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.

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.

Nobel Prize for quantum optics

The award of the 2012 Nobel Prize in Physics to Serge Haroche and David Wineland is the latest in a series of Nobel Prizes honoring elegant experiments using light to illuminate fundamental physics. The Swedish Academy of Sciences cited the two "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems." By examining individual photons and atoms, they resolved big questions about quantum mechanics.

Physicists long wondered how seriously they should take the paradoxes that arise from applying quantum mechanics rigorously to the behavior of individual particles. Albert Einstein famously called the concept of entangled particles "spooky action at a distance," but recent experiments have shown that such entanglement is real, and can be used for quantum encryption. Other recent experiments have observed quantum behavior of individual particles, and manipulated that behavior so that quantum states can be superposed for purposes such as quantum computing.

Haroche and Wineland developed complementary techniques for quantum manipulation of single particles. Haroche pioneered cavity quantum electrodynamics, which studies how an electromagnetically resonant cavity can affect quantum properties of an atom contained inside it, including spontaneous and stimulated emission. Working with microwave and optical cavities, his group measured photon properties without destroying the quantum states. Wineland and his colleagues used light to trap ions in ways that allowed them to transfer and superpose states of an ion. They were able to create single-quantum "Schrödinger's cat" states in the laboratory and watch them change from a quantum superposition to a classical mixture. Their work has opened the door to quantum computing and new types of optical clocks. 

Haroche holds the chair in Quantum Physics at the Collége de France (Paris, France), and is well-known for his research in quantum optics and quantum computing, and for his major contributions to cavity quantum electrodynamics, the behavior of atoms and light in high-Q cavities. He is work has earned him a long list of awards, including the Townes Award in 2007 from the Optical Society of America and the Herbert Walther Award from the German Physical Society and OSA in 2010. His deep roots in the optics community include doing his doctoral dissertation under Claude Cohen-Tannoudji and postdoctoral research under Arthur Schawlow, both future Nobel laureates. 

Wineland wrote his doctoral dissertation at Harvard University under Norman Ramsay, another Nobel Laureate, and heads the ion-storage group at the National Institute of Standards and Technology (Boulder, CO). He demonstrated the first laser cooling in 1978, and has used that technique to study quantum mechanics and develop applications. He demonstrated the first single-atom quantum logic gate in 1995, showing the potential of quantum computing, and later demonstrated entanglement of two and four ions. Other achievements include demonstrating quantum teleportation and a quantum logic atomic clock, which is now the world's most precise atomic clock. His long list of awards includes the Schawlow award in laser science from the American Physical Society, OSA's Frederick Ives award, and the first Herbert Walther award in 2008.

David Wineland has won the 2012 Nobel Prize in Physics, along with Serge Haroche. (Image courtesy of
Geoffrey Wheeler/NIST)