Efficient light-controlled quantum gates in semiconductors with single spins

Image: Quantum gates on a single electron spin in diamond are implemented by changing the characteristics of a two-color optical pulse illuminating the electron. The color plot shows the simulated time-dependent light emission from the diamond sample and represents a phase space for the continuously tunable quantum gates G, visualized as a rotation of the spin.

Quantum computing uses the unique properties of quantum bits – or “qubits” –  to tackle problems intractable by ordinary computers. Like classical computing, quantum computing requires performing operations, or “gates,” on individual or pairs of bits. However, qubits can be subject to an infinite array of operations and quickly lose their information with time, making implementing quantum gates challenging. Now, researchers at the Institute for Molecular Engineering have demonstrated the ability to perform any single-qubit gate on the quantum state of an electron by operating with only one extremely fast pulse of light.

A lone classical bit has just one possible operation: flip 0 to 1 or 1 to 0. Individual qubits, on the other hand, have a continuous range of states. “If a coin is like a qubit, we not only have the option to flip it, but we can rotate it to any orientation. Each rotation represents a distinct quantum gate and requires performing a different experimental control,” explained Professor David Awschalom.

In their demonstration, the researchers’ quantum “coin” was a single electron trapped by a nitrogen-vacancy (NV) center in diamond, which occurs when a single nitrogen impurity sits next to a missing carbon atom in the lattice. As well as charge, the localized electron carries “spin”, a quantum variable that characterizes its intrinsic rotation. The quantum nature of the electron allows it to simultaneously rotate both clockwise and anticlockwise, occupying a so-called spin “superposition” state.

The researchers were able to implement a full range of quantum gates to rotate the spin superposition by applying a single well-controlled laser pulse containing two frequencies or “colors”. The key discovery lies in controlling the amplitude and phase of each color in the pulse, as well as its frequency at a resolution better than one part per billion. This frequency control enabled arbitrary quantum gates to be implemented in a single pulse, rather than in two pulses as in previous protocols. “Optical control offers clear advantages: we achieve significantly better spatial resolution and lower power consumption, which are important for scaling to larger quantum devices,” commented Brian Zhou, a postdoctoral researcher in the Awschalom group.

The IME team, including graduate student Paul Jerger and research scientist F. Joseph Heremans, collaborated with Professor Guido Burkard’s group at the University of Konstanz to theoretically interpret the experiment. “In the future, we envision performing all-optical multiple-qubit gates and generalizing our method to efficiently control quantum systems with similar structure to the NV center,” said Paul Jerger. The team’s findings were recently reported in Physical Review Letters (Oct. 2017), accessible at https://doi.org/10.1103/PhysRevLett.119.140503.

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