Quantum Privacy Achieved: Algorithms Run Securely on Remote Servers

A new demonstration of blind quantum computation addresses the key need for data and algorithmic privacy in quantum computing. Yongxin Song and colleagues at Paul Scherrer Institute successfully executed a measurement-based blind quantum computation protocol on a superconducting processor built from two flip-chip-bonded modules. The demonstration showcases how a client can perform calculations without revealing information about the task or result to the server. By implementing a three-qubit Deutsch-Jozsa algorithm and verifying negligible information leakage, the team demonstrates the feasibility of blind protocols in superconducting-circuit architectures and suggests that near-term improvements in gate fidelities could enable intermediate-scale implementations.

Modular superconducting processor enables secure client-only quantum computation

A deterministic blind quantum computation protocol was demonstrated on a modular superconducting quantum processor, comprising server and client modules. Researchers performed arbitrary quantum computations through measurements and adaptive single-qubit rotations after the server generated a resource cluster state and transmitted it to the client. Within this framework, measurement-based single- and two-qubit gates, alongside a three-qubit instance of the Deutsch-Jozsa algorithm, were realised.

The protocol is blind, ensuring the computation and its results remain known only to the client, assuming no information leakage to the server. Characterisation of the server’s quantum state after each rotation of a measurement-based single-qubit gate quantified information leakage, revealing negligible information about the client’s state. This proof-of-principle demonstration validates the core concepts of blind quantum computation in a modular superconducting architecture, with implications for secure remote computation and the development of a quantum internet.

The superconducting quantum device hosts three flux-tunable transmon qubits in both the server and client modules. These modules are flip-chip bonded to a common carrier chip integrating control and readout circuits, mirroring a previously presented device. Each qubit features individual drive and flux lines, enabling independent state manipulation and frequency tuning, as well as a dedicated frequency-multiplexed readout line for dispersive readout.

Researchers implement static qubit couplers both within and between the modules to enable flux-activated two-qubit gates. The system processes individual qubit readout signals by a real-time feedforward control system at the client, instructing drive electronics to generate feedforward pulses according to user-programmed logic. Measurement-based quantum computation is implemented by generating cluster states using the server module and measuring them sequentially on the client module.

A computation is specified by a measurement pattern, defined by the topology of the cluster state and the chosen measurement bases, which must be updated in real time according to previous measurement outcomes to obtain a deterministic result. Two-dimensional cluster states of sizes w × d, with a width w ∈{1, 2, 3} and a depth d ∈Z+, were realised. Each step of the computation is specified by a set of three projection angles, {αk, βk, γk}. Measurement-based quantum computation can also be performed with subsets of the qubits in these cluster states.

An equivalent gate-based circuit, comprising nested layers of CZ, single qubit z-rotations and Hadamard gates, can be mapped onto each measurement-based computation. The implemented protocol involves creating a w × 1 cluster state at the server module, transferring it to the client via CNOT gates, applying adaptive basis rotation pulses and measuring the qubits in the computational basis. To prepare for the next state transfer, X gates are applied conditioned on the measurement result to reset the client qubits, while the server generates another layer of the cluster state.

This iterative protocol enables measurement-based quantum computation with cluster state sizes n = w × d, limited by gate and decoherence errors. Finally, a circuit swaps the quantum state of the server register with that of the client, and the client applies the final feedforward pulses to generate the target output state deterministically. The fidelity of the cluster state directly impacts the fidelity of the measurement-based quantum computation, so it was characterised using an efficient fidelity estimation method based on a random selection of stabilizers evaluated via Pauli measurements. This method is well-suited for stabilizer states, enabling unbiased fidelity estimation of arbitrarily large cluster states.

Secure delegation of quantum tasks validated with superconducting qubits

Researchers, led by David Awschalom, are building the foundations for a quantum internet, where sensitive calculations can be outsourced without revealing their secrets. This new demonstration shows an important component, blind quantum computation, allowing a client to delegate tasks to a remote quantum processor while maintaining complete data privacy. The current demonstration relies on a specific architecture utilising a cluster state, a complex arrangement of entangled qubits, which presents a challenge for scalability.

Despite employing a complex cluster state architecture, the significance of this demonstration remains substantial. The successful execution confirms data privacy is achievable even when outsourcing processing, validating the core concepts of blind quantum computation using current superconducting technology. Although scaling this specific approach presents hurdles, it demonstrates a viable pathway towards secure quantum computation and expands the set of tools for building a quantum internet.

This demonstration of a blind quantum computation protocol, executed on a superconducting processor, confirms the feasibility of secure remote computation. Researchers achieved a computation where the server remained unaware of the task or result by separating processing between server and client modules. This modular approach, employing adaptive measurements and single-qubit rotations, represents a departure from previous implementations reliant on photons or alternative qubit technologies. The successful execution of the three-qubit Deutsch-Jozsa algorithm validates the core principles of information-theoretic privacy in a superconducting architecture.

This research successfully demonstrated blind quantum computation on a superconducting processor comprising two modules. This means a client can delegate a three-qubit Deutsch-Jozsa algorithm to a remote server without revealing any information about the calculation itself. Researchers verified negligible information leakage to the server through analysis of the quantum state after each step, confirming the one-way flow of information necessary for privacy. The authors suggest that improvements in gate fidelities could enable larger-scale implementations of these blind protocols.