'Quantum criticality': Ultracold experiments heat up quantum research

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This false color image shows the average density of cesium atoms taken during multiple experimental cycles for studying quantum criticality in the ultracold laboratory of Cheng Chin, associate professor in physics at UChicago. The density is lowest in the white area on the outside, highest toward the center, where higher numbers of atoms are blocking the incoming infrared laser light. Xibo Zhang collected these data in connection with his recently completed doctoral research at UChicago. (Xibo Zhang and Cheng Chin)

(PhysOrg.com) -- University of Chicago physicists have experimentally demonstrated for the first time that atoms chilled to temperatures near absolute zero may behave like seemingly unrelated natural systems of vastly different scales, offering potential insights into links between the atomic realm and deep questions of cosmology.

This ultracold state, called “quantum criticality,” hints at similarities between such diverse phenomena as the gravitational dynamics of black holes or the exotic conditions that prevailed at the birth of the universe, said Cheng Chin, associate professor in physics at UChicago. The results could even point to ways of simulating cosmological phenomena of the early universe by studying systems of atoms in states of quantum criticality.

“Quantum criticality is the entry point for us to make connections between our observations and other systems in nature,” said Chin, whose team is the first to observe quantum criticality in ultracold atoms in optical lattices, a regular array of cells formed by multiple laser beams that capture and localize individual atoms.

UChicago graduate student Xibo Zhang and two co-authors published their observations online Feb. 16 in Science Express and in the March 2 issue of Science.

Quantum criticality emerges only in the vicinity of a quantum phase transition. In the physics of everyday life, rather mundane phase transitions occur when, for example, water freezes into ice in response to a drop in temperature. The far more elusive and exotic quantum phase transitions occur only at ultracold temperatures under the influence of magnetism, pressure or other factors.

“This is a very important step in having a complete test of the theory of quantum criticality in a system that you can characterize and measure extremely well,” said Harvard University physics professor Subir Sachdev about the UChicago study.

Physicists have extensively investigated quantum criticality in crystals, superconductors and magnetic materials, especially as it pertains to the motions of electrons. “Those efforts are impeded by the fact that we can’t go in and really look at what every electron is doing and all the various properties at will,” Sachdev said.

Sachdev’s theoretical work has revealed a deep mathematical connection between how subatomic particles behave near a quantum critical point and the gravitational dynamics of black holes. A few years hence, offshoots of the Chicago experiments could provide a testing ground for such ideas, he said.

There are two types of critical points, which separate one phase from another. The Chicago paper deals with the simpler of the two types, an important milestone to tackling the more complex version, Sachdev said. “I imagine that’s going to happen in the next year or two and that’s what we’re all looking forward to now,” he said.

Other teams at UChicago and elsewhere have observed quantum criticality under completely different experimental conditions. In 2010, for example, a team led by Thomas Rosenbaum, the John T. Wilson Distinguished Service Professor in Physics at UChicago, observed quantum criticality in a sample of pure chromium when it was subjected to ultrahigh pressures.

Zhang, who will receive his doctorate this month, invested nearly two and a half years of work in the latest findings from Chin’s laboratory. Co-authoring the study with Zhang and Chin were Chen-Lung Hung, PhD’11, now a postdoctoral scientist at the California Institute of Technology, and UChicago postdoctoral scientist Shih-Kuang Tung.

In their tabletop experiments, the Chicago scientists use sets of crossed laser beams to trap and cool up to 20,000 cesium atoms in a horizontal plane contained within an eight-inch cylindrical vacuum chamber. The process transforms the atoms from a hot gas to a superfluid, an exotic form of matter that exists only at temperatures hundreds of degrees below zero.

“The whole experiment takes six to seven seconds and we can repeat the experiment again and again,” Zhang said.
The experimental apparatus includes a CCD camera sensitive enough to image the distribution of atoms in a state of quantum criticality. The CCD camera records the intensity of laser light as it enters that vacuum chamber containing thousands of specially configured ultracold atoms.

“What we record on the camera is essentially a shadow cast by the atoms,” Chin explained.

The UChicago scientists first looked for signs of quantum criticality in experiments performed at ultracold temperatures from 30 to 12 nano-Kelvin, but failed to see convincing evidence. Last year they were able to push the temperatures down to 5.8 nano-Kelvin, just billionths of a degree above absolute zero (minus 459 degrees Fahrenehit). “It turns out that you need to go below 10 nano-Kelvin in order to see this phenomenon in our system,” Chin said.

Chin’s team has been especially interested in the possibility of using ultracold atoms to simulate the evolution of the early universe. This ambition stems from the quantum simulation concept that Nobel laureate Richard Feynman proposed in 1981. Feynman maintained that if scientists understand one quantum system well enough, they might be able to use it to simulate the operations of another system that can be difficult to study directly.

For some, like Harvard’s Sachdev, quantum criticality in ultracold atoms is worthy of study as a physical system in its own right. “I want to understand it for its own beautiful quantum properties rather than viewing it as a simulation of something else,” he said.

More information: “Observation of Quantum Criticality with Ultracold Atoms in Optical Lattices,” by Xibo Zhang, Chen-Lung Hung, Shih-Kuang Tung, and Chen Chin, Science, March 2, 2012, Vol. 335, No. 6072, pp. 1070-1072, and online Feb. in Science Express Feb. 16.

Provided by University of Chicago (news : web)

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IBM Scalable Quantum Computing

IBM Paves The Way Towards Scalable Quantum Computing

Alex Knapp, Forbes Staff

Three superconducting qubits. (Credit: IBM Research)

IBM has announced today that it’s achieved a breakthrough in its work to develop scalable quantum computing by developing a superconducting qubit made from microfabricated silicon that maintains coherence long enough for practical computation.

And now that I’ve thrown a ton of information at you in one tiny sentence, let’s break it all down. I had a chance to talk with IBM scientist Matthias Steffen about this new technology, and he broke it down for me. Let’s start with the qubit. Classical computing, as you probably know, is based on the bit. A bit can exist in one of two possible states, which are typically referred to as “0″ or “1″. A qubit is the equivalent of a bit for quantum computing. It can be in three possible states – “0″ or “1″ or both. The “both” state is known as the superposition. Now, the difference may seem subtle, but mathematically, it’s huge. A few hundred qubits can contain more classical bits of information than the the universe has atoms.

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What makes quantum computing challenging is the problem of decoherence. When a qubit is moved from the 0 state to either 1 or the superposition, it will decohere to state 0 due to interference from other parts of the computer. In order for quantum computing to be scalable and practical, the qubits have to be coherent for a long enough time that error-correction techniques can be employed to make sure that the decoherence doesn’t prevent accurate computation.

“In 1999, coherence times were about 1 nanosecond,” Steffen told me. “Last year, coherence times were achieved for as long as 1 to 4 microseconds. With these new techniques, we’ve achieved coherence times of 10 to 100 microseconds. We need to improve that by a factor of 10 to 100 before we’re at the threshold we want to be. But considering that in the past ten years we’ve increased coherence times by a factor of 10,000, I’m not scared.”

 

Alex Knapp Forbes Staff
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The IBM team has taken two approaches to quantum computing, both of which factor into the breakthroughs announced here. The first approach is building a 3-D qubit made from superconducting, microfabricated silicon. Steffen notes that the benefit of using silicon for these qubits is that the manufacturing equipment and know-how already exists – new techniques don’t have to be developed. 3-D qubits were pioneered by the Schoelkopf Lab at Yale, and Steffen expressed his admiration for that work. Building on the Yale techniques, the IBM team was able to maintain coherence for 95 microseconds. (“But you could round that to 100 for the piece if you want,” Steffen joked.)

How To Make A Cheaper Quantum Computer
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 The second approach involved a traditional 2-D qubit, which IBM’s scientists used to build a “Controlled NOT gate” or CNOT gate, which is a building block of quantum computing. A CNOT gate connects two qubits such that the second qubit will change state if the first qubit changes its state to 1. For example, if qubit A’s state is changed from 0 to 1, and qubit B’s state is 1, it will flip to state 0. But if qubit A’s state is changed from 1 to 0, qubit B is unaffected. That seems simple enough, but when you scale multiple logic gates like this together, you have a very real basis for computation. The CNOT gates were able to maintain coherence times of 10 microseconds, which is long enough to show a 95% accuracy rate. The previous accuracy record for CNOT gates was 81% accuracy, so this is a huge step.  Of course, Steffen was quick to note that there’s still a ways to go before this can be implemented as a computing solution. That makes common sense, since 95% is accurate, but in the long run you need the accuracy to be as close to 100% as possible. 
 

The Inner Workings of a Quantum von Neumann Computer Alex Knapp Forbes Staff

Given the rapid progress that IBM has made, scalable quantum computing is starting to look like a real possibility. As error-correction protocols improve and coherence times lengthen, accurate quantum computing becomes a real possibility. But don’t expect to have a quantum smartphone anytime soon using this technique. In order to get the results the IBM team has seen in either the 2-D or 3-D configuration, the qubits have to be cooled down to less than a degree above absolute zero.

“There’s a growing sense that a quantum computer can’t be a laptop or desktop,” said Steffen. “Quantum computers may well just being housed in a large building somewhere. It’s not going to be something that’s very portable.  In terms of application, I don’t think that’s a huge detriment because they’ll be able to solve problems so much faster than traditional computers.”

The next steps for the team is to improve coherence and error-correction protocols to the point where the accuracy is over 99.9%. That means they’ll have achieved a “logical qubit” – one that, for practical purposes, doesn’t experience decoherence. From that point, the next step is to develop a quantum computing architecture. IBM is considering some possibilities here, including developing some quantum memory architechture. But what encourages Steffen in these endeavors is that these are questions of engineering, not of theory.

“We are very excited about how the quantum computing field has progressed over the past ten years,” he told me. “Our team has grown significantly over past 3 years, and I look forward to seeing that team continue to grow and take quantum computing to the next level.”

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