APS News

July 2017 (Volume 26, Number 7)

Decoherence Eases Bit by Bit

By Rachel Gaal

2017 APS Division of Atomic, Molecular, and Optical Physics Meeting — Between a binary bit and a quantum bit, cryptographers and code crackers alike would choose the quantum route every time. The ability of quantum systems, making use of entities called qubits, to carry out powerful computations through superposition and entanglement, is compelling. But it has also driven physicists up the wall trying to implement them in actual applications. Correcting quantum errors in particular poses a large problem because of the sensitivity of these bits to noise and loss of coherence.

"We all know there is a problem, that quantum information processing suffers from decoherence … [with] a loss of quantum information due to interaction of a system and environment," explained Florentin Reiter of Harvard University at the 2017 DAMOP Meeting in Sacramento, California. Formerly a postdoc at the University of Innsbruck, he has taken on a common problem that most quantum systems face: dissipation, or loss of energy. "There are some people who came up with the idea to do quantum information not against dissipation, but quantum information by dissipation," Reiter said. In effect, dissipation drives the system to a desired outcome.

To deal with errors, most correction schemes map the information of one qubit onto highly entangled multiple qubits whose coherence has to be carefully maintained. By intentionally coupling a qubit to a specifically designed dissipative environment, surprisingly, several advantages like enhanced coherence lifetime can be achieved. "The goal [here] is to realize dissipation, without the use of time-dependent unitary gates, and without use of classical measurements," said Reiter.

To fully achieve this, Reiter and his team created a combined correction system that harnesses both dissipation and a trapped-ion scheme, a well-known workhorse in the quantum computing world.

They start with three trapped ions, each one storing a single qubit in the ions’ stable electronic states. The dissipative error correction acts like a watchdog to correct any random errors on the qubits that occur during information processing, such as spin flips. This scheme immensely improved the coherence of quantum system, meaning enhanced precision for quantum measurements.

"A single qubit [is known to] decay exponentially, whereas if we use three decaying qubits, and use error correction on top of that, it can stabilize very well, close to [a fidelity] of 0.9," Reiter presented. "We've achieved a suppressed decay rate, and increased lifetime … a huge improvement for quantum information processing."

The challenge of implementing these types of innovative strategies has recently led to novel theoretical proposals for superconducting circuits, one of which was presented by Yiwen Chu of Yale University.

"These systems do a lot of nice things, ranging from making complex circuits, doing error correction, [or] using fancy states of microwave resonators," Chu explained. "But unfortunately the things we [quantum researchers] are most infamous for is convincing people to spend a lot of money on what may or may not be a quantum computer."

Chu explained that despite the typical challenges faced in quantum computing today, like decoherence, she wants to create a quantum system that is useful for long distance quantum communication. "I believe that one of the possible solution is to use a hybrid system, basically [to] couple a quantum circuit to a mechanical oscillator," Chu said.

"To generate more complex states, like Schrodinger cat states, you need a source of nonlinearity," said Chu. "That’s where the qubit comes in … we have still been looking for a more robust, scalable, easily fabricated system that allows us to increase the complexity and performance of these kinds of devices." The team’s approach is to use a small acoustic resonator coupled to the qubit.

"[The] mechanical resonator in our system is basically the sapphire substrate where we base our qubits," said Chu as she showed a figure of their experimental setup. "When you think about the two polished faces of the sapphire wafer, it forms an enclosed cavity for bulk acoustic waves."

Chu admitted that her system was "embarrassingly simple," and agreed that there is significant room for improvement, but the first steps to a quantum acoustic platform have opened doors that could lead to enhanced coherence and strong coupling interactions in the system. By re-imagining the limits of quantum physics, the results may help qubits become the basis for powerful computers.

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Editor: David Voss
Staff Science Writer: Rachel Gaal
Contributing Correspondent: Alaina G. Levine
Publication Designer and Production: Nancy Bennett-Karasik