Carlos A. Perez-Delgado
Lecturer in Computer Science

Thank you very much everyone for being here. My name is Carlos Perez Delgado and I'm going to talk about what is a quantum computer. So, before I introduce myself, why do we even care what a quantum computer is? Here's a few news pieces that I've taken off the web in the past three months. This is the beginning of 2018, and it refers to large important companies that we all know about: Google, IBM, Intel, NEC, they're all jumping into this area of quantum computing. They're pouring millions of dollars, millions of euros and millions of pounds and they're getting there and they're achieving very important stuff. 

Now, who am I? I'm a lecturer here at the University of Kent in computer science, have 10 years of experience working on quantum computing and quantum computing research, I have a PhD from the Institute of Quantum Computing at the University of Waterloo in Canada. I worked for a while at the Center for Quantum Technologies at the National University of Singapore in Singapore. Now the reason I say all this is to make it clear to all of you here in the audience and at home that I'm not going to lie to you, everything that I say here is going to be perfectly accurate. Now I will simplify stuff, I will simplify some... you won't see any mathematics up there and this is a very technical subject but what I promise is that I will simplify
as much as I can but no more than that.

So, let's get started. What is a quantum computer? To understand what a quantum computer is first we need to understand what quantum theory is. In the words of Richard Feynman, one of the most famous physicists of our time, quantum theory is the most successful theory of all time. It is so successful in fact that it's similar in its predictive powers to measuring the distance between Los Angeles and New York down to the breath of a human hair. Now all this accuracy has a cost and the cost has been the erosion of our human intuition. Quantum theory is simply not intuitive. 

Now we can summarise the basic tenets of quantum theory into three basic tenets: the first one is that both matter and energy behave like particles and waves; the second is that quantum systems can be in, what we call super positions, these are complicated states and the best example of this is something that I'm sure all of you have heard of which is Schrodinger's cat. It's a cat that's in a box and as long as I don't look at it that cat can be both dead and alive at the same time. It's not that it's either dead or alive, it's both or neither and I'll explain more of that in a second; a third tenet of quantum theory is that when I measure a quantum system I invariably alter its state. I cannot measure a quantum system without disturbing it. 

Now let's focus on the second tenet because we can use that for quantum computation. Much like Schrodinger's cat can be both dead and alive, a quantum bit can be both in the state 0 and 1 at the same time. To remind you all a classical bit is the essential building block, the fundamental building block of computers. A bit can be either 0 or 1. A quantum bit can be in both and that means that if I have two quantum bits I can have the state 0 1 2 & 3 simultaneously. 

Let's try to explain this with a little bit of help from some pictures. Think of the Cartesian co-ordinates your XY plane north-south-east-west. Now, a classical bit will always be on one of these axis, so we can represent a bit being in the state 0 by being on the x axis and in the a bit in the state 1 by being on the y axis and that's a classical bit you can only either be on one of those two lines, one of those two blue lines. A quantum bit, however, can be an on a diagonal and it can be in any direction whatsoever. Important to remember is that if I measure that quantum bit I'll only ever measure a 0 or a 1, which one will I measure. Well, if we were to cast a torch from the very top that line that blue line will cast a shadow onto the X axis and that shadow that you see there is proportional to the probability of me measuring a zero. Similarly, if I shine the torch from the far right the shadow that it casts onto the y-axis is proportional to the probability of measuring a 1. So, if I measure a quantum bit I only measure 0 or a 1 but as long as I don't measure it, it can be in this strange diagonal state which is neither 0 or 1. We can use this quantum computing power to speed up some computations. The punchline is that you can't solve all problems faster on a quantum computer but you can solve some problems somewhat faster and you can solve a fewer problems a lot faster. 

What are quantum computers specifically good for? Well they're very good at simulation, they're very good at solving some hard optimisation problems, some hard mathematics problems, they're also good at machine learning and they're also very good at one other thing that I won't mention quite yet. Before I get there let's talk about the internet. We all use the internet here, we'll go online, shop online, go on Facebook, post pictures of ourselves, we all care about our privacy in the security of our information and all that in all that information is safeguarded by what we call cryptography. There's a particular type of cryptography which we call public key cryptography, which is what we use to authenticate people on the internet and to create private communication. By authentication I mean that you can prove that you are who you say you are, or for example when you go to your bank's webpage you know that that webpage actually belongs to your bank and you're not being scammed out of your money. Together public key cryptography is used in all sorts of applications, from online banking to your private communication, your email, your RSA two-factor keys etc etc. Without public key cryptography literally the internet would stop working tomorrow. 

Now public key cryptography as we know it is all based on certain types of problems that are very easy to perform but very difficult to undo and a good example of that is multiplication versus factoring. If I give you two numbers, for example 23 and 11, and ask you to multiply them, that's generally easy for you if you're thinking 253 that's the right answer. But if I give you a large number and ask you what two numbers multiply to that number that's harder, if I give you the number 221 and ask you what two numbers multiplied together give you 221, that's harder. The answer, by the way, is 13 and 17. It's important to know though that this isn't just harder for you and me as people, it's also harder for computers in fact the difficulty of factoring the difficulty of finding the factors is an intrinsic part of the security of public key cryptography. The most used out there public key cryptographic tool is RSA 2048, it is the gold standard and it's based on factoring. Now that introduces a problem because quantum computers are really good, really good at factoring. How good are they? Well, let's put up an example. If I gave you the most powerful classical computers today, take your pick, it would take them around several times the age of the universe to break RSA 2048, again RSA 2048 it's the gold standard of security today and the reason is because your classical computers would take millions and millions of years to break it. What a quantum computer, a modest quantum computer bigger than any quantum computer we have today but not nowhere near as powerful as the classical computers that we have today that quantum computer would only take 14 minutes to break RSA 2048. So you can see the problem here.
Now, fortunately as I said quantum computers today are simply not large enough to break RSA 2048 but some of us predict some of my colleagues and myself have predicted that there's a good probability that by the year 2030 there will be quantum computers large enough to break RSA 2048. Fortunately there's a plan, in fact there's two plans, there's a two-tiered approach, the first one consisting of what we call post quantum cryptography. Post quantum cryptography simply refers to cryptographic systems that are not known to be breakable by quantum computers, so there's no attack, there's no quantum computing attack for post quantum crypto. But another idea is to use the power of quantum theory itself to create security. If you remember towards the beginning of my talk I mentioned how when you measure a quantum system you invariably alter its state so I can prepare quantum systems in particular states and if someone else comes in and observes them, just looks at them I'll know because they'll alter its state. It's like nature is providing its very own intrusion detection system and we can use this, we can use this to build cryptographic tools to regain the security that we'll be losing and create even better security tools unlike you've ever seen before, Now if you want to learn more about that you'll have to stay tuned on this channel. 

For today that's all for me. Thank you all very much for your attention.