As discussed in the main text of "Seeing with Superconductors," superconducting detectors are revolutionizing a wide variety of research and technology fields, including nuclear nonproliferation and homeland security, analysis of microchips, submillimeter band astronomy and the study of the cosmic microwave background. Here I talk about two more: quantum cryptography and the search for dark matter.

Quantum information. The promise of quantum information is high, including quantum computers and unbreakable cryptography. The practical realization of recent theoretical advances in the field, however, requires many experimental breakthroughs. Because one of the leading techniques is the manipulation of small numbers of photons, one critical need has been for a highly efficient photon counter that can measure the number of photons in a pulse of light.

In quantum cryptography, the security of information exchange over a fiber-optic cable is, in principle, protected by the laws of quantum mechanics. In practice, however, the security of quantum-cryptography systems can be compromised by noise in the detectors. The realization of long-distance quantum-cryptographic networks requires sources and detectors operating at telecommunication wavelengths in the near infrared. Conventional near-infrared semiconductor detector systems are limited by low sensitivity and high error rates and cannot count numbers of photons.

A team led by Sae Woo Nam of the National Institute of Standards and Technology in Boulder, Colo., and Blas Cabrera of Stanford University has developed superconducting detectors that can measure photon numbers efficiently at telecommunication wavelengths with a negligible error rate, opening the way to secure quantum cryptography over a distance of 100 kilometers. These detectors are also the enabling technology for other quantum information applications. They allow researchers to prove that single-photon sources, another key element of many optical quantum information systems, reliably emit only one photon at a time. With further development of efficiency and speed, superconducting detectors may also be an important component in photonic quantum computers.

Particle Physics. Over the past decade, evidence has mounted that only about one sixth of the matter in the universe is the ordinary baryonic matter with which we are familiar. The remaining five sixths is in the form of dark matter. Dark matter has never been directly detected, but its existence is strongly suggested from its gravitational influence on the matter that we can observe. Recent dramatic evidence for dark matter was provided by observations of two colliding galaxy clusters. The most important dark matter candidates are weakly interacting massive particles (WIMPs), which are stable relic particles from the big bang. If WIMPs exist, they are very common, but they usually pass through ordinary matter without interaction.

The Cryogenic Dark Matter Search (CDMS) is an experiment that uses superconducting detectors to search for rare WIMP interactions. A team led by Cabrera of Stanford and Bernard Sadoulet of the University of California, Berkeley, developed the experiment. The detectors consist of thick semiconductor wafers with a superconducting detector on one side and charge detectors on the other. When a particle (such as a WIMP) collides with the detector, it creates crystal lattice vibrations (phonons) and releases electrons. The energy in the phonons is measured as heat in the superconducting detectors, and the charge signal is measured by the charge detectors. The experiment is now operating a half mile underground in the Soudan iron mine in northeastern Minnesota, where the effects of cosmic ray particles are largely eliminated. By a careful analysis of the charge and heat signals, scientists discriminate real WIMP interactions from interfering radioactive background sources.

No experiment has yet conclusively detected WIMPs, but CDMS has set the most stringent limits of any experiment on the strength of WIMP interactions with ordinary matter. Over the next several years, the size of CDMS will be increased, and it has a good chance of discovering WIMPs.