Blind quantum computation (BQC) enables clients with limited quantum capabilities to protect the privacy of inputs, outputs and algorithms during the computation process. However, if a client’s private information is exposed to a server after the computation, the server can deduce the client’s output or even input. Quantum encryption with certified deletion (QECD) offers a potential solution by enabling the data owner to generate a deletion certificate, making the original plaintext inaccessible, provided the certificate is valid. Nevertheless, current QECD can only handle classical data and cannot be applied to BQC with quantum inputs. This paper first introduces the concept of certified deletion for quantum states and then proposes a single-client BQC protocol with certified deletion, where the client can use the classical certificate generated by the server to confirm whether her quantum inputs have been deleted after computation.We also give a specific example and simulate it using Qiskit to show its feasibility. In addition, the proposed protocol can also be extended to a multi-client environment in which honest clients request certified deletion if any client disconnects or behaves maliciously, thereby terminating the protocol and protecting their privacy.

Variational quantum algorithms (VQAs) can train parameterized quantum circuits via classical optimizers to find approximate solutions to some important problems. They can overcome the limitations of existing quantum technologies only allowing for a few qubits and small circuit depth and are considered as one of the most promising methods for achieving quantum advantages in the noisy intermediate-scale quantum (NISQ) era. In this paper, we propose secure delegated VQAs by utilizing quantum homomorphic encryption (QHE) for users with limited quantum power to delegate the task of running VQAs to remote quantum servers while still keeping the training data private. Firstly, a client-friendly QHE scheme that allows quantum servers to perform calculation on encrypted data is proposed to be suitable for VQAs. Then, delegated VQAs based on the given QHE scheme are presented, where servers can train the ansatz circuit using the encrypted data. Finally, a delegated variational quantum classifier to identify handwritten digit images is given as a specific example of delegated VQAs and simulated on the cloud platform of Original Quantum to show the feasibility. Secure delegated VQAs will provide significant technical support for future quantum cloud services.

Blind quantum computation (BQC) provides a solution for clients with limited quantum capabilities to delegate their quantum computational tasks to remote quantum servers while keeping their own data private. In this paper, we first propose three multi-party verifiable blind quantum computation (MPVBQC) protocols, each of which can handle one type of clients with certain simple quantum capabilities such as making single-qubit measurements, preparing single qubits, or performing a few single-qubit gates. Then a flexible and hybrid MPVBQC framework for multi-type clients in quantum networks is given by combining the three proposed MPVBQC protocols. It simultaneously allows at least three types of clients in quantum networks to achieve BQC depending on their own quantum devices. Furthermore, all the proposed protocols can achieve identity authentication, resist both insider and outsider attacks, and be verifiable which means that the clients can verify the correctness of their computational results.

Quantum blockchain is expected to offer an alternative to classical blockchain to resist malicious attacks laughed by future quantum computers. Although a few quantum blockchain schemes have been constructed, their efficiency is low and unable to meet application requirements due to the fact that they lack of a suitable consensus mechanism. To tackle this issue, a consensus mechanism called quantum delegated proof of stake (QDPoS) is constructed by using quantum voting to provide fast decentralization for the quantum blockchain scheme at first. Then an efficient scheme is proposed for quantum blockchain based on QDPoS, where the classical information is initialized as a part of each single quantum state and these quantum states are entangled to form the chain. Compared with previous methods, the designed quantum blockchain scheme is more complete and carried out with higher efficiency, which greatly contributes to better adapting to the challenges of the quantum era.