Spin measured without destruction

16 Nov 2006

US researchers have exploited polarized laser light to make the first non-destructive measurements of the spin state of a single electron in a quantum dot.

David Awschalom and colleagues at University of California, Santa Barbara, determined the electron's spin state by reflecting polarized laser light from a quantum dot. Making such non-destructive measurements of the electron spin is an important step towards the development of quantum computers that exploit the quantum properties of single electrons (Sciencexpress 9 November 2006).

The idea behind quantum computers is that a quantum particle can be in two states at the same time - spin up or spin down in the case of an electron. The state of these quantum particles can therefore can represent a one or a zero, and so the particles have become known as quantum bits (qubits).

According to the rules of quantum mechanics, these qubits can be combined or "entangled" to achieve parallel processsing of information on a massive scale. However, the realization of a quantum computer involves fundamental challenges such as how to read the logical state of a qubit without destroying the state, and how to entangle the qubits.

Quantum dots containing a single electron could be used as qubits, but existing optical and electronic schemes for reading the spin state destroy the state as part of the process.

The Santa Barbara group have solved this problem by shining plane-polarized laser light on a quantum dot made from galiium arsenide. The spin state of the electron was determined from the direction of rotation of the polarization of the reflected light -- the so-called Kerr rotation.

According to Awschalom, a Kerr rotation measurement is inherently non-destructive because it involves photons that have reflected from the sample without absorption. "If a photon was absorbed by the dot (thereby disturbing the system), then Kerr rotation would not be observed," he explained. The researchers minimized the chances of absorption occurring by using photons with energy sufficiently far from any optical transitions in the quantum dot.

Awschalom explained that the Santa Barbara work represents an important step towards the optical entanglement of single-electron quantum dots. The reason for this is that the reflected photon and the dot are entangled in the same quantum state. If the photon is then reflected from a second dot, all three become entangled. When the polarization of the photon is measured non-destructively, the two quantum dots remain entangled.