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The MacroPhoton is an interactive, quantum physics simulation which uses the concept of photons scaled to the macroscale to demonstrate the potential of quantum mechanics for uncompromisingly secure, encrypted communications.

Visitors to the MacroPhoton exhibit are introduced to the concept of light polarisation and the fact that in general, apart from very specific cases, light is irreversibly changed by passing through a polariser. Via this, visitors are also introduced to the idea that, at the quantum light level of photons, Nature introduces randomness into the results if we simply measure photons in a polarisation that we choose. Once visitors are aware of these concepts, they can use the MacroPhoton exhibit to understand how these can be utilised to generate a secret shared string of bits (1s and 0s) in two remote locations and then how this is used to enable quantum secured communications in the real world.  

The MacroPhoton consists of three small, portable units with large arcade-style buttons, illuminated by LED lights. The units include: a sender unit, “Alice”; a receiver unit, “Bob”; and a third unit, “Eve”, which represents a potential eavesdropper. The demonstration also includes up to 16 small poker chip-like disks representing the photons that Alice is sending to Bob, but scaled up – literally macro-photons. The tokens have a small amount of memory that can be encoded using any of the three units.

During the simulation, Alice uses her unit to encode a polarisation of her choice (horizontal/vertical or diagonal/anti-diagonal) and a bit value (0 or 1) onto a macrophoton. Eve, the eavesdropper, uses her unit to measure the polarisation (of her choice, because she does not know what Alice chose) of an intercepted macrophoton –  giving her the correct bit if she guesses Alice’s polarisation correctly and a random bit otherwise – before re-encoding the photon with values of her own choosing. The receiver, Bob, also uses his unit to measure the polarisation of his choice for each macrophoton – again giving him the correct bit if he correctly chooses consistently with Alice and a random bit otherwise.

Once the encoding of disks has taken place, each unit displays a record of the visitors’ choices and measurements allowing the participants in the roles of “Alice” and “Bob” to generate their shared bit string (comparing only their polarisation choices, but not their bit values) as well as to detect whether “Eve” was indeed up to no good and trying to eavesdrop (comparing a small sample of their bit values).

The MacroPhoton was developed as a joint investment by the Quantum Communications Hub and the School of Engineering and Physical Sciences at Heriot-Watt University. It has been demonstrated at a variety of outreach events involving thousands of participants across all age ranges and education backgrounds. Since its inception in 2018, it has been met with an enthusiastic response and positive feedback throughout, winning the Principal’s Public Engagement Prize in the Public Engagement Partnership category from Heriot-Watt University in 2019.

A video of how the demonstration works can be viewed here: