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This interactive demonstration, created by Robert Woodward, former Hub research associate, uses a Raspberry Pi connected with LEDs, switches and light-dependent resistors to clearly explain the principle behind measurement-device-independent quantum key distribution (MDI QKD).

MDI QKD is a novel quantum communication protocol which allows two users (“Alice” and “Bob”) to communicate with perfect security, even if the (measurement) hardware they are using has been tampered with (e.g. by an eavesdropper). This is achieved by measuring correlations between signals from Alice and Bob, rather than the actual signals themselves.

In practice, we use polarisation or phase as the property of light that our two users change to indicate a bit 0 or bit 1. For illustrative purposes, this demonstration uses colour instead: a blue particle of light = bit 0 & and red packet of light = bit 1. The aim of MDI QKD is to generate a random string of bits for both Alice and Bob that only they know. They can then use this key to encrypt messages they send to each other, knowing that only they share the keys to encrypt/decrypt the message and this “correspondence” cannot be compromised, even if an eavesdropper has unlimited computational power (e.g. an advanced quantum computer).

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