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WSEAS Transactions on Circuits and Systems archive

Brief Notes on The Possibility of Copying Qubits in Quantum Systems

Artyom M. Grigoryan, Sos С. Agaian

Published Feb 10, 2022• 0 citations
Publisher

Abstract

Copying the quantum states is contradictory to classical information processing since the fundamental difference between classical and quantum information is that while classical information can be copied perfectly, quantum information cannot. However, this statement does not rule out the risk of building a device that can reproduce a set of quantum states. This paper investigates the naturally arising question of how well or under what conditions one can copy and measure an arbitrary quantum superposition of states. The CNOT and XOR operation-based quantum circuit is presented that exhibits entanglement of states and allows for measuring the doubled qubits.

DOI
10.37394/23201.2022.21.2

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Abstract An arbitrary quantum state cannot be “cloned” perfectly, i.e. reproduced with “fidelity” F = 1 into M > 1 states identical to the original by any conceivable physical device. The main root of this impossibility resides in the linearity of quantum mechanics. A second “quantum impossibility” process, based on the complete positivity character of any quantum operation and conceivably subtly related to the first one, forbids the realization of a universal NOT gate i.e. one that flips exactly any input qubit into an orthogonal one. A detailed investigation of these results, representing the most fundamental difference between classical and quantum information, indeed can reveal the detailed structure of the latter one. We report an experimental demonstration of the process of cloning of N = 1 input qubit into M = 2 output qubits by a quantum‐injected optical parametric amplifier OPA. By the same apparatus the realization of a universal NOT gate is also demonstrated. The two processes will be found to be “universal” and optimal, i.e. the measured fidelity of both processes F < 1 will be found close to the theoretical values.

Fortschritte der Physik•Apr 24, 2003•2 citations
Copying the quantum states is contradictory to classical information processing since the fundamental difference between classical and quantum information is that while classical information can be copied perfectly, quantum information cannot. However, this statement does not rule out the risk of building a device that can reproduce a set of quantum states. This paper investigates the naturally arising question of how well or under what conditions one can copy and measure an arbitrary quantum superposition of states. The CNOT and XOR operation-based quantum circuit is presented that exhibits entanglement of states and allows for measuring the doubled qubits.
Copying the quantum states is contradictory to classical information processing since the fundamental difference between classical and quantum information is that while classical information can be copied perfectly, quantum information cannot. However, this statement does not rule out the risk of building a device that can reproduce a set of quantum states. This paper investigates the naturally arising question of how well or under what conditions one can copy and measure an arbitrary quantum superposition of states. The CNOT and XOR operation-based quantum circuit is presented that exhibits entanglement of states and allows for measuring the doubled qubits.
Copying information is an elementary operation in classical information processing. However, copying seems rather different in the quantum regime. Since the discovery of the universal quantum cloning machine, much has been found from the fundamental point of view about quantum copying. But a basic question as to the utility of universal quantum cloning remains. We have considered its application in quantum state restoration by using cloning circuit for state estimation. It might be expected that classical information from the state estimation might help restore the quantum state that was disturbed during transmission. We find that the fidelity of the final state is, interestingly, independent of error probabilities inside the channel. However, this also turns out to impose a severe constraint on our original aims.
Information processing based on the laws of quantum mechanics promises to be a revolutionary new avenue in information technology. This emerging field of quantum information processing (QIP) is however challenged by the fragile nature of the quantum bits (qubits) in which quantum information is stored and processed. An error in even a single qubit makes the quantum processor go off-track, corrupting the calculation as a whole. Therefore, the chance for an erroneous outcome increases with the number of qubits in the processor. Large-scale QIP thus hinges on the ability to correct for these errors. Classical information processing often uses error correction algorithms to identify errors by checking whether information is consistent in multiple copies. This strategy is unfortunately not applicable to QIP as quantum states cannot be copied. Moreover, direct measurements on qubits collapse their quantum states, reducing them to classical information. Fortunately, the theory of quantum error correction (QEC) overcomes these complications by encoding quantum information in entangled states of many qubits and performing parity measurements to identify errors in the system without destroying the encoded information. Implementing these codes is challenging as it requires many qubits and quick interleaving of operations and measurements. Moreover, to not introduce more errors in the system than QEC can solve for, these operations and measurements need to be of sufficient fidelity and speed...
Over the last decade a new way to compute has been defined which, unlike conventional methods, is based on quantum mechanics rather than classical physics. This fundamental change in paradigm can, in principle, solve some important and hitherto intractable problems such as prime factorization of large numbers. This tutorial presentation will review the basics of quantum computation and the design of circuits to implement quantum algorithms. The starting point is the notion of a quantum bit or qubit. Because of the superposition property of quantum states, n qubits can store 2n numbers simultaneously, implying a type of massive parallelism. Furthermore, quantum states allow powerful forms of interaction such as entanglement that have no classical counterparts. Qubits are fragile, however, and are altered by measurement; hence quantum circuits must follow very different rules from classical ones. The differences between quantum and classical logic circuits will be discussed and illustrated. Finally, the physical implementation of quantum devices will be considered, along with the prospects for practical quantum computers.
Abstract An arbitrary quantum state cannot be “cloned” perfectly, i.e. reproduced with “fidelity” F = 1 into M > 1 states identical to the original by any conceivable physical device. The main root of this impossibility resides in the linearity of quantum mechanics. A second “quantum impossibility” process, based on the complete positivity character of any quantum operation and conceivably subtly related to the first one, forbids the realization of a universal NOT gate i.e. one that flips exactly any input qubit into an orthogonal one. A detailed investigation of these results, representing the most fundamental difference between classical and quantum information, indeed can reveal the detailed structure of the latter one. We report an experimental demonstration of the process of cloning of N = 1 input qubit into M = 2 output qubits by a quantum‐injected optical parametric amplifier OPA. By the same apparatus the realization of a universal NOT gate is also demonstrated. The two processes will be found to be “universal” and optimal, i.e. the measured fidelity of both processes F < 1 will be found close to the theoretical values.