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# Quantum computer may be scalable

*Amy Walsh (Author) Published Date : Mar 04, 2016 22:04 IST*

#### Science

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's success depends on a computer with a large number of quantum bits. Now, in a paper published today in the journal Science, 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's 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's algorithm.

We show that Shor's 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, says Isaac Chuang, professor of physics and professor of electrical engineering and computer science at MIT.

In classical computing, numbers are represented by either 0s or 1s, and calculations are carried out according to an algorithm's instructions, which manipulate these 0s and 1s to transform an input to an output. In contrast, quantum computing relies on atomic-scale units, or qubits, that can be simultaneously 0 and 1 -- 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.

Once you had too many atoms, it was like a big forest -- it was very hard to control one atom from the next one, Chuang says. The difficulty is to implement [the algorithm] in a system that's sufficiently isolated that it can stay quantum mechanical for long enough that you can actually have a chance to do the whole algorithm.

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 logic gates, or components of Shor's algorithm, on four of the five atoms. The results are then stored, forwarded, extracted, and recycled via the fifth atom, thereby carrying out Shor's algorithm in parallel, with fewer qubits than is typically required.

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.

Chuang's 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 -- the smallest number that can meaningfully demonstrate Shor's algorithm. Without any prior knowledge of the answers, the system returned the correct factors, with a confidence exceeding 99 percent.

What will all this eventually mean for encryption schemes of the future

Well, one thing is that if you are a nation state, you probably don't want to publicly store your secrets using encryption that relies on factoring as a hard-to-invert problem, Chuang says. Because when these quantum computers start coming out, you'll be able to go back and unencrypt all those old secrets.

## Quantum computer may be scalable