A team of researchers at the University of Waterloo has developed a protocol that allows perfect copies of quantum information to be created, provided the copies remain encrypted and only one can ever be decrypted. The work, published in Physical Review Letters in January 2026, sidesteps the no-cloning theorem, a foundational rule in quantum mechanics that has long prohibited exact duplication of unknown quantum states. The result could reshape how quantum computers handle data backup and error recovery, two problems that have dogged the field for decades. According to a university summary reported by Phys.org, the researchers emphasize that the key innovation is limiting how many readable copies can exist, rather than how many physical duplicates can be produced.
How Encryption Breaks the Cloning Barrier
The no-cloning theorem has stood since the 1980s as one of quantum information science’s firmest constraints. It states that no physical process can produce an identical copy of an arbitrary, unknown quantum state. Researchers have spent years probing the edges of that rule. The best-known approximate method, the optimal universal quantum cloner, achieves a maximum fidelity of 5/6 when copying one qubit into two, meaning the copies are close but never exact. Other approaches have relaxed assumptions about determinism or restricted the class of states being copied, but none produced perfect duplicates of a truly unknown qubit, leaving a seemingly hard boundary between what quantum theory allows and what engineers might want for robust computation.
The Waterloo protocol changes the terms of the problem. Rather than trying to copy a bare quantum state, it first wraps the qubit in a layer of encryption, then generates any number of identical encrypted clones through a unitary operation. Decryption requires a single-use key, and the protocol’s design ensures that only one plaintext state can ever be recovered, even if many encrypted copies exist. The clones themselves are real and perfect, but they are useless without the key, and the key works exactly once. In the Physical Review Letters report, the authors argue that the no-cloning theorem constrains accessible information rather than encrypted carriers, and that their scheme respects the theorem by ensuring that the underlying unknown state is never available in more than one readable instance at a time.
From Theory to Hardware on IBM Processors
A separate experimental preprint has already tested whether this encrypted cloning scheme holds up on real quantum hardware. Researchers ran the protocol on IBM’s superconducting Heron-R2 processors, scaling the demonstration to 154 qubits in a programmable architecture. The experiment checked whether encrypted cloning remained stable when composed as a modular building block in parallel, series, and interleaved configurations, mimicking how it might be embedded into larger algorithms. It also verified that the process preserves pre-existing entanglement between qubits, a property that would be essential for integrating encrypted cloning into multi-qubit circuits that rely on delicate correlations for speedups.
That hardware validation matters because quantum processors are notoriously sensitive to noise. A protocol that works on paper but collapses under real-world error rates would have limited practical value. The fact that encrypted clones survived composition on a current-generation processor suggests the technique could be deployed as a subroutine inside fault-tolerant quantum programs, not just as a standalone curiosity. The IBM tests indicate that the encryption and cloning steps can be implemented with realistic gate depths and connectivity constraints, hinting that future devices with better error correction could scale the method to much larger registers without fundamentally redesigning the protocol.
Prior Loopholes and Why This One Is Different
Physicists have found ways to stretch the no-cloning theorem before, but those methods came with significant trade-offs. Probabilistic cloning, for instance, can surpass the deterministic cloning limit for specific state families like coherent light pulses, but it works only some of the time and only on restricted inputs. Experimental attacks on quantum money schemes have similarly used cloning strategies that copy limited sets of states with success rates bounded by linear optics, meaning the adversary must accept a nonzero chance of failure and detection. These approaches relaxed the theorem’s assumptions (by narrowing the allowed input states or tolerating randomness), rather than satisfying them outright for arbitrary unknown qubits.
The encrypted cloning protocol is deterministic, applies to arbitrary unknown qubits, and produces perfect copies. Its constraint is not on the quality or probability of cloning but on access: the encryption layer guarantees that duplicating the information does not duplicate the ability to read it. That distinction aligns the new method more closely with cryptographic primitives than with earlier cloning experiments. Research into uncloneable encryption has explored the opposite direction, designing schemes where ciphertexts cannot be meaningfully copied at all because any duplication attempt degrades the data. The Waterloo protocol flips that logic, allowing unlimited duplication while restricting decryption to a single instance, and it does so in a way that can be precisely formulated within standard quantum information theory.
What Encrypted Cloning Means for Quantum Security
The most immediate consequence is practical: quantum computers could finally store backup copies of fragile quantum data. Classical computers rely on redundant copies for error correction and disaster recovery, but the no-cloning theorem blocked that strategy for quantum systems, forcing engineers to rely on more elaborate codes that spread information across many entangled qubits. Encrypted cloning offers a complementary workaround. A quantum computer could generate multiple encrypted backups of a critical state, distribute them across different storage nodes, and recover the original by decrypting exactly one copy when needed. The single-use key constraint prevents anyone, including an attacker who intercepts the backups, from extracting more than one readable copy, preserving a fundamental scarcity of accessible information.
That built-in access control could also strengthen quantum cryptographic protocols. If encrypted clones can be freely distributed without increasing the risk of unauthorized decryption, then systems designed around single-decryptor guarantees gain a new tool for verifiable key management and auditability. The Physical Review Letters analysis establishing the theoretical foundation, combined with the IBM hardware demonstration, suggests the protocol is not merely an abstract possibility but a near-term engineering option. Quantum money schemes, secure communication channels, and distributed quantum computing all depend on the assumption that quantum states cannot be copied in a way that multiplies usable information. Encrypted cloning does not violate that assumption so much as refine it: the states can be copied as encrypted carriers, but the information they carry remains locked behind a one-time gate that enforces a hard cap on how many readable instances can exist.
Open Questions and Limits of the Approach
Several gaps remain between the current results and widespread deployment. The experimental preprint tested stability on current IBM hardware, but no published data yet compares the fidelity of encrypted clones against traditional approximate cloners under identical noise conditions, leaving open whether encryption adds hidden overheads in realistic devices. Scalability beyond 154 qubits in fault-tolerant architectures will depend on how the protocol interacts with error-correcting codes, which already require substantial qubit overhead and complex syndrome extraction. Engineers will need to determine whether encrypted backups can be maintained within logical qubit spaces without introducing new error channels or compromising the guarantees those codes provide.
There are also conceptual and security questions still to be explored. The single-use decryption key is central to the protocol’s safety, but implementing that property in a full-stack system (spanning quantum hardware, classical control, and networked infrastructure) will require careful design to avoid side channels that effectively allow multiple decryptions. Researchers will need to analyze how the scheme behaves under composition with other cryptographic tools, and whether adversaries could exploit timing, measurement strategies, or partial information about the key to circumvent the one-decryption limit. As work progresses, follow-up studies drawing on both theoretical models and larger-scale experiments will be crucial for understanding how this striking loophole in the no-cloning theorem can be turned into a reliable building block for future quantum technologies.
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*This article was researched with the help of AI, with human editors creating the final content.
