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Physical Review A

Engineered noisy environment for studying decoherence

愛 岩倉, Yuichiro Matsuzaki, Yasushi Kondo

Published Sep 1, 2017• 10 citations
Publisher

Abstract

The largest obstacle to perform quantum information processing is decoherence of a system. In order to overcome this, various techniques, such as dynamical decoupling and quantum Zeno effects, have been proposed and demonstrated. Here, we present an NMR model with which various decoherence suppression techniques can experimentally be evaluated. By changing the conditions in the sample preparation, we can engineer an environment to interact the system that contains the information. Moreover, we can efficiently describe the dynamics by the operator-sum representation due to the simplicity of our model. As concrete examples, we have investigated the performance of dynamical decoupling with several molecules. Our model provides a useful test bench to understand the mechanism of decoherence induced by a noisy environment and to examine various ideas of decoherence suppression techniques.

DOI
10.1103/physreva.96.032303

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The largest obstacle to perform quantum information processing is decoherence of a system. In order to overcome this, various techniques, such as dynamical decoupling and quantum Zeno effects, have been proposed and demonstrated. Here, we present an NMR model with which various decoherence suppression techniques can experimentally be evaluated. By changing the conditions in the sample preparation, we can engineer an environment to interact the system that contains the information. Moreover, we can efficiently describe the dynamics by the operator-sum representation due to the simplicity of our model. As concrete examples, we have investigated the performance of dynamical decoupling with several molecules. Our model provides a useful test bench to understand the mechanism of decoherence induced by a noisy environment and to examine various ideas of decoherence suppression techniques.
Due to omnipresent environmental interferences, quantum coherences inevitably undergo irreversible transformations over certain time scales, thus leading to the loss of encoded information. This process, known as decoherence, has been a major obstacle in realizing efficient quantum information processors. Understanding the mechanism of decoherence is crucial in developing tools to inhibit it. Here we utilize a method proposed by Teklemariam et al. [Phys. Rev. A 67, 062316 (2003)] to engineer artificial decoherence in the system qubits by randomly perturbing their surrounding ancilla qubits. Using a two-qubit nuclear magnetic resonance quantum register, we characterize the artificial decoherence by noise spectroscopy and quantum process tomography. Further, we study the efficacy of dynamical decoupling sequences in suppressing the artificial decoherence. Here we describe the experimental results and their comparisons with theoretical simulations.
[Abstract] A quantum computer, that is able to calculate much faster than a conventional one, has been attracting a lot of attentions, recently. The largest obstacle against its realization is a decoherence of a system. We implement a model system with NMR technique of which decoherence can be controlled by varying the concentration of magnetic impurity and apply a dynamical decoupling technique on it. [概要] 量子コンピュータは,通常のコンピュータでは事実上不可能な計算を実行できる可能性を秘めており注目されている. この量子コンピュータの実現を阻む最大の原因は位相緩和である. NMRを用いて磁性不純物濃度を変えることにより位相緩和を制御できるモデルを実装し, さらにそのモデルに対して動的位相緩和抑制を試みた.
Entangled sensors have been attracting a lot of attention recently because they can achieve the sensitivity beyond that of the classical sensors. To exploit entanglement as a resource, it is important to understand the effect of noise because the entangled state is fragile against noise. Here, we provide a Nuclear Magnetic Resonance (NMR) model of an entangled sensor under engineered noise: one can implement an entangled sensor under various noisy environments. In particular, we experimentally investigate the performance of the entangled sensor under the effect of time-inhomogeneous noisy environment with which the entangled sensor holds potential to beat the classical sensors. Our "entangled sensor" consists of a multi-spin molecule solved in isotropic liquid, and we can perform the quantum sensing by using NMR techniques.
Introduction and overview As mentioned in Chapter 4, dynamical decoupling (DD) techniques have developed decades of tradition in the context of NMR spectroscopy, where they continue to play a pervasive role throughout all aspects of imaging and coherent control of multi-spin dynamics in the liquid and solid state. A dedicated experimental characterization and exploitation of DD as a tool for high-fidelity dynamical control and decoherence suppression in different quantum information processing (QIP) platforms has been undertaken only in more recent years, largely enabled by impressive laboratory progress in fast and ultrafast coherent control capabilities over the past decade. Without attempting to provide a comprehensive in-depth account, it is our goal in this chapter to highlight some key experimental advances to illustrate the significance of DD strategies for near-term practical quantum error control in QIP; and justify why, quoting from a recent experimental work [MTA+06], they “will likely form a quintessential element in real quantum computers.” The large majority of dynamical error-suppression implementations in QIP have aimed thus far to validate DD as a tool for enhanced quantum memory in different qubit devices in the presence of various decoherence mechanisms. Likewise, with the exception of a recent work reporting the suppression of collisional decoherence using a “continuous spin echo” close in spirit to Eulerian DD [SAD10], the majority of experiments have employed (sufficiently) “hard” pulses, such that any evolution other than that due to the control field could be taken as negligible and the bang-bang (BB) limit formalized in Chapter 4 invoked as an adequate approximation.
Methods that preserve coherence broadly impact all quantum information processing and metrology applications. Dynamical decoupling methods accomplish this by protecting qubits in noisy environments but are typically constrained to the limit where the qubits themselves are non-interacting. Here we consider the alternate regime wherein the inter-qubit couplings are of the same order as dephasing interactions with the environment. We propose and demonstrate a multi-pulse protocol that protects transverse spin states by suitably Hamiltonian engineering the inter-spin coupling while simultaneously suppressing dephasing noise on the qubits. We benchmark the method on 13C nuclear spin qubits in diamond, dipolar coupled to each other and embedded in a noisy electronic spin bath, and hyperpolarized via optically pumped NV centers. We observe effective state lifetimes of 13C nuclei $T_2^{\prime}\approx$2.5s at room temperature, an extension of over 4700-fold over the conventional T2∗T_2^{\ast}T2∗​ free induction decay. The spins are continuously interrogated during the applied quantum control, resulting in 13C NMR line narrowing and an $>$500-fold boost in SNR due to the lifetime extension. Together with hyperpolarization spin interrogation is accelerated by >1011>10^{11}>1011 over conventional 7T NMR. This work suggests strategies for the dynamical decoupling of coupled qubit systems with applications in a variety of experimental platforms.