The MacroPhoton

Conceived in 2018 by Heriot Watt researcher Dr Robert Collins, the MacroPhoton is a demonstrator using the concept of photons scaled to the macroscale to demonstrate how quantum secure communications work. It is an interactive, easily transportable teaching aid using the duality of photons (light particles) and light polarisation to highlight the potential of quantum mechanics for uncompromisingly secure, encrypted communications.

The MacroPhoton consists of a sender unit (“Alice”) in the form of a small, portable suitcase with big bright buttons in primary colours and flashing lights, representing a laser (photon source). Alongside Alice, there are eight equally portable, glossy plastic receiver units, each approximately the size of a fizzy drink can, representing the photons that Alice is sending but scaled up – literally macro-photons. Communication scenarios are enacted involving two (or more) people performing the role of sender and receiver. The person handling the Alice unit is asked to make a series of eight random choices by choosing a light polarisation state (linear or diagonal) and a bit state (0 or 1) each time and pressing the encoding button in the middle of the unit to wirelessly send part of the secret key as encoded information to the receiver. A second person, representing the receiver (“Bob”) is then being handed the MacroPhoton units, one by one, and is asked to make an equally random choice regarding the polarisation, i.e. how to measure the encoded information that is being received. At the end of the experiment, Alice is asked to reveal to Bob the eight polarisation states used each time and results between the two parties are compared. Due to quantum effects such as the uncertainty principles and superposition, the two parties will have randomly chosen the same option some times (e.g. both chose linear). By comparing the screens of Alice and the “correctly encoded” macro-photons, Alice and Bob can see that they have correctly transmitted parts of the secret key as 0s and 1s in the case where they made the same choice of polarisation state. The rest of the macro-photons, where different polarisations states were used, are then discarded.

Since its inception, the MacroPhoton has been used in a number of outreach events involving thousands of participants across all age ranges and education backgrounds. It has been met with enthusiastic response and hugely positive feedback throughout and in early 2019, it won the Heriot Watt University Principal’s Public Engagement Prize in the Public Engagement Partnership category.

The MacroPhoton was developed as a joint investment by the Quantum Communications Hub and the School of Engineering and Physical Sciences, Heriot Watt University.

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