Posted in | Quantum Physics

Accelerated Dynamics of Quantum Particles Enable Rapid Switching of Quantum Bit

Intricately shaped pulses of light pave a speedway for the accelerated dynamics of quantum particles, enabling faster switching of a quantum bit. Credit: Peter Allen

Today’s technology, ranging from laptops to cellphones, advances through the continuously increasing speed at which electric charges are directed through circuits. Likewise, the speeding up control over quantum states in nanoscale and atomic systems could lead to leaps for the growing field of quantum technology.

Recently, a new framework for faster control of a quantum bit was demonstrated by a global collaboration between physicists at the University of Chicago, Argonne National Laboratory, McGill University, and the University of Konstanz. Their experiments were initially published online on Nov. 28, 2016, in Nature Physics. These experiments on a single electron in a diamond chip led to the development of quantum devices that are less prone to errors when made to work at high speeds.

Accelerating Quantum Dynamics

Their experiment can be understood by considering the ultimate setting for speed in classical dynamics: the oval racetracks at the Indianapolis or Daytona 500. The pavement of the racetrack is "banked" by up to 30 degrees in order to enable the racecars to navigate the turns at excellent speeds.

A student in Newtonian mechanics will be able to explain that this inward slope of the pavement permits the normal force provided by the road to assist in cancelling the car's centrifugal acceleration, or its tendency to outwardly slide from the turn. Increased speed results in the requirement of a greater bank angle.

The dynamics of quantum particles behave analogously. Although the equations of motion are different, to accurately change the state of a quantum particle at high speeds, you need to design the right track to impart the right forces.

Aashish Clerk, Professor of Theoretical Physics, McGill University

A new method to enable faster quantum dynamics by precisely absorbing detrimental accelerations felt by the quantum particle was formulated by Clerk in collaboration with McGill postdoctoral fellows Alexandre Baksic and Hugo Ribeiro. These accelerations, unless compensated, would redirect the particle from its intended trajectory in the space of quantum states, very much like how the centrifugal acceleration deflects the racecar from its proposed racing line on the track.

David Awschalom, professor in spintronics and quantum information at the Institute for Molecular Engineering in the University of Chicago, studied the possibility of using the new theory to speed up the diamond-based quantum devices in his labs. However, experimentally executing the control sequences predicted by Clerk and co-workers presented challenges in quantum engineering just like how constructing the banked speedways presented challenges in civil engineering.

The quantum fast track was built by shining intricately-shaped, synchronized laser pulses on single electrons that were trapped at defects present inside their diamond chips. This experimental achievement was achieved by lead author Brian Zhou, working with Christopher Yale, F. Joseph Heremans, and Paul Jerger.

"We demonstrated that these new protocols could flip the state of a quantum bit, from 'off' to 'on,' 300% faster than conventional methods," said Awschalom, also a senior scientist at Argonne National Laboratory. "Shaving every nanosecond from the operation time is essential to reduce the impact of quantum decoherence," he explained, referring to the process by which quantum information is lost to the environment.

Professor Guido Burkard and Adrian Auer from the University of Konstanz collaborated with the Awschalom and Clerk groups to analyze the data from the experiments.

What is promising for translating these techniques beyond the laboratory is that they are effective even when the system is not perfectly isolated.

Guido Burkard, Professor, University of Konstanz

The researchers predict that their techniques can be further used for fast and accurate control over the transfer of quantum states between different systems or the physical motion of atoms. These techniques according to them will transmit benefits to quantum applications, such as secure communications and simulation of complex systems.

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