A long-time quantum memory capable of storing and measuring quantum information at the single-qubit level is an essential ingredient for practical quantum computation and com-munication [1,2]. Recently, there have been remarkable progresses of increasing coherence time for ensemble-based quantum memories of trapped ions [3,4], nuclear spins of ionized donors [5] or nuclear spins in a solid [6]. Until now, however, the record of coherence time of a single qubit is on the order of a few tens of seconds demonstrated in trapped ion systems [7][8][9]. The qubit coherence time in a trapped ion is mainly limited by the increasing magnetic field fluctuation and the decreasing state-detection efficiency associated with the motional heating of the ion without laser cooling [10,11]. Here we report the coherence time of a single qubit over 10 minutes in the hyperfine states of a 171 Yb + ion sympathetically cooled by a 138 Ba + ion in the same Paul trap, which eliminates the heating of the qubit ion even at room temperature. To reach such coherence time, we apply a few thousands of dynamical decoupling pulses to suppress the field fluctuation noise [5,6,[12][13][14][15][16]. A long-time quantum memory demonstrated in this experiment makes an important step for construction of the memory zone in scalable quantum computer architectures [17,18] or for ion-trap-based quantum networks [2,19,20]. With further improvement of the coherence time by techniques such as magnetic field shielding and increase of the number of qubits in the quantum memory, our demonstration also makes a basis for other applications including quantum money [21,22].The trapped ion system constitutes one of the leading candidates for the realization of large-scale quantum computers [1]. It also provides a competitive platform for the realization of quantum networks which combines long-distance quantum communication with local quantum computation [2]. One scalable architecture for iontrap quantum computer is to divide the system into operation and memory zones and to connect them through ion shuttling [17,18]. For this architecture, the basic unit of operation zone has been demonstrated [23,24]. As the size of the system scales up, the needed storage time of the qubits in the memory zone will correspondingly increase. To keep the qubit error rates below a certain threshold for fault-tolerant computation, it is crucial to extend the coherence time of qubits. For the quantum network based on probabilistic ion-photon mapping [25], the basic units of ion-photon and ion-ion entanglement have been demonstrated [26][27][28]. The required coherence time of qubits increases in this approach as the size of the system grows. A long-time quantum memory is therefore important for both quantum computation and communication [2,29].For trapped ion qubits, the main noise is not relaxation with time T 1 but instead dephasing with time T * 2 induced by fluctuation of magnetic fields. The current records of single-qubit coherence time in trapped ion systems are around tens of se...
Realizing a long coherence time quantum memory is a major challenge of current quantum technology. 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. However, it was not clear what prohibited further enhancement. Here, we identify and suppress the limiting factors, which are the remaining magnetic-field fluctuations, frequency instability and leakage of the microwave reference-oscillator. 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. 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. Our experimental demonstration will accelerate practical applications of quantum memories for various quantum information processing, especially in the noisy-intermediate-scale quantum regime.
Here, we present the first quantum device that generates a molecular spectroscopic signal with the phonons in a trapped ion system, using SO2 as an example.
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