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Quantum coherence decoherence12/10/2023 ![]() ![]() Furthermore, with the capability of full control on a single qubit, we systematically study the decoherence process of the quantum memory by quantum process tomography. ![]() This is achieved by identifying and suppressing the three dominant error sources: magnetic-field fluctuation, the phase noise of the local oscillator, and microwave leakage for qubit operation. Here we address this challenge by improving the coherence time of a 171Yb + ion-qubit memory from 10 min to over one hour. While the fundamental limit is far beyond 10 min however, it remains a major technological challenge to further enhance the quality of a trapped-ion quantum memory. The problem was addressed by sympathetic cooling by other species of ion, which allowed further improvements of coherence time to over 10 min with the support of dynamical decoupling 28. For the coherence time of a minute, the limitation mainly came from the qubit-detection inefficiency 25, 26, 27 due to the motional heating of qubit-ions without Doppler laser-cooling. For a single qubit quantum memory, which is the essential building-block for quantum computers 18, 19 and quantum repeaters 20, 21, records of coherence time have been reported to the time scale of a minute in trapped ion qubit 22, 23, 24, 25. With ensembles of trapped ions and nuclear spins in a solid, coherence time of 10 min 13, 14, and 40 min at room temperature 15, 16 and a few hours at 4 K 17 have been reported, respectively. Numerous experimental attempts have been made to enhance the coherence time of quantum memory in a variety of quantum systems. It is thus of practical importance to have a stable quantum memory with a long-coherence time. Limited coherence time may also undermine quantum-information applications such as quantum money 11, 12. In practice, decoherence, loss of coherence in the computational basis, in the quantum system comes from the coupling with the surrounding environment and fluctuations of control parameters in quantum operations, which can lead to the infidelity of quantum-information processing, the low sensitivity of quantum sensors, and the inefficiency of quantum repeater based protocols in quantum communication networks. Quantum coherence is a vital component for scalable quantum computation 1, 2, 3, quantum metrology 4, 5, and quantum communication 6, 7, 8, 9, 10. Our experimental demonstration will accelerate practical applications of quantum memories for various quantum information processing, especially in the noisy-intermediate-scale quantum regime. We also systematically study the decoherence process of the quantum memory by using quantum process tomography and analyze the results by applying recently developed resource theories of quantum memory and coherence. Then, we observe the coherence time of around 5500 s for the 171Yb + ion-qubit, which is the time constant of the exponential decay fit from the measurements up to 960 s. Here, we identify and suppress the limiting factors, which are the remaining magnetic-field fluctuations, frequency instability and leakage of the microwave reference-oscillator. However, it was not clear what prohibited further enhancement. Until now, the longest coherence-time of a single qubit was reported as 660 s in a single 171Yb + ion-qubit through the technical developments of sympathetic cooling and dynamical decoupling pulses, which addressed heating-induced detection inefficiency and magnetic field fluctuations. Realizing a long coherence time quantum memory is a major challenge of current quantum technology. ![]()
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