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A scheme to verify gates of a quantum computer without examining devices
Quantum computers, systems that process information using the principles of quantum mechanics, could solve some problems that cannot be tackled by the classical computers currently used worldwide. Despite their potential, verifying that these computers are working correctly and can reliably perform computations remains challenging.
Shubhayan Sarkar, a researcher at the University of Gdansk, recently introduced a new scheme for certifying that quantum chips (unitary gates) in a quantum computer are operating correctly without relying on assumptions about their internal components. This scheme, introduced in a paper published in Physical Review Letters, uses an approach referred to as almost device-independent (DI) certification.
"Consider the computer you are using right now," Sarkar told Phys.org. "If it provides the answer to a mathematical problem, how do you know that the computation is correct? In practice, we rarely verify every calculation ourselves.
"Instead, we trust the manufacturer, the hardware, the operating system, and the well-established engineering principles on which the computer is built. If necessary, the result can often be checked independently using another classical computer."
Before they are integrated into commercial portable and desktop computers, classical processors undergo extensive testing as part of their design and manufacturing. This testing process ensures that the processors operate as intended before they are deployed in real-world settings.
"Our confidence in a processor ultimately relies on trusting the testing methods, the manufacturer, and the underlying hardware," said Sarkar.
"Quantum computers, however, present a fundamentally different challenge. Since they are expected to solve problems that are beyond the reach of classical computers, there may be no efficient classical method to benchmark them. This raises two natural questions. First, how can we be sure that the reported result was genuinely produced by exploiting quantum phenomena rather than by some classical process? Second, how can we trust that the computation itself was carried out correctly?"
The potential of device-independent certification
A promising approach to testing quantum computers entails asking them carefully designed questions. Based on a computer's answers to these questions, engineers can certify that its underlying hardware is genuinely exploiting principles of quantum mechanics without requiring knowledge of the system's components and how they perform computations.
"This approach is known as device-independent (DI) certification, as it makes only minimal physical assumptions about the device being tested," said Sarkar. "All device-independent certification protocols are rooted in the phenomenon of quantum nonlocality, whose experimental verification was recognized with the 2022 Nobel Prize in Physics. Since any quantum computer is based on quantum states, measurements and unitary gates, certifying them in a DI way is the first step to self-test a quantum computer."
As part of his previous research, Sarkar developed a new approach to self-test any quantum state and measurement. Building on this earlier study, he set out to demonstrate the potential of the new approach for self-testing any unitary gate (i.e., any mathematical operation that preserves all quantum information and represents an ideal quantum gate).
"Since any quantum computer will be based on quantum chips that are basically unitary gates, self-testing unitary gates is certifying the 'bricks' of a quantum computer," said Sarkar.
"This issue is particularly significant for developing countries. Many may not have the resources or infrastructure to manufacture large-scale quantum processors domestically and will instead rely on importing quantum chips and integrating them into their own quantum computing platforms. In such cases, having reliable methods to certify the imported hardware becomes especially important."
The newly developed DI scheme
The work consists of two different schemes. The first, the Almost-DI scheme, is much simpler than a conventional DI scheme. This scheme, developed by Sarkar, could, in principle, be used to self-test any quantum unitary gate operation, subject to a technical assumption that the gate does not alter the initial subspace of the states. Remarkably, this could be achieved without the need to model a processor's underlying hardware in detail.
"Consider a large network like a star, with a single node inside and all the other external nodes connected to it, like a server connected to various other systems," explained Sarkar.
"In between the server and the external nodes, there is a switch (or gate) that can be either on or off. The server can perform joint operations on the signals received from all links while external systems act only on their own links. When the switch is off, we follow the approach presented in our previous paper; that is, using a family of Bell inequalities, we first certify the external nodes and their links to the server."
This certification approach, under specific conditions, can be used to verify a server's joint operations. Once the gate is turned on again, its correct operation can be verified using the certified server, links and external nodes.
"It is important to note that all phases are carried out in a single experiment, and then the statistics from the entire experiment are used to certify all the components together," explained Sarkar.
The main advantage of the newly developed scheme is that it could be used to verify any quantum processor. In addition, it relies on a star-shaped network that was already successfully implemented in experiments with a small number of external systems.
"The DI scheme is more involved, as one also requires invoking additional teleportation links between the server and the switch," said Sarkar. "Consequently, in this scheme, one has to consider other Bell inequalities to certify these teleportation links, after which the scheme follows as for Almost-DI."
Toward more reliable quantum computers
This recent study resolves a long-standing problem in quantum science and computing. Specifically, it proves the existence of a unique set of correlations for every quantum operation that can only be attained by each of these operations, up to some degrees of freedom that cannot be ruled out by experiments.
Initial tests performed by Sarkar suggest that these correlations could be used as a resource to verify quantum unitary gate operations. While the new scheme has not yet been implemented experimentally, the team hopes to demonstrate its potential in the future.
"From an application perspective, we provided a way to certify any quantum unitary gate, which are the bricks of any quantum computer," said Sarkar.
"Since the assumptions in the device-independent case are minimal, with an additional assumption of independent sources in the network, the security and trust in such protocols will be maximal. Thus, we make a significant step toward certifiable use of future quantum devices. Even the recent controversy regarding Microsoft's Majorana chip could have been avoided if we had a setup as suggested in the work."
The scheme developed at the University of Gdansk is still a proof of principle, so it will need to be optimized and tested further before it can be used to verify real quantum processors. Sarkar will now try to make the scheme better suited for practical applications, for instance by lowering the number of Bell tests required to verify a system and ensuring that it is robust against disturbances or imperfections.
"We would also like to adapt the scheme to realize a near-term quantum computing platform that is publicly accessible, like IBM, such that the gates can be certified and thus certify the quantum operations by IBM without trusting IBM," added Sarkar.
"This would not just benchmark the quantum computing of these companies but also increase the trust of the general public in quantum computing. Moreover, collaborating on experiments to build the network as suggested in the work is a major direction that should be explored."
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Publication details
Anonymous, Any unitary gate can be certified device-independently in a quantum network, Physical Review Letters (2026). DOI: 10.1103/m1tx-9mx1.
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Citation: A scheme to verify gates of a quantum computer without examining devices (2026, July 17) retrieved 17 July 2026 from https://phys.org/news/2026-07-scheme-gates-quantum-devices.html
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