QKD where entanglement is a sufficient but not a necessary resource. Entanglement becomes necessary in device-independent and ideal QKD, where discord still provides an upper bound to the secret-key rates. Future work may involve the derivation of a direct mathematical relation between the amount of quantum discord in Alice and Bob’s output state and the optimal secret-key rates which are achievable in device-dependent QKD.

*Acknowledgments*.– S.P. was supported by a Leverhulme Trust research fellowship and EPSRC (grants EP/J00796X/1 and EP/L011298/1).

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