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arXiv:1906.00609

Side information -driven quantum composite control for protecting a qubit

Ya Cao, Fei Gao, Dandan Li, Qiaoyan Wen

Published Jun 3, 2019• 1 citation
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Abstract

We study protection of a qubit that transfer through a decoherence noise by quantum control technique. In this work, we assume that the communication participants have some side information about the qubit. Our aim is to take fully advantage of given side information to design a quantum control scheme that makes the output state close to the input state as better as possible. Especially, in this paper, we consider the quantum composite control (QCC) structure, which has shown to capture a better balance between fidelity and success probability, i.e., two essential measures for protection. Comparison has been made between our scheme and previous ones. We show there's advantages in two aspects. On one hand, when compared with the scheme that has taking fully consideration of classical side information but not quantum one, our scheme achieves higher fidelity. Inversely, when the scheme with quantum side information but not classical one is considered, our scheme also has nontrivial improvement.

DOI
10.48550/arxiv.1906.00609

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We study protection of a qubit that transfer through a decoherence noise by quantum control technique. In this work, we assume that the communication participants have some side information about the qubit. Our aim is to take fully advantage of given side information to design a quantum control scheme that makes the output state close to the input state as better as possible. Especially, in this paper, we consider the quantum composite control (QCC) structure, which has shown to capture a better balance between fidelity and success probability, i.e., two essential measures for protection. Comparison has been made between our scheme and previous ones. We show there's advantages in two aspects. On one hand, when compared with the scheme that has taking fully consideration of classical side information but not quantum one, our scheme achieves higher fidelity. Inversely, when the scheme with quantum side information but not classical one is considered, our scheme also has nontrivial improvement.
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One of the crucial tasks in quantum systems is to reduce the effects of decoherence due to the unavoidable interactions between a system and its environment. Many protection schemes have been proposed recently, among them the weak measurement quantum measurement reversal (WMQMR), weak measurement-based quantum feedback control (QFBC) and quantum feedforward control (QFFC) are reviewed in this paper. By considering weak measurement, the aim is to find a balance between information gain and disturbance of the system caused by the measurement. We classify different types of measurement and give the definition of noise sources and their effects on the state of the system. Finally, we compare and analyze the performance of the discussed protection schemes for different noise sources by numerical simulations.
The goal of quantum control is to enable robust quantum information processing in noisy environments. Quantum feedback control is one of the most established strategies, but its reliance on measurement and real-time feedback imposes intrinsic limitations from measurement backaction and latency. Here, we propose and experimentally realize a measurement-free quantum feedforward control scheme, together with a composite framework that sequentially combines quantum feedforward control and quantum feedback control. We demonstrate both theoretically and experimentally that quantum feedforward control effectively protects nonorthogonal quantum states without measurement, while the composite strategy achieves superior performance when measurement is available, which makes it a practical alternative to quantum feedback control. These results establish quantum feedforward control as an independent control approach beyond quantum feedback control and highlight the advantages of the composite scheme, providing a new route toward high-fidelity quantum information processing in noisy environments.
NNN-Qubit State Protection Against Amplitude Damping by Quantum Feed-Forward Control and its Reversal
We propose a state protection scheme of quantum feed-forward control and its reversal (QFFCR) by quantum weak measurement and quantum control to protect the arbitrary N-qubit state in amplitude-damping channel (ADC). Before the state enters the ADC, a weak measurement and an offsetting operation are made to make the state almost immune to the effect of ADC. After the state passes the ADC, the reversed offsetting operation and a correction rotation are applied to recover the state. Unlike most previous state protection schemes, ours does not use weak measurement reversal, but a correction rotation to bring back the state to its initial amount. Our protection scheme can also protect entangled states even for heavy damping probabilities effectively. Fidelity and success probability are used to evaluate the effect of the proposed scheme. The general formula for complete recovery and the total success probability of N-qubit system are derived. The effectiveness of our control scheme is demonstrated by simulation results.
We discuss protection of a quantum state that goes through a noise process by measurements and operations before and after the noise process. In our previous work, we showed the nonexistence of ``truly quantum'' protocols that protect an unknown qubit state against depolarizing noise better than ``classical'' ones. Toward identifying the class of noise processes that is optimally suppressed by such a ``classical'' protocol, we extend our previous result in two directions. First, we show that the statement is also true in any finite-dimensional Hilbert spaces, which was previously conjectured; the optimal protocol is either the do nothing protocol or the discriminate and reprepare protocol, depending on the strength of the noise. Second, in the case of a single qubit, we show that essentially the same conclusion holds for any unital noise. Thus, the noise must be nonunital for a control protocol beyond ``classical'' ones to exist.