Quantum Cloning Machine Research Articles

Simple construction of Rydberg quantum cloning machines via nonadiabatic geometric quantum operations

A theoretical scheme based on the nonadiabatic geometric quantum computation (NGQC) is proposed to realize quantum cloning in the Rydberg atom system. In contrast to previous schemes, the present one utilizes the NGQC to construct the clone machine and thus is more robust against control errors. Meanwhile, to implement the desired cloning framework in the quantum clone machine, the scheme reported by Zhu and Ye [M.-Z. Zhu and L. Ye, Phys. Rev. A 91, 042319 (2015)] based on cavity QED requires a series of single(two)-qubit operations. For the present NGQC-based scheme, through modulating the rotation angle of geometric operations, we can greatly simplify the process of realizing the cloning framework. In addition, through adjusting the relevant parameters, the scheme can perform symmetrical (asymmetrical) universal cloning, optimal symmetrical (asymmetrical) phase-covariant quantum cloning, and optimal symmetrical (asymmetrical) real-state quantum cloning. The present attempt to optimize quantum cloning using geometric quantum control in Rydberg atoms provides a path to robust and simplified quantum cloning, which is meaningful for experiments and has implications for quantum cloning in other quantum systems as well.

Process-optimized phase-covariant quantum cloning

After the appearance of the no-cloning theorem, approximate quantum cloning machines (QCMs) has become a well-studied subject in quantum information theory. Among several measures to quantify the performance of a QCM, single-qudit fidelity and global fidelity have been most widely used. In this paper we compute the optimal global fidelity for phase-covariant cloning machines via semidefinite programming optimization, thereby completing a remaining gap in the previous results on QCMs. We also consider optimal simulations of the cloning and transpose cloning map, both by a direct optimization and by a composition of component-wise optimal QCMs. For the cloning map the composition method is suboptimal whereas for the transpose cloning map the method is asymptotically optimal.

Quantum Switchboard with Coupled-Cavity Array

The ability to control the flow of quantum information is deterministically useful for scaling up quantum computation. In this paper, we demonstrate a controllable quantum switchboard which directs the teleportation protocol to one of two targets, fully dependent on the sender’s choice. Importantly, the quantum switchboard also acts as a optimal quantum cloning machine, which allows the receivers to recover the unknown quantum state with a maximal fidelity of . This protects the system from the complete loss of quantum information in the event that the teleportation protocol fails. We also provide an experimentally feasible physical implementation of the proposal using a coupled-cavity array. The proposed switchboard can be utilized for the efficient routing of quantum information in a large quantum network.

On optimal cloning and incompatibility

We investigate the role of symmetric quantum cloning machines (QCMs) in quantifying the mutual incompatibility of quantum observables. Specifically, we identify a cloning-based incompatibility measure whereby the incompatibility of a set of observables maybe quantified in terms of how well a uniform ensemble of their eigenstates can be cloned via a symmetric QCM. We show that this new incompatibility measure is faithful since it vanishes only for commuting observables. We prove an upper bound for for any set of observables in a finite-dimensional system and show that the upper bound is attained if and only if the observables are mutually unbiased. Finally, we use our formalism to obtain the optimal quantum cloner for a pair of qubit observables. Our work marks an important step in formalising the connection between two fundamental concepts in quantum information theory, namely, the no-cloning principle and the existence of incompatible observables in quantum theory.

Experimental demonstration of entanglement-enabled universal quantum cloning in a circuit

No-cloning theorem forbids perfect cloning of an unknown quantum state. A universal quantum cloning machine (UQCM), capable of producing two copies of any input qubit with the optimal fidelity, is of fundamental interest and has applications in quantum information processing. This is enabled by delicately tailored nonclassical correlations between the input qubit and the copying qubits, which distinguish the UQCM from a classical counterpart, but whose experimental demonstrations are still lacking. We here implement the UQCM in a superconducting circuit and investigate these correlations. The measured entanglements well agree with our theoretical prediction that they are independent of the input state and thus constitute a universal quantum behavior of the UQCM that was not previously revealed. Another feature of our experiment is the realization of deterministic and individual cloning, in contrast to previously demonstrated UQCMs, which either were probabilistic or did not constitute true cloning of individual qubits.

Quantum process randomness

We argue that the optimization of a quantum process based on the customarily used fidelity between the actual and the expected outputs may not be sufficient. We introduce a quantity namely the “quantum process randomness”, which is the standard deviation of the distribution of Born probabilities corresponding to the orthonormal measurement that projects the expected output onto the eigenstates of the actual output, and obtain a trade-off between fidelity and randomness for an arbitrary quantum process. We consider approximate quantum cloning and teleportation processes to illustrate the concept. We find that the optimal approximate universal and state-dependent quantum cloning machines obtained by maximizing the fidelity is the same as that obtained by optimizing the difference between randomness and fidelity.

Continuous variable direct secure quantum communication using Gaussian states

Continuous variable one-way and controlled-two-way secure direct quantum communication schemes have been designed using Gaussian states. Specifically, a scheme for continuous variable quantum secure direct communication and another scheme for continuous variable controlled quantum dialogue are proposed using single-mode squeezed coherent states. The security of the proposed schemes against a set of attacks (e.g., Gaussian quantum cloning machine and intercept resend attacks) has been proved. Further, it is established that the proposed schemes do not require two-mode squeezed states which are essential for a set of existing proposals. The controlled two-way communication scheme is shown to be very general in nature as it can be reduced to schemes for various relatively simpler cryptographic tasks like controlled deterministic secure communication, quantum dialogue, quantum key distribution. In addition, it is briefly discussed that the proposed schemes can provide us tools to design quantum cryptographic solutions for several socioeconomic problems.

Broadcasting of entanglement via orthogonal and non-orthogonal state-dependent cloners

In this work, we extensively study the problem of broadcasting of entanglement as state dependent versus state independent cloners. We start by re-conceptualizing the idea of state dependent quantum cloning machine (SD-QCM), and in that process, we introduce different types of SD-QCMs, namely, orthogonal and non-orthogonal cloners. We derive the conditions for which the fidelity of these cloners will become independent of the input state. We note that such a construction allows us to maximize the cloning fidelity at the cost of having partial information of the input state. In the discussion on broadcasting of entanglement, we start with a general two qubit state as our resource and later we consider a specific example of Bell diagonal state. We apply both state dependent and state independent cloners (orthogonal and non-orthogonal), locally and non locally, on input resource state and obtain a range for broadcasting of entanglement in terms of the input state parameters. Our results highlight several instances where the state dependent cloners outperform their state independent counterparts in broadcasting entanglement. Our study provides a comparative perspective on the broadcasting of entanglement via cloning in two qubit scenario, when we have some knowledge of the resource ensemble versus a situation when we have no such information.

Multipurpose Quantum Simulator Based on a Hybrid Solid-State Quantum Device

This paper proposes a scheme to enhance the fidelity of symmetric and asymmetric quantum cloning using a hybrid system based on nitrogen-vacancy (N-V) centers. By setting different initial states, the present scheme can implement optimal symmetric (asymmetric) universal (phase-covariant) quantum cloning, so that the copies with the assistance of a Current-biased Josephson junction (CBJJ) qubit and four transmission-line resonators (TLRs) can be obtained. The scheme consists of two stages: cjhothe first stage is the implementation of the conventional controlled-phase gate, and the second is the realization of different quantum cloning machines (QCM) by choosing a suitable evolution time. The results show that the probability of success for QCM of a copy of the equatorial state can reach 1. Furthermore, the | W 4 ± ⟩ entangled state can be generated in the process of the phase-covariant quantum anti-cloning. Finally, the decoherence effects caused by the N-V center qubits and CBJJ qubit are discussed.

High-fidelity quantum cloning of two nonorthogonal quantum states via weak measurements

We propose a scheme to enhance the fidelity of symmetric quantum cloning machine using a weak measurement. By adjusting the intensity of weak measurement parameter $p$, we obtain the copies with different optimal fidelity. Choosing proper value of $p$, we can obtain the perfect copies for initial qubits. In this paper, we focus on $1-2$ quantum cloning for two nonorthogonal states. Sets containing more than two linear independent states are also discussed briefly. Due to weak measurements being probabilistic, we obtain high fidelity at risk of probability. If the weak measurement successes, we do the following operations to obtain copies with high fidelity, otherwise, the cloning process fails and we need do nothing. From this perspective, the scheme we propose is economical for saving quantum resource and time, which may be very useful in quantum information processing.

Coherence and entanglement under three-qubit cloning operations

Coherence and entanglement are the two most crucial resources for various quantum information processing tasks. Here, we study the interplay of coherence and entanglement under the action of different three qubit quantum cloning operations. Considering certain well-known quantum cloning machines (input state independent and dependent), we provide examples of coherent and incoherent operations performed by them. We show that both the output entanglement and coherence could vanish under incoherent cloning operations. Coherent cloning operations on the other hand, could be used to construct a universal and optimal coherence machine. It is also shown that under coherent cloning operations the output two qubit entanglement could be maximal even if the input coherence is negligible. Also it is possible to generate a fixed amount of entanglement independent of the nature of the input state.

Duplicating classical bits with universal quantum cloning machine

It seems there is a large gap between quantum cloning and classical duplication since quantum mechanics forbid perfect copies of unknown quantum states. In this paper, we prove that a classical duplication process can be realized by using a universal quantum cloning machine (QCM). A classical bit is encoded not on a single quantum state, but on a large number of single identical quantum states. Errors are inevitable when copying these identical quantum states due to the quantum no-cloning theorem. When a small part of errors are ignored, i.e., errors as the minority are automatically corrected by the majority, the fidelity of duplicated copies of classical information will approach unity infinitely. In this way, the classical bits can be duplicated precisely with a universal QCM, which presents a natural transition from quantum cloning to classical duplication. The implement of classical duplication by using QCM shines new lights on the universality of quantum mechanics.

Secure Coding for Type II Quantum Wiretap Channel

This paper considers the problem of secure coding design for a type II quantum wiretap channel, where the main channel and the eavesdropper channel are both quantum channel, and the eavesdropper overhears the legitimate communication through a quantum cloning machine. Based on Calderbank-Shor-Steane (CSS)-type quantum low-density parity-check (QLDPC) codes, we show that it is possible to construct linear-time decodable codes for type II quantum wiretap channels to achieve security.

On an explicit representation of the Łukasiewicz sum as a quantum operation

The aim of this work is to introduce a quantum operation able to implement, in an approximate way, the Łukasiewicz sum in the framework of quantum computation. Different techniques for improving this approximation are studied, and in particular, the use of quantum cloning machine is considered.

Fast implementation of the orbital state quantum cloning machine

We present a scheme to implement a orbital state quantum cloning machine assisted by quantum Zeno dynamics. By constructing shortcuts to adiabatic passage with transitionless quantum driving, we can complete this scheme effectively and quickly in one step. The effects of decoherence, including spontaneous emission and the decay of the cavity, are also discussed. The numerical simulation results show that high fidelity can be obtained and the feasibility analysis indicates that this can also be realized in experiments.

Performance of quantum cloning and deleting machines over coherence

Coherence, being at the heart of interference phenomena, is found to be an useful resource in quantum information theory. Here we want to understand quantum coherence under the combination of two fundamentally dual processes, viz., cloning and deleting. We found the role of quantum cloning and deletion machines with the consumption and generation of quantum coherence. We establish cloning as a cohering process and deletion as a decohering process. Fidelity of the process will be shown to have connection with coherence generation and consumption of the processes.

True Single-Photon Stimulated Four-Wave Mixing

Single-photon stimulated four-wave mixing (StFWM) is an important nonlinear optical phenomenon at the single-photon level. In this process, signal and idler photons are stimulated by seeded single photons when two pump photons are annihilated. The generated photons are correlated with each other, and more importantly, one of them is in the same mode with the seeded photons. Hence, StFWM has great potential to realize complex functions of photonic quantum information processing, such as quantum state manipulation and quantum cloning. In this paper, we provide the first experimental demonstration of single-photon StFWM in a piece of optical fiber. By using heralded single photons as the seeded photons, a true single-photon StFWM process is realized at the telecom band. It is confirmed by time-resolved four-photon coincidence measurement and variation of four-photon coincidence counts under different seed–pump delays. According to the experiment results, the potential performance of a quantum cloning machine...

High-dimensional quantum cloning and applications to quantum hacking.

Attempts at cloning a quantum system result in the introduction of imperfections in the state of the copies. This is a consequence of the no-cloning theorem, which is a fundamental law of quantum physics and the backbone of security for quantum communications. Although perfect copies are prohibited, a quantum state may be copied with maximal accuracy via various optimal cloning schemes. Optimal quantum cloning, which lies at the border of the physical limit imposed by the no-signaling theorem and the Heisenberg uncertainty principle, has been experimentally realized for low-dimensional photonic states. However, an increase in the dimensionality of quantum systems is greatly beneficial to quantum computation and communication protocols. Nonetheless, no experimental demonstration of optimal cloning machines has hitherto been shown for high-dimensional quantum systems. We perform optimal cloning of high-dimensional photonic states by means of the symmetrization method. We show the universality of our technique by conducting cloning of numerous arbitrary input states and fully characterize our cloning machine by performing quantum state tomography on cloned photons. In addition, a cloning attack on a Bennett and Brassard (BB84) quantum key distribution protocol is experimentally demonstrated to reveal the robustness of high-dimensional states in quantum cryptography.

Two-qubit quantum cloning machine and quantum correlation broadcasting

Due to the axioms of quantum mechanics, perfect cloning of an unknown quantum state is impossible. But since imperfect cloning is still possible, a question arises: "Is there an optimal quantum cloning machine?" Buzek and Hillery answer to this question and construct their famous B-H quantum cloning machine. The B-H machine clones state of an arbitrary single qubit in optimal manner and hence it is universal. Generalizing this machine for two-qubit system is straightforward, but this procedure does not preserve quantum correlation existing in bipartite state in optimal manner and also, during this procedure, this machine loses its universality and becomes a state-dependent cloning machine. In this paper we propose an optimal universal local quantum state cloner for two qubit systems. Also we present two classes of state-dependent local quantum copying machine. Furthermore, we investigate local broadcasting of two aspects of quantum correlations, i.e., quantum entanglement and quantum discord defined, respectively, within the entanglement-separability paradigm and from an information-theoretic perspective. The results show that although quantum correlation is in general very fragile during broadcasting procedure, quantum discord is broadcasted more robustly than quantum entanglement.

Optimal quantum cloning based on the maximin principle by usinga prioriinformation

We propose an optimal $1\ensuremath{\rightarrow}2$ quantum cloning method based on the maximin principle by making full use of a priori information of amplitude and phase about the general cloned qubit input set, which is a simply connected region enclosed by a ``longitude-latitude grid'' on the Bloch sphere. Theoretically, the fidelity of the optimal quantum cloning machine derived from this method is the largest in terms of the maximin principle compared with that of any other machine. The problem solving is an optimization process that involves six unknown complex variables, six vectors in an uncertain-dimensional complex vector space, and four equality constraints. Moreover, by restricting the structure of the quantum cloning machine, the optimization problem is simplified as a three-real-parameter suboptimization problem with only one equality constraint. We obtain the explicit formula for a suboptimal quantum cloning machine. Additionally, the fidelity of our suboptimal quantum cloning machine is higher than or at least equal to that of universal quantum cloning machines and phase-covariant quantum cloning machines. It is also underlined that the suboptimal cloning machine outperforms the ``belt quantum cloning machine'' for some cases.