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Physical Review A

Entangling and cloning machine with increasing robustness against decoherence as the number of qubits increases

Shi‐Biao Zheng, Guang‐Can Guo

Published Dec 9, 2005• 17 citations
Publisher

Abstract

We propose a machine for entangling multiple qubits and implementing cloning and anti-cloning. The operation speed increases with the number of qubits without a limitation and thus the operation becomes more and more robust against decoherence with the increase of the number of qubits. This is contrary to usual entangling machines and quantum information processors whose performance would deteriorate with the increase of the number of qubits due to decoherence. We illustrate the idea in the ion trap system, however it can also be realized in other systems, opening a new perspective for building a scalable machine for entanglement generation and quantum information processing.

DOI
10.1103/physreva.72.064303

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Communications in Theoretical Physics•Jan 1, 2008•1 citation
We propose a machine for entangling multiple qubits and implementing cloning and anti-cloning. The operation speed increases with the number of qubits without a limitation and thus the operation becomes more and more robust against decoherence with the increase of the number of qubits. This is contrary to usual entangling machines and quantum information processors whose performance would deteriorate with the increase of the number of qubits due to decoherence. We illustrate the idea in the ion trap system, however it can also be realized in other systems, opening a new perspective for building a scalable machine for entanglement generation and quantum information processing.
Universal quantum-cloning machines (UQCMs), sometimes called quantum cloners, generate many outputs with identical density matrices, with as close a resemblance to the input state as is allowed by the basic principles of quantum mechanics. Any experimental realization of a quantum cloner has to cope with the effects of decoherence which terminate the coherent evolution demanded by a UQCM. We examine how many clones can be generated within a decoherence time. We compare the time that a quantum cloner implemented with trapped ions requires to produce M copies from N identical pure state inputs and the decoherence time during which the probability of spontaneous emission becomes non-negligible. We find a method to construct an $\stackrel{\ensuremath{\rightarrow}}{N}M$ cloning circuit, and estimate the number of elementary logic gates required. It turns out that our circuit is highly vulnerable to spontaneous emission as the number of gates in the circuit is exponential with respect to the number of qubits involved.
We propose a physically realizable machine which can either generate multiparticle W-like states, or implement high-fidelity 1-->M (M=1,2,...infinity) anticloning of an arbitrary qubit state, in a single step. This universal machine acts as a catalyst in that it is unchanged after either procedure, effectively resetting itself for its next operation. It possesses an inherent immunity to decoherence. Most importantly in terms of practical multiparty quantum communication, the machine's robustness in the presence of decoherence actually increases as the number of qubits M increases.
Entangling operations are a necessary tool for large-scale quantum information processing, but experimental imperfections can prevent current schemes from reaching sufficient fidelities as the number of qubits is increased.Here it is shown numerically how multitoned generalizations of standard trapped-ion entangling gates can simultaneously be made robust against noise and mis-sets of the frequencies of the individual qubits.This relaxes the degree of homogeneity required in the trapping field, making physically larger systems more practical.