Boson sampling can be thought of as a quantum version of a classical device called the bean machine. In that device, balls are dropped onto rows of pegs, which they bounce off of, landing in slots at the bottom. The random motion of the balls typically leads to a normal distribution in the slots: most balls fall near the center, and fewer fall toward the sides, tapering off at the edges. Classical computers can easily simulate random motion to predict this result.
Quantum computers rely on fundamentally different principles to today’s computers, in which each bit represents either a zero or a one. In quantum computing, each bit can be both a zero and a one simultaneously. So while three conventional bits can represent any of eight values (2^3), three qubits, as they’re called, can represent all eight values at once. That means calculations can theoretically be performed at much higher speeds.
Quantum computers must overcome the challenge of detecting and correcting quantum errors before they can fulfill their promise of sifting through millions of possible solutions much faster than classical computers.
Detecting quantum errors is anything but straightforward. Classical computers can detect and correct their bit-flip errors by simply copying the same bit many times and taking the correct value from the majority of error-free bits. By comparison, the fragility of quantum states in qubits means that trying to directly copy them can have the counterproductive effect of changing the quantum state.
And in any case, because this trick works using only 4 qubits, it can easily be reproduced on any classical computer. So it’s not so useful after all.
Here’s a paper on this subject:
Abstract. Time travel has captured the public imagination for much of the past century, but little has been done to actually search for time travelers. Here, three implementations of Internet searches for time travelers are described, all seeking a prescient mention of information not previously available. The first search covered prescient content placed on the Internet, highlighted by a comprehensive search for specific terms in tweets on
The second search examined prescient inquiries submitted to a search engine, highlighted by a comprehensive search for specific search terms submitted to a popular astronomy web site. The third search involved a request for a direct Internet communication, either by email or tweet, pre-dating to the time of the inquiry. Given practical verifiability concerns, only time travelers from the future were investigated. No time travelers were discovered. Although these negative results do not disprove time travel, given the great reach of the Internet, this search is perhaps the most comprehensive to date.
The team behind the project believes that the limited availability of quantum computers would deter extensive research and there would be shortage of skilled quantum researchers, engineers and programmers once quantum computers actually make it to the main stream. Through Qcloud the team is keen to open up quantum computing research and make it available to as many researchers, engineers, entrepreneurs and engineers as possible.
Physicists have attempted to solve the problem by sending photons through a shared fibre along a ‘quantum channel’ at one characteristic wavelength. The trouble is that the fibre scatters light from the normal data traffic into that wavelength, polluting the quantum channel with stray photons. Andrew Shields, a physicist at the Toshiba Cambridge Research Laboratory, UK, and his colleagues have now developed a detector that picks out photons from this channel only if they strike it at a precise instant, calculated on the basis of when the encoded photons were sent. The team publishes its results in Physics Review X.
Still, 90 kilometres is a “world record that is a big step forward in demonstrating the applicability of quantum cryptography in real-world telecommunications infrastructures”, says Vicente Martín, a physicist at the Technical University of Madrid.
Laing’s team is also working towards that end. As part of the recycling technique, the researchers chained together two logic gates – a first for an optical quantum computer and an essential step in terms of hardware for building more complex devices. “To me, this achievement is more important than factoring 21, although to do so is an excellent demonstration of the technique’s power,” says Browne.
Artificial intelligence researchers at Google regularly log into a D-Wave computer over the Internet to try it out, and 2011 also saw the company sign its first customer. Defense contractor Lockheed Martin paid $10 million for a computer for research into automatically detecting software bugs in complex projects such as the delayed F-35 fighter (see “Tapping Quantum Effects for Software that Learns“). Questions remain about just how its technology works, but D-Wave says more evidence is forthcoming. It is readying an improved processor that Rose calls the company’s first true product rather than a piece of research equipment. D-Wave is expected to announce other major customers in coming months.
“At an engineering level they’ve put together a setup that’s impressive in various ways,” says Scott Aaronson, an MIT professor who studies the limits of quantum computation. “But in terms of the evidence that they’re solving problems using quantum mechanics faster than you could classically, I don’t think it’s there yet.” A fierce critic of D-Wave in the years following its 2007 demo, Aaronson softened his stance last year after the company’s Nature paper showing quantum effects. “In the past there was an enormous gap between the marketing claims and where the science was and that’s come down, but there’s still a gap,” says Aaronson, who visited the company’s labs in February. “The burden of proof is on them and they haven’t met the burden yet.”
Imagine you have a product of two prime numbers, say, 221. Now, we set that number to be an endpoint—for the purposes of our game, there are no higher integers. If we multiply two numbers together and get a number larger than 221, it wraps around, so 15 times 15 results in 225-221 = 4. If we multiply two by itself, we only get four, which doesn’t wrap, and we can do that 7 times before it wraps. But 28 results in 35. Got that? Great.
Let’s consider a consequence of using phase to calculate prime factors: 221 has prime factors 17 and 13, and factors 1 and 221. We can eliminate the latter in the classical part of our algorithm. But, what about two and 111? “Wait,” you say. “That is not a factor. The product is 222.” Nevertheless, we need to think about it, because quantum algorithms are probabilistic. 17 and 13 have the highest probabilities, but two and 111 only have a phase error of 0.5 percent. The probability of Shor’s algorithm returning the incorrect result is rather high. Unfortunately, a near miss (though easy to spot, since it is very quick to calculate that 2×111=222 not 221). This is likely not very useful in terms of decrypting a message, so we need to do something to increase the chance of getting the correct answer.