Quantum Computing Advances with Verifiable Blind Technology, Oxford-led Team Reports

Verifiable Blind Quantum Computing (BQC) is a groundbreaking technology that allows quantum computations to be performed on a server while keeping the client’s data and algorithm hidden. This technology, based on quantum mechanics principles, offers information-theoretic security and can detect server malfunctions or attacks. BQC requires a universal quantum computer server and a quantum link, ideally using photons, to connect to the client. Recent experiments have implemented BQC using a trapped-ion quantum processor server. Despite its potential, BQC faces challenges such as noise and technical difficulties in combining different platforms. The research involves several institutions, including the University of Oxford and the University of Maryland.

What is Verifiable Blind Quantum Computing?

Verifiable Blind Quantum Computing (BQC) is a revolutionary technology that allows quantum computations to be performed on a server, while keeping the client’s input, output, and algorithm hidden from the server. This technology is based on the principles of quantum mechanics, which state that quantum information cannot be copied and measurements irreversibly change the quantum state. This means that information stored in these systems can be protected with information-theoretic security, and any incorrect operation of the server or attempted attacks can be detected. This is a surprising possibility that has no equivalent in classical computing.

The BQC technology requires not only a universal quantum computer as the server but also a quantum link connecting it to the client. Photons are a natural choice to provide that link. However, unavoidable photon loss, either due to limited photon detection efficiencies or absorption in the link, results in potential security risks and places hard limits on the scalability of this approach due to the resource overhead incurred by post-selection.

Ideally, quantum information at the server should be stored in a stable quantum memory that can be manipulated with high fidelity yet readily interfaced to a photonic link. The ability to retain quantum information on the server then enables the client to perform adaptive mid-circuit adjustments in order to execute the target computation deterministically and securely.

How is BQC Implemented?

In a recent experiment, BQC was implemented using a trapped-ion quantum processor server that integrates a robust memory qubit encoded in 43Caþ with a single-photon interface based on 88Srþ to establish a quantum link to the client photon detection system. The client can remotely prepare single-qubit states on the server adaptively from shot to shot using real-time classical feedforward control.

The complexity needed for universal quantum computation is contained entirely within the server, while the client is a simple photon polarization measurement device that is independent of the size and complexity of the algorithm and supports near-perfect blindness by construction. The client and the server are controlled by independent hardware and connected only by a classical signaling bus and an optical fiber.

What are the Implications of BQC?

The implications of BQC are profound. Quantum computers are poised to outperform the world’s most powerful supercomputers, with applications ranging from drug discovery to cybersecurity. These computers harness quantum phenomena such as entanglement and superposition to perform calculations that are believed to be intractable with classical computers.

However, as quantum processors control delicate quantum states, they are necessarily complex and physical access to high-performance systems is limited. Cloud-based approaches, where users can remotely access quantum servers, are likely to be the working model in the near term and beyond. Many users already perform computations on commercially available devices for state-of-the-art research.

What are the Challenges and Future Directions?

Despite the promising potential of BQC, there are still challenges to be addressed. Combining two completely different platforms at the single-quantum level is technically challenging. So far, quantum network nodes with integrated memory qubits have been realized with solid-state systems and trapped atoms.

In the presence of noise, even a faithfully operating server produces erroneous results that are indistinguishable from nefarious modifications to the honest protocol. Blindness allows the client to secretly test the quantum resources provided by the server. The protocol implemented here achieves this by interleaving computation and test rounds.

Looking forward, the experiment demonstrates a path to fully verified quantum computing in the cloud. The system achieves noise levels below a certain threshold for which arbitrary improvements to the protocol security and success rate robustness are theoretically possible. This opens up exciting possibilities for the future of quantum computing.

Who are the Key Players in BQC Research?

The research on BQC is a collaborative effort involving several institutions. The team includes researchers from the Department of Physics at the University of Oxford, Laboratoire d’Informatique de Paris 6, CNRS Sorbonne Université, Joint Center for Quantum Information and Computer Science at the University of Maryland, and the School of Informatics at the University of Edinburgh. The research was received in May 2023, accepted in January 2024, and published in April 2024.