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In this paper, we further research the quantum private comparison protocol with the semihonest participant, in which two parties can test the equality of their information without revealing the content to anyone. Considering both the success of protocol and the security of information, we¯rst give a reasonable and signi¯cant description about the semi-honest model in quantum secure communications. Moreover, based on the feature of four-qubit cluster state which has the great robust against decoherence, we propose a new protocol for the quantum private comparison. Our protocol ingeniously utilizes the special symmetry of the four-qubit cluster state to enhance the e±ciency of comparison. This new protocol is secure. The participants only know the result of comparison; cannot know each other's private information. TP cannot learn anything about the private information, even about the comparison result and the length of secret inputs.

To mitigate commutation failures (CFs) and minimize the instability of HVDC systems due to AC faults or control mode ambiguity, a constant extinction voltage-time area based control strategy with virtual resistance is proposed. In this strategy, using sine-cosine components detector and considering zerocrossing phase shift of commutation voltage, the extinction angle setting value can be adjusted dynamically. Meanwhile, to reflect the characteristics of DC current during the fault and recovery process, a virtual resistance is introduced into the control system and DC voltage considering the voltage drop of the virtual resistance is taken as the input of voltage-dependent current order limiter (VDCOL). Through theoretical analysis, the proposed strategy not only reduces the firing angle dynamically, but also reduces DC current by lowering the current order on the rectifier side immediately when the AC voltage disturbance is detected, thereby further reducing the occurrence of CFs. Therefore, the control strategy can effectively suppress successive and intermittent CFs. The effectiveness of the proposed control strategy is verified by simulation of the single HVDC and multi-infeed HVDC model based on the CIGRE HVDC benchmark system, in which AC-DC current criterion of identifying CF and suppression ratio index are adopted. The simulation results show that the proposed control strategy can effectively mitigate CFs under single-phase and three-phase faults to a certain extent. Comparing with the existing control strategies based on controller modification, i.e. commutation failure prevention, DC current predictive control, smooth logic switching control and DC current limitation control strategy based on virtual resistance, the proposed control strategy is superior in mitigation effects. The average suppression rates of these strategies are 1.66%, 3.21%, 6.11%, 3.35%, and 8.33% under single-phase fault, respectively; with the rates of 1.4%, 2.72%, 4.97%, 0%, and 6.8% under three-phase fault, respectively.

In this paper, we propose an asymmetric controlled bidirectional transmission protocol. In the protocol, by using the thirteen-qubit entangled state as the quantum channel, Alice can realize the transmission of a two-qubit equatorial state for Bob and Bob can transmit a four-qubit equatorial state for Alice under the control of Charlie. Firstly, we give the construction of the quantum channel, which can be done by performing several H and CNOT operations. Secondly, through implementing the appropriate measurements and the corresponding recovery operations, the desired states can be transmitted simultaneously, securely and deterministically. Finally, we analyze the performance of the protocol, including the efficiency, the necessary operations and the classical communication costs. And then, we describe some comparisons with other protocols. Since our protocol does not require auxiliary particles and additional operations, the classic communication costs less while achieving the multi-particle bidirectional transmission, so the overall performance of the protocol is better.