Volume 5 Issue 10, October 2009

The burgeoning field of quantum information science is not only about building a working device. Already we can learn a lot by thinking about how computation works under the rule of quantum mechanics.

Formulating a consistent framework for relativistic thermodynamics has been the subject of intense debate over the past century. Defining quantities with respect to the observer's past lightcone could open new vistas.

In semiconductor quantum dots, single electron spins are surrounded by a bath of nuclear spins. Controlling the nuclear magnetization is difficult, but two experiments now demonstrate locking of the average nuclear field to a particular value and narrowing of the nuclear randomness.

Bound entanglement represents an irreversible form of quantum correlation: bound-entangled states require entanglement for their preparation, but no pure-state entanglement can be extracted from them. A recent experiment reports the first experimental realization of a bound-entangled state.

Large ensembles of atoms can be buffer-gas loaded into a magnetic trap and further evaporatively cooled all the way down to quantum degeneracy. The approach has now been shown to provide an alternative — and potentially general — route to Bose–Einstein condensation.

When two identical photons hit a half-silvered mirror, quantum mechanics requires that both pass through or both be reflected in the same direction. Previously, this effect had only been demonstrated with photons from similar light sources. It has now been repeated with photons generated by two completely different physical processes.

Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy are complementary probes, and yet the results of recent studies using these techniques on quasiparticle excitations in the copper oxide superconductors seem to be contradictory. In fact, there is no contradiction.

One of the many unusual characteristics of graphene is that it shows ‘puddles’ of positive and negative charge throughout. A systematic scanning tunnelling microscope study shows that these puddles are not a consequence of ripples in graphene’s structure as originally thought, but are due to charged impurities below its surface.

Magnetic switching is typically accomplished by using a driving field that stays on until the magnetization is rotated to its final position. An experiment demonstrates that, in antiferromagnets, inertial effects can be harnessed, such that only a short ‘kick’ is required to transfer sufficient momentum to the spin system for it to reorient.

Using arguments from computational complexity theory, fundamental limitations are found for how efficient it is to calculate the ground-state energy of many-electron systems using density functional theory.

Radiofrequency spectroscopy provides a microscopic probe of fermionic pairing in ultracold Fermi gases. Calculations now suggest that there is a one-to-one correspondence between the theory of these spectra and the theory of paraconductivity fluctuations in superconductors, that is, the effect of enhanced conductivity even before the system enters the superconducting state.

Formulating a consistent framework for relativistic thermodynamics has been a subject of controversy over the past century. A new approach for defining thermodynamic quantities makes predictions that are, in principle, testable, and which might lead to a natural extension of thermodynamics to general relativity.

Bound entanglement is a particular class that is not distillable—that is, it cannot be converted into a pure maximally entangled state by means of local operations and classical communication. A four-qubit bound entangled state, or Smolin state, has now been created experimentally.

The identification of the magnetic-fluctuation mode at a quantum phase transition of the archetypical heavy-fermion compound Ce_1−xLa_xRu₂Si₂ indicates that quantum criticality in this system is governed by collective antiferromagnetic behaviour, rather than by local magnetic moments as has been suggested.

In semiconductor quantum dots, interactions between the confined electrons and the surrounding reservoir of nuclear spins limit the attainable electron-spin coherence. But the nuclear-spin reservoir can also take a constructive role, as it facilitates the locking of the optical quantum-dot resonance to the changing frequency of an external driving laser, as an experiment now demonstrates.

When electrons are transported through a semiconductor quantum dot, they interact with nuclear spin in the host material. This interaction—often considered to be a nuisance—is now shown to provide a feedback mechanism that actively pulls the electron-spin Larmor frequency into resonance with that of an external microwave driving field.

Understanding the mechanical properties of DNA helps us to predict protein–DNA and DNA–DNA interactions. It is now shown that—with the aid of statistical physics—the melting temperature of DNA can be used to extract very detailed information about local flexibility.