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arXiv:1102.5576 · Physical Review Letters

Anomalous Decoherence Effect in a Quantum Bath

Nan Zhao, Zhenyu Wang, Ren‐Bao Liu

Published May 26, 2011• 84 citations
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Abstract

Decoherence of quantum objects in noisy environments is important in quantum sciences and technologies. It is generally believed that different processes coupled to the same noise source have similar decoherence behaviors and stronger noises cause faster decoherence. Here we show that in a quantum bath, the case can be the opposite. We predict that the multitransition of a nitrogen-vacancy center spin-1 in diamond can have longer coherence time than the single transitions, even though the former suffers twice stronger noises from the nuclear spin bath than the latter. This anomalous decoherence effect is due to manipulation of the bath evolution via flips of the center spin.

DOI
10.1103/physrevlett.106.217205

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Decoherence of quantum objects in noisy environments is important in quantum sciences and technologies. It is generally believed that different processes coupled to the same noise source have similar decoherence behaviors and stronger noises cause faster decoherence. Here we show that in a quantum bath, the case can be the opposite. We predict that the multitransition of a nitrogen-vacancy center spin-1 in diamond can have longer coherence time than the single transitions, even though the former suffers twice stronger noises from the nuclear spin bath than the latter. This anomalous decoherence effect is due to manipulation of the bath evolution via flips of the center spin.
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We study decoherence of a single nitrogen-vacancy (NV) center induced by the 13C nuclear spin bath of diamond. By comparing Hahn-Echo experiments on single and double-quantum transitions of the NV triplet ground state we demonstrate that this bath can be tuned into two different regimes. At low magnetic fields, the nuclei behave as a quantum bath which causes decoherence by entangling with the NV central spin. At high magnetic fields, the bath behaves as a source of classical magnetic field noise, which creates decoherence by imprinting a random phase on the NV central spin.
We theoretically investigate spin decoherence of a single nitrogen-vacancy (NV) center in diamond. Using the spin coherent-state PPP-representation method, we simulate coherence evolution of the NV center coupled to surrounding nitrogen electron (N) spins. In the system, the strength of N-N coupling is the same order as that of NV-N coupling (the strong intrabath coupling regime). We find that spin decoherence time as well as free-induction decay of the NV center depend on the spatial configuration of N spins. Both the spin decoherence rate (1/T21/{T}_{2}1/T2​) and dephasing rate (1/T2∗)1/{T}_{2}^{*})1/T2∗​) of the NV center increase linearly with the concentration of the N spins. Using the PPP-representation method, we also demonstrate extracting the noise spectrum of the N spin bath. The capability to calculate the noise spectrum will provide promising pathways to suppress decoherence of spin systems in the strong intrabath coupling regime.
Nitrogen-vacancy (NV) centers in diamond have emerged in the last decade as a prominent platform for quantum technologies. As for any qubit system, a good understanding of their local environment is crucial to build quantum devices protected from detrimental noise. Here, we describe in detail a method to spectroscopically characterize the spin bath around an NV center, even when the NV coherence time is short, and identify the coherent coupling with the nearest nuclear spins. In the regime of weak qubit-bath coupling, the acquired knowledge of the bath reliably predicts the qubit dynamics under different controls.
We theoretically study the decoherence and the dynamical decoupling control of nitrogen vacancy center electron spins in high-purity diamond, where the hyperfine interaction with 13{}^{13}13C nuclear spins is the dominating decoherence mechanism. The decoherence is formulated as the entanglement between the electron spin and the nuclear spins, which is induced by nuclear spin bath evolution conditioned on the electron spin state. The nuclear spin bath evolution is driven by elementary processes such as single spin precession and pairwise flip flops. The importance of different elementary processes in the decoherence depends on the strength of the external magnetic field.
Decoherence is the biggest bottleneck in all quantum technologies. For nitrogen-vacancy (NV) centers in diamond, the loss of coherence is caused by the electron and nuclear spin bath of the diamond lattice. Here, we demonstrate that the spin bath - that typically causes decoherence - entangles the spin states of the NV electron and the host 14^{14}14N nucleus. The many-body interaction between the 14^{14}14N nucleus - electron - bath spins at an energy level anti-crossing occurring for an applied magnetic field orientation perpendicular to the NV axis is responsible for this effect. This is observed experimentally in NV ensembles via electron spin-echo measurements, where the echo envelope is modulated at the frequency of a 14^{14}14N nuclear spin transition. Using numerical simulations, we show that the spin bath coupling to the NV centers is essential for observing this modulation. Due to the zero first-order Zeeman effect at the anti-crossing, the observed oscillations have long spin-echo coherence times, 2--3 times those at the parallel magnetic field orientation. The oscillation frequency is highly stable and robust against environmental fluctuations. These findings provide new opportunities for fundamental studies of many-body physics and quantum sensing.