A group of researchers at the University of Waterloo has achieved a significant advancement in quantum computing that skillfully circumvents the inherent "no cloning" issue. The study was published in Physical Review Letters.
Image created by Waterloo researcher Dr. Koji Yamaguchi that shows how encryption enables the first copying mechanism for quantum information. Image Credit: University of Waterloo.
Quantum computing represents a thrilling technological frontier, where information is stored and processed in minuscule units known as qubits. Qubits can be stored, for instance, in individual electrons, photons (light particles), atoms, ions, or small currents.
Universities, industries, and governments globally are investing billions of dollars to refine the technology for managing these qubits, enabling their integration into large, dependable quantum computers. This technology is poised to have substantial applications, including in cybersecurity, materials science, medical research, and optimization.
This breakthrough will enable quantum cloud storage, like a quantum Dropbox, a quantum Google Drive, or a quantum STACKIT, that safely and securely stores the same quantum information on multiple servers, as a redundant and encrypted backup. It’s an important step in enabling the buildup of quantum computing infrastructure.
Dr. Achim Kempf, Dieter Schwarz Chair, Physics of Information and AI, Department of Applied Mathematics, University of Waterloo
“Quantum computing has tremendous potential, particularly for solving very hard problems, but it also poses unique challenges. One of the most challenging issues facing quantum computing is called the no-cloning theorem, which states that quantum information cannot be copied, at least not directly. This is because of the delicate way in which quantum information is stored,” said Kempf.
Kempf, who is also an associate at the Institute for Quantum Computing in Waterloo and an associate member of the Perimeter Institute, elaborates that quantum information functions similarly to dividing a password. If one person possesses the first half of the password and another has the second half, neither can utilize it independently. However, when the two halves are combined, the valuable password is obtained.
In a comparable manner, qubits are remarkable because they can exchange information in a manner that expands as they are combined. A single qubit does not retain much by itself, but when qubits are interconnected, they can store an immense amount of information that only manifests when they are linked. This distinctive capability to maintain shared information across multiple qubits is referred to as quantum entanglement.
Kempf states that 100 qubits can share information in 2100 different ways at the same time. This enables them to share such a vast amount of entangled information that it exceeds the storage capacity of all contemporary classical computers.
Despite the vast potential of quantum computing, the no-cloning theorem imposes restrictions on its applications. This limitation arises from the fact that, in contrast to classical computing, where information copying, used for sharing and backups, is a frequently employed technique, quantum computing does not allow for straightforward copying and pasting.
We have found a workaround for the no-cloning theorem of quantum information.
Dr. Koji Yamaguchi, Research Assistant Professor, Kyushu University
“It turns out that if we encrypt the quantum information as we copy it, we can make as many copies as we like. This method is able to bypass the no-cloning theorem because after one picks and decrypts one of the encrypted copies, the decryption key automatically expires; that is, the decryption key is a one-time-use key. But even a one-time key enables important applications, such as redundant and encrypted quantum cloud services,” said Yamaguchi, who co-discovered the new method with Kempf while working as a post-doctoral researcher in Kempf’s lab.
This significant advancement strengthens Waterloo's position as a global leader in quantum science and its commercialization efforts. The Institute for Quantum Computing is globally acknowledged for its ability to combine top-tier fundamental research with robust commercialization assistance, a strategy that has successfully facilitated the launch of over 23 quantum startups that are revolutionizing sensing, security, and computing.
Journal Reference:
Yamaguchi, K., et al. (2025) Encrypted Qubits Can Be Cloned. Physical Review Letters. DOI: 10.1103/y4y1-1ll6. https://journals.aps.org/prl/abstract/10.1103/y4y1-1ll6.