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Physical Review A

Optimal control of a spin bath

Lihong Dong, Hang Liang, Chang‐Kui Duan, Ya Wang, Zhaokai Li, Xing Rong, Jiangfeng Du

Published Jan 22, 2019• 18 citations
Publisher

Abstract

Harnessing the dynamics of a microscopic system lies at the core of quantum information processing. For quantum systems with a small number of spins, optimal control has been shown to be an efficient method to realize high-fidelity quantum control. As the number of the spins increases, it becomes challenging to steer the dynamics of the quantum system due to the exponentially increasing dimension of Hilbert space. Herein, we propose a method to realize optimal control of a spin bath consisting of hundreds of spins. A central spin is used to monitor the dynamics of the spin bath under a control Hamiltonian. The information obtained from the central spin is fed back to the control Hamilton to realize optimal control over the spin bath. We take a nitrogen vacancy (NV) center in diamond as an example, and our numerical simulation shows that the dephasing time of the NV center can be improved by more than 2 orders of magnitude. Furthermore, our method is compatible with dc magnetometry, whose sensitivity can be improved by an order of magnitude compared to conventional Ramsey interference. Our method is also feasible for various other spin-based systems, such as quantum dots, phosphorus-doped silicon, and defects in silicon carbide.

DOI
10.1103/physreva.99.013426

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Harnessing the dynamics of a microscopic system lies at the core of quantum information processing. For quantum systems with a small number of spins, optimal control has been shown to be an efficient method to realize high-fidelity quantum control. As the number of the spins increases, it becomes challenging to steer the dynamics of the quantum system due to the exponentially increasing dimension of Hilbert space. Herein, we propose a method to realize optimal control of a spin bath consisting of hundreds of spins. A central spin is used to monitor the dynamics of the spin bath under a control Hamiltonian. The information obtained from the central spin is fed back to the control Hamilton to realize optimal control over the spin bath. We take a nitrogen vacancy (NV) center in diamond as an example, and our numerical simulation shows that the dephasing time of the NV center can be improved by more than 2 orders of magnitude. Furthermore, our method is compatible with dc magnetometry, whose sensitivity can be improved by an order of magnitude compared to conventional Ramsey interference. Our method is also feasible for various other spin-based systems, such as quantum dots, phosphorus-doped silicon, and defects in silicon carbide.
The nitrogen-vacancy (NV) center in diamond is a quantum defect in diamond with unique properties for use in high-sensitive, high-resolution quantum sensors of magnetic fields. One of the most interesting and challenging application of NV quantum sensors is nanoscale magnetic resonance imaging (nano-MRI), which would enable to address single biomolecules. To this goal, improving the sensitivity of the NV sensor is a crucial task. Here, we present a quantum optimal control method that optimizes the sensitivity of NV sensor to specific weak magnetic signals with biologically-relevant, complex spectrum. The method, based on the mapping of the sensing problem on a problem of energy optimization of an Ising chain, allows us to improve sensitivity by three orders of magnitude compared to standard control sequences.
Nitrogen-vacancy (NV) centers in diamond are a promising platform for ambient, high-spatial resolution magnetometry relevant to biological signals such as cardiac and muscle activity. One of the challenges is the development of pulse sequences tailored to biological signals in the near DC-range. The spatial environment of NV centers within the diamond lattice is highly complex, e.g. due to magnetic field inhomogeneities, limiting the sensors performance. To overcome these experimental constraints, quantum optimal control (QOC) aims at a precise manipulation of the systems dynamics via optimally shaped microwave pulses increasing the overall robustness of spin-based quantum sensors.
Coupling-selective quantum optimal control in weak-coupling NV-13^{13}13C system
Abstract Quantum systems are under various unwanted interactions due to their coupling with the environment. Efficient control of quantum system is essential for quantum information processing. Weak-coupling interactions are ubiquitous, and it is very difficult to suppress them using optimal control method, because the control operation is at a time scale of the coherent life time of the system. Nitrogen-vacancy (NV) center of diamond is a promising platform for quantum information processing. The 13^{13}13 13 C nuclear spins in the bath are weakly coupled to the NV, rendering the manipulation extremely difficulty. Here, we report a coupling selective optimal control method that selectively suppresses unwanted weak coupling interactions and at the same time greatly prolongs the life time of the wanted quantum system. We applied our theory to a 3 qubit system consisting of one NV electron spin and two 13^{13}13 13 C nuclear spins through weak-coupling with the NV center. In the experiments, the iSWAP †^{\dagger }† † gate with selective optimal quantum control is implemented in a time-span of TctrlT_{ctrl}Tctrl​ T ctrl = 170.25 μ\muμ μ s, which is comparable to the phase decoherence time T2T_2T2​ T 2 = 203 μs\mu sμs μ s . The two-qubit controlled rotation gate is also completed in a strikingly 1020(80) μ\muμ μ s, which is five times of the phase decoherence time. These results could find important applications in the NISQ era.
Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Here, we demonstrate quantum control of a mesoscopic spin bath in diamond at room temperature that is composed of electron spins of substitutional nitrogen impurities. The resulting spin bath dynamics are probed using a single nitrogen-vacancy (NV) centre electron spin as a magnetic field sensor. We exploit the spin bath control to dynamically suppress dephasing of the NV spin by the spin bath. Furthermore, by combining spin bath control with dynamical decoupling, we directly measure the coherence and temporal correlations of different groups of bath spins. These results uncover a new arena for fundamental studies on decoherence and enable novel avenues for spin-based magnetometry and quantum information processing.
The nitrogen-vacancy (NV) center spin represents an appealing candidate for quantum information processing. Besides the widely used microwave control, its coherent manipulation may also be achieved using a laser as mediated by the excited energy levels. Nevertheless, the multiple levels of the excited state of the NV center spin make the coherent transition process become complex and may affect the fidelity of coherent manipulation. Here, we adopt the strategy of optimal quantum control to accelerate coherent-state transfer in the ground-state manifold of the NV center spin using a laser. The results demonstrate improved performance in both the speed and the fidelity of coherent-state transfer which will be useful for optical control of the NV center spin in diamond.
The decoherence of a central electron spin due to the dynamics of a coupled electron-spin bath is a core problem in solid-state spin physics. Ensemble experiments have studied the central spin coherence in detail, but such experiments average out the underlying quantum dynamics of the bath. Here, we show the coherent back-action of an individual NV center on an electron-spin bath and use it to detect, prepare, and control the dynamics of a pair of bath spins. We image the NV-pair system with subnanometer resolution and reveal a long dephasing time [ T 2 * = 44 ( 9 ) ms] for a qubit encoded in the electron-spin pair. Our experiment reveals the microscopic quantum dynamics that underlie the central spin decoherence and provides new opportunities for controlling and sensing interacting spin systems.