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All-photonic Quantum Repeaters: a Major Step Towards a Worldwide Quantum Internet

Researchers at the University of Toronto and the Nippon Telegraph and Telephone Corporation (NTT) in Japan have suggested a new method for extending the distance over which current and future quantum networks can transmit photons encoded with encryption keys. They published their research in the 15 April issue of Nature Communications. The proposal was also the subject of an extended communiqué released by NTT the same day.

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Laser-printed polysilicon transistors on paper

Printed electronics have opened up applications—flexible circuits and rollable displays, to name two—that were impossible with conventional electronics. Usually, printed electronics are created using organic or metal-oxide inks whose electronic properties often pale in comparison to silicon. Now scientists have discovered a new way to print silicon, potentially ousting its erstwhile usurpers.

The ability to print silicon onto substrates has existed for some time, but producing solid silicon from liquid polysilane ink required exposing the silicon to temperatures upwards of 350 degrees Celsius—far too hot for many of the flexible surfaces onto which one might want to print. The new technique, from Delft University of Technology in the Netherlands and the Japan Advanced Institute of Science and Technology in Ishikawa, completely bypasses this step. The collaborators detailed their findings in the 21 April online edition of the journal Applied Physics Letters.

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Disney Does Better Dubbing

Bad dubbing on foreign films could one day be replaced with better lip-synched audio with the aid of software from Disney Research.

Speech redubbing, used for the translation of movies, television shows and video games into another language, or the removal of offensive language for television networks, usually involves careful scripting to choose words that match lip motions and a subsequent re-recording by actors. The weakness of dubbing lies in how easy it is to detect even subtle discrepancies between spoken words and facial motions. 

To overcome this challenge, scientists at Disney Research Pittsburgh and the University of East Anglia in England are developing automated video redubbing strategies that find plausible word sequences to match actors' speech motions. They relied on the extreme level of ambiguity inherent in reading lips to increase the number of word possibilities that dubbing could place in people's mouths.

The scientists will present their findings on 23 April at the IEEE International Conference on Acoustics, Speech and Signal Processing in Brisbane, Australia.

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Theory Lowers the Speed Limit for Quantum Computing

Today’s quantum computing systems have just begun hinting at how future versions might outperform classical computers at solving certain complex problems. But new research has lowered the theoretical speed limit that future quantum computers will eventually run up against.

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New Theory Leads to Gigahertz Antenna on a Chip

It’s well understood that a dipole or monopole metal antenna’s length has to be at least one eighth of the wavelength of the wireless signal in order to transmit enough power. For transmission in the gigahertz range, where most mobile communication takes place, wavelengths between 15 and 30 centimeters had set a limit for miniaturization of transmitter and receiver antennas even as the silicon chips on which they must be integrated got ever smaller.

Now researchers have found a way to reduce the size of GHz antennas by modifying an existing technique, the use of antennas made from a dielectric or insulating material instead of a conductor.  In a proof of concept experiment, researchers at the department of engineering at the University of Cambridge and at the National Physical Laboratory in Teddington, Middlesex, UK, have shown that they can reduce the size of a GHz antenna without significant transmission loss by using dielectric materials as the radio wave emitting material.  They reported the results earlier this month in Physical Review Letters.

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How Much Did Early Transistors Cost?

In our coverage of the 50th anniversary of Moore’s Law, you might have noticed a few different numbers being thrown around for the price of transistors in the early days of the integrated circuit.

There’s Carver Mead’s recollection that Caltech students were buying discrete transistors for about a dollar or so around 1960 – about $8 today. 

A similar figure crops up in Dan Hutcheson’s beautiful plot of transistor prices and production levels since 1955. According to his data, the average transistor price in 1965 wasn’t very far off from that $8 mark.

And in Chris Mack’s piece on why Moore’s Law has lasted for so long, he quotes a price of $30 (in present-day dollars) for the integrated circuit transistors of 50 years ago.

These numbers aren’t necessarily in conflict. Then, as always, price depended on the particulars of the product and how many were manufactured.

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IBM Will Harness Fitness and Health Data from Apple Devices

All the health data on a single person collected by personal fitness trackers, mobile apps and electronic medical records could add up to more than 1 million gigabytes of health data in the average lifetime – the equivalent of about 300 million books. IBM has partnered with Apple and other companies in an effort to harness that data to improve personalized medical care for patients, as well as provide new insights for both medical research and health insurers.

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Electronic Gate Built For Silicon Quantum Computers

Borrowing a page from transistor design, a team led by Andrea Morello at the University of New South Wales in Australia has created an electronic gate that can be used to control especially long-lived quantum bits in silicon. This could help pave the way for full-scale, silicon-based quantum computers.

Quantum computing using superconducting circuits has garnered a lot of press over the years thanks to exciting resultsGoogle’s deep pockets and the controversy over D-Wave’s computing systems. But a number of researchers are eying silicon instead, in part because the material could potentially be used to build computers with especially compact and stable quantum bits, or qubits. 

One way to build silicon qubits is to add impurities, or “donors”, such as phosphorus atoms to silicon. Embedded in relative isolation, such qubits have been breaking longevity records; in some incarnations, they can store information for minutes or even hours before losing their delicate quantum state.

But until now, says team member Arne Laucht, researchers could only control—and thus set the state of and perform logic operations on—donor qubits by hitting them with short pulses of a magnetic field that happens to be oscillating at the right frequency.

This scheme works very well for a single qubit, where one qubit is located next to an on-chip microwave antenna, and one microwave source is used to generate the high frequency pulses,” Laucht told IEEE Spectrum in an e-mail. “But imagine scaling up to 100 or more qubits.” Magnetic fields, he says, are difficult to keep confined to a small space, so they’re liable to affect other qubits in the vicinity. What’s more, Laucht says, each qubit would need its own microwave source, and “each of these sources costs more than $100,000 apiece.” 


Fortunately there is another way. In 1998, physicist Bruce Kane laid out a recipe for a silicon-based quantum computer in which each donor gets its own gate. In Kane’s scheme, a single source would wash an oscillating magnetic field over all the qubits in the computer. This field would ordinarily leave the qubits unaffected. But by applying a voltage, one of the donor atom’s electrons could be drawn slightly toward the gate. This would shift the frequencies of oscillating magnetic field to which both the atom’s electron and the atom’s nucleus would respond.

Designed correctly, this arrangement could push an atom’s nucleus into resonance so that its state could be changed. (I say state, but I mean more specifically spin, a property of fundamental particles that can be made to point in either one of two directions, or, like Schrodinger’s cat, a superposition of both). 

In this new work, Laucht and colleagues show it’s possible to use this basic approach to control the spins of both an electron and the nucleus of a donor phosphorus atom. The results appeared on Friday in the open-access journal Science Advances. 

Laucht says the key to implementing Kane’s gate idea was having silicon that had been isotopically purified to further isolate the donor atoms from magnetic interference from stray spins. This narrowed the linewidth of resonance of the electron, allowing the researchers to move the qubit in and out of resonance with a voltage small enough to avoid disturbing the environment around the qubit. 

“There’s no other technique we know of to control individual [qubits] that are very close together,” says John Morton of the University College London. (Morton has worked with the University of New South Wales researchers on previous demonstrations, and he co-authored a survey of silicon quantum computing for IEEE Spectrum last year).

But Morton adds there are a lot of open questions left to be sorted out as researchers contemplate making systems with many such qubits. “What we really need to do is figure out how we’re going to make arrays of dopants in silicon that are able to communicate,” Morton says. “No one has a perfect blueprint yet.”

Kane’s proposal also included gates that would be able to control interactions between qubits. But such gates would be difficult to make since they would have to be made “very, very narrow and positioned exactly between the two atoms”, Laucht says. Fortunately, he says, there are other promising ways to get qubits to interact, and he and his colleagues are on the case. 


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