Quantum Computational Supremacy

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by Adriaan Rol

Last week Google and collaborators published a paper in which they claim to have achieved Quantum Supremacy, one of the major milestones in quantum computing. The idea of quantum supremacy is to use a programmable quantum device to perform a task that is out-of-reach for any classical computer. Google claims to have solved a problem in seconds that would take tens of thousands of years on a state of the art supercomputer. The quantum supremacy experiment has been a long-standing milestone in the field of quantum computation, and as such, skepticism has arised; soon after publication of the article a group in IBM research has challenged the results [1].

Rather than joining in on the controversy of whether or not Google has really achieved quantum supremacy , I want to focus on some more basic questions: what is quantum supremacy, how does one demonstrate quantum supremacy and why is this such an important milestone?
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Quantum internet: at the verge of an emerging technology

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by Bas Dirkse

The internet as we know it today has become an integral part of our lives. We use this piece of technology on a daily, hourly, almost continuous basis. We use it at work, to relax, to socialize, to fact check our friends during an argument and even to control the thermostat at our homes. We can definitely claim that, despite the internet bubble in the 90’s, the technology has far outperformed its expectations of the early 60’s and 70’s in the societal and financial benefits it provided. Will the same bright future be reserved for the quantum internet? Will it deliver the same amount of societal and financial benefits to the world as its classical sibling has done? Continue reading Quantum internet: at the verge of an emerging technology

A quantum network stack?

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by Axel Dahlberg

Summer is approaching fast! The days until vacation are getting fewer and fewer. But before you can relax at a beach with a cold drink you need to send a bunch of emails. However for some reason the software allowing your computer to connect to the Internet has suddenly vanished. What can you do? Well, maybe you can just manually do whatever this software does. It can’t be too hard right? Or can it…?

The network stack, a collection of of software used by computers to connect to each other and run applications over a network, such as e-mail, social media, file sharing, video streaming etc., used by today’s Internet is crucial to its operation. You use it everyday, but do you know what it actually does? When you send an email to your colleague, how is your email actually transmitted across to a different computer?

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Who’s afraid of Majorana qubits?

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by Guanzhong Wang

Don’t hide it; don’t deny it. I know every time you, my dear friend in or related to the quantum computing community, hear about the words “topological qubits”, you raise your eyebrows slightly and say to yourself, “weird”… Pretend no more! We know you are puzzled why anyone would want to embark on the journey of making a topological qubit and how on earth they go about doing it. In this new series on topological qubits, we will try to explain to you why building such a seemingly unconventional qubit is rather fun and is even one of the natural choices when it comes to quantum computing.

I will start in this post with a virtual lab tour, hoping to give you an overview on where and how we look for the basic building blocks of a topological qubit—a Majorana bound state in condensed matter systems. From the particle that Ettore Majorana envisioned on a piece of paper to the nanowire devices and then back to the blueprints for a topological qubit, this will be a journey linking seemingly strange ideas to real, tangible chips in cryostats. If you’ve ever got curious about a Majorana qubit, gone through some reviews and tutorials but still wonder how experimentalists try to build them, this article is totally for you. If you haven’t, I hope it will arouse your interest in doing so! Continue reading Who’s afraid of Majorana qubits?

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…

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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?

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?

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

Looking back at the DiVincenzo criteria

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by David DiVincenzo

The first time that I heard that there were “DiVincenzo criteria” was when Richard Hughes of Los Alamos contacted me in the fall of 2001, telling me that ARDA (predecessor of IARPA – a funding agency of the US intelligence services) had commissioned him to form a roadmap committee to forecast the future of quantum information technology [1]. Before that, I just thought of them as a list that I showed in various talks and wrote down in a few essays. So the fact that they have become a “thing” is basically because some government bureaucrats found them a handy way to draw up metrics for the progress of their quantum computing programs.

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The first Delft qubit

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By Hans Mooij

Introduction

When did we have our first quantum bit? To answer, one needs to agree on the definition. When does a two-level system become a qubit? In my view, only when coherent quantum dynamics is demonstrated. In the summer of 2002, Rabi oscillations of a superconducting flux qubit were observed in our laboratory. They were published in Science [1]; the primary authors were Irinel Chiorescu (postdoc, now professor at Florida State University) and Yasunobu Nakamura (on sabbatical from NEC Japan, now professor at University of Tokyo). As we all know, much has happened in the years after. Here I want to describe what happened before. How did we come to this point? I concentrate on my personal story and on superconducting circuits. In our Quantum Transport group we had the parallel research line on semiconductor quantum dots by Leo Kouwenhoven and his people that led to our first spin qubit in 2006.

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