Power/Performance Bits: Aug. 25

Speeding up quantum computing

A team of physicists from the University of Vienna and the Austrian Academy of Sciences demonstrated a new quantum computation scheme in which operations occur without a well-defined order. The researchers used this effect to accomplish a task more efficiently than a standard quantum computer. Moreover, these ideas could set the basis for a new form of quantum computing, potentially providing quantum computers with an even larger computational speed-up.

The team realized that superimposing the order of quantum gates, an idea which had been proposed theoretically, could be implemented in the laboratory. In a superposition of quantum gate orders, it is impossible – even in principle – to know if one operation occurred before another operation, or the other way around. This means that two quantum logic gates A and B can be applied in both orders at the same time. In other words, gate A acts before B and B acts before A.

Quantum mechanics does not only allow superposition of quantum states but also superposition of quantum gates. It was shown that superimposing two quantum gates A and B, an unordered quantum computation can run more efficiently than a well-defined order quantum computation. (Source: Jonas Schmöle/University of Vienna)

The results of their experiment confirmed that it is impossible to determine which gate acted first – but the experiment was not simply a curiosity. “In fact, we were able to run a quantum algorithm to characterize the gates more efficiently than any previously known algorithm,” says Lorenzo Procopio, lead author of the study. From a single measurement on the photon, they probed a specific property of the two quantum gates thereby confirming that the gates were applied in both orders at once. As more gates are added to the task, the new method becomes even more efficient compared to previous techniques.

Reshaping the solar spectrum

A team of chemists at the University of California, Riverside found a way to make solar energy conversion more efficient. By combining inorganic semiconductor nanocrystals with organic molecules, they succeeded in “upconverting” photons in the visible and near-infrared regions of the solar spectrum.

“The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today’s solar cells,” said Christopher Bardeen, a professor of chemistry at UC Riverside. “This is energy lost, no matter how good your solar cell. The hybrid material we have come up with first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted.”

Bardeen added that these materials are essentially “reshaping the solar spectrum” so that it better matches the photovoltaic materials used today in solar cells. The ability to utilize the infrared portion of the solar spectrum could boost solar photovoltaic efficiencies by 30 percent or more.

In experiments, the researchers directed 980-nanometer infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nanometer light, almost doubling the energy of the incoming photons. The researchers were able to boost the upconversion process by up to three orders of magnitude by coating the nanocrystals with organic ligands.

Besides solar energy, the ability to upconvert two low energy photons into one high energy photon has potential applications in biological imaging, data storage and organic light-emitting diodes.

Flexing light

Researchers from National Chiao Tung University, Taiwan created highly flexible, efficient white LEDs with potential use in wearable displays and non-flat surfaces, such as curved and flexible television screens. While the design itself is new, the LED was completely fabricated from pre-existing technologies, allowing others to easily replicate and build on the platform.

The researchers’ off-the-shelf LED device gets its flexibility from its two primary materials, polyimide and polydimethylsiloxane. To construct it, the team first covered a polyimide substrate with copper foil shielding tape. In a process known as flip-chip bonding, which reduces thermal resistance and results in higher heat dissipation than traditional wire bonding, they mounted 81 Blue LED chips, measuring 1.125 mm x 1.125 mm, to the foil in an upside down position.

The large area warm white light source with bending capability. (Source: Chin-Wei Sher, Chien-chung Lin, Hao-Chung Kuo/National Chiao Tung University)

To provide a warm white-yellow light, the researchers then added another layer consisting of a yellow phosphor film that had been mixed and spin-coated in polydimethylsiloxane, or PDMS, a widely used silicone-based organic polymer. It was chosen for its high degree of transparency, stability, and flexibility. The final film measured five centimeters by five centimeters, but there is no reasonable limitation to the size of the film.

The researchers ran the device for a standard 1,000 hours, to test its durability, finding that its emission decayed by only 5%. Its potential for use in wearables was demonstrated when subjected to bending tests. It held its power output when bent to a curvature with a 1.5-cm radius. It also exhibited a light efficiency of 120 lumens per watt.

Future work includes reducing the LED device’s thickness, as well as increasing its functional lifetime and energy efficiency.