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Quantum Bit Commitment with a Composite Evidence
Entanglement-based attacks, which are subtle and powerful, are usually
believed to render quantum bit commitment insecure. We point out that the no-go
argument leading to this view implicitly assumes the evidence-of-commitment to
be a monolithic quantum system. We argue that more general evidence structures,
allowing for a composite, hybrid (classical-quantum) evidence, conduce to
improved security. In particular, we present and prove the security of the
following protocol: Bob sends Alice an anonymous state. She inscribes her
commitment by measuring part of it in the + (for ) or (for
) basis. She then communicates to him the (classical) measurement outcome
and the part-measured anonymous state interpolated into other, randomly
prepared qubits as her evidence-of-commitment.Comment: 6 pages, minor changes, journal reference adde
How to Build Unconditionally Secure Quantum Bit Commitment Protocols
The ``impossibility proof'' on unconditionally secure quantum bit commitment
is critically analyzed. Many possibilities for obtaining a secure bit
commitment protocol are indicated, purely on the basis of two-way quantum
communications, which are not covered by the impossibility proof formulation.
They are classified under six new types of protocols, with security proofs for
specific examples on four types. Reasons for some previously failed attempts at
obtaining secure protocols are also indicated.Comment: 10 pages, revtex4; significant changes (see footnote
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