Quantum nondemolition detection of microwave photons in three papers

Short answer: A quantum nondemolition (QND) measurement reads out a quantity without destroying it, so the measurement can be repeated. For microwave photons, three papers cover a 2010 on-chip photon-number detector that the authors call 90 percent QND, a 2014 proposal for detecting single travelling photons, and a 2018 experiment on propagating photons with 50 percent external detection fidelity. These are laboratory results, and the 2014 paper is theory.

What is QND photon detection?

The 2010 Nature Physics abstract says many measurements are destructive, removing more information from a system than they obtain. QND measurements allow repeated measurements that give the same eigenvalue, and the authors list possible uses: error correction, preparation by measurement and one-way quantum computing. They say QND measurement of photons is especially challenging because the detector must be completely transparent to the photons while still acquiring information about them.

What did the 2010 experiment show?

The authors demonstrate a QND detection scheme that measures the number of photons inside a high-quality-factor microwave cavity on a chip. The scheme maps the photon number onto a qubit state in a single shot, using qubit-photon logic gates. They verify its operation for zero and one photons by analysing average correlations of repeated measurements, and show it is 90 percent QND.

What did the 2014 proposal say?

The 2014 Physical Review Letters paper is theoretical. The authors note that nondestructively detecting a single travelling photon has been a long-standing goal, complicated by the weakness of photon-photon interactions, though very strong effective interactions in a waveguide have recently been shown at microwave frequencies. They propose using this to realise a QND measurement of a single propagating microwave photon, with a chain of three-level transmons cascaded through circulators that suppress backscattering. Their analysis suggests detection with fidelity around 90 percent can be realised with existing technologies. That figure is a calculation, not a measurement.

What did the 2018 experiment show?

The 2018 Physical Review X paper realises a QND detector for propagating microwave photons and characterises it with a single-photon source. A cavity-assisted conditional phase gate acts between the incoming photon and a superconducting artificial atom, whose state is read out in a single shot. The authors reach an external photon-detection fidelity of 50 percent and an internal fidelity of 71 percent, limited by the transmission efficiency between source and detector (75 percent) and by the coherence of the qubit. They demonstrate the QND nature by characterising the coherence and average photon number of the field reflected off the detector. They envisage heralded remote entanglement between qubits and logic gates between propagating photons, which are stated prospects.

How to read this

The 2010 device counts photons trapped in a cavity, while the 2014 and 2018 papers concern photons travelling in a line, which is a harder task. External fidelity includes transmission losses, and internal fidelity does not. This post is based on abstracts, and full texts were not checked. It makes no claim about products.

FAQ

What does QND mean?

A measurement that can be repeated and gives the same result, per the 2010 abstract.

What did the 2018 experiment achieve?

External detection fidelity of 50 percent and internal fidelity of 71 percent for propagating microwave photons.

Was the 90 percent figure measured?

In 2010 it is a measured QND figure for cavity photons, and in 2014 it is a theoretical estimate.

What are the stated uses?

Error correction, remote entanglement and photon logic gates, as prospects.