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No-cloning theorem

The no-cloning theorem is a fundamental principle in quantum mechanics asserting that it is impossible to create an identical, independent copy of an arbitrary unknown quantum state using any physical process.^[1] This theorem arises from the linearity of quantum evolution and applies to all quantum systems, distinguishing quantum information from classical bits, which can be perfectly cloned.^[2] The theorem was independently proven in 1982 by William K. Wootters and Wojciech H. Zurek in their paper "A Single Quantum Cannot Be Cloned," published in Nature,^[1] and by Dennis Dieks in "Communication by EPR Devices," published in Physics Letters A.^[3] The proof relies on the linearity of quantum mechanics: for a cloning machine to work, it must map an input state |\psi\rangle to |\psi\rangle |\psi\rangle, but when applied to a superposition \alpha |\psi\rangle + \beta |\phi\rangle, linearity produces \alpha |\psi\rangle |\psi\rangle + \beta |\phi\rangle |\phi\rangle rather than the required (\alpha |\psi\rangle + \beta |\phi\rangle)^2 = \alpha^2 |\psi\rangle |\psi\rangle + 2\alpha\beta |\psi\rangle |\phi\rangle + \beta^2 |\phi\rangle |\phi\rangle, which cannot be achieved without prior knowledge of the states.^[2] Perfect cloning is possible only for orthogonal states, but arbitrary non-orthogonal states cannot be cloned exactly.^[1] The no-cloning theorem has profound implications for quantum information science, underpinning the security of quantum cryptography protocols like quantum key distribution (QKD), where it prevents eavesdroppers from copying qubits without introducing detectable errors.^[4] It also informs quantum money schemes, which use non-clonable quantum states to prevent counterfeiting, and limits error correction in quantum computing by prohibiting straightforward duplication of quantum information for redundancy.^[4] Furthermore, the theorem highlights quantum monogamy, where information shared between parties cannot be fully replicated elsewhere, enhancing privacy in quantum networks.^[4]

Background

Quantum States and Superposition

In quantum mechanics, the state of a physical system is mathematically represented as a vector in a complex Hilbert space, a complete inner product space that provides the foundational framework for quantum descriptions. Pure states, which describe systems with maximal knowledge, are denoted by normalized ket vectors |ψ⟩ satisfying ⟨ψ|ψ⟩ = 1, where the normalization ensures the total probability is unity. For more general cases involving mixtures of states or partial knowledge, quantum states are described using density operators ρ, which are positive semi-definite Hermitian operators with trace Tr(ρ) = 1. These operators generalize pure states, as a pure state |ψ⟩ corresponds to the projector ρ = |ψ⟩⟨ψ|.^[5]^[6] A hallmark of quantum states is the principle of superposition, which allows a quantum system to exist as a coherent linear combination of basis states. For a single qubit, the computational basis states are |0⟩ and |1⟩, and a general superposition takes the form |ψ⟩ = α|0⟩ + β|1⟩, where α and β are complex amplitudes satisfying |α|^2 + |β|^2 = 1. This state embodies probabilities |α|^2 for measuring |0⟩ and |β|^2 for |1⟩, but unlike classical bits that are definitively 0 or 1, the qubit cannot be described by a classical mixture of definite states without invoking probabilities; the superposition represents an intrinsic quantum interference that persists until measurement.^[7] Entanglement extends superposition to composite systems, creating states that cannot be factored into individual subsystems. A prototypical example is the Bell state |Φ^+⟩ = \frac{1}{\sqrt{2}} (|00⟩ + |11⟩), a maximally entangled two-qubit state where the overall wavefunction is non-separable, meaning it cannot be expressed as a tensor product |ψ_A⟩ ⊗ |ψ_B⟩ of local states for qubits A and B. Such entangled states exhibit correlations that transcend classical descriptions, as the measurement outcome on one qubit instantaneously determines the state of the other, regardless of separation. The mathematical structure relies on the inner product ⟨φ|ψ⟩, which quantifies the overlap between states |φ⟩ and |ψ⟩; distinct basis states, such as |0⟩ and |1⟩, are orthogonal, satisfying ⟨0|1⟩ = 0, ensuring mutual exclusivity in measurements.^[8]^[9] Measurement of a quantum state in superposition introduces a fundamental irreversibility: upon interaction with a measuring apparatus, the state collapses probabilistically to one of the basis states, destroying the coherent superposition and any associated entanglement. For instance, measuring the qubit |ψ⟩ = α|0⟩ + β|1⟩ yields |0⟩ with probability |α|^2 or |1⟩ with |β|^2, after which the system resides definitively in that outcome, precluding reversal to the original superposition without additional quantum resources. This collapse contrasts sharply with classical bits, where copying or reading a definite 0 or 1 is reversible and nondestructive. These quantum features—superposition, entanglement, and measurement-induced collapse—underlie limitations like the no-cloning theorem.^[10]^[11]

Classical versus Quantum Information

In classical information theory, copying or duplicating data is a straightforward and deterministic process. A classical bit, which can be in one of two definite states (0 or 1), can be replicated perfectly without disturbing the original or introducing errors. For instance, a bit string representing a coin flip outcome—heads (1) or tails (0)—can be copied using simple duplication mechanisms, such as logic gates like the controlled-NOT (CNOT) gate applied to basis states, allowing multiple identical copies to be created and distributed without loss of fidelity. This capability enables broadcasting, where the same classical information can be sent to multiple receivers simultaneously, forming the basis of reliable data transmission in classical computing and communication systems. In contrast, quantum information encoded in qubits exhibits fundamentally different behavior due to the principles of quantum mechanics, particularly the linearity of quantum evolution and the backaction associated with measurement. A qubit, which can exist in a superposition of states such as \alpha |0\rangle + \beta |1\rangle where |\alpha|^2 + |\beta|^2 = 1, cannot be copied noiselessly or perfectly for an arbitrary unknown state. Attempting to duplicate such a superposition, for example, using a CNOT gate on the unknown qubit and a blank ancilla qubit initialized to |0\rangle, results in an entangled state \alpha |00\rangle + \beta |11\rangle rather than the desired product state (\alpha |0\rangle + \beta |1\rangle) \otimes (\alpha |0\rangle + \beta |1\rangle), inevitably introducing errors or correlations that prevent independent copies. This impossibility stems from the no-cloning theorem's core insight, which highlights how quantum superposition—introduced in the prior discussion of quantum states—precludes the non-duplicative replication seen in classical systems. The key distinction lies in the nature of information itself: classical information is orthogonal and deterministic, permitting perfect cloning and broadcasting to multiple parties without degradation, whereas quantum information for unknown states forbids such operations, ensuring that copies cannot be made without measurement-induced collapse or distortion. Consider the cloning of a classical coin flip versus a qubit in an unknown superposition; the classical case yields flawless replicas of heads or tails, while the quantum case always incurs fidelity loss, as no unitary process can map the input to identical outputs across all possible superpositions. The ideal cloning operation, conceptualized as a unitary transformation U satisfying U |\psi\rangle |0\rangle = |\psi\rangle |\psi\rangle for any pure state |\psi\rangle, functions seamlessly in the classical limit for basis states but fails universally in the quantum domain due to these inherent constraints.90613-7)

Historical Development

Early Motivations and Precursors

The conceptual foundations of the no-cloning theorem trace back to early investigations in quantum measurement theory, particularly those addressing the limitations of extracting and replicating information from quantum systems. In 1935, Einstein, Podolsky, and Rosen highlighted non-local correlations in entangled quantum states through their famous paradox, raising fundamental questions about whether such states could be fully characterized and reproduced without disturbing their inherent uncertainties. This work underscored the tension between quantum mechanics and classical intuitions of information copying, setting the stage for later no-go results on state replication. A pivotal precursor emerged in 1970 with a no-go result by James L. Park, which demonstrated the impossibility of perfectly replicating an unknown quantum state due to limits on information extraction in quantum theory.^[12] His analysis, rooted in the formalism of quantum information content, emphasized that any attempt to empirically ascertain and reproduce an arbitrary quantum state violates the principles of quantum mechanics, primarily motivated by constraints in quantum communication protocols where perfect replication would enable unbounded information transfer. This result highlighted the intrinsic limits of quantum information processing, though it focused more on measurement-reconstruction processes than direct cloning. Further motivation arose in the context of quantum communication and signaling. In 1981, GianCarlo Ghirardi provided an unpublished proof in a referee report for Nick Herbert's proposed "FLASH" device, arguing that cloning arbitrary quantum states is impossible and thus precludes using quantum correlations for superluminal information transmission. This argument, a special case of the later no-cloning theorem, was driven by efforts to resolve whether quantum non-locality could facilitate faster-than-light communication, reinforcing that early explorations centered on transmission limitations rather than standalone cloning devices. The immediate catalyst for formalizing the no-cloning theorem came in 1982 from Nick Herbert's proposal of a "quantum copying" machine within his FLASH device, intended to exploit entangled photons for superluminal signaling by creating replicas of polarization states.^[13] Herbert's scheme, which aimed to distinguish and duplicate unknown quantum states to enable instantaneous message relay, inadvertently exposed the need for rigorous disproofs, as it assumed a level of replicability incompatible with quantum linearity. These precursors collectively shifted focus from classical copying ideals to quantum-specific barriers, paving the way for the theorem's explicit statement while prioritizing information-theoretic implications over general duplication.

Formulation and Proofs

The no-cloning theorem was independently formulated and proved in 1982 by William K. Wootters and Wojciech H. Zurek, as well as by Dennis Dieks, in response to proposals suggesting quantum amplification could enable superluminal communication.^[1]^[14] Their work established that it is impossible to create a perfect copy of an arbitrary unknown quantum state, particularly for non-orthogonal states, due to the fundamental linearity of quantum evolution.^[14] Wootters and Zurek's proof relied on the linearity of quantum mechanics to demonstrate a contradiction in attempting universal cloning. They considered a cloning operation that would map an input state |\psi\rangle to |\psi\rangle|\psi\rangle on two output systems, while preserving the original for known basis states. For orthogonal states such as |0\rangle and |1\rangle, linearity implies the cloner would produce |0\rangle|0\rangle and |1\rangle|1\rangle correctly. However, for a superposition \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle), linearity would yield \frac{1}{\sqrt{2}}(|0\rangle|0\rangle + |1\rangle|1\rangle), which is entangled and not equal to the desired \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle) \otimes \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle), violating the cloning requirement. This argument highlights how the no-cloning theorem arises directly from the superposition principle and linear evolution in quantum theory. Dieks extended the analysis by showing that even imperfect cloning of unknown states cannot surpass classical fidelity limits. His proof emphasized that any attempt to clone non-orthogonal states via entanglement or amplification fails to produce identical copies without introducing errors exceeding those achievable by classical measurement and repreparation.^[14] Dieks' contribution underscored the theorem's implications for quantum measurement and information processing, proving that universal quantum cloners do not exist.^[14] The two papers appeared almost simultaneously—Wootters and Zurek in Nature and Dieks in Physics Letters A—marking a pivotal moment in quantum information theory. Later, Asher Peres clarified the theorem's foundation in linear algebra, emphasizing how the structure of Hilbert space and unitary transformations preclude perfect copying and directly addressing Nick Herbert's earlier proposal for a "FLASH" device.^[15] This resolution revealed the paradox's root in assumed clonability.

Core Theorem

Formal Statement

The no-cloning theorem asserts that it is impossible to create a perfect copy of an arbitrary unknown quantum state using a universal process that works for all possible inputs. Formally, there does not exist a unitary operator U on the tensor product Hilbert space \mathcal{H}_A \otimes \mathcal{H}_B, where \mathcal{H}_A and \mathcal{H}_B are isomorphic Hilbert spaces, such that for an arbitrary pure state |\psi\rangle_A on system A and a fixed initial "blank" state |e\rangle_B on system B,

U (|\psi\rangle_A |e\rangle_B) = |\psi\rangle_A |\psi\rangle_B

holds for all |\psi\rangle. This formulation requires the output states to have perfect fidelity F = 1, meaning the cloned state is identical to the original with probability 1. The theorem was independently established in its original form for unitary operations by Wootters and Zurek, and by Dieks.^[1] The impossibility specifically applies to non-orthogonal quantum states, as the linearity of quantum evolution prevents a single operation from mapping superpositions to the required tensor product form without distortion. For orthogonal states, perfect cloning is possible via classical measurement and preparation, but such a process is not universal and requires prior knowledge of the basis. A universal cloner must succeed for any unknown input state without such specificity, which is precluded by the theorem. The scope of the theorem is limited to pure states in the original proofs, but it extends to general quantum operations described by completely positive trace-preserving (CPTP) maps, or quantum channels, since any such map can be dilated to a unitary evolution on an enlarged Hilbert space via the Stinespring theorem. Thus, no CPTP map exists that perfectly clones arbitrary pure states in this universal sense.

Proof for Pure States

The proof of the no-cloning theorem for pure states proceeds by contradiction, relying on the linearity and unitarity of quantum evolution. Suppose there exists a unitary operator U acting on the tensor product Hilbert space \mathcal{H} \otimes \mathcal{H}_A, where \mathcal{H} is the Hilbert space of the system to be cloned and \mathcal{H}_A is the space of the ancillary system prepared in a fixed state |e\rangle. This operator is assumed to perfectly clone any unknown pure state |\psi\rangle \in \mathcal{H} via

U (|\psi\rangle |e\rangle) = |\psi\rangle |\psi\rangle

for all normalized |\psi\rangle. To derive a contradiction, consider two arbitrary normalized states |\phi\rangle, |\chi\rangle \in \mathcal{H} that are linearly independent but non-orthogonal, so $0 < |\langle \phi | \chi \rangle| < 1. By the cloning assumption,

U (|\phi\rangle |e\rangle) = |\phi\rangle |\phi\rangle, \quad U (|\chi\rangle |e\rangle) = |\chi\rangle |\chi\rangle.

Since U is unitary, it preserves inner products:

\langle U (\phi e) | U (\chi e) \rangle = \langle \phi e | \chi e \rangle = \langle \phi | \chi \rangle.

The left side is

\langle \phi \phi | \chi \chi \rangle = \langle \phi | \chi \rangle \langle \phi | \chi \rangle = |\langle \phi | \chi \rangle|^2.

Thus,

\langle \phi | \chi \rangle = |\langle \phi | \chi \rangle|^2.

Letting z = \langle \phi | \chi \rangle, this equation z = |z|^2 has no solution for complex z with $0 < |z| < 1, because |z|^2 < |z| in that range (and z = |z|^2 would require z real and non-negative, but even then, solutions are only z=0 or z=1). This contradiction implies that no such unitary cloner exists for arbitrary non-orthogonal states.^[1]

Generalizations

Extension to Mixed States

The no-broadcasting theorem generalizes the no-cloning theorem to mixed quantum states, asserting that it is impossible to perfectly broadcast an arbitrary mixed state \rho of a quantum system onto two or more separate quantum systems, meaning there exists no quantum operation that transforms a single copy of \rho into multiple identical copies \rho \otimes \rho \otimes \cdots.^[16] This theorem, introduced by Barnum et al. in 1996, highlights that the prohibition on cloning extends beyond pure states to the more general case of density operators describing mixed states, which arise from partial traces over entangled systems or due to environmental decoherence.^[16] The proof relies on purification: any mixed state \rho on a system can be purified to a pure state |\Psi\rangle in an enlarged Hilbert space by introducing an auxiliary system, such that tracing out the auxiliary recovers \rho.^[16] If broadcasting \rho to \rho \otimes \rho were possible, this would imply the ability to clone the purification |\Psi\rangle to |\Psi\rangle \otimes |\Psi\rangle, which contradicts the no-cloning theorem for pure states.^[16] Thus, the impossibility follows directly from the pure-state result, without requiring a separate linearity argument for mixed states. An important caveat is that broadcasting is possible for certain restricted families of mixed states, specifically those that commute—i.e., [\rho_i, \rho_j] = 0 for all pairs in the family—corresponding to classical probability distributions over orthogonal states.^[16] For non-commuting mixed states, however, no such operation exists, underscoring the inherently quantum nature of the restriction.^[16] Formally, a broadcasting channel \Lambda would act on \rho via an isometry or unitary U on the system tensored with an ancilla initialized in a fixed state |e\rangle\langle e|_B, followed by a partial trace:

\Lambda(\rho) = \operatorname{Tr}_B \left[ U (\rho \otimes |e\rangle\langle e|_B) U^\dagger \right].

No such channel can satisfy \Lambda(\rho) = \rho \otimes \rho for all mixed states \rho, as this would violate the linearity of quantum evolutions and the purification argument.^[16] The ancilla's role is to facilitate the interaction but cannot bypass the fundamental prohibition, as the overall process remains constrained by unitarity and trace preservation.^[16]

Broader Quantum Operations

The no-cloning theorem extends beyond individual quantum states to arbitrary quantum channels, which are completely positive trace-preserving (CPTP) maps describing the evolution of quantum systems. Specifically, there does not exist a universal CPTP map that can perfectly replicate the action of an arbitrary unknown quantum channel on all possible input states, as this would contradict the fundamental no-cloning limit for states themselves. This generalization underscores that quantum operations cannot universally broadcast or duplicate dynamical processes without distortion, maintaining the theorem's role as a cornerstone of quantum information restrictions. Recent theoretical advances have derived cloning bounds using the no-signaling principle alone, without relying on the full apparatus of quantum mechanics. In a 2024 study, researchers established upper limits on the fidelity of cloning machines by considering constraints from relativistic no-signaling conditions, which prevent superluminal information transfer and yield bounds matching those from quantum theory for specific cases like Choi states of unitary channels. These results demonstrate that even minimal physical principles enforce no-cloning-like restrictions, providing a pathway to understand the theorem's origins in broader physical frameworks. For d-dimensional systems, the worst-case fidelity for such channel cloning is bounded by F \leq \frac{d+1}{2d}. A key insight linking these generalizations is that relativistic causality, through the no-signaling principle, directly implies the no-cloning theorem, as violating cloning limits could enable faster-than-light communication. This connection extends to the black hole information paradox, where preserving quantum information during evaporation requires respecting no-cloning to avoid contradictions between unitarity and event horizons; proposed resolutions, such as holographic complementarity, must navigate these causality-enforced bounds to reconcile general relativity with quantum mechanics. In 2025, investigations into topological obstructions revealed that even classical mechanical systems can exhibit no-cloning-like barriers due to geometric constraints, suggesting the theorem's essence transcends quantum superposition in certain structured environments. These findings, discussed in seminars at the Perimeter Institute, highlight topological features as additional limits on replication in both classical and quantum operations, broadening the theorem's applicability.^[17]

Implications and Applications

Quantum Computing and Error Correction

The no-cloning theorem fundamentally prohibits the creation of identical copies of arbitrary quantum states, which prevents the use of classical-style redundancy in quantum computing where qubits could be duplicated for backup purposes. This limitation exacerbates challenges posed by decoherence, as quantum information cannot be redundantly stored through simple replication, forcing reliance on alternative strategies to mitigate noise and errors during computation. Instead of copying, quantum systems must preserve information through entangled encodings that distribute data across multiple physical qubits without violating the theorem's constraints.^[18] To address these issues, early quantum error-correcting codes like the Shor code, introduced in 1995, encode a single logical qubit into nine physical qubits using a combination of bit-flip and phase-flip corrections based on entangled states rather than copies. Similarly, the Steane code, developed in 1996, utilizes seven physical qubits to protect one logical qubit by leveraging the classical Hamming code structure adapted for quantum operations, again relying on entanglement to detect and correct errors without cloning. These codes enable the recovery of quantum information from partial errors, ensuring that the logical state remains intact despite environmental noise. The consequences of the no-cloning theorem extend to the quantum threshold theorem, which establishes that fault-tolerant quantum computing is achievable if the physical error rate remains below a certain threshold, relying on iterative error correction cycles that amplify logical qubits without direct copying. This theorem underpins scalable quantum algorithms by guaranteeing arbitrary precision in computations through concatenated codes, provided noise is sufficiently low. Recent computational separations, such as those demonstrated in 2023 analyses published in 2024, further illustrate how no-cloning imposes inherent limits on the efficiency of quantum simulations, separating complexity classes like clonableQMA from classical analogs and highlighting barriers to efficient state duplication in oracle-based models.^[19] Additionally, the no-cloning theorem facilitates secure multi-party quantum computation by ensuring that quantum states cannot be illicitly duplicated or intercepted during collaborative protocols, thereby enhancing privacy in distributed quantum tasks. However, it also complicates the design of scalable quantum algorithms, as the inability to broadcast or replicate states across parties increases overhead in synchronization and error management for large-scale systems.^[20]

Quantum Cryptography and Security

The no-cloning theorem serves as a foundational principle for the security of quantum cryptographic protocols, ensuring that quantum states cannot be duplicated without detection.^[21] In the BB84 protocol, introduced in 1984, the theorem underpins the security by making eavesdropping detectable: any attempt to intercept and clone polarized photons disturbs their quantum states, leading to measurable error rates in the shared key that Alice and Bob can identify and abort the session if necessary.^[22] More broadly, quantum key distribution (QKD) relies on the no-cloning theorem to prevent perfect interception of quantum information without introducing disturbances, as an eavesdropper cannot create identical copies of unknown qubit states to forward unaltered signals.^[21]^[23] Advancements in twin-field QKD protocols, such as the 2023 demonstration achieving secure key generation over 1002 kilometers, leverage the no-cloning theorem, where intermediate nodes perform interference measurements without cloning the transmitted states, thereby maintaining information-theoretic security against collective attacks.^[24] Additionally, metamodeling techniques introduced in 2025 enforce the no-cloning theorem in quantum software engineering through formal frameworks like UML and OCL constraints, ensuring that quantum program designs prevent cloning operations on non-orthogonal states during development and verification.^[25] The theorem's implications extend to the no-communication theorem, which prohibits signaling through cloning attempts in quantum cryptography, as any measurement or copying process on one party's qubits cannot convey information to another without a classical channel, preserving the protocol's unconditional security. In 2025 studies on quantum agency, the no-cloning theorem demonstrates that reliable copying of world-models in AI-quantum hybrid systems is impossible, as duplicating the quantum states representing an agent's internal model would violate the theorem, thereby limiting purely quantum realizations of decision-making agency.^[26]

Approximate and Optimal Cloning

Bounds on Fidelity

The no-cloning theorem implies that perfect replication of unknown quantum states is impossible, but approximate cloning is feasible with fidelity bounded above by values less than 1. For universal 1-to-2 cloning of qubits, where the cloner treats all pure input states equally, the optimal fidelity between the input state and each output clone is \frac{5}{6} \approx 0.833. This bound was established through the construction and optimality proof of the Bužek-Hillery cloner, which achieves this fidelity symmetrically for both clones.^[27] For restricted ensembles, higher fidelities are possible. In phase-covariant cloning, which targets equatorial states on the Bloch sphere (e.g., states of the form \frac{1}{\sqrt{2}} (|0\rangle + e^{i\phi} |1\rangle)), the optimal 1-to-2 fidelity reaches \frac{1}{2} + \sqrt{\frac{1}{8}} \approx 0.854, exceeding the universal bound due to the reduced symmetry requirements. This improvement arises from exploiting the phase invariance of the input set, as derived in the analysis of state-dependent cloners.^[28] In higher dimensions, the optimal fidelity for symmetric universal 1-to-2 cloning of qudits generalizes to F = \frac{1}{2} + \frac{1}{d+1}, where d is the dimension of the Hilbert space. For d=2, this recovers \frac{5}{6}; as d increases, F approaches \frac{1}{2}, reflecting the classical limit. This bound, which is achievable, follows from covariance arguments and has been proven optimal using methods including semidefinite programming to maximize the fidelity under unitarity and symmetry constraints. Approximate cloning inherently involves trade-offs: enhancing the fidelity of one clone typically increases disturbance to the original state or reduces the fidelity of the other clone. In asymmetric cloning, where the two output fidelities F_A and F_B differ, no-signaling conditions impose relations such as F_A + F_B \leq \frac{5}{3} for qubits, preventing superluminal communication while allowing optimization for specific tasks like eavesdropping. Although perfect cloning remains impossible, probabilistic schemes can succeed exactly with probability p \leq \frac{1}{2} for qubits, failing otherwise without disturbing the input on failure. Recent advances (2023–2024) have refined these bounds using no-signaling principles for general probabilistic and asymmetric cloners in arbitrary dimensions, deriving tight worst-case fidelity limits via linear programming that align with known optima and reveal new constraints for multipartite outputs. These results confirm that no-signaling alone suffices to derive the no-cloning theorem and its quantitative extensions without invoking full unitarity.^[29]

Universal Cloning Machines

Universal cloning machines provide an explicit construction for approximate quantum cloning that achieves the same fidelity for any input pure state, distinguishing them from state-dependent cloners optimized for specific subsets of states, such as equatorial states. These machines operate via unitary transformations on the input qubit, a blank ancilla qubit prepared in a fixed state (typically |0⟩), and an auxiliary system, producing two output qubits with correlated imperfections. The seminal example is the Bužek-Hillery cloner, introduced in 1996, which performs 1-to-2 cloning through a unitary operator U acting on the three-qubit Hilbert space.^[30] The transformation matrix for the Bužek-Hillery cloner is defined in the computational basis, with explicit actions such as U|0⟩_A|0⟩_B|0⟩C = √(2/3)|00⟩{AB}|↑⟩C + √(1/3)|+⟩{AB}|↓⟩_C and U|1⟩_A|0⟩_B|0⟩C = √(2/3)|11⟩{AB}|↓⟩C + √(1/3)|+⟩{AB}|↑⟩_C, where |+⟩ = (1/√2)(|01⟩ + |10⟩) and |↑⟩, |↓⟩ form an orthonormal basis for the ancilla. For a general input state |ψ⟩ = α|0⟩ + β|1⟩, tracing out the ancilla yields the two-qubit output state whose reduced density matrix for each clone is \rho = \frac{2}{3} |\psi\rangle\langle\psi| + \frac{1}{3} \frac{I}{2}, reflecting symmetric shrinking toward the maximally mixed state. This achieves a cloning fidelity of F = 5/6 for each output qubit, independent of the input.^[30]^[31] This fidelity represents the optimal value for symmetric universal 1-to-2 cloning of qubits, saturating the bounds derived from quantum information theory. In practical applications, such universal cloners facilitate tasks in quantum repeaters, where approximate cloning enables signal amplification and distribution of entangled states over lossy channels without direct measurement, preserving quantum coherence to some extent.^[32] More recent integrations, as of 2025, incorporate universal symmetric cloners into repeater-type quantum networks for multi-party transmission and approximate state estimation, allowing efficient resource sharing without requiring perfect cloning.^[33] Asymmetric variants of universal cloning machines extend this framework by producing clones with unequal fidelities, optimizing one output for higher accuracy while degrading the other; these are particularly useful in eavesdropping strategies for quantum key distribution, where an intercept-resend attack can balance the eavesdropper's information gain against minimal disturbance to the legitimate channel.

References

[1]
A single quantum cannot be cloned - Nature
Oct 28, 1982 · We show here that the linearity of quantum mechanics forbids such replication and that this conclusion holds for all quantum systems.
[2]
[PDF] The no-cloning theorem - UMD Physics
Mar 1, 2009 · The no-cloning theorem. William K. Wootters and Wojciech H. Zurek. People gather, copy, and distribute information all the time. But in the.
[3]
[PDF] Quantum cloning, Quantum Money, and Quantum Monogamy
The no-cloning theorem is really at the heart of the security of the quantum cryptography scheme discussed above, since when an eavesdropper reads a message and ...
[4]
[PDF] Foundations of quantum statistical mechanics - Assets - Cambridge ...
). The operator ρ is called the density operator, representing a state consisting of a ... therefore identifies the pure states with vectors of the Hilbert space, ...
[5]
[PDF] The Quantum Density Matrix and Its Many Uses - arXiv
Aug 16, 2023 · For a pure state in an n-dimensional Hilbert space, the density matrix is parametrised by 2n − 2 real parameters. The maximum number of ...
[6]
[PDF] The quantum superposition principle - arXiv
The quantum superposition principle states that any linear combination of quantum states is also a state of that system, and it is simultaneous and coherent.
[7]
[PDF] Chapter 4 Quantum Entanglement - John Preskill
Entangled states are interesting because they exhibit correlations that have no classical analog. We will begin the study of these correlations in this chapter.Missing: non- | Show results with:non-
[8]
[PDF] Entanglement and mixed states - UMD Physics
basis of Bell states,. |φ±i = (|00i±|11i)/. √. 2,. |ψ±i = (|01i±|10i)/. √. 2. (3). Most states are nonseparable. In fact, (2) is separable if and only if ad − ...Missing: non- | Show results with:non-
[9]
Fluctuation Theorems for Continuous Quantum Measurement and ...
Jul 15, 2018 · From this relation, we demonstrate that measurement-induced wave-function collapse exhibits absolute irreversibility, such that Jarzynski and ...
[10]
[PDF] Lecture 2: Quantum Math Basics 1 Complex Numbers
Sep 14, 2015 · The inner product (or dot product) of two quantum states |x1i and |x2i is defined as hx1|·|x2i, which can be further simplified as hx1|x2i ...
[11]
The empirical determination of quantum states
James L. Park. 233 Accesses. 59 Citations. Explore all metrics. Abstract. A common approach to quantum physics is enshrouded in a jargon which treats state ...
[12]
FLASH—A superluminal communicator based upon a new kind of ...
FLASH—A superluminal communicator based upon a new kind of quantum measurement. Published: December 1982. Volume 12, pages 1171–1179, (1982); Cite this article.Missing: copying | Show results with:copying
[13]
Noncommuting Mixed States Cannot Be Broadcast | Phys. Rev. Lett.
Apr 8, 1996 · We show that, given a general mixed state for a quantum system, there are no physical means for broadcasting that state onto two separate quantum systems.
[14]
https://www.sciencedirect.com/science/article/pii/0375960182900846
[15]
Is the no-cloning theorem truly quantum? Topological Obstructions ...
Jan 27, 2025 · The no-cloning theorem is an essential result in quantum information on top of which many quantum cryptography protocols are built.Missing: versus | Show results with:versus<|control11|><|separator|>
[16]
[PDF] Chapter 7 Quantum Error Correction
In our discussion of error recovery using the nine-qubit code, we have assumed that each qubit undergoes either a bit-flip error or a phase-flip error (or both) ...
[17]
A Computational Separation Between Quantum No-cloning and No ...
Feb 3, 2023 · Two of the fundamental no-go theorems of quantum information are the no-cloning theorem (that it is impossible to make copies of general quantum ...
[18]
[PDF] Secure Multi-party Quantum Computation - People | MIT CSAIL
Now a corollary of the no cloning theorem of the previous section is that no quantum code with n components can withstand the erasure of dn/2e components.
[19]
[2507.23192] Quantum Key Distribution - arXiv
Jul 31, 2025 · Abstract:Quantum Key Distribution (QKD) is a technology that ... quantum mechanics, such as the no-cloning theorem and quantum uncertainty.
[20]
[PDF] The No-cloning Theorem and its Implications in Quantum ...
More concretely, the no-cloning theorem states that it is impossible to measure in- formation that distinguishes between two non-orthogonal quantum states ...
[21]
What Is Quantum Cryptography? | NIST
Apr 24, 2025 · This “no-cloning rule” means that anyone trying to intercept or copy a code based on one or more qubits will destroy the code instead of ...
[22]
Quantum copying: Beyond the no-cloning theorem | Phys. Rev. A
Sep 1, 1996 · We analyze the possibility of copying (that is, cloning) arbitrary states of a quantum-mechanical spin-1/2 system.
[23]
Optimal universal and state-dependent quantum cloning | Phys. Rev. A
Quantum cloning produces two clones from one input. Universal cloners have a maximal fidelity of 5/6, while state-dependent cloners operate on two ...
[24]
Fundamental limits on quantum cloning from the no-signaling principle
The no-cloning theorem states that within the framework of quantum mechanics there does not exist any universal procedure that is able to replicate an unknown ...
[25]
https://www.sciencedirect.com/science/article/pii/S154622182500654X
[26]
Universal optimal cloning of qubits and quantum registers - arXiv
Jan 7, 1998 · Abstract: We review our recent work on the universal (i.e. input state independent) optimal quantum copying (cloning) of qubits.Missing: 1996 5/6
[27]
[1301.2956] Quantum Cloning Machines and the Applications - arXiv
Jan 14, 2013 · Specifically, quantum cloning machines can be designed to analyze the security of quantum key distribution protocols such as BB84 protocol, six- ...