The peculiar principles of quantum theory mean that, in a quantum state, a many-particle system can exhibit behaviour that would be inconceivable within classical physics. Superfluids, such as liquid helium at very low temperatures, can flow with no viscosity; superconductors can carry electricity with absolutely zero resistance.
These systems fall into states with so-called quantum coherence — unusually strong correlations of behaviour among many particles, which can only be interpreted with quantum mechanics. Quantum coherence underlies many of the most exciting phenomena of modern physics, including high-temperature superconductivity and the quantum Hall state, in which several fundamental particles collectively form a composite particle with a fraction of the elementary charge.
Historically, most of these phenomena were discovered by accident; however, scientists have made enormous progress in learning to control electronic behaviour in solids, and to create genuine many-body phenomena with cold atoms. Further progress in techniques for this nascent field of quantum engineering over the next decade will advance both fundamental physics and new technologies.
Control of quantum coherence in solids has progressed farthest in semiconductors, in which modern fabrication techniques make it possible to grow nearly perfect crystalline materials and to confine electrons in constricted spaces of zero, one or two dimensions, creating respectively quantum dots, wires or wells. Such precise confinement has allowed scientists to probe fundamentally new physics. A quantum dot, for example, is akin to an artificial atom in which the force binding the electrons, and the atom’s energy levels, can be carefully tuned. Consequently, quantum dots have been used to create powerful devices in electronics and laser optics.
Going further, researchers have recently learned to create quantum dots, wires and wells that interact with one another — or with other nanostructures such as carbon nanotubes — in a controlled way. This capability has revolutionized understanding of fundamental many-particle phenomena, including the decades-old mystery of the Kondo effect, which describes an increase in electrical resistance at low temperatures in the presence of magnetic ion impurities in non-magnetic crystals. Progress in fabricating more precise semiconductor structures will create further opportunities to probe similar quantum-coherent phenomena with unprecedented precision.
Similar dramatic progress has been made in atomic physics. Fifteen years ago, after long effort, physicists finally observed the Bose–Einstein condensation — an archetypal example of quantum coherence in which all the atoms in an extremely cold gas behave identically. This landmark advance kick-started a new era of many-body physics and researchers have since learned to control the strength of atomic interactions in these gases over a wide range. By imposing periodic laser fields on dilute gases, to influence atomic motion in a similar way to how internal fields influence electrons in crystalline solids, researchers have also been able to probe many-particle phenomena previously seen only in solid matter1.
Such experiments illustrate how solid materials and systems of cold atoms provide complementary opportunities for exploring novel quantum many-body physics. The diversity of atomic elements offers a virtually unlimited variety of solid compounds for the possible discovery of many-body states2, and atomically precise nanostructures can be built out of increasingly complex compounds. Physicists using cold-atom systems can tune how strongly the atoms interact and the shape of the optical-defined energy landscapes in which they reside. This flexibility presents opportunities for testing some of the most basic models of solid-state theory in ways not possible with real solids.