Thursday, May 22, 2014

Quantum manipulation: Filling the gap between quantum and classical world

Quantum manipulation: Filling the gap between quantum and classical world

Quantum superposition is a fundamental and also intriguing property of the quantum world. Because of superposition, a quantum system can be in two different states simultaneously, like a cat that can be both "dead" and "alive" at the same time. However, this anti-intuitive phenomenon cannot be observed directly, because whenever a classical measuring tool touches a quantum system, it immediately collapse into a classical state. On the other hand, quantum superposition is also the core of quantum computer's enormous computational power. A quantum computer can easily break the widely used RSA (Rivest, Shamir and Adleman) security system with Shor's algorithm. But for now, quantum computation still suffers from the decoherence induced by environment. Obviously, the key to manipulate a quantum system is to make it stay coherent as long as possible, to achieve this, one need to isolate the system from its environment. "For ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems", Serge Haroche and David Wineland won the 2012 Nobel Prize in Physics.

[caption id="attachment_307" align="aligncenter" width="400"]Quantum manipulation: Filling the gap between quantum and classical world Quantum manipulation - Filling the gap                                         between quantum and classical world[/caption]

This review begins by introducing the interesting property of quantum superposition, explaining its physical meaning, potential applications and main obstacles ahead. Then the author goes on to introduce the work of the two 2012 Nobel Prize Laureates – Serge Haroche and David Wineland. Instead of manipulating a neutral atom or a photon, Wineland and his team focused on controlling a charged atom, the ion, in an electromagnetic well. In order to break the limit of Doppler cooling, a new cooling technique – Side-Band cooling was used to reach extreme low temperature. The well cooled ions made an ideal platform for building optical clock and quantum computer. Since 2001, Wineland and his team had realized several optical clocks with very high precision. They had also realized basic quantum logic gate in ion trap and demonstrated the scalability of ion system, proving their system is promising for practical quantum computation. This article covers the above topics and gives detailed review.

In the fourth section, the author introduces the work of Haroche and his collaborators. Haroche et al managed to build a high-Q microwave cavity with superconducting materials and cooled it down to superconducting phase. According to Meissner effect, photons in the cavity cannot penetrate the superconducting mirror and will be trapped inside, thus isolate the photons from its environment. Since the cavity has extremely high-Q, the Rydberg atoms inside the cavity are strongly correlated to the photon field, which makes a perfect platform for testing the fundamental principles of quantum mechanics. With the aid of quantum non-demolition measurement, quantum processes can be observed without destroying the state. Using this platform, Haroche et al had directly observed decoherence, quantum jump and several other quantum information processes.

Finally, the review introduces recent developments and further applications of quantum manipulation, and then ends with a discussion of the relationship between quantum and classical world. With advanced quantum manipulation techniques, people are able to investigate fundamental quantum mechanics. In return, a better understanding of quantum mechanics makes it possible to develop new technologies that will change our classical world.



A new epoch of quantum manipulation. Yongjian Han, Zhen Wang, and Guang-Can Guo Natl Sci Rev (March 2014) 1 (1): 91-100 DOI:10.1093/nsr/nwt024


News Release Source : Quantum manipulation: Filling the gap between quantum and classical world

Thursday, May 8, 2014

Superconducting qubit array points the way to quantum computers

Superconducting qubit array points the way to quantum computers

A new 5-qubit array from UCSB's Martinis Group is on the threshold of making a quantum computer technologically feasible to build

A fully functional quantum computer is one of the holy grails of physics. Unlike conventional computers, the quantum version uses qubits (quantum bits), which make direct use of the multiple states of quantum phenomena. When realized, a quantum computer will be millions of times more powerful at certain computations than today's supercomputers.

[caption id="attachment_301" align="alignleft" width="400"]Superconducting qubit array points the way to quantum computers These are control signals for all five qubits.[/caption]

A group of UC Santa Barbara physicists has moved one step closer to making a quantum computer a reality by demonstrating a new level of reliability in a five-qubit array. Their findings appear Thursday in the journal Nature.

Quantum computing is anything but simple. It relies on aspects of quantum mechanics such as superposition. This notion holds that any physical object, such as an atom or electron — what quantum computers use to store information — can exist in all of its theoretical states simultaneously. This could take parallel computing to new heights.

"Quantum hardware is very, very unreliable compared to classical hardware," says Austin Fowler, a staff scientist in the physics department, whose theoretical work inspired the experiments of the Martinis Group. "Even the best state-of-the-art hardware is unreliable. Our paper shows that for the first time reliability has been reached."

While the Martinis Group has shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error. "Qubits are faulty, so error correction is necessary," said graduate student and co-lead author Julian Kelly who worked on the five-qubit array.

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

The unique configuration of the group's array results from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named Xmons. Superconductivity results when certain materials are cooled to a critical level that removes 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 John Martinis, a professor in UCSB's Department of Physics. "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."

According to Martinis, it was Fowler's surface code that pointed the way, providing an architecture to put the qubits together in a certain way. "All of a sudden, we knew exactly what it was we wanted to build because of the surface code," Martinis said. "It took a lot of hard work to figure out how to piece the qubits together and control them properly. The amazing thing is that all of our hopes of how well it would work came true."

News Release Source :  Superconducting qubit array points the way to quantum computers