, inaugurated our brand new Blog with a warm welcoming post, where he very illustratively explains why we chose the name “quanta rei”,

All entities move and nothing remains still. […]
This is remarkably true once one gets down to the smallest components of the fabric of the world, in the realm of quantum mechanics: even if something appears still to our macroscopic perception, the particles inside it never stop moving, fluctuating, exchanging energy and getting correlated with the others nearby, even when cooled down to the absolute zero temperature.  From here comes the inspiration for the name of our blog, κβάντα ρει” (quanta rei).

We certainly see many headlines reporting quanta fluctuating, even empty space itself,

But is our claim, that quantum particles are in constant motion, really true?

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