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Acta Physica Sinica

Single-ion qubit with coherence time exceeding 10 minutes

Ye Wang, Jing-Ning Zhang, Kihwan Kim

Published Jan 1, 2019• 3 citations
Publisher

Abstract

Quantum memory device capable of storing quantum information for a long period of time is one of the fundamental ingredients to realize large-scale quantum computation and quantum communication. Comparing with other quantum computation platforms, one of the advantages of the trapped-ion system is the long intrinsic coherence time. Before our work, the longest single-qubit coherence time in trapped-ion systems has been achieved to be less than 1 minute. It is discovered that the main limitation for the coherence time is the motional mode heating and the environment noise that includes the contributions from the magnetic field fluctuation and the phase noise of the microwaves. In a hybrid trapping system simultaneously trapping 171Yb+ and 138Ba+ ions, single-qubit quantum memories with coherence time longer than 10 minutes can be realized by applying sympathetic cooling and dynamical decoupling. This technique may have some value as the building blocks for quantum cryptography protocols and hybrid quantum computation platforms.

DOI
10.7498/aps.68.20181729

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Quantum memory device capable of storing quantum information for a long period of time is one of the fundamental ingredients to realize large-scale quantum computation and quantum communication. Comparing with other quantum computation platforms, one of the advantages of the trapped-ion system is the long intrinsic coherence time. Before our work, the longest single-qubit coherence time in trapped-ion systems has been achieved to be less than 1 minute. It is discovered that the main limitation for the coherence time is the motional mode heating and the environment noise that includes the contributions from the magnetic field fluctuation and the phase noise of the microwaves. In a hybrid trapping system simultaneously trapping 171Yb+ and 138Ba+ ions, single-qubit quantum memories with coherence time longer than 10 minutes can be realized by applying sympathetic cooling and dynamical decoupling. This technique may have some value as the building blocks for quantum cryptography protocols and hybrid quantum computation platforms.
Abstract 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 171 Yb + 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 171 Yb + 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.
Quantum systems promise to revolutionize information processing science and technology [1-3]. The preservation of quantum coherence, the defining property of qubits, fundamentally constrains the performance of quantum information processing with quantum memories [4]. While trapped atomic ions theoretically support million-year coherence based on spontaneous emission [5-7], experimental demonstrations have reached far less, only about an hour [8-13]. Here we combine clock-state qubits with decoherence-free subspace (DFS) encoding to achieve coherence exceeding ten hours. Using correlation-based phase tracking in 171Yb+ ion pairs sympathetically cooled by 138Ba+ ion, we demonstrate this without magnetic shielding or enhanced microwave phase stabilization that previously limited coherence times. DFS encoding references the qubit phase to the inter-ion energy difference to reject microwave phase noise and common-mode magnetic fluctuations, while clock states provide environmental insensitivity. Throughout measurements extended to 1600 seconds, we observe minimal coherence decay, with exponential fits yielding a coherence time of (3.77 +/- 1.09) x 10^4 seconds. Our results establish DFS encoding as a form of passive error correction that eliminates technical noise constraints, unlocking the million-year coherence potential of atomic ions for scalable quantum information processing.
Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species and constantly monitoring for erasure errors, we demonstrate a coherence time of 136(42) seconds with a qubit encoded in the metastable 5D5/25D_{5/2}5D5/2​ state of a single 137^{137}137Ba+^++ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once erasure errors are removed.
We demonstrate long-lived coherence in internal hyperfine states of a single \Ca{43} trapped-ion qubit $[T_2=1.2(2)\s]$, and in external motional states of a single \Ca{40} trapped-ion qubit $[T_2'=0.18(4)\s]$, in the same apparatus. The motional decoherence rate is consistent with the heating rate, which was measured to be 3(1) quanta/sec. Long coherence times in the external motional states are essential for performing high-fidelity quantum logic gates between trapped-ion qubits. The internal-state T2T_2T2​ time that we observe in \Ca{43}, which has not previously been used as a trapped-ion qubit, is about one thousand times longer than that of physical qubits based on \Ca{40} ions. Using a single spin-echo pulse to ``re-phase'' the internal state, we can detect no decoherence after 1\s, implying an effective coherence time $T_2^{\mbox{\tiny SE}} \gtish 45\s$. This compares with timescales in this trap for single-qubit operations of \ish 1\us, and for two-qubit operations of \ish 10\us.
Stable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. We demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature. The qubit consists of a single (13)C nuclear spin in the vicinity of a nitrogen-vacancy color center within an isotopically purified diamond crystal. The long qubit memory time was achieved via a technique involving dissipative decoupling of the single nuclear spin from its local environment. The versatility, robustness, and potential scalability of this system may allow for new applications in quantum information science.