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EMPIR project develops portable single photon sources along with new quantum measurement paradigm

Blue sparks of light

Developing quantum computers, communications and sensors requires the ability to generate, manipulate, and measure single photons

The ability to create sources of single photons would greatly advance many quantum-based technologies currently in development, such as quantum computing, quantum communications and enhanced imaging and sensing technologies. However, producing sources that emit one photon on demand with an adjustable repetition is not a simple task.

Projects within the European Metrology Research Programmes (EMRP and EMPIR) supported tackling this challenge.

The EMRP project Single-photon sources for quantum technologies (EXL02, SIQUTE) provided the first comprehensive metrological characterisation of a single-photon source based on nitrogen valency centre in diamonds.

Building on this the EMPIR project Single-photon sources as new quantum standards (17FUN06, SIQUST) developed a new suite of diamond single-photon emitters including ones implanted with ions such as tin (Sn), lead (Pb), silicon (Si) and  germanium (Ge). Single photon quantum dots were also investigated – including the first ever ones based on indium gallium arsenide (InGaAs) which were used to calibrate a single-photon detector.  From this work the project developed two portable single-photon sources, which as well as use in calibration, can also be used in quantum key distribution testbeds.

New quantum Protective Measurement method

In addition to the first standardised technique to characterise single-photon sources and how to provide common uncertainty estimation procedures the project also provided the first realisation of  a new quantum measurement paradigm with the potential to have a great impact in the quantum field.

At the quantum level a photon can exist in a ‘superposition’ of more than one state. Measuring a photon causes this ‘decoherence’ to collapse into one state or the other. In other words, measuring a quantum system affects the outcome of the system. The result is that it is necessary to detect a statistically significant number of photons to obtain a faithful estimate of the measured observable of interest.

The new method takes advantage of a phenomena termed the quantum Zeno effect. This predicts that if a series of repeated measurements are made quickly and smoothly enough then this preserves superposition rather than collapsing it.

In the new quantum ‘protective measurements’ paradigm a single polarised photon is ‘prepared’ and sent through a series of instrumentation, such as birefringent crystals and polarising plates. The photon interacts weakly with each, undergoing an ‘interaction-interference plus selection’ process. Providing it survives it emerges as a new ‘prepared’ coherent photon. As the knowledge of the surviving photon’s characteristic is known (e.g. polarisation) this acts as an indirect observable of interest giving information of the photon state without a direct measurement, and simply exploiting the information of its position in the x/y plane provides a ‘pointer’ on the state.

The photon can, theoretically, be sent through an infinite number of these ‘selection’ stages - gaining knowledge on its state at each stage.

This is the first realisation of protective measurements, demonstrating its capability to preserve the system state coherence, and at the same time, extract the expectation value of an observable from a single photon measurement event.
Although the new paradigm is only at the proof-of-principle stage this work paves the way for its use in quantum metrology and sensing.

In addition, the project published 46 open access articles, presented at 121 conferences and provided 9 training courses.

Work has continued in the project Single- and entangled photon sources for quantum metrology (20FUN05, SEQUME) which focused on protocols and instrumentation required for measurements at the single-photon level.

The work of these EURAMET projects is expected to strengthen Europe’s position in the field of quantum technologies and stimulate new developments in areas such as cryptography, communication, astronomy and health, and contribute to the improvement of the mise en pratique for the SI unit of light, the candela.

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

The EMRP joint research project was part of EURAMET’s European Metrology Research Programme. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

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