Single-use key unlocks perfect qubit copies, scientists find

Researchers found encrypted cloning stable under hardware noise, even when used in complex configurations, establishing it as a versatile quantum primitive. This finding requires a revision to their understanding of quantum limits: the constraint isn’t duplication itself, but the single-use nature of the decryption.

Encrypted Cloning Circumvents No-Cloning with Single-Use Keys

The ability to create redundant copies of quantum data, once forbidden by the no-cloning theorem, is now demonstrably possible through a process called encrypted cloning, according to recent experiments. This circumvention relies on simultaneously encrypting replicated qubits with a single-use decryption key, a detail that preserves the fundamental laws of quantum mechanics while enabling practical applications. The research establishes that quantum information can be spread without degradation, provided the decryption process is limited to a single instance.

This approach addresses a critical challenge in quantum information processing: the dilution of signals due to hardware noise. Spreading quantum information across multiple channels previously risked catastrophic fidelity decay, but the experiments show encrypted cloning remains stable even when used in complex configurations. Researchers tested this modularity by preparing Greenberger-Horne-Zeilinger (GHZ) states, with up to 15 qubits, and creating three encrypted clones of each qubit.

The single-use nature of the decryption key is fundamental. Once used, it renders all remaining encrypted clones indecipherable, ensuring adherence to the no-cloning theorem. “The actual constraint is that the decryption mechanism must be single-use,” the research concludes, refining the understanding of quantum limits and establishing encrypted cloning as a versatile quantum primitive for future technologies.

IBM Heron-R2 Qubit Platform Validates Encrypted Cloning

The researchers find that encrypted cloning is stable under hardware noise, even when used as a module, namely in parallel, series or interleaved, while preserving pre-existing entanglement. Since this prohibition against perfect duplication presents a significant obstacle in quantum information processing, various strategies for achieving at least some level of approximate quantum cloning have been developed. Among these, the universal quantum cloning machine enables deterministic generation of two approximate copies of an arbitrary qubit with a fidelity of 5/6.

This value was proven to be approximate cloning’s maximum. Even this maximum performance is incapable, however, to provide the clones with sufficient quantumness.

For example, they cannot violate the Clauser-Horne-Shimony-Holt (CHSH) inequalities (see Supplementary Information). Subsequent studies have extended approximate cloning to include schemes that exploit prior knowledge of the states to be copied, such as state-dependent and probabilistic cloning, as well as phase-covariant generalizations (see, e.g., Refs.

Encrypted Cloning Maintains Fidelity Across Multiple Copies

Encrypted cloning maintains fidelity even when applied to multiple entangled qubits, a finding that expands the potential of this recently proposed quantum technique. Experiments demonstrate that the fidelity of recovered quantum states remains robust even when cloning qubits within a genuine multipartite entanglement, such as a GHZ state, up to four qubits. This signifies that encrypted cloning isn’t limited to simple, two-qubit systems and can be integrated into more complex circuits without immediately losing coherence.

The observed fidelity degradation isn’t driven by the sheer number of encrypted clones created, but rather by the circuit depth, the number of operations performed, a critical distinction for building practical quantum devices. The team also investigated the correlation strength between an ancillary qubit and the decrypted encrypted clone, finding that the correlation remains strong even after encryption and decryption. “Encrypted cloning can serve as a candidate quantum primitive in the sense that it respects not only bipartite but also genuine multipartite entanglement of input qubits embedded in larger circuits,” the paper states.

Encrypted Cloning Preserves Pre-Existing Entanglement

Within quantum systems, a finding demonstrated through experiments maintaining entanglement fidelity even after multiple cloning iterations. The work establishes that pre-existing two-partite and multi-partite entanglement survives the encryption and decryption processes inherent to the protocol, validating its potential as a modular component in larger systems. This preservation is important because the dilution of entanglement is a primary obstacle to scaling quantum technologies. The ability to maintain it during cloning was unexpected given the inherent fragility of quantum states.

Experiments specifically tested the protocol’s resilience by measuring entanglement of a single qubit after decrypting one of up to seven encrypted clones, confirming that prior entanglement is recoverable. Further tests assessed the protocol’s performance with multipartite entanglement, specifically a six-partite GHZ state, demonstrating its applicability beyond simple qubit pairs. The ability to operate in parallel, series, or interleaved configurations further solidifies encrypted cloning’s versatility as a quantum primitive, allowing it to be smoothly integrated into diverse quantum architectures.

Comparison of Encrypted Cloning to Approximate Cloning Methods

This efficiency is notable because it suggests that the computational cost of generating multiple encrypted clones does not increase linearly with the desired number of copies, offering a potential advantage over methods that require exponentially more resources. The experimental setup involved evaluating the fidelity of both Bell states and r-qubit GHZ states, employing two distinct methods for fidelity estimation.

Experimental Demonstration of Encrypted Cloning Module Integration

The ability to create multiple, encrypted copies of a qubit has been experimentally verified using IBM Heron-R2 processors, demonstrating resilience even within complex circuits. Researchers evaluated the protocol’s performance by iteratively applying encrypted cloning, while preserving pre-existing entanglement. The team’s work demonstrates that encrypted cloning is a versatile quantum primitive suitable for practical use.

Quantum Cloud Storage Application of Encrypted Cloning

Quantum cloud storage benefits from a newly demonstrated redundancy technique; encrypted cloning allows for perfect data recovery even if some storage servers fail. Experiments specifically tested the protocol’s viability as such a module, demonstrating its stability under realistic conditions. The team performed tests to measure sensitivity to hardware noise, and to demonstrate operation in parallel, series and interleaved configurations, all while preserving entanglement.

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