Single-qubit quantum memory exceeding $10$-minute coherence time (1701.04195v1)

Abstract: 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. Recently, there have been remarkable progresses of increasing coherence time for ensemble-based quantum memories of trapped ions, nuclear spins of ionized donors or nuclear spins in a solid. 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. 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. Here we report the coherence time of a single qubit over $10$ minutes in the hyperfine states of a \Yb ion sympathetically cooled by a \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. A long-time quantum memory demonstrated in this experiment makes an important step for construction of the memory zone in scalable quantum computer architectures or for ion-trap-based quantum networks. 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.

• The paper demonstrates a record single-qubit coherence time over 10 minutes using optimized dynamical decoupling in a dual-species ion trap.
• It employs sympathetic cooling with a Ba⁺ ion and tailored CPMG and KDDxy pulse sequences to effectively mitigate motional heating and environmental noise.
• This advancement achieves 99.994% gate fidelity, paving the way for scalable quantum memory and robust quantum computing applications.

An Overview of "Single-qubit quantum memory exceeding 10-minute coherence time"

The paper presents a significant advancement in quantum information technology, specifically addressing the challenge of extending quantum memory coherence times at the single-qubit level. The authors have achieved a coherence time of over 10 minutes for a single qubit stored in the hyperfine states of a $^{171}$Yb$^+$ ion. This breakthrough was facilitated by sympathetic cooling using a $^{138}$Ba$^+$ ion in the same Paul trap, an innovation that effectively suppresses motional heating effects and consequently the associated noise, even at room temperature.

Experimental Setup and Methodology

The paper utilizes a dual-species ion trap system within a linear Paul trap—comprising $^{171}$Yb$^+$ and $^{138}$Ba$^+$ ions, both laser-cooled but with the cooling lasers influencing only the $^{138}$Ba$^+$ ion. This configuration allows the manipulation and measurement of the $^{171}$Yb$^+$ ion's qubit states without disturbance from cooling operations. The authors successfully implemented dynamical decoupling sequences, a technique that counters the dephasing effects induced by magnetic field fluctuations, which are a major source of decoherence in trapped ion systems.

To address environmental noise, the research delineates the use of the Carr-Purcell-Meiboom-Gill (CPMG) and KDD$_{xy}$ pulse sequences, both essential in managing and extending coherence by filtering noise frequencies. By profiling the noise spectrum, the authors demonstrate how specific pulse intervals significantly mitigate noise at frequencies aligned with power line harmonics, which is crucial for maintaining extended coherence.

Results and Implications

A standout result is a coherence time of over 600 seconds for certain initial qubit states, marking a substantial improvement over previous records that capped at a few tens of seconds. This was largely achieved through the optimization of dynamical decoupling pulse sequences tailored to the ion trap system's noise environment. The fidelity of single-qubit gates was rigorously tested, achieving a high value of 99.994%, reinforcing the robustness of the applied techniques.

The demonstration of this extended coherence time is highly relevant for the construction of scalable quantum computers. It offers a potential path to creating reliable quantum memory zones that can store qubits for durations long enough to support fault-tolerant quantum computation. Moreover, it has implications for quantum networks where qubit storage time scales with network size. These findings could facilitate the advancement of quantum cryptography applications, including quantum money.

Future Directions

Further research efforts could center on enhancing coherence times beyond the current achievements by deploying additional noise suppression techniques such as magnetic shielding or employing qubits that are intrinsically insusceptible to magnetic fluctuations. Moreover, expanding this methodology to a multi-qubit architecture presents an intriguing avenue, with sympathetic cooling providing a feasible means to incorporate more qubits without introducing excessive decoherence from ion heating.

In conclusion, the paper successfully demonstrates a critical step forward in quantum memory development. The results provide a foundational basis for numerous practical applications in both quantum computing and quantum communication systems, highlighting significant progress in addressing one of the key challenges in advancing quantum information technology.