Insider Brief
Researchers at the University of Waterloo demonstrated that quantum information can be redundantly backed up by encrypting qubits into multiple entangled systems that can be decrypted only once, preserving the no-cloning theorem.
The method, reported in Physical Review Letters, relies on entanglement-based encryption that renders each stored qubit locally unreadable while allowing perfect recovery of the original state using a one-time decryption key.
The approach enables a new form of quantum redundancy with potential applications in quantum cloud storage and distributed infrastructure, while remaining constrained by fundamental limits on repeat access and hardware fragility.
For decades, the no-cloning theorem has stood as one of quantum physics’ immovable rules. This theorem — which states an unknown quantum state cannot be copied — is not a technical limitation or a matter of engineering maturity. It is a consequence of how quantum mechanics itself works.
The new result from researchers at the University of Waterloo does not overturn that rule. Instead, it steps around it.
In a paper published in Physical Review Letters and announced in a news release, Achim Kempf and Koji Yamaguchi show that quantum information can be redundantly backed up — not by copying qubits directly, but by encrypting them into multiple entangled systems that can later be unlocked exactly once. The distinction matters, both technically and conceptually.
What emerges, though, could have real impact in the real world, the researchers suggest.
“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,” said Dr. Achim Kempf, the Dieter Schwarz Chair for Physics of Information and AI at the University of Waterloo, in the news release. “It’s an important step in enabling the buildup of quantum computing infrastructure.”
Why Quantum Information Cannot Be Copied
The work hits right at a core obstacle in quantum computing. While classical data can be copied freely, quantum information cannot be duplicated without being disturbed. That limitation has constrained everything from quantum networking to cloud storage, making redundancy — a basic feature of modern computing — difficult to achieve.
Quantum computers store information in qubits, which behave differently from classical bits. A qubit can exist in a combination of states, and its full information content is revealed only when it is measured, a process that generally alters the state itself.
This behavior is intrinsic to the no-cloning theorem, which states that no physical process can create an identical copy of an unknown quantum state.
One way to think of it is to consider an unknown quantum state like a perfectly balanced spinning coin sealed inside a box.
You can’t open the box to see how it is spinning without stopping the spin. And because you do not know its exact motion in advance, there is no way to build a second box with an identical spin just by looking at the first one. So, any attempt to “copy” it requires interacting with it — and that interaction inevitably changes it.
The theorem, in other words, is not a technical hurdle but a direct consequence of quantum mechanics’ mathematical structure.
As a result, operations taken for granted in classical computing — copying files, creating backups, mirroring data across servers — do not carry over cleanly into the quantum domain. A hardware failure or transmission loss can permanently destroy quantum data.
The Waterloo team did not attempt to bypass this constraint directly. Instead, the researchers reframed the problem by separating copying from accessibility.
Encrypting a Qubit Before It Is Stored
The new protocol begins by encrypting the quantum information so thoroughly that it becomes locally unreadable everywhere it is stored.
The method starts with a single qubit whose state is unknown. That qubit is made to interact, through a carefully designed quantum operation, with a collection of other qubits that are already entangled in pairs. After this interaction, each of the storage qubits contains a complete imprint of the original quantum information.
Each storage qubit on its own appears as pure noise. Measurements performed on any single stored qubit reveal nothing about the original state. In technical terms, each encrypted qubit is in a maximally mixed state.
The encryption relies on entanglement rather than mathematical scrambling. The “noise” masking the information is not random in the classical sense but is instead recorded in a separate set of qubits that never interacted with the original data. These qubits collectively function as the decryption key.
“We have found a workaround for the no-cloning theorem of quantum information,” Dr. Koji Yamaguchi, who co-discovered the method while a postdoctoral researcher in Kempf’s lab and is now a research assistant professor at Kyushu University, said in the release. “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.”
Why Only One Copy Can Ever Be Recovered
The expiration of the decryption key is not a design choice but a physical requirement.
To recover the original quantum state, all of the key qubits must be used together in a second quantum operation that removes the encryption noise from one selected storage qubit. That operation consumes the key. Afterward, the remaining encrypted copies become permanently unreadable.
At no point do two accessible versions of the same unknown quantum state exist simultaneously. This property preserves consistency with the no-cloning theorem, as well as with related constraints such as entanglement monogamy and quantum secret sharing.
The study formally proves that the encrypted copies can be generated through unitary operations and that the recovery of the original state succeeds with perfect fidelity — but only once. If even a single key qubit is missing, the recovery fails entirely.
The team also shows that the resources required to perform the encryption and decryption scale linearly with the number of encrypted copies, making the protocol compatible with near-term experimental systems. The paper reports early experimental implementations of the method using up to ten encrypted copies on a quantum processor.
Implications and Limits
The team suggests in the paper that the most direct application described in the study is encrypted quantum cloud storage. A quantum data owner could distribute encrypted versions of a qubit across multiple remote systems while keeping the decryption key on-site. If one system fails, the data can still be recovered — at least once — from another.
The method does not replace quantum error correction, which protects against noise during computation, nor is it designed to regulate access among multiple parties. Instead, it allows redundancy without duplication, an essentially new capability.
The researchers note connections to quantum networking, distributed quantum computing, and even theoretical models of information recovery in black hole physics. They also emphasize that encrypted cloning does not allow repeated access to stored data and does not eliminate the fragility of quantum hardware.
There is still work to do, according to the researchers. The result does not make quantum cloud services routine or inexpensive. It does not remove the need for error correction or fault tolerance. But it resolves a structural impossibility that has limited how quantum systems can be organized.
For an industry attempting to move quantum computing out of isolated laboratories and into scalable infrastructure, that distinction matters.
Just a note: While the paper was published in the peer-reviewed Physical Review Letters, this article relied on the open-access arXiv version of the study.