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.
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.
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’.
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.
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.
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.
Sweet Grandmother’s Spatula! After pulling out your fresh cookies from the oven and taking your first taste you notice that in creating your cookie dough, you accidentally mixed up the sugar and salt. Now your dreams of enjoying that oh so sweet sugar cookie have been dashed away and you are left with a confectionary calamity. However, all is not lost…. your seemingly imperfect dough with salty defects can be fixed to result in the masterpiece of baked goods. Thus I present to you, the challenge of baking the chocolate chip and sea salt cookie. When balanced correctly, these two flavours can enhance each other to create complex layers of pure deliciousness.
Now you may be asking yourself, what does this have to do with quantum mechanics? Picture your salty cookie, full of defects within its dough, seemingly useless. However, if you add the right amount of chocolate chips in places as accurately as possible, you will achieve the right balance of flavour and achieve the perfect chocolate chip and sea salt cookie, a true baking delicacy. Likewise, in experiment, we begin with a crystal that contains lattice defects with quantum properties. In the general scheme of things, the goal is to add other materials within or near the substrate in as precise a location and concentration as possible so that we may enhance, control and measure these defects.
When people talk about physics, and in particular the human side of it, the ‘doing’ physics, they will usually point out that there exist two main forms of physicists. There are experimentalists, who spend their days gathering data in labs or tinkering with huge particle accelerators. These physicists, although rarely actually wearing white lab coats – at least in my experience – seem to be the closest to the pop culture stereotype of a scientist: wedding a strong analytical spirit to a practical, do-it-yourself mindset and a work ethic that often borders on obsession. They form the majority of physics practitioners and often speak with mild disdain about the ‘other’ type of physicist: the theorist. Theorists differ from experimentalist in that they mostly do, well, theory. Their days are usually not spent tinkering with equipment or analysing data but rather studying literature and diving into the complicated mathematics needed to describe modern physics. They often eschew the practical in favour of a generalist, axiomatic mindset; using as few assumptions as possible to describe the largest possible piece of the physical puzzle.
Throughout history these two strands of physics were usually not distinct professions but merely reflected the interests of a singular physicist. Even Newton, the prototype of a theoretical physicist, regularly performed experiments using prisms and even built one of the earliest reflecting telescopes. In my understanding of the history of physics these two strands of physicist started splitting into true professions in the late 19th century and early 20th century in response to the ever growing complexity of physics. Over time they grew further apart until the present day where among many theorists it is considered a point of pride to have never performed any experiments at all. Entire careers can be wholly devoted to the understanding of ‘physical theories’ that are decades away from being subjected to experimental verification. On the other hand, as the scale and complexity of experiments has grown, many experimentalists find themselves spending most of their time not doing physics but the cutting edge engineering work necessary to perform modern experiments to begin with. This has lead both groups to develop language and practices which differ immensely and can lead to almost Babylonic misunderstandings in the occasions where theorists and experimentalists do meet.
Have you ever dreamt about teleportation? You wonder if it is possible, or if we can use it to travel faster than light, or at least to communicate instantaneously. Then you are at the good place. Here I will explain what quantum teleportation is. Behind this very attractive name that reminds us of science fiction, a communication protocol is hidden which uses the mysterious quantum mechanics.
Why do I talk about quantum teleportation ?
I wanted to write about this protocol because we hear a lot about it and a lot of information and explanations can be found about it. But sometimes those are partially wrong, or are a complete nonsense. For example we can read that quantum teleportation is an instantaneous transfer of information at a distance which respects special relativity… Well this is a contradiction.
These misconceptions of the protocol are not surprising since it relies on one of the most ununderstandable and less well understood phenomenon of quantum mechanics: the famous entanglement.
That’s why I will try to clarify what this notorious quantum teleportation actually is. For that I will have to introduce a little bit of quantum mechanics. Therefore there will be some mathematical expressions, but I’ll guide you through it to make you understand what is going on. It shouldn’t be too difficult since you should have already seen all the mathematical concepts in high school (vectors), and I think it is worth it and permits to really understand what quantum teleportation is.
So what is that quantum teleportation ?
The first thing to understand is that we only teleport a quantum state (we will call it ), and not a particle nor any other kind of matter. Only information is transmitted from one place to another. So somehow we will scan the physical system in the first place (which will destroy the state), send the information (by phone, internet or any other way of communication) to an other place and there reconstruct the state. But to do that there are some obstacles. The main obstacle the “scanning phase” is not trivial since we cannot (by the law of quantum mechanics) get all the
information on a state by measuring it, as I will explain later, but only partial information. To overcome that we will need to use a correlated state which will somehow compensate the lack of information.
It is an honor to write the first blog post here and being conscious of that certainly influenced what I was going to write about. They say write what you know, but this is a blog so I’m going to write what I think. The blog will hopefully be a place for opinions and discussions. So I’ll begin with a question:
Do physics institutes need blogs? Certainly it is a neat additional way to communicate with other scientists, especially to share more provocative thoughts and give people a chance to discuss in the comments. But science is kind of a gated community and a blog is a nice way to open it more. For communication with the rest of society, journalists often come in whenever some piece of science has an air of general interest. But especially in a field receiving a lot of interest and a lot of funding from the public, we should try to explain what we do directly to anybody who is interested enough to end up on our website. A blog is a chance for us to share and discuss our perspective on the story of quantum computing as it is being written.
Quantum computers and the media
There are news article on quantum computing almost weekly somewhere on the internet and one can use them to follow the story of the quantum computer. But the news has a certain inertia and a need to fit complicated arguments into a single sentence or paragraph. Some of the one-liners are productive simplifications, but they can also be misleading. Exploring all the misconceptions about quantum computing requires more than one blog article. I considered going through the list found here and fact-checking it, but this blog article would not have been very serious then. I thought it better for the first blog article to be a link from the past to today and focus on a single aspect that annoys me in the way the quantum computer story is told: I will try to give a more nuanced view on the relationship between the classical and quantum computer. Maybe later there will be more blog articles on other common misconceptions about quantum computers.