Quantum Secret Sharing with Classical and Quantum Shares

In quantum secret sharing, a quantum secret state is mapped to multiple shares such that shares from qualified sets can recover the secret state and shares from other forbidden sets reveal nothing about the secret state; we study the setting where there are both classical shares and quantum shares. We show that the quantum secret sharing problem with both classical and quantum shares is feasible if and only if any two qualified sets have some quantum share in common. Next, for threshold quantum secret sharing where there are N₁ classical shares, N₂ quantum shares and qualified sets consist of any K₁ (or more) classical shares and any K₂ > N₂/2 (or more) quantum shares, we show that to share 1 qubit secret, each classical share needs to be at least 2 bits and each quantum share needs to be at least 1 qubit. Finally, we characterize the minimum share sizes for quantum secret sharing with at most 2 classical shares and at most 2 quantum shares. The converse proofs rely on quantum information inequalities and the achievable schemes use classical secret sharing, (encrypted) quantum secret sharing with only quantum shares, superdense coding, treating quantum digits as classical digits, and their various combinations.