Solid-state electronic spin coherence time approaching one second (1211.7094v2)

Abstract: Solid-state electronic spin systems such as nitrogen-vacancy (NV) color centers in diamond are promising for applications of quantum information, sensing, and metrology. However, a key challenge for such solid-state systems is to realize a spin coherence time that is much longer than the time for quantum spin manipulation protocols. Here we demonstrate an improvement of more than two orders of magnitude in the spin coherence time ($T_2$) of NV centers compared to previous measurements: $T_2 \approx 0.5$ s at 77 K, which enables $\sim 10^7$ coherent NV spin manipulations before decoherence. We employed dynamical decoupling pulse sequences to suppress NV spin decoherence due to magnetic noise, and found that $T_2$ is limited to approximately half of the longitudinal spin relaxation time ($T_1$) over a wide range of temperatures, which we attribute to phonon-induced decoherence. Our results apply to ensembles of NV spins and do not depend on the optimal choice of a specific NV, which could advance quantum sensing, enable squeezing and many-body entanglement in solid-state spin ensembles, and open a path to simulating a wide range of driven, interaction-dominated quantum many-body Hamiltonians.

• The paper demonstrates that applying CPMG dynamical decoupling increases NV center spin coherence by over two orders of magnitude, reaching nearly 0.5 s at 77 K.
• The study employs high-purity, isotopically engineered diamond and multi-pulse sequences to mitigate magnetic noise and isolate phonon-induced decoherence.
• The extended coherence time supports up to 10^7 coherent quantum operations, advancing practical applications in quantum metrology, sensing, and processing.

Solid-State Electronic Spin Coherence Time Approaching One Second

The paper presents substantial advancements in the spin coherence time of negatively charged nitrogen-vacancy (NV) color centers in diamond, extending the coherence time to approximately 0.5 seconds at 77 K. This represents an improvement of more than two orders of magnitude over previous measurements and is achieved through the implementation of dynamical decoupling pulse sequences to mitigate magnetic noise-induced decoherence. This paper holds implications for quantum information processing, sensing, and metrology, positioning NV centers at the forefront of solid-state quantum technologies.

The NV center in diamond, a well-established platform in quantum research due to its optically addressable spin states and long coherence times, serves as the focus of this investigation. The authors explore spin coherence, a critical parameter for quantum operations, emphasizing that achieving a long coherence time relative to quantum manipulation durations is essential for practical applications. Here, the coherence time ($T_2$) is shown to reach up to 0.5 seconds, facilitated by using Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling sequences across various temperatures, notably enhancing the potential for coherent NV spin manipulations, now nearing $10^7$ operations before decoherence.

The improvement in $T_2$ is attributed to the suppression of magnetic noise, yet the coherence time appears to be constrained by half of the longitudinal relaxation time ($T_1$) across the temperature range investigated. The authors suggest that this limitation is due to phonon-induced decoherence, a significant insight that could inform future studies on solid-state spin defects, such as phosphorus donors in silicon. The finding that $T_2 \approx 0.5 T_1$ invites further exploration of decoherence mechanisms and the role of phonons in solid-state quantum systems.

Experimentally, the research employs high-purity, isotopically engineered diamond samples and utilizes multi-pulse control sequences within a tightly controlled temperature environment. The results exhibit striking consistency, showing that the NV spin coherence extends significantly with the CPMG pulse number, although increasingly limited by pulse errors at extreme sequences. At 77 K, coherence times achieve 580 ms with an 8192-pulse CPMG, establishing a new benchmark for NV center performance.

The implications of this extended coherence time are considerable. Increased coherence times directly translate to enhanced quantum metrology and improved magnetic field sensitivity, which scales inversely with the square root of both the number of NV centers and $T_2$. These findings could propel advances in precision measurement technologies, such as magnetometry and rotation sensing.

In terms of future directions, the authors outline potential for forming non-classical states within NV ensembles, given sufficiently long coherence times. This could facilitate novel quantum metrology and information protocols, such as collective spin squeezing, leveraging the reduced decoherence rates relative to NV-NV interaction frequencies.

The research not only advances the practical capabilities of NV-based quantum devices but also provides a pathway towards studying interaction-dominated topological quantum phases in the solid state. The work underscores the NV center's scalability and practicality, making it a compelling candidate for diverse quantum applications, particularly as techniques are developed to achieve long coherence times in high-density NV environments.

In summary, this paper makes a significant contribution to the field by demonstrating extended NV spin coherence times and identifying the underlying decoherence mechanisms, paving the way for innovation in quantum technology applications and advancing our understanding of quantum spin systems within solid-state environments.