Quantum computing is featured on the cover of this week’s Time Magazine, the February 17th issue. The story is of particular interest in our department, since Aaron Reinhard and his student coworkers are building an atomic physics experiment that aims to understand the interactions among ultracold, highly excited atoms. These interactions could someday be used to build a scalable quantum computer.
The article in Time focuses on the D-Wave Two, a controversial new commercial product which claims to implement an adiabatic quantum computer which consists of 512 “qubits,” or quantum bits. A quantum computer is a device where the individual bits, or the “ones” and ”zeros“ which encode all the information in the computer, are stored in quantum systems. In a classical computer, each bit can be in either the “zero” state or the “one” state. In a quantum computer, owing to the strange properties of quantum mechanics, each bit can be either in the zero state, the one state, or zero and one at the same time. There are several very clever computing algorithms that harness this strange behavior to perform massively parallel operations.
An example of such an process, called Shor’s algorithm, involves the factorization of a large number into its prime factors. Factorization is at the heart of nearly all protocols for data encryption and secure communication. The increase in the speed at which a quantum computer would be able to factor a number compared to a classical computer scales exponentially with the number of bits. This means that numbers which might take years to factor on the best modern classical computers might take seconds to factor using a quantum computer. Numbers that would take the age of the universe to factor on a classical computer, could be factored in months.
The reason the $10 million D-Wave Two is controversial is that it purports to implement adiabatic quantum computation, or a very special kind of quantum computation. Adiabatic quantum computers can only be used for optimization problems, or problems related to finding the fastest or most efficient way to do something. They cannot be used to factor numbers. Gate-type quantum computers are necessary for this task. Adding to the controversy is the fact that several independent tests of the D-Wave Two have not shown any improvement in speed over a classical computer1,2.
Dr. Reinhard’s group is building an experiment where they will trap rubidium atoms and cool them to a few millionths of a degree above absolute zero. They will then excite the atoms’ valence electrons to very high internal states (principal quantum numbers of 40-100) and study the interactions among these neutral atoms. The goal is to understand the properties of the interactions so that they might someday be used to build a gate-based quantum computer made of neutral atoms. The fundamental science involved in the interactions among neutral atoms is of significant interest, but the potential application to the important problem of quantum information is very exciting.
1 Boixo et. al. (April 2013). Quantum annealing with more than one hundred qubits
2 Ronnow et. al. (January 2014). Defining and detecting quantum speedup