Investigadores de MIT (USA) y de la Universidad de Innsbruck (Austria) informaron haber dise\u00f1ado y construido una computadora cuantica.<\/p>\n
What are the prime factors, or multipliers, for the number 15? Most grade school students know the answer \u2014 3 and 5 \u2014 by memory. A larger number, such as 91, may take some pen and paper. An even larger number, say with 232 digits, can (and has) taken scientists two years to factor, using hundreds of classical computers operating in parallel.<\/p>\n
Because factoring large numbers is so devilishly hard, this \u201cfactoring problem\u201d is the basis for many encryption schemes for protecting credit cards, state secrets, and other confidential data. It\u2019s thought that a single quantum computer may easily crack this problem, by using hundreds of atoms, essentially in parallel, to quickly factor huge numbers.<\/p>\n
In 1994, Peter Shor, the Morss Professor of Applied Mathematics at MIT, came up with a quantum algorithm that calculates the prime factors of a large number, vastly more efficiently than a classical computer. However, the algorithm\u2019s success depends on a computer with a large number of quantum bits. While others have attempted to implement Shor\u2019s algorithm in various quantum systems, none have been able to do so with more than a few quantum bits, in a scalable way.<\/p>\n
Now, in a paper published today in the journal Science<\/em>, researchers from MIT and the University of Innsbruck in Austria report that they have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor\u2019s algorithm on each atom, to correctly factor the number 15. The system is designed in such a way that more atoms and lasers can be added to build a bigger and faster quantum computer, able to factor much larger numbers. The results, they say, represent the first scalable implementation of Shor\u2019s algorithm.<\/p>\n \u201cWe show that Shor\u2019s algorithm, the most complex quantum algorithm known to date, is realizable in a way where, yes, all you have to do is go in the lab, apply more technology, and you should be able to make a bigger quantum computer,\u201d says Isaac Chuang, professor of physics and professor of electrical engineering and computer science at MIT. \u201cIt might still cost an enormous amount of money to build \u2014 you won\u2019t be building a quantum computer and putting it on your desktop anytime soon \u2014 but now it\u2019s much more an engineering effort, and not a basic physics question.\u201d<\/p>\n Seeing through the quantum forest<\/strong><\/p>\n In classical computing, numbers are represented by either 0s or 1s, and calculations are carried out according to an algorithm\u2019s \u201cinstructions,\u201d which manipulate these 0s and 1s to transform an input to an output. In contrast, quantum computing relies on atomic-scale units, or \u201cqubits,\u201d that can be simultaneously 0 and 1 \u2014 a state known as a superposition. In this state, a single qubit can essentially carry out two separate streams of calculations in parallel, making computations far more efficient than a classical computer.<\/p>\n In 2001, Chuang, a pioneer in the field of quantum computing, designed a quantum computer based on one molecule that could be held in superposition and manipulated with nuclear magnetic resonance to factor the number 15. The results, which were published in Nature<\/em>, represented the first experimental realization of Shor\u2019s algorithm. But the system wasn\u2019t scalable; it became more difficult to control the system as more atoms were added.<\/p>\n \u201cOnce you had too many atoms, it was like a big forest \u2014 it was very hard to control one atom from the next one,\u201d Chuang says. \u201cThe difficulty is to implement [the algorithm] in a system that\u2019s sufficiently isolated that it can stay quantum mechanical for long enough that you can actually have a chance to do the whole algorithm.\u201d<\/p>\n \u201cStraightforwardly scalable\u201d<\/strong><\/p>\n Chuang and his colleagues have now come up with a new, scalable quantum system for factoring numbers efficiently. While it typically takes about 12 qubits to factor the number 15, they found a way to shave the system down to five qubits, each represented by a single atom. Each atom can be held in a superposition of two different energy states simultaneously. The researchers use laser pulses to perform \u201clogic gates,\u201d or components of Shor\u2019s algorithm, on four of the five atoms. The results are then stored, forwarded, extracted, and recycled via the fifth atom, thereby carrying out Shor\u2019s algorithm in parallel, with fewer qubits than is typically required.<\/p>\n The team was able to keep the quantum system stable by holding the atoms in an ion trap, where they removed an electron from each atom, thereby charging it. They then held each atom in place with an electric field.<\/p>\n \u201cThat way, we know exactly where that atom is in space,\u201d Chuang explains. \u201cThen we do that with another atom, a few microns away \u2014 [a distance] about 100th the width of a human hair. By having a number of these atoms together, they can still interact with each other, because they\u2019re charged. That interaction lets us perform logic gates, which allow us to realize the primitives of the Shor factoring algorithm. The gates we perform can work on any of these kinds of atoms, no matter how large we make the system.\u201d<\/p>\n Chuang\u2019s team first worked out the quantum design in principle. His colleagues at the University of Innsbruck then built an experimental apparatus based on his methodology. They directed the quantum system to factor the number 15 \u2014 the smallest number that can meaningfully demonstrate Shor\u2019s algorithm. Without any prior knowledge of the answers, the system returned the correct factors, with a confidence exceeding 99 percent.<\/p>\n \u201cIn future generations, we foresee it being straightforwardly scalable, once the apparatus can trap more atoms and more laser beams can control the pulses,\u201d Chuang says. \u201cWe see no physical reason why that is not going to be in the cards.\u201d<\/p>\n Mark Ritter, senior manager of physical sciences at IBM, says the group\u2019s method of recycling qubits reduces the resources required in the system by a factor of 3 \u2014 a significant though small step towards scaling up quantum computing.<\/p>\n \u201cImproving the state-of-the-art by a factor of 3 is good,\u201d says Ritter. But truly scaling the system \u201crequires orders of magnitude more qubits, and these qubits must be shuttled around advanced traps with many thousands of simultaneous laser control pulses.\u201d<\/p>\n If the team can successfully add more quantum components to the system, Ritter says it will have accomplished a long-unrealized feat.<\/p>\n \u201cShor’s algorithm was the first non-trivial quantum algorithm showing a potential of \u2018exponential\u2019 speed-up over classical algorithms,\u201d Ritter says. \u201cIt captured the imagination of many researchers who took notice of quantum computing because of its promise of truly remarkable algorithmic acceleration. Therefore, to implement Shor’s algorithm is comparable to the \u2018Hello, World\u2019 of classical computing.\u201d<\/p>\n What will all this eventually mean for encryption schemes of the future?<\/p>\n \u201cWell, one thing is that if you are a nation state, you probably don\u2019t want to publicly store your secrets using encryption that relies on factoring as a hard-to-invert problem,\u201d Chuang says. \u201cBecause when these quantum computers start coming out, you\u2019ll be able to go back and unencrypt all those old secrets.\u201d<\/p>\n This research was supported, in part, by the Intelligence Advanced Research Project Activity (IARPA), and the MIT-Harvard Center for Ultracold Atoms, a National Science Foundation Physics Frontier Center.<\/p>\n<\/div>\n<\/div>\n<\/div>\n