An Overview of Quantum Computers. Confused about the NSA’s quantum computing project? This MIT computer scientist can explain. My Washington Post colleagues have reported on an National Security Agency program to to build a quantum computer. In principle, the unique capabilities of a quantum computer could allow it to easily crack cryptographic codes that cannot be cracked by even the most powerful conventional computers. But right now, quantum computing is more a theoretical research topic than a practical technology. To understand how quantum computers could work and what the implications would be if they did, I talked to Scott Aaronson. Quantum Information Science Seth Lloyd. Quantum mechanics is the branch of physics that describes how systems behave at. Welcome to Oxford Quantum! Oxford University is the UK's largest and most diverse centre for quantum research. We have 38 separate research teams, with a total of around 200 researchers. Quantum Mechanics on the Personal Computer Hardcover – December. This book presents the most up-to-date approach to elementary quantum mechanics. Based on the interactive program Interquanta (included on a 5 1/4' MS-DOS. Reddit: the front page of. Researchers have written a Computer Program to Choose Quantum Experiments. That was one of the first confusing results that led down the path of quantum mechanics. Quantum weirdness is hard for humans to grasp, so researchers wrote a program to suggest experimental setups. Optical components configured to produce photons in a high-dimensional GHZ entangled state, in an. Aaronson is a Computer Science professor at the Massachusetts Institute of Technology who has written extensively about quantum computation and its implications. We spoke by phone on Wednesday. The transcript has been edited for length and clarity. ![]() ![]() Timothy B. Lee: Let's start at the beginning, how does a quantum computer differ from a conventional computer? Scott Aaronson: The easiest way to say it is that a quantum computer would exploit quantum mechanics, laws of physics that are not familiar in everyday life but have been familiar to physics for . It's hard to get across with newspaper- friendly analogies. Quantum mechanics is a framework for subatomic physics which is probabilistic. You can only calculate the probability that an electron or proton will be in a certain place when you make a measurement. That's not the most important part of it. We use probability all the time in everyday life. But quantum mechanics has a completely different way to find probability. People talk about a 2. But nobody talks about a negative 2. Quantum Monte Carlo (QMC) is an exciting, modern computational technique which allows us to approximately solve the equations of quantum mechanics – which are far too complicated to solve exactly – and in most cases get. We are a data storage company that provides a unique combination of specialized storage solutions and unmatched value for traditional, virtual and cloud environments. That would be nonsense. Amplitudes can be positive or negative, or even complex numbers. What's important is there are different ways that something can happen, and some of those ways have positive amplitude and some have negative amplitude. They can cancel each other out. That's the thing that's totally unfamiliar to us. If you close one of the slits, you do see light there because you no longer have this interference. By decreasing the number of ways that a photon can get to a certain point, you can increase the chance that it will be found at a point. That's what interference means. The idea with quantum computing is to exploit the phenomenon of interference which is the core of quantum mechanics on a massive scale. To try to choreograph a whole computation, not just two possibilities with two slits of light to go through, but 2 to the 1. What you could try to do is arrange things for each so of the wrong answers, some have positive amplitude and some have negative amplitude. So those would cancel each other, . So that's the idea. This is different from what most of the popular articles describe. Most of them take this lazy way out, they say a quantum computer will be unlike a classical one because it will explore all the possible answers in parallel. That's not a good way to describe it because you have to measure the computer. While you can in some sense try every possibility in parallel, there's a sense in which that's true, but as soon as you make a measurement, you're going to see one of these answers, not all of them. You could get a random answer. So the only hope of getting a benefit with a quantum computer relies on this interference effect. So it's really something subtle. How long have people been thinking about quantum computation? The idea of quantum computing was proposed in the 1. Richard Feynman and David Deutsch, but it wasn't obvious that a quantum computer would be good for anything. The only application people could see immediately was you could use a quantum computer to simulate quantum mechanics. That's sort of obvious. The big discovery that sort of got people excited about this field: Peter Shor discovered in . That's a practical problem we don't know how to solve with . If you can do that you can break most of the cryptography on the Internet. It is important to . So even quantum computers have significant limitations. There's a famous class of NP- complete problems . We don't know if quantum computer will be able to . There's something special about factoring. So it's not a matter of trying every possible divisor in quantum superposition, that wouldn't work. You have to do something more clever to arrange this interference . For modern cryptography, you need special algorithms to make it work. Can you give me some concrete details about how a quantum computer works? Conventional computers are built with switches made out of transistors. Is there something similar for quantum computers? The reason I haven't been concrete about it is that there's a lot of different ideas on the table about what a quantum computer could be built out of. We don't know which of these ideas is going to be the best one. Regardless of which one, they lead to the same theory. It's very much like if you're a classical computer programmer, if you're building a computer program, you don't need to know the physics of the transistor, if that's what you're concerned about. I can tell you some of the ideas. Basically what you need is some physical system. You have to be able to place it in a quantum superposition of two states. If you have such a system, you call that a quantum bit or a qubit. You have to be able to set them, you have to be able to do operations on them, and you have to be able to make pairs of the qubits talk to each other. You have to be able to . You have to be able to measure the qubits at the end to read out an answer. Finally you have to do this while keeping the qubits . That's a little misleading. It doesn't have to be a conscious being. But if the system leaks out information into its external environment, then it loses its quantum characteristics. As soon as the system becomes too correlated with the rest of the world, then you no longer see the quantum characteristics. This is why we don't notice these quantum effects in day to day life. It's why they were only discovered in the 1. They only come to predominate at the atomic scale. What makes it so hard is you've got these requirements that you've got to be able to do these operations while keeping things isolated from the environment. So what ideas have people had for how to build qubits? Different ideas are being explored in parallel. One is that you could use ions that are trapped in a magnetic field. You could use a laser to manipulate them. Another one is superconducting qubits. You'd have current that is in a superposition of flowing clockwise and counterclockwise. These coils would be large enough that you can see them with a magnifying glass. Use optical elements like beam splitters to move information around. There are a dozen of other proposal. You describe quantum computers as a mostly theoretical concept, but a company called D- Wave claims to have created a practical quantum computer. What's their story? I've been writing about D- Wave on my blog for the last decade. They've been the leaders on generating hype and generating press. For a long time they were literally a black box. We didn't really know what was going on, they would make press announcements with these deals, they'd sell these machines to Lockheed Martin and Google. We didn't really know. Academics . They hadn't given evidence that they were doing anything beyond what you could do with a conventional computer. They were making extravagant claims, but we didn't see any evidence that there were really quantum effects going on or that we had a speed- up. We know a lot more in the last year about what's going on with these devices. After they sold to Lockheed, an independent group led by Matthias Troyer actually did independent investigations of this machine. What they found briefly is that there is pretty good circumstantial evidence that there's some kind of quantum annealing behavior, which means that there's a little bit of quantumness there. Not even D- Wave is claiming that what they have or what they't trying to do is a full- scale quantum computer. They're not even trying to get universal quantum computer. A universal one is one that can do any quantum computation, like Shor's factoring algorithm. D- Wave is aiming to build something much more limited. Even if you could build this adiabatic optimization approach totally perfectly, you still wouldn't know if there's a practically important speedup using quantum computers. It's very different than the situation with Shor's factoring algorithm. If you could do it, we're extremely confident we could get this speedup. With the optimization problem, we don't really know if we'd get a speedup. It's going to boil down to people trying it out and seeing what happens. Another issue: D- Wave's hardware is nowhere close to the theoretical ideal. It's mostly classical with a little bit of quantumness. Most of the scientists have focused on getting qubits that really work. Most people view this as basic research at this stage. That's how I think about it. If you can't even build one qubit that you can really control and make work, it seems ridiculously premature to be trying to build commercial devices. But D- Wave's approach is very different. Where the rubber meets the road and supposing you do that and find you don't get a speedup. Unfortunately, D- Wave has taken the path of obfuscating what the issues are and counting on journalists and people in the business world not caring enough to understand. Would today's cryptographic algorithms be in danger? Almost all of the public- key encryption that is currently used would be breakable in principle by a quantum computer. Public- key algorithms are widely used online. That accounts for 9. That's all breakable by a quantum computer. On the other side, if you look at private- key cryptography, the kind where you have to agree on the key in advance, then most private- key cryptography you don't know how to break with a quantum computer. Even with public- key cryptography, there are proposals out there for public- key cryptosystems that could resist quantum computers.
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