Coherent single photons

Stimulated emission makes lasers excellent sources of large numbers of coherent photons, which is fine for most applications. But quantum information networks are a problem because they work best with coherent photons that come one at a time, and lasers generally are not amenable to generating single photons. Single-photon sources have been developed for quantum computing, but they lack the coherence needed to create quantum entanglement at a distance using quantum entanglement.

Now, a team at the Cavendish Laboratory at Cambridge University (Cambridge, England) led by Mete Atature has found a way to generate single photons with laser-like coherence. Their starting point was optical pumping of quantum dots, which is one way of producing single photons. They first fabricated a Schottky diode containing self-assembled indium-arsenide quantum dots, which could be individually addressed with a pump laser to generate single photons by resonance fluorescence. Resonant fluorescence does not optically excite the host material, reducing interactions in the solid that decrease coherence of emitted photons, but charge fluctuations and other interactions remain to degrade coherence.

In Nature Communications they report avoiding photon decoherence by weak laser excitation, which generates photons primarily by elastic scattering. This avoided charge fluctuations, and allowed them to generate single photons from one quantum dot that remained coherent with the excitation laser for more than three seconds. Taking advantage of this mutual coherence, they report they could "synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field." That, in turn, let them show that as long as the photons emitted by the quantum dot remained coherent with the pump laser field, the separate photons were "fundamentally indistinguishable," so quantum interference among them can create quantum entanglement at a distance. That makes it possible to combine quantum computing with quantum communications, producing a more powerful tool for tasks such as quantum cryptography.

Ways to encode a qubit.

"The ability to generate quantum entanglement and  perform quantum teleportation between distant quantum-dot spin qubits with very high fidelity is now only a matter of time," says Atature. That's still a long way from science-fiction teleportation. However, the ability to generate single photons that maintain coherence well enough that they can be combined to produce novel waveforms may lead to real-world capabilities almost as attractive as avoiding airport lines.

Laser asteroid defense

Could lasers protect the Earth from wayward asteroids? A number of schemes have been proposed for pushing asteroids gradually to move their orbits away from the planet. Now, two California professors are proposing a bold scheme to build solar-powered space lasers powerful enough to evaporate a 500 m asteroid in about a year--or to make short work of a 17 m asteroid like the one that exploded near Chelyabinsk, Russia, on February 15.

Philip Lubin of the University of California (Santa Barbara, CA) and Gary Hughes of California Polytechnic State University (San Luis Obispo, CA) began planning the project they call DE-STAR--for Directed Energy Solar Targeting of Asteroids and exploRation--a year ago. On February 14, they issued a press release timed to the close approach by asteroid 2012 DA14. They were as stunned by the Russian explosion as everyone else.

Their bold proposal seeks to take advantage of the dramatic improvements in high-power diode lasers and solid-state lighting to build giant orbital phased arrays of lasers powered by electricity from huge solar panels. They envision starting with a desktop 1 m array called DE-STAR 0, then scaling up to a 10 m array called DE-STAR 1. They have proposed that NASA support a conceptual study of scaling up to a 10 km DE-STAR 4 array, powerful enough to vaporize a half-kilometer asteroid 150 million kilometers away. Even bigger versions could be used for laser propulsion; they estimate that a 1000 km DE-STAR 6 array could accelerate a 10 ton spacecraft close to the speed of light.

Future DE-STAR array samples composition of an asteroid as it propels an interplanetary spacecraft. (Courtesy of Philip Lubin)

The scheme may sound fantastic, but Lubin says it violates no laws of physics and requires no "technological miracles." It merely envisions continuing technological progress at the rate of the past 50 years, which took us from the feeble LEDs and diode lasers of 1963 to today's powerful emitters. They assume photovoltaic cells that can convert 70% of incident solar energy into electricity, and diodes which can convert 70% of the input electrical power into light.

Lubin doesn't think it will be easy. He worries about issues including the mass needed to build the giant array, and controlling output phase across the array with the precision needed to tightly focus the emission. But he predicts his assumptions will be considered "extraordinarily conservative and modest" in 30 to 50 years.

That remains to be seen, but space-based solar-powered diode arrays are worth investigating. They could go beyond asteroid defense to could help move asteroids, collect valuable materials from them, or provide power resources in space--as well as inspiring some fun science-fiction stories.

Bright future for silicon

The Wiley-VCH journal ChemPhysChem issued an embargoed press release embargoed early on the morning of November 21, 2012, heralding "a bright future for silicon." Just eight hours later, they lifted the embargo, citing "early reporting" of the research by Brian Korgel of the University of Texas (Austin, TX) and colleagues.

Embargo breaks often indicate hot stories, and the headline hinted at an important step toward the elusive goal of efficient light emission from silicon. Yet the next line was more muted: "Ordered nanocrystal arrays may provide a new platform to study and tailor the light-emitting properties of silicon." What is the real story?

Silicon is a wonderful material for electronics, but its photonic uses have been hobbled by an indirect bandgap that makes it very hard for electrons dropping into the valence band to release their energy as photons. That leaves silicon far behind III-V compounds like gallium arsenide for LEDs and diode lasers. Yet silicon is far ahead of other semiconductors in electronics, and companies like Intel (Santa Clara, CA) want to integrate photonics into their integrated circuits.

So far they have demonstrated "silicon lasers" by optically pumping Raman lines in silicon and III-V diode laser chips bonded to silicon. Both were important advances. But neither met the real goal--electrically powered emitters based on silicon that could be integrated into standard semiconductor chip production processes.

In their ChemPhysChem paper, Korgel and colleagues take a different approach, tapping the bright luminescence produced by silicon quantum dots. They write that their major achievement is devising a chemical technique that causes self-assembly of "the first colloidal Si nanocrystal superlattices." Self-assembly is essential because individual dots are too small to fabricate by conventional photolithography, and transmission electron microscope images show the dots are closely spaced in regular face-centered-cubic arrangements (see photo).

TEM image silicon nanocrystals in the 111-oriented (c) and 112-oriented (d) plans, with depictions of the crystalline structures shown in insets. (Courtesy Yixuan Yu et al., ChemPhysChem, Wiley-VCH Verlag GmbH & Co. KGaA, http://dx.doi.org/10.1002/cphc.201200738 [2012]. Reproduced with permission)

The authors say that covalent bonds with the hydrocarbon solvent make the silicon-nanocrystal superlattices stable to 350 degrees Celsius, higher than other similar superlattices. That's encouraging news, because self-organized nanocrystals are a promising fresh approach to structuring silicon to emit light more efficiently. But so far electrical excitation--sought for integrated optoelectronics--has far to go to match the efficiency of optical excitation of isolated silicon quantum dots. So Korgel is understandably optimistic about having "a new playground for understanding and manipulating the properties of silicon in new and unique ways," and is appropriately cautious in not claiming silicon lasers are just around the corner.

IEEE recognizes fiber laser milestone

Fiber lasers and amplifiers can do incredible things, but the technology is not as new as you think. Half a century ago, Elias Snitzer and a handful of colleagues at the American Optical Company's Research Center in Southbridge, MA pioneered both technologies. On October 26, 2012, I attended the dedication of plaque recognizing the achievement as a Milestone in electrical technology by the Institute of Electrical and Electronics Engineers.

Founded in the 19th century to make spectacles, American Optical in 1954 became the first company to try to develop practical fiber-optic imaging bundles, which were first demonstrated by academics and an independent inventor working on shoestring budgets. Initially funded by the Central Intelligence Agency to develop image scrambling bundles for secure messaging, AO later developed imaging bundles.

AO hired Snitzer to work on fiber optics in 1959. At his job interview, he recognized the puzzling patterns in a fiber bundle as evidence of lateral modes, and later published the first analysis of single-mode transmission. Interested in the laser, Snitzer took advantage of AO's glass expertise to make a solid-state laser of glass rather than crystals. He formed barium crown glass doped with 2% neodymium oxide into a three-inch rod thinner than a millimeter, covered with a low-index glass cladding to improve light transmission.  Pumping with a coiled flashlamp like the one Theodore Maiman used in the ruby laser, Snitzer demonstrated pulsed lasing in the stiff neodymium-doped fiber at room temperature in 1961.

In 1963, Snitzer and Charles Koester amplified pulses by up to a factor of 50,000 in a meter-long fiber laser without reflective end coatings, coiled around a linear flashlamp. Their goal was to measure gain dynamics, but the demonstration also showed the potential of fiber amplifiers.

The lack of good pump diodes kept fiber lasers and amplifiers from being practical until the 1980s. Snitzer played an important role in that development, developing doped fiber sensors, demonstrating 1480 nm diode pumping for erbium-fiber amplifiers and developing dual-core fibers now used in high-power fiber lasers. Snitzer died in May, but lived to see developments including multi-kilowatt fiber lasers and high-speed communications through fiber amplifiers in the global telecommunications network. Four of his children who attended the Milestone dedication were pleased by the recognition of the man they knew as "dad."

IEEE Milestone for fiber lasers and amplifiers, across the street from former American Optical headquarters in Southbridge, MA. (Courtesy of Dick Whitney)