# Experimental Quantum Message Authentication with Single Qubit Unitary Operation
## 5 Conclusions and Discussion  
We have proposed a new quantum message authentication protocol including quantum encryption for improving security against an existential forgery. Additionally, a practical implementation of the proposed protocol has been developed and its robustness against existential forgery has been verified experimentally. It consists of wave plates and the Hong-Ou-Mandel interferometer. The measurement results for each function--QMAC generation and recovery, [PERSON]'s encryption and decryption, and quantum encryption and decryption--successfully show the feasibility of robustness against [PERSON]'s forgeries.  
The system loss and the optical channel loss, etc., should be considered when applying our protocol to real implementation. Let us assume that [PERSON] and [PERSON] use the single photon detector with 20% efficiency and are connected by 30-km single-mode fiber with 0.2 dB/km loss. In a result, the total efficiency becomes 0.08% because the qubits are pass through total 100 km, and if the QMAC pairs are generated at 100 MHz, [PERSON] can receive \(8\times 10^{4}\) pairs/s. As we mentioned in Section 2, the size of the quantum state sequence should be more than 15. Therefore, [PERSON] must generate at least \(1.9\times 10^{4}\) QMAC pairs, i.e., \((1.9\times 10^{4})\times 0.08\%=15\) that is quite implementable number, and send them to [PERSON] to ensure this accuracy of the swap test.  
Our protocol can be used as an arbitrated quantum signature protocol if a trusted center (TC) is added in the communication channel used by [PERSON] and [PERSON] [19]. In addition, if freshness property is added to our protocol, it can be used for quantum entity authentication as well [1, 52]. In conclusion, we have proposed the base technology for a complete quantum cryptosystem that provides confidentiality, authentication, integrity, and nonrepudiation.  
[PERSON] conceived the main idea. [PERSON] wrote the manuscript. [PERSON] and [PERSON] developed the experimental setup and performed the experiment. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] analyzed the results. [PERSON] supervised the whole project. All authors reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.  
Korea Institute of Science and Technology (2E30620); National Research Foundation of Korea (2019 M3E4A107866011, 2019M3E4A1079777, 2019R1A2C2 006381); Institute for Information and Communications Technology Promotion (2020-0-00947, 2020-0-00972).  
Figure 6: Coincidence probability by existential forgery. Red bars denote the case where [PERSON] attempts original Gao’s Forgery when operator \(H\) is not used in the quantum encryption scheme \(\left(k_{H}^{i}=0\right)\). The blue bars show the case of attempting improved Gao’s Forgery when operator \(H\) is used in the quantum encryption scheme \(\left(k_{H}^{i}=1\right)\). \(P_{c}\) is the coincidence probability. The black bars indicate the standard deviation of the coincidence counts for 1 s. \(k_{AB}^{i}\) is the same as in Figure 1. \(e_{i}\in\left\{00,\,01,\,10,\,11\right\}\) corresponds to the Pauli operator \(\sigma_{\alpha_{i}}\in\left\{1,\,\sigma_{\mathrm{x}},\,\sigma_{\mathrm{y}},\, \sigma_{z}\right\}\) that [PERSON] uses to attempt Gao’s Forgery 1.