Centre updates

Feynman inspires new CQC2T Nature Communications paper

UNSW physics researcher Sam Gorman

Director of CQC2T, Scientia Professor Michelle Simmons said her team’s approach to building a quantum computer “from the ground up, atom by atom” is inspired by physicist Richard Feynman who said: ‘what I cannot create, I do not understand’. Centre researchers create their atom qubits by precisely positioning and encapsulating individual phosphorus atoms within a silicon chip. Information is stored on the quantum spin of a single phosphorus electron. Simmons’ team use a scanning probe to directly measure the atom’s wave function to show the exact physical location in the chip. “We are the only group in the world who can actually see where our qubits are,” said Prof Simmons.

In the new paper, the team show they can control the interactions between two of these atom qubits so the quantum spins of their electrons become correlated. Building on two other recent results, these three papers collectively confirm the extremely promising prospects for building multi-qubit systems using Centre atom qubits.

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CQC2T demonstrates Bell inequality with Light Wave

Centre research associate Oliver Thearle in the ANU Quantum Optics lab

A new CQC2T paper in Physical Review Letters has demonstrated the first observation of Bell correlations in a continuous variable system, thereby showing the strength of photon number correlations when inferred through homodyne measurements.

Lead author from ANU Oliver Thearle said the paper’s significance is in the fact that “it is the first demonstration of a Violation of Bell’s inequality using light fields as opposed to photon counting as was originally proposed by Bell. This is possible through the wave particle duality of light”.

Violation of Bell inequality is a fundamental test to rule out local hidden variable model descriptions of correlations between two physically separated systems. There have been a number of experiments in which a Bell inequality has been violated using discrete-variable systems in recent years. The ANU-led Centre team demonstrated a violation of Bell’s inequality using continuous variable quadrature measurements. This means that the wave nature of light also leads to the same conclusion that local hidden variable is an insufficient description of reality.

By creating a four-mode entangled state with homodyne detection, they recorded a clear violation with a Bell value of B = 2.31±0.02, where B ≤ 2 validates local hidden variables. This opens new possibilities for using continuous variable systems for a number of quantum communication applications, such as a source independent quantum random number generator.

Centre Chief Investigator Prof Ping Koy Lam from the ANU said, “Our experiment using bright laser beams complements very well with the recent successful loophole free Bell test demonstrations in the discrete variable regime.”

Four CQC2T Chief Investigators from the ANU and the University of Queensland collaborated on this paper.

Full paper is here