The spin of an electron is in many ways the archetypical quantum system. If you measure the spin, it will take one of two values: spin up or spin down. It's a perfect binary, which makes it good for quantum computing systems, as well as in the nascent field of spintronics. However, implementing things based on spin has proven to be far more complicated: single electron spins in atoms interact with the environment so that they forget their original state. Measurement of the spin state brings its own difficulties.

Researchers have now made some progress by manipulating the electronic spin state in a phosphorous atom embedded in silicon. Jarryd J. Pla and colleagues exploited the properties of both atom types to isolate the spin of a single electron in the phosphorous atom. At 0.3 Kelvin (0.3°C above absolute zero) the spin state stayed stable for relatively long amounts of time. While single electron spins are insufficient to build quantum devices, this experiment is a reasonable proof-of-principle, and shows how multi-spin systems could be developed.

The spin of an atom is mostly determined by the electrons that orbit furthest from the nucleus, since they have a higher response to external stimuli (spin states of atomic nuclei are more stable but more difficult to work with, though another experiment made some progress along those lines). As a result, if the atom has the proper electronic configuration, the spin state of an atom is often the same as the spin state of a single electron.

Phosphorous turns out to be useful in that sense, having a highly exposed electron, while silicon's electrons are less easily manipulated. (Phosphorous is sometimes used as a "dopant" atom in semiconductor devices, since it donates its electron to enhance conduction properties.) By embedding a phosphorous atom in silicon, the researchers used this contrast to isolate the properties of a single electron orbiting the atom. They massaged the electron spin into a particular state using microwaves to drive the system. Varying the microwave pulses, the researchers could cause the electron spin to flip in a controlled fashion.

The entire system was kept at very low temperatures, and subjected to strong magnetic fields. The researchers measured several coherent oscillations of the spin orientation before interactions with the electrons in the silicon damped them out. They found the electron spin maintained its coherent behavior for as much as 0.2 milliseconds—short on human terms, but long enough for electronic devices.

One appealing aspect to this system is that it's silicon-based, so it bears strong resemblance to the common semiconductor devices that are the backbone of modern electronics. In other words, quantum spin devices could be constructed that from the same materials as familiar transistors and logic circuits.

The researchers also noted that a particular isotope of silicon, 28Si, has zero nuclear spin. If the substrate for the device contains a higher fraction of this isotope, earlier experiments have shown the phosphorous electron can maintain its spin coherence for longer than a second. They propose future experiments to use "enriched" silicon samples in spin control systems.

Exciting as these results are, they comprise only one spin—one qubit. Real devices require multiple qubits, and the transfer of quantum states between them in a coherent manner. Based on its simplicity and the relative ease of control, the use of single phosphorous atoms seems as promising as any for spin-based quantum computing.

Nature, 2012. DOI: 10.1038/nature11449 (About DOIs).