erwise intractable problems. In principle, computers could be built to take advantage of genuine quantum phenomena such as entanglement and interference that have no classical analogue and that offer otherwise impossible capabilities and speeds. Computers that thrive on entangled quantum information could thus run exponentially faster than classical computers, say Brassard et al.2

Brassard et al. explain that quantum parallelism arises because a quantum operation acting on a superposition of inputs produces a superposition of outputs. The unit of quantum information is the quantum bit, or qubit. Classical bits can take a value of either 0 or 1, but qubits can be in a linear superposition of the two classical states.

The quartet assert that implementation of quantum computers presents a profound experimental challenge. Quantum computer hardware must satisfy fundamental constraints. First, qubits must interact very weakly with the environment to preserve their superpositions. Second, the qubits must interact very strongly with one another to make logic gates and transfer information. Lastly, the states of the qubits must be able to be initialized and read out with high efficiency. Although few physical systems can satisfy these seemingly conflicting requirements, a notable exception is a collection of charged atoms (ions) held in an electromagnetic trap. Here, each atom stores a qubit of information in a pair of internal electronic levels. Each atom’s levels are well protected from environmental influences, which is why such energy levels also are used for atomic clocks.

For the moment, however, no large-scale quantum computation has been achieved in the laboratory. Nevertheless, several teams around the globe are working at small-scale prototypes, and quantum computing may be possible within the decade.


Genomic Medicine

New kinds of diagnostic and therapeutic treatments will likely be derived from an enhanced understanding of the human genome. As Fields et al. see it, the emerging field of functional genomics—the term refers to a gene’s inner workings and interplay with other genes—seeks to contribute to the elucidation of some fundamental questions. The three ask: How does the exact sequence of human DNA differ between individuals? What are the differences that result in disease or predisposition to disease? What is the specific role of each protein


Frontiers of Science/1997. Gilles Brassard, Isaac Chuang, Seth Lloyd, and Christopher Monroe, at <>.

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