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Science

Putting a Quantum Computer to Work in a Cup of Coffee

Gary Taubes

Published Jan 17, 1997• 2 citations
Publisher

Abstract

A quantum computer, tapping into the quantum weirdness of the microscopic world, could in theory collapse calculations that would take billions of years on mundane supercomputers into a few seconds. Experimentally, though, quantum computers have been daunting. Now two groups of researchers have come up with a way to realize at least some of that potential by applying nuclear magnetic resonance—a standard technique in medical imaging and chemical analysis—to a liquid as simple as coffee (see Research Article on p. 350 ). By flipping, coupling, and reading out the spins of individual nuclei in the coffee, the strategy can turn it into a quantum logic gate and even a quantum integrated circuit.

DOI
10.1126/science.275.5298.307

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A quantum computer, tapping into the quantum weirdness of the microscopic world, could in theory collapse calculations that would take billions of years on mundane supercomputers into a few seconds. Experimentally, though, quantum computers have been daunting. Now two groups of researchers have come up with a way to realize at least some of that potential by applying nuclear magnetic resonance—a standard technique in medical imaging and chemical analysis—to a liquid as simple as coffee (see Research Article on p. 350 ). By flipping, coupling, and reading out the spins of individual nuclei in the coffee, the strategy can turn it into a quantum logic gate and even a quantum integrated circuit.
We show that quantum computation is possible with mixed states instead of pure states as inputs. This is performed by embedding within the mixed state a subspace that transforms like a pure state and that can be identified by labelling it based on logical (spin), temporal, or spatial degrees of freedom. This permits quantum computation to be realized with bulk ensembles far from the ground state. Experimental results are presented for quantum gates and circuits implemented with liquid nuclear magnetic resonance techniques and verified by quantum state tomography.
If a computer could be built from quantum elements, which can exist in superpositions of many states rather than the 1s and 0s of conventional binary logic, then important hard problems could be solved rapidly. One example is the prime number factorization problem at the heart of modern encryption: given the product of two large primes, find the primes themselves. Another important problem is exhaustive search: given an unstructured list of items, quickly find the one that satisfies a certain property. A pair of Research Commentaries in this issue discuss some important new results in which simple quantum computers have been constructed with liquids probed by nuclear magnetic resonance techniques. The first, by L. K. Grover of Bell Labs, describes the theoretical situation and the implementation of an algorithm that he designed for exhaustive search. The second, by J. A. Jones of Oxford University, UK, discusses the recent experimental approaches to quantum computing with nuclear magnetic resonance.
Nuclear magnetic resonance (NMR) is a direct macroscopic manifestation of the quantum mechanics of the intrinsic angular momentum of atomic nuclei. It is best known for its extraordinary range of applications, which include molecular structure determination, medical imaging, and measurements of flow and diffusion rates. Most recently, liquid-state NMR spectroscopy has been found to provide a powerful experimental tool for the development and evaluation of the coherent control techniques needed for quantum information processing. This burgeoning new interdisciplinary field has the potential to achieve cryptographic, communications, and computational feats far beyond what is possible with known classical physics. Indeed, NMR has made the demonstration of many of these feats sufficiently simple to be carried out by high school summer interns working in our laboratory (see the last two authors). In this paper the basic principles of quantum information processing by NMR spectroscopy are described, along with several illustrative experiments suitable for incorporation into the undergraduate physics curriculum. These experiments are spin–spin interferometry, an implementation of the quantum Fourier transform, and the quantum simulation of a harmonic oscillator.
Machines that make the slightest possible motion could lead to wild new technologies and help reveal why the weird rules of the microscopic realm don9t apply to our everyday world. At least four groups hope to reach the quantum limit of motion within months. The feat could open the way for tiny, fingerlike force detectors with the highest possible sensitivity. Such detectors might enable researchers to quickly decode DNA and other large molecules, and someday they might serve as the guts of superfast quantum computers.
Nuclear magnetic resonance is arguably both the best available quantum technology for implementing simple quantum computing experiments and the worst technology for building large scale quantum computers that has ever been seriously put forward. After a few years of rapid growth, leading to an implementation of Shor's quantum factoring algorithm in a seven-spin system, the field started to reach its natural limits and further progress became challenging. Rather than pursuing more complex algorithms on larger systems, interest has now largely moved into developing techniques for the precise and efficient manipulation of spin states with the aim of developing methods that can be applied in other more scalable technologies and within conventional NMR. However, the user friendliness of NMR implementations means that they remain popular for proof-of-principle demonstrations of simple quantum information protocols.