id Quantique’s Optical Platform: Plug-&-Play QKD

The SwissQuantum network uses id Quantique’s Plug-&-Play auto-compensating set-up. Whereas in the Raw Key Exchange section, the principle of this exchange is explained using single photons, note that the Plug-&-Play implementation uses attenuated laser pulses. Attenuated laser pulses are not single photons, but are a very good approximation if their optical intensity is very weak.

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Autocompensated Plug&Play Quantum Cryptography Set-up

The Plug-&-Play optical platform is well known for its robustness against polarization and phase fluctuations thanks to its auto-compensation property [1]. Qubits are encoded in phase. To implement the BB84 and SARG protocols, Alice, the emitter, applies a phase shift of 0 or π and π/2 or 3π/2. Bob, the receiver, chooses the measurement base by applying a 0 or π/2 shift. The qubits travel from Bob to Alice and back: this is a go-&-return implementation. Notice that this implies that the laser is located in Bob’s device. For convenience, Alice will still be referred to as the emitter.

As shown in the Autocompensated Plug&Play Quantum Cryptography Set-up figure, a laser pulse (at 1550 nm) emitted at Bob is separated into two pulses at a first 50/50 beam splitter (BS). The two optical pulses impinge on the input ports of a polarization beam splitter (PBS), after having traveled through a short arm and a long arm. All fibers and optical elements at Bob are polarization maintaining. The linear polarization is turned by 90° in the long arm, therefore the two pulses exit Bob’s set-up at the same port of the PBS. The two pulses traveling to Alice are reflected by a Faraday mirror, modulated in phase (only one of the two is modulated), attenuated and return orthogonally polarized. In turn, thanks to the combination of Alice’s Faraday mirror and Bob’s PBS, both pulses now take the other path at Bob. The one taking the long path is phase modulated in order to choose the base of measurement. They both arrive at the same time at the BS where they interfere. They are then detected either in D1, or after passing through the Circulator, C, in D2. Since the pulses take the same path, inside Bob in reversed order, this interferometer is auto-compensated in phase. Furthermore, thanks to the combination of the PBS and the Faraday mirror, the system is auto-compensated in polarization fluctuations of the quantum link.

The phases applied by Alice and Bob are randomly chosen using a Quantis Random Number Generators.

This optical platform is used to generate a raw key. This key can then be processed using Key Sifting and Key Distillation to obtain a secure key.