How to make artificial atoms out of electrical circuits



Part 1: Superconductivity saves the day

By Christian Dickel

In a series of blog posts, I want to introduce the bread and butter of the DiCarlo group within QuTech: Studying quantum effects in superconducting electrical circuits. In the title, I suggest that we are building artificial atoms, but that depends on the definition of “atomness”. I hope to give the reader some insight to judge for him or herself whether our work comes short of this or goes beyond it. Also, I want to convey some of the amazement I feel working on a subject that brings together electrical engineering, superconductivity, and quantum mechanics in its purest form.

This blog post is rather long, but I have marked non-essential sections with a *.

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Playing the Quantum Ballgame


By Jonas Helsen

One of the things that is often repeated about quantum computing is the idea that a quantum computer is somehow more powerful than regular computers because, when considering a problem it can “try all possible solutions at once”. Let’s get this out of the way first and say that this is not exactly the case. While we would very much love a computer that tries all solutions at once (this would be extremely useful) quantum computers sadly aren’t quite this powerful. Of course, as with all good clichés it does contain a grain of truth. In this blog post I will try to explain in a (sort of) simple way what makes quantum computers more powerful than classical computers.

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Playing cards with quantum entanglement


by Gláucia Murta

You have probably heard that entanglement is a very strong correlation way beyond anything we can conceive classically. However, as we’ve seen from Jeremy’s post , these strong correlations by itself do not allow us to send any information to the other part. So what can we use entanglement for?… To play games!

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My Quantum Kitchenette


by James Kroll

There are many things that might pop in to your mind when I propose that you may be able to do quantum mechanics in the comfort of your own home. A ‘quantum kitchenette’ is probably not one of them.

This may have been a bit facetious, but it is true that many of the things you find in your kitchen such as a fridge, a microwave and beer bottles are perfectly analogous to the tools that are used in labs around the world to perform cutting edge experiments in quantum mechanics – in particular with applications in quantum computing.

These tools are technically challenging to fully understand, very expensive and equally impressive in their capabilities. As an experimental physicist, one of the most enjoyable parts of the job is using this equipment, understanding fully how it works so we can use and repair it if need be, but also the small idiosyncrasies that each specific piece of equipment acquires over time.

On a personal level, you really do develop an intimate relationship with your equipment, such that in some cases you are the only one who can use it reliably. A shorter way to summarise the connection might be: “Boys and their toys”, or whatever phrase would convey the same meaning in a more egalitarian manner.

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Progression of technology for quantum control or: jumping and drifting of NV centres



Technological sophistication is a cornerstone of our society. Apart from a few outstanding examples, technology has always advanced towards a new echelon, which in turn enabled further advance. Whether one investigates the height of the tallest skyscrapers, or the timeline from the first transistor to today’s computers, the principle remains the same: inventions are being made with increasingly faster strides. Of course this trend should hold true for our favourite qubit! Along these lines I will delve into a technological aspect of my favourite qubit: the nitrogen-vacancy (NV) centre in diamond.

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Nanoscale Superstition


by Michiel de Moor

In our cleanroom, we use nanofabrication techniques to combine materials in a precise and controlled way in order to study the wonders of quantum physics. For a nice introduction on the topic, I recommend reading Madelaine Liddy’s blog post. This post is not about nanofabrication specifics, but more about the people involved in the process.

Doing nanofabrication takes up a significant amount of time. Often it’s very difficult to understand what the important parameters are, and outcomes can seem random. As scientists, we should be rational and analyze the problem, then test possible solutions until we understand what is happening. But as people, we are susceptible to the same kind of magical thinking that makes people believe lightning strikes are a sign of Zeus’ displeasure.

As with cooking food, every research group will have their own fabrication recipes (which are, of course, the very best in the world). Continue reading Nanoscale Superstition

Can you tell your grandma the weather using only entanglement?


by Jérémy Ribeiro

Entanglement may seem mysterious. It permits us to have correlations between two separate systems that are arbitrarily far from each other. Moreover these correlations are stronger than any (non causal) classical correlation we can think of. In some ways it looks like the two quantum systems can communicate between each other. This is why some people think that it might be possible to use it to devise an instantaneous communication system. I will try here to give you an intuition as to why this is not possible. But before we see why using only entanglement does not permit you to communicate, we have to understand what we really mean by ‘communicate’.

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Programming for the quantum computer


by Christian Dickel

The general purpose programmable computer has been an enabling technology that has exceeded the original expectations in countless ways. From the humble beginnings of the original transistor, we now have devices that contain several billion transistors all working perfectly in unison in the smartphones we keep in our pocket. Our great hopes for the quantum computer are partially based on the belief that this could happen once again with the quantum computing paradigm.

The main challenge for realizing the quantum computer is certainly finding a suitable ‘quantum hardware’, that’s why it is still mainly a physics effort. However, it will also require a significant amount of computer programming and design. This makes our field interdisciplinary and soon computer scientists and engineers will likely play important roles in the further development of the quantum computer.

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Climbing the Ivory Tower


by Julia Cramer

After some years of doing science in the dark basement of a physics building, one may wonder: ‘who am I?’ Searching for answers at Google Images, there turns out to be a distinct difference between the stereotypes ‘scientist’ and ‘physicist’. Surprisingly, a ‘scientist’ always wears a lab-coat plus safety glasses. The ‘scientist’ works in a clean lab environment, handling chemicals and a microscope. According to Google, the ‘scientist’ is happy and young, can be male or female, black or white.

How large is the contrast to Google’s ‘physicist’: an old, somewhat otherworldly, serious man wearing thick glasses. The man standing in front of a whiteboard is writing down equations and drawing spheres on a blackboard. It is interesting to notice that Google barely makes a distinction between ‘physicist’ and ‘professor’. Leaving behind the fact that both a ‘scientist’ and a ‘professor’ can be an academic in any kind of field, I wonder why the ‘physicist’ never conducts any experiments.

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Entanglement distillation


by Filip Rozpedek

Entanglement, is it always pure?

You have probably already heard about entanglement. Entanglement is this fascinating phenomenon, in which two distant objects can manifest correlations, even if they are far far away from each other. You may have also heard that remote entanglement is a necessary ingredient for many quantum information processing tasks. For example, in quantum cryptography, two people who hold entangled particles can use those correlations to obtain shared secret keys, whose security is guaranteed by the laws of quantum mechanics. Today, we will not discuss how to use remote entanglement, but rather, what to do if our entanglement is too weak.

Unfortunately, fully entangled states which are perfectly correlated are a great idealization and from an experimental perspective almost impossible to create. In general, there can be many reasons for this, e.g. our experimental equipment isn’t perfect or we cannot maintain our quantum system long enough. All those things combined lead to various forms of contamination of the entanglement. That is, the correlations become weaker and completely diluted in a mixture of various other quantum states.
So what do we do with those so-called “partially entangled states”? Let us say that two parties working at QuTech, whom we call Alice and Bob, share those partially entangled states and would like to use them to generate shared secret keys. Let us also say that their experimental setup allows them to produce partially entangled states very fast, but the amount of entanglement in each of them is insufficient to generate shared secret keys. It is known from Quantum theory that it is not possible to increase the amount of entanglement in a given quantum state by only performing operations on the entangled particles locally and exchanging classical messages. It seems that there is no choice for Alice and Bob, but to go home without a key.

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