BB84 - QuantumCryptography QingqingYuan Outline...

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    Quantum Cryptography Qingqing Yuan
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Outline No-Cloning Theorem BB84 Cryptography Protocol Quantum Digital Signature
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One Time Pad Encryption Conventional cryptosystem: Alice and Bob share N random bits b 1 …b N Alice encrypt her message m 1 …m N  b 1 m 1 ,…,b N m N   Alice send the encrypted string to Bob Bob decrypts the message: (m j b j ) b j  = m j   As long as b is unknown, this is secure Can be passively monitored or copied
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Two Qubit Bases Define the four qubit states: {0,1}(rectilinear) and {+,-}(diagonal) form an orthogonal  qubit state. They are indistinguishable from each other. - = - + = + ) 1 0 ( ) 1 0 ( 1 0 2 1 2 1
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No-Cloning Theorem |q  =  α |0 + β |1 To determine the amplitudes of an unknown qubit, need  an unlimited copies It is impossible to make a device that perfectly copies an unknown qubit. Suppose there is a quantum process that implements: |q,_ ⟩→ | q,q Contradicts the unitary/linearity restriction of quantum physics
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Wiesner’s Quantum Money A quantum bill contains a serial number N, and 20 random  qubits from {0,1,+,-} The Bank knows which string {0,1,+,-} 20  is associated with  which N The Bank can check validity of a bill N by measuring the  qubits in the proper 0/1 or +/- bases A counterfeiter cannot copy the bill if he does not know the  20 bases
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BB84 - QuantumCryptography QingqingYuan Outline...

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