Stitching Qubits

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How to add qubits to an array one by one.

by Anne-Marije Zwerver

So it is winter and it is cold. Cold? It is freezing! But the air is nice and dry outside, so you decide to take a wintery walk in the forest. If you’re in a part of the world where you can currently fry an egg on the street, just wander along in your head – this is a small gedanken experiment. The walk is nice, yet cold and by the time you arrive home, the only thing you want, is to take a nice and warm shower. You turn on the tap and you feel the water running, splashing on your arms and shoulders, slowly defrosting your fingers. But then, for goodness sake, your roommate turns on her (cold) tap and your water temperature rises instantly. In a reflex, you jump out of the water jet, your skin already showing red stains. Luckily it was just an instant and soon you can go back into the shower. But then, of course, your other roommate needs some hot water and with a scream you, again, jump out of the now ice-cold shower. Time for a cup of tea…

Continue reading Stitching Qubits

Once upon a time in the Netherlands …

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by Freeke Heijman

Usually this blog is about research topics. When I was asked to write an item on the political aspects of quantum, I was excited but also a bit hesitant: would this parallel universe, in which politicians, executives and government officials rule the world, be interesting to the research community? What could I share without breaking rules of confidentiality while keeping some of the juicy details that make life interesting? How could I bridge the gap between these two worlds that are so completely different in their values, methods and people?

I decided to take up the challenge. After all, these dilemmas are the very same ones people with two legs in two different communities have to deal with every day. It is about making an effort to understand and find your way in different cultures and trying to get across some glimpse of these worlds to the people you work with on both ends. The last part almost feels like a mission impossible to me – the more reason to give it a shot in this blog! Continue reading Once upon a time in the Netherlands …

QuTech interview: Prof. Wolfgang Tittel

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It is a cold Monday afternoon when we have our appointment with Wolfgang Tittel, professor in physics and specialized in photon entanglement, quantum teleportation, and quantum memory. As we expect from a busy professor, he is still in another meeting when we arrive, so we decide to wait two meters outside his door. When we go and check if he’s almost done with this other meeting, his office suddenly appears deserted. In a vivid demonstration of his expertise, he seems to have teleported away from his office…

Luckily, he reappears quickly and lets us into his spacious office. One wall is completely covered by a large whiteboard, covered in scribbled equations and diagrams. Clear signs of occupation by a physics professor.  Professor Tittel himself welcomes us with a smile, clearly relishing the opportunity to talk about his work. What follows is an interview with professor Tittel, shortened and lightly edited for clarity.

What type of research do you do?

My research lies in the framework of the quantum internet. More precisely, it is about quantum key distribution (QKD) and the creation of quantum key distribution systems over very long links. This requires quantum repeaters. To create the quantum internet, we send photons down an optical fiber, but, just as in standard telecommunication, these photons get lost at some point. In standard telecommunication, you can use amplifiers to boost the signal level, but for quantum internet this doesn’t work because of the no-cloning theorem. Instead, we can use a so-called quantum repeater.

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How can we speed up the quantum internet?

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by Suzanne van Dam

What if we could make an internet, but quantum? This would allow us to connect distant nodes (e.g. computers) not with classical information, but with quantum information. We can already imagine cool applications, such as secure communication
guaranteed by the laws of quantum physics. But many of the applications may not even have been dreamed up yet,
as happened before with new technology.
Continue reading How can we speed up the quantum internet?

Farewell, Editors Emeriti; Welcome, New Editors

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Autumn (or Spring for our readers in the Southern hemisphere) is a time of change, and things are changing as well for Bits of Quantum. Editorial duties on this blog are performed on a volunteer basis by PhD students (in what little remains of their free time), and this means that any editor’s tenure is inherently limited by his or hers PhD track. This is why, with some sadness, we announce the departure of James and Suzanne, who have handed in their editorial powers to finish up their doctoral track. They were great members of the team and we would like to thank them for the time they have spent making this blog an amazing place for quantum computing.

Luckily change also brings renewal and we are very happy to announce that Bits Of Quantum has two new editors: Guan and Anne-Marije. They have been unofficially part of the team for a while now and we figured it was high time to formalize their editor-ship. Continue reading Farewell, Editors Emeriti; Welcome, New Editors

Dead or Alive: Can you be both?

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by Jérémy Ribeiro

At the heart of Quantum Mechanics lies quantum superposition. This strange phenomenon is often described as the capacity of a quantum system to be in multiple incompatible states at the same time. The most famous example of this is Schrödinger’s cat, which would be both dead and alive at the same time. But how can this be? How can we humanly make sense of that apparent contradiction? Well… I think we cannot! More precisely, I think there is a problem of language in here. Exactly what a quantum scientist means by being “in superposition”, I think, is quite far from what the layman has in mind.

A simple analogy

To start explaining what a quantum scientist has in mind when he/she says that a state is in superposition I will use a simple analogy: Shapes.

What? How is that related to the topic?

You’ll see! How would you describe or draw a shape that is both a disk and a rectangle?

That does not make any sense! Maybe something like this:

 

 

 

 

Yeah you see, it does not make sense to you, and you struggle to draw anything because I said something that does not make sense. This is exactly what happens when someone says that Schrödinger’s cat is both dead and alive! It is not clear what he/she means, and stated like that it is non-sense. When a quantum scientist says that a physical system is in superposition of two states (dead and alive), he/she means that it is in a state that is neither the first (dead) nor the second (alive) but it is in another state that possesses some of the characteristics of both (dead and alive).

Hmm…This is quite hard to visualize for me. Don’t you have an example?

Yes! For the example of the shape it could look like this:

 

 

 

 

 

 

Oh I see!

This is a relatively good analogy. This shape is neither a rectangle nor a disk, however it has some of the properties of both. Moreover I like this analogy because in quantum mechanics you cannot “see” the quantum state the physical system is in. In other words, if someone gives you a system in a certain unknown state, you cannot learn the state. If you try to measure it, you will only see a “projection” of it… Continue reading Dead or Alive: Can you be both?

A Quantum Internet made of Diamonds

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How cool is that? A Quantum Internet. Made of Diamonds.

by Matteo Pompili

We are constantly connected to Internet. With our computers, our smartphones, our cars, our fridges (mine is not, yet, but you get the idea). In its very first days, the Internet was a very rudimentary, yet revolutionary, connection between computers [1]. It enabled one computer on the network to send messages to any other computer on the network, whether it was directly connected to it (that is, with a cable) or not. Some of the computers on the network acted as routing nodes for the information, so that it could get directed toward the destination. In 1969 there were four nodes on the then-called ARPANET. By 1973 there were ten times as many. In 1981 the number of connected computers was more than 200. Last year the number of devices capable of connecting to Internet was 8.4 billion (with a b!) [2].
Computers on their own are already great, but there is a whole range of applications that, without a network infrastructure, would be inaccessible. Do you see where I am going?
Continue reading A Quantum Internet made of Diamonds

A Cloud Quantum Computer Business Plan

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by Christian Dickel

Quantum computing and nuclear fusion are potential 21st century technologies based on 20th century physics and neither of them is currently market ready. But while they are sometimes bunched together as fascinating concepts that will at any time be twenty years away from being realized, some estimate the timescale for the commercialization of the quantum computer to be much shorter now. Quantum computing is currently in a hype phase: The company D-wave has already sold a few quantum annealers based on flux qubits for millions of euros. They can solve certain optimization problems, but their computational advantages are a topic of debate. Google, IBM, Intel, and Microsoft are major commercial companies investing in quantum technologies right now. Several startups such as Rigetti Computing and IonQ have been founded recently with the goal of commercializing quantum computing. A list of such companies can be found here.
Continue reading A Cloud Quantum Computer Business Plan

Making quantum computers with spin qubits

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In one of the previous blog posts, David DiVincenzo reviewed his criteria. Here we will follow this theme and look how these criteria translate onto a physical system. Currently, there are a few qubit implementations that look quite promising. The most prominent examples are superconducting qubits, ion traps and spin qubits. We will focus on the latter one, since that’s the one I’m working on. All the platforms mentioned above fulfill the so called DiVincenzo criteria. These criteria, defined in 2000 by David DiVincenzo, need to be fulfilled for any physical implementation of a quantum computer:

  1. A scalable physical system with well characterized qubits.
  2. The ability to initialize the states of the qubits to a simple state, such as |000⟩.
  3. Long relevant coherence times, much longer than the gate operation time.
  4. A “universal” set of quantum gates.
  5. A qubit-specific measurement capability.

In this article we will go through all these criteria and show why spin qubits fulfill these criteria, but before doing that, let’s first introduce spin qubits.

Spin qubits are qubits where the information is stored in the spin momentum of an electron. A spin of a single electron in a magnetic field can either be in the spin down (low energy) or in the spin up (high energy) state. Comparing to a classical bit, the spin down state will be the analogue to a zero and spin up to a one.

The two quantum states of an fermion.

Continue reading Making quantum computers with spin qubits

Science on the beach

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by Jonas Helsen

A few weeks ago I found myself boarding a series of planes that would take me from the Netherlands to pretty much the furthest point reachable from this starting point that still includes dry land: Sydney, Australia. I wasn’t going there entirely of my own accord. Rather I had been invited to speak at a quantum information theory workshop called Coogee. For those knowledgeable of the Sydney region, it is indeed named after a beach and yes the conference takes place less than a stone’s throw away from said beach.

Continue reading Science on the beach