Sunday, November 24, 2013

NIST demonstrates how losing information can benefit quantum computing

NIST demonstrates how losing information can benefit quantum computing

BOULDER, Colo -- Suggesting that quantum computers might benefit from losing some data, physicists at the National Institute of Standards and Technology (NIST) have entangled—linked the quantum properties of—two ions by leaking judiciously chosen information to the environment.

Researchers usually strive to perfectly shield ions (charged atoms) in quantum computing experiments from the outside world. Any "noise" or interference, including heat generated by the experiment and measurements that cause fragile quantum states to collapse, can ruin data and prevent reliable logic operations, the conventional approach to quantum information processing.

Turning bug into feature, a collaboration of physicists from NIST and the University of Copenhagen in Denmark decided to think and work outside the box. They cleverly linked the experiment to the outside world to establish and maintain the entanglement of two ions. Entanglement is a curious feature of the quantum world that will be necessary to process and transport quantum data or correct errors in future quantum computers.

The new research is described in a Nature paper posted online Nov. 24,* along with similar work at Yale University using superconducting circuits.

"These new methods might be used to create entangled states that would be a resource in a traditional, logic-based quantum computer," NIST postdoctoral researcher John Gaebler says. "But there are also alternative architectures in which, for example, one couples a quantum computer to a specific noise environment and the resulting state of the computer contains the solution to the target problem."

The NIST experiments used two beryllium ions as quantum bits (qubits) to store quantum information and two partner magnesium ions, which were cooled with three ultraviolet laser beams to release heat.

The qubits were entangled by two ultraviolet laser beams and induced to "leak" any unwanted quantum states to the environment through continuous application of microwaves and one laser beam. The unwanted data were coupled to the outgoing heat in such a way that the qubits were left in only the desired entangled state—which happens to be the point of lowest motional energy, where no further heat and information is released to the environment.

Unlike a logic operation, the process can be started from any state of the ions and still yield the same final state. The scheme also can tolerate some kinds of noise that might cause a traditional logic gate to fail. For instance, the lasers and microwaves had no negative effects on the target entangled state but reshuffled any unwanted states.

All operations applied at the same time quickly drove the two qubits into a specific entangled state and kept them in that state most of the time. The qubits approached the target state within a few milliseconds and were found to be in the correct entangled state 75 percent of the time. The qubit state deteriorated slightly over longer times as the qubits were disturbed by errant laser emissions. By applying about 30 repetitions of the four steps in a particular order, scientists boosted the success rate to 89 percent in a separate experiment.

Co-authors of the paper include two collaborators at QUANTOP, The Niels Bohr Institute, University of Copenhagen. The work was supported in part by the Intelligence Advanced Research Projects Activity, Office of Naval Research, and the European Union's Seventh Framework Program.

* Y. Lin, J.P. Gaebler, F. Reiter, T.R. Tan, R. Bowler, A.S. Sorensen, D. Leibfried and D.J. Wineland. Dissipative production of a maximally entangled steady state. Nature. Posted online Nov. 24, 2013.

Sidebar: How Lost Data Generates Entanglement

The NIST process for using lost data to generate entanglement works like this: Two ultraviolet laser beams entangle the two ion qubits' internal "spins," analogous to tiny bar magnets pointing up or down. The lasers are carefully tuned to couple the ions' synchronized, back-and-forth sideways motion to their spins, entangling this motion with the spins.

The spins have three possible correlations: Both qubits spin up, both spin down, or one is up and one is down. The desired entangled state is a superposition of spins up-down and down-up at the same time. Superposition is another special feature of the quantum world. A measurement of this state with another special-purpose laser beam causes quantum states to collapse, resulting in spins up-down, or the opposite, spins down-up. Such measurements are made by detecting light signals; spin up scatters laser light, whereas spin down does not.

If the two spins are in the desired entangled state and the lowest motional energy state, they are unaffected by all laser and microwave fields. But microwaves and one ultraviolet laser beam reshuffle all other spin states and at the same time boost the qubits to an intermediate state with higher motional energy. This energy is then removed from the qubits by three cooling laser beams applied to the magnesium ions. This continuous feedback loop alters the qubits spins until they settle into the entangled state that is no longer affected by the driving fields.

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Friday, November 15, 2013

Overcoming a Key Barrier Towards Building Ultrafast Quantum Computers

Overcoming a Key Barrier Towards Building Ultrafast Quantum Computers

15 November 2013

A normally fragile quantum state has been shown to survive at room temperature for a world record 39 minutes, overcoming a key barrier towards building ultrafast quantum computers.

[caption id="attachment_196" align="aligncenter" width="500"]Overcoming a Key Barrier Towards Building Ultrafast Quantum Computers Overcoming a Key Barrier Towards Building Ultrafast Quantum Computers[/caption]

An international team including Stephanie Simmons of Oxford University, UK, report in this week’s Science a test performed by Mike Thewalt of Simon Fraser University, Canada, and colleagues. In conventional computers data is stored as a string of 1s and 0s. In the experiment quantum bits of information, ‘qubits’, were put into a ‘superposition’ state in which they can be both 1s and 0 at the same time – enabling them to perform multiple calculations simultaneously.

In the experiment the team raised the temperature of a system, in which information is encoded in the nuclei of phosphorus atoms in silicon, from -269 °C to 25 °C and demonstrated that the superposition states survived at this balmy temperature for 39 minutes – outside of silicon the previous record for such a state’s survival at room temperature was around two seconds. The team even found that they could manipulate the qubits as the temperature of the system rose, and that they were robust enough for this information to survive being ‘refrozen’ (the optical technique used to read the qubits only works at very low temperatures).

‘39 minutes may not seem very long but as it only takes one-hundred-thousandth of a second to flip the nuclear spin of a phosphorus ion – the type of operation used to run quantum calculations – in theory over 20 million operations could be applied in the time it takes for the superposition to naturally decay by one percent. Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer,’ said Stephanie Simmons of Oxford University’s Department of Materials, an author of the paper.

‘This opens up the possibility of truly long-term coherent information storage at room temperature,’ said Mike Thewalt of Simon Fraser University.

The team began with a sliver of silicon doped with small amounts of other elements, including phosphorus. Quantum information was encoded in the nuclei of the phosphorus atoms: each nucleus has an intrinsic quantum property called ‘spin’, which acts like a tiny bar magnet when placed in a magnetic field. Spins can be manipulated to point up (0), down (1), or any angle in between, representing a superposition of the two other states.
The team prepared their sample at just 4 °C above absolute zero (-269 °C) and placed it in a magnetic field. Additional magnetic field pulses were used to tilt the direction of the nuclear spin and create the superposition states. When the sample was held at this cryogenic temperature, the nuclear spins of about 37 per cent of the ions – a typical benchmark to measure quantum coherence – remained in their superposition state for three hours. The same fraction survived for 39 minutes when the temperature of the system was raised to 25 °C.

‘These lifetimes are at least ten times longer than those measured in previous experiments,’ said Stephanie Simmons. ‘We've managed to identify a system that seems to have basically no noise. They're high-performance qubits.’

There is still some work ahead before the team can carry out large-scale quantum computations. The nuclear spins of the 10 billion or so phosphorus ions used in this experiment were all placed in the same quantum state. To run calculations, however, physicists will need to place different qubits in different states. ‘To have them controllably talking to one another – that would address the last big remaining challenge,’ said Simmons.

For more information contact Stephanie Simmons of Oxford University on mobile; +44 (0)7823 333960 or email


An artistic rendition of a 'bound exciton' quantum state used to prepare and read out the state of the qubits [credit: © 2013 Stef Simmons with CC BY]:

A phosphorus atom qubit in silicon can preserve quantum information for over 3 hours at cryogenic temperatures or 39 minutes at room temperature [credit: © 2013 Karl G. Nyman with CC BY]:

Alternatively contact the University of Oxford Press Office on +44 (0)1865 283877 or email


  • A report of the research, entitled ‘Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28’, is published in this week’s Science.

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