Verifying a Quantum State More Efficiently
To confirm that a quantum computer has correctly encoded a specific quantum state, researchers can verify the computer’s output against a trusted reference device. This verification is especially important for quantum computers that comprise multiple modules, which must be checked against one another for consistency. However, current verification methods quickly become too costly as the number of qubits increases. Kieran Dalton of the Swiss Federal Institute of Technology (ETH) Zurich and colleagues have now demonstrated a quantum-state-verification method that should work efficiently on quantum computers comprising arbitrary numbers of qubits [1].
In some approaches to quantum-state verification, researchers measure the overlap between two quantum computers’ states via a quantity called the inner product. Conventionally, the inner product is determined after the quantum states have been read out as classical information. This process involves making a series of measurements whose number rises exponentially with the number of qubits.
Dalton and colleagues determined the inner product for two modules of three superconducting qubits bonded to a shared chip. These modules were connected via a quantum link, which allowed them to use a protocol that does not require the states to first be translated into classical bits. This protocol was previously predicted to require the same number of measurements regardless of the number of qubits involved.
The researchers prepared each module in a maximally entangled quantum state, then estimated the inner product for these states via an additional quantum process. For the three-qubit modules tested, reading the result of this quantum process needed one quarter of the number of measurements required by conventional methods. For larger devices, the new protocol is expected to offer an even greater advantage.
–Sophia Chen
Sophia Chen is a freelance science writer based in Columbus, Ohio.
References
- K. Dalton et al., “Resource-efficient cross-platform verification with modular superconducting devices,” PRX Quantum 6, 040365 (2025).