Qubit architectureA new level of reliability in a five-qubit array has been achieved by a team of physicists at UC Santa Barbara. This moves us a step closer to making a quantum computer a reality.

A functional quantum computer is a dream of many physicists. Contrasted with regular computers, the quantum computer would use quantum bits, or qubits, which make use of the multiple states of quantum phenomena.

When built, a quantum computer would have millions of times power at certain computations than today’s supercomputers.

Quantum computing relies on complex facets of quantum mechanics such as superposition. This idea holds that any physical object, such as an atom or electron, what quantum computers use to store information, may exist in all of its theoretical states simultaneously. This could raise parallel computing to new levels.

Qubit Error Correction

“Quantum hardware is very, very unreliable compared to classical hardware,” said UCSB staff scientist Austin Fowler. “Even the best state-of-the-art hardware is unreliable. Our paper shows that for the first time reliability has been reached.”

“Qubits are faulty, so error correction is necessary,” said co-lead author Julian Kelly.

Although the team has shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error.

“We need to improve and we would like to scale up to larger systems,” said lead author Rami Barends. “The intrinsic physics of control and coupling won’t have to change but the engineering around it is going to be a big challenge.”

Xmon Power

Qubit control signalsThe novel configuration of the group’s array stems from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named Xmons.

Superconductivity comes when certain materials are cooled to a critical level that eliminates electrical resistance and eliminates magnetic fields. The team chose to place five Xmons in a single row, with each qubit talking to its nearest neighbor, a simple but effective arrangement.

“Motivated by theoretical work, we started really thinking seriously about what we had to do to move forward,” said physics professor John Martinis. “It took us a while to figure out how simple it was, and simple, in the end, was really the best.”

“If you want to build a quantum computer, you need a two-dimensional array of such qubits, and the error rate should be below 1 percent,” said Fowler. “If we can get one order of magnitude lower — in the area of 10-3 or 1 in 1,000 for all our gates — our qubits could become commercially viable. But there are more issues that need to be solved. There are more frequencies to worry about and it’s certainly true that it’s more complex. However, the physics is no different.”

Reference:

R. Barends, J. Kelly, A. Megrant, A. Veitia, D. Sank, E. Jeffrey, T. C. White, J. Mutus, A. G. Fowler, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, C. Neill, P. O’Malley, P. Roushan, A. Vainsencher, J. Wenner, A. N. Korotkov, A. N. Cleland, John M. Martinis.
Superconducting quantum circuits at the surface code threshold for fault tolerance.
Nature, 2014; 508 (7497): 500 DOI:10.1038/nature13171

Images courtesy of Erik Lucero, UCSB

self lensing binary starThe first self-lensing binary star system, one in which the mass of the closer star can be measured by how powerfully it magnifies light from its more distant companion star, has been confirmed by researchers at the University of Washington.

The possibility of such a system was predicted by an astronomer in 1973, working with stellar evolution models of the time.

The discoveries happened mainly by accident. What initially appeared to be a sort of upside-down planet instead uncovered a new technique to study binary star systems. Our own sun stands alone, but around 40 percent of similar stars are in binary two-star (binary) or multi-star systems, orbiting their companions in a gravitational dance.

Star System KOI-3278

Astronomers become aware of planets too far away for direct observation via the dimming of light when a world passes in front of, or transits, its home star. The UW’s Ethan Kruse, a doctoral student, was searching for transits other astronomers may have overlooked, using data from the Kepler Space Telescope. He saw something in binary star system KOI-3278 that didn’t make sense.

“I found what essentially looked like an upside-down planet,” Kruse said. “What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite — it looks like an anti-transit.”

KOI-3278’s two stars are about 2,600 light-years (one light-year being equal to 5.88 trillion miles) away from Earth, in the Lyra constellation. They take turns being nearer to Earth as they orbit each other every 88.18 days.

The stars are about 43 million miles apart, around same the distance the planet Mercury is from the sun. The white dwarf, a cooling star thought to be in the final stage of life, is about Earth’s size but 200,000 times more massive.

Gravitational Lensing

The increase in light, instead of than the dip Kruse thought he’d see, was the white dwarf bending and magnifying light from its more distant neighbor through gravitational lensing, like a magnifying glass.

“The basic idea is fairly simple,” UW astronomer Eric Agol said. “Gravity warps space and time and as light travels toward us it actually gets bent, changes direction. So, any gravitational object — anything with mass — acts as a magnifying glass,” though a weak one. “You really need large distances for it to be effective.”

“The cool thing, in this case, is that the lensing effect is so strong, we are able to use that to measure the mass of the closer, white dwarf star. And instead of getting a dip now you get a brightening through the gravitational magnification.”

A common tool in astronomy, gravitational lensing has been used to detect planets around distant stars within the Milky Way galaxy, and was among the first methods used to confirm Albert Einstein’s general theory of relativity. Lensing within the Milky Way galaxy, such as this, is called microlensing.

But until now, the process had only been used in the fleeting instances of a nearby and distant star, not otherwise associated in any way, aligning just right, before going their separate ways again.

“The chance is really improbable,” said Agol. “As those two stars go through the galaxy they’ll never come back again, so you see that microlensing effect once and it never repeats. In this case, though, because the stars are orbiting each other, it repeats every 88 days.”

White dwarfs are significant in astronomy. They are used as indicators of age in the galaxy, according to the UW researchers. Essentially embers of burned-out stars, white dwarfs cool off at a specific rate over time. With this lensing, astronomers can learn with much greater precision what its mass and temperature are, and follow-up observations may yield its size.

Reference:

E. Kruse, E. Agol.
KOI-3278: A Self-Lensing Binary Star System.
Science, 2014; 344 (6181): 275 DOI: 10.1126/science.1251999

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