The first two-party Quantum Key Agreement (QKA) protocol, based on quantum teleportation, was proposed by Zhou et al. (Electron. Lett. 40(18) (2004) 1149). In this protocol, to obtain the key bit string, one of the parties uses a device to obtain the inner product of two quantum states, one being unknown, and the other one performs Bell measurement. However, in this paper, we show that it is not possible to obtain a device that would output the inner product of two qubits even when only one of the qubits is unknown. This is so because the existence of such a device would imply perfectly distinguishing among four different states in a two-dimensional vector space. This is not permissible in quantum mechanics. Furthermore, we argue that the existence of such a device would also imply a violation of the “No Signaling Theorem” as well.

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评论《量子密钥协议协议》

Zhou等人提出了第一个基于量子隐形传态的两方量子密钥协议（QKA）协议。（电子。Lett. 40(18) (2004) 1149）。在该协议中，为了获得密钥比特串，其中一方使用设备获得两个量子态的内积，一个是未知的，另一个进行贝尔测量。然而，在本文中，我们表明即使只有一个量子位未知，也无法获得能够输出两个量子位内积的设备。之所以如此，是因为这种设备的存在意味着可以完美地区分二维向量空间中的四种不同状态。这在量子力学中是不允许的。此外，我们认为这种设备的存在也意味着违反“无信号定理”。