The World's Smallest Diamond Ring

Measuring only 5 microns (millionths of a meter) in diameter and 300 nanometers (billionths of a meter) in thickness, has been made by scientists at the University of Melbourne. The ring is a component in a device for producing and detecting single photons. A picture of the ring (see image at http://www.aip.org/png/2008/299.htm) was shown by Steven Prawer (s.prawer@unimelb.edu.au) at a session devoted to circuitry based on artificial diamonds at this week’s March Meeting of the American Physical Society (APS) in New Orleans. For more on the Australian work see http://www.qcaustralia.org/home.htm.

DIAMOND QUBITS. The APS session featured several additional striking quantum information processing (QIP) results. But first: why diamond? Diamond is an excellent heat conductor and electrical insulator, and it looks as if it will be an excellent host for qubits. Qubits are a special kind of bit. Unlike the bits (with a value of “1" or “0") used in ordinary digital computers, qubits can have a value of 1 and 0 at the same time. That’s because a qubit is manifested in the form of a quantum system that exists in a superposition of two different states. Examples include photons that can be in either of two polarization states, or Cooper pairs that can reside on either side of a Josephson junction, or the net spin (up or down) of a quantum dot.

A relatively new form of qubit utilizes the two spin orientations of an unpaired electron circulating around a strange kind of “molecule” at the heart of an artificially created diamond film. The molecule consists of a nitrogen atom (present as in impurity amid all those carbon atoms) and a nearby vacancy, a place in the crystal containing no atom at all. The advantages of employing this NV color center (so named since the molecule, when excited, re-emits photons one at a time) include the fact that it is easily excited or polarized by laser light; it stays polarized for as long as a millisecond, compared to mere nanoseconds for most electrons in a semiconductor; and all of this occurs at room temperature. Putting the electron into each of two spin states simultaneously makes it into a long-lived qubit. With further optical networking this qubit might be entangled (brought into coherence) with other nearby qubits, creating a logic gate or processor for a future quantum computer. (The article by David Awschalom awsch@physics.ucsb.edu in the Oct 2007 Scientific American provides excellent background.)
In quick order, here is some of the other diamond news from the APS meeting.

SINGLE-ELECTRON ESR. Ronald Hanson (Kavli Institute, Delft, r.hanson@tudelft.nl) reported results from the University of California at Santa Barbara revealing the ability to flip the spin of an electron (associated with the NV center) in a few nanoseconds and observe that electron as it loses its assigned polarization through interactions with the surrounding diamond environment; this environment he referred to as a “spin bath” since it consisted of many surrounding nitrogen atoms whose spins could be adjusted. Hanson argued that he and his colleagues had achieved electron spin resonance (ESR, essentially the electron equivalent of nuclear magnetic resonance, NMR) with single-electron sensitivity. The results were also reported online in Science on March 13.

CONTROLLING SINGLE NUCLEAR SPINS. Mikhail Lukin of Harvard (lukin@fas.harvard.edu) described the effect of a magnetic carbon-13 nucleus on the observed behavior of color centers in diamond. Carbon-13, an isotope present in very pure diamond at the 1% level, is magnetic, whereas as ordinary carbon-12 nuclei are not. Lukin said that he hopes to entangle several such NV/C-13 qubits, creating a potential register for performing quantum processing (see Dutt et al., Science, 1 June 2007, for background). A single C-13 atom could be located to within a space of 1 nm and its spin could remain stable for periods as long as 1 second. Furthermore, the NV/C-13 interaction provides a way to perform NMR spectroscopy on a single isolated nuclear spin and to sense very weak magnetic fields. Lukin and his colleagues have performed experiments in whichan NV site in a tiny diamond mounted on the end of a probe was used to sense the magnetic signature of a sample lying close underneath. Fields as small as 10 nano-tesla were sensed. In effect, Lukin said, this setup performed as a one-atom magnetometer.

PHOTONIC QUBIT NETWORK. Finally, Charles Santori (Hewlett Packard, charles.santori@hp.com) reported the creation of qubits in diamond at room temperature without the need for any external magnetic field (for polarizing electrons) or microwaves (for flipping the polarization). All these tasks, he said, could be accomplished with a visible-light laser modulated at two frequencies. The all-optical approach to manipulating spins, using optical waveguides and cavities, was a necessary step toward streamlining and scaling up the process of creating and linking many qubits in a workable quantum computer.