Archive for October 12th, 2011

October 12, 2011

Driving a Hard Bargain with Diamond Qubits

In order to make a quantum computer, it’s first necessary to have a good way to implement qubits, the basic unit of quantum information. Although this can be done experimentally with photons or ions at low temperatures, photons and ions are hard to control in practice. Solid state implementations, using traditional semiconductors, for example, should make control easier, but there are both theoretical and practical difficulties.

One related possibility that has received some attention involves a well-known material: diamonds. A specific type of point defect in diamond crystals – nitrogen-vacancy (NV) color centers – may be well-suited for qubit implementation. The NV center is a defect in the crystal lattice in which an included nitrogen atom occurs next to a lattice vacancy that would ordinarily contain a carbon atom. Such a defect can emit photons (photoluminescence) under stimulation by electric or magnetic fields.

NV defects also have their own quantum spin states, which can be initialized, manipulated, and read optically. The problem of using these spin states as qubits is the practical one of coupling one to another, since isolated qubits aren’t useful for quantum computing. Most of the total spin derives from the spin of nitrogen nuclei, but these aren’t close enough together in diamond crystals to couple effectively. New research presents a theoretical argument for producing indirect coupling of NV sites through electron spin interactions under the influence of an externally applied electromagnetic field.

Driving a Hard Bargain with Diamond Qubits

Since the nuclear spin at each site (bearing a qubit) is able to “see” its local electron spin partner via the hyperfine interaction, it follows that if the electron spins at neighboring sites can interact sufficiently strongly, then the two nuclei will be able to communicate indirectly through their electron brothers. While the idea is simple, it turns out to be more complex than one might imagine, in fact, Bermudez et al. start by establishing that the electron-mediated interaction between nuclei at adjacent NV sites will be far too weak to be useful! They estimate that even quite closely neighboring sites, 10 nanometers (nm) apart, will have an effective nuclear-nuclear coupling strength of only 0.1 hertz—requiring several seconds to achieve a useful exchange of information. This is impractical, since nuclear spins suffer dephasing (losing any stored qubit) in less than a second.

Happily, the Ulm team has discovered a solution. Their analysis predicts that when the spins are resonantly driven by externally applied electromagnetic fields, the effective strength of the interaction increases dramatically. In essence, they introduce a new energy scale into the problem, replacing the effect of the crystal field splitting (which acts to suppress the effective nuclear-nuclear coupling) with the Rabi frequencies of the driven spins—a parameter that is under experimental control. Remarkably, with a suitable choice for this parameter, the coupling between nuclei is enhanced a thousandfold, becoming entirely practicable as a channel to exchange quantum information. As an added bonus, the act of driving the spins serves to protect the quantum state from the decohering effects of the surroundings. The effect is equivalent to “dynamic decoupling,” in which a spin that is periodically inverted at a frequency faster than the local magnetic field fluctuations acquires an aggregate zero phase.

Further reading:

Electron-Mediated Nuclear-Spin Interactions between Distant Nitrogen-Vacancy Centers

October 12, 2011

Astrophysicists find evidence of black holes’ destruction of stars

If you were to find yourself falling into a black hole, it would be quite an uncomfortable experience – not when you actually cross the event horizon of the black hole, but on the outside, just before crossing. The reason is that the gravitational force just outside the event horizon is so strong, the mere difference between the forces on the parts of you closest and farthest away from the horizon would be strong enough to tear you apart. The same is true for a star that got close enough, even if it was on a path that could actually avoid falling into the black hole.

Astrophysicists find evidence of black holes’ destruction of stars

Cosmologists have calculated that, on occasion, a star’s orbit will be disturbed in such a way that it passes very near the super-massive black hole at the center of its galaxy—but not so close that it is captured whole. Such a star will be torn apart by the extreme tidal forces it experiences: the force of gravity on the near side of the star is so much stronger than that on the far side that the gravitational force holding the star together is overwhelmed, causing the star to simply come apart. While some of the star’s matter falls into the black hole, much of it continues in chaotic orbits, crashing into itself and producing intense radiation lasting days to months. These phenomena are called stellar tidal disruption flares, or TDFs.

In spite of the large number of stars that typically orbit near a galaxy’s central black hole, TDF events are estimated to be very infrequent – about once per 100,000 years per galaxy. The present research involved a ground-based search involving observations of more than 2 million galaxies over a period of 10 years. Although 342 intense, well-measured flares were observed, it was necessary to exclude events that could have been supernovae or flares in active galactic nuclei instead of TDFs. (Supernovae are thought to occur 1000 times as frequently as TDFs.) Out of all the detected events, in only 2 cases was it possible to determine that the probability was extremely small the event was not a TDF.

Interestingly enough, two other recent research reports have shown good evidence for TDFs, using space-based equipment. The first suggested that repeated TDF events right in our own galaxy are responsible for gamma-ray bubbles extending 25,000 light-years above and below the galactic plane. The model used assumed that a TDF would occur once every 10,000 to 100,000 years.

The other recent case involved a flare that was bright enough to be a gamma-ray burst, except it lasted much longer. Instead, that event was more likely the result of a TDF that produced a very energetic jet of gamma rays and relativistic particles aimed straight in our direction.

Further reading:

Optical discovery of probable stellar tidal disruption flares