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Ultrafast logic gate made from quantum dot

Posted: 03 Apr 2013     Print Version  Bookmark and Share

Keywords:optical communications  quantum dot  Controlled-NOT  qubit 

Inside a computer processor there are billions of transistors switching back and forth between two states. In optical communications, data from these switches are encoded onto light that travels along glass fibre. Joint Quantum Institute (JQI) researchers together with the Department of Electrical and Computer Engineering are studying ways on how to control the quantum nature of light and semiconductors to develop further the capabilities of computers.

All computers, even the future quantum versions, use logic operations or "gates" that are the fundamental building blocks of computational processes. JQI scientists, led by Professor Edo Waks, have performed an ultrafast logic gate on a photon, using a semiconductor quantum dot.

Photons are a proven transit system for information. In quantum devices, they are the ideal information carriers that relay messages between quantum bits (qubits) such as neutral atoms, ion traps, superconducting circuits, nitrogen vacancy centres, and of course the device used here: quantum dots. A quantum dot (QD) is a semiconductor structure that acts like an atom. This means it has allowed energy levels that can be populated and even shifted around using lasers and magnetic fields. Quantum dots are an attractive platform for quantum processing because they live inside a semiconductor material, thus the technology for integration with modern electronics already exists.

Waks' team has implemented a conditional logic gate called a Controlled-NOT (CNOT).

Here's how a generic CNOT gate works: If a control qubit is in what we will call state 1, then the gate flips the state of a second qubit. If the control qubit is in state 0, nothing happens.

Waks explained the importance of this gate, "Although this logic operation sounds simple, the CNOT gate has the important property that it is universal, which means that all computational algorithms can be performed using only this simple operation. This powerful gate can thus be seen as important step towards implementing any quantum information protocol."

In this experiment, a quantum dot plays the role of the control qubit. The second qubit is a photon that has two polarization states. Polarization can be thought of as an orientation of the traveling light waves. For instance, polarized sunglasses can shield your eyes from light having certain orientations. Here, photons can be oriented horizontally or vertically with respect to a defined direction. Just like energy levels for a quantum dot constitute a qubit, the two available polarizations make up a photonic qubit.

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