Precision in quantum network operations: Finely tuned laser beams for addressing and resonance reading illuminate the GeV quantum memory. Credit: Katharina Senkalla.
The colored centers of diamond are the subject of an increasing number of studies due to their potential for the development of quantum technologies. Some work has particularly explored the use of negatively charged group IV diamond defects, which exhibit an efficient spin-photon interface, as quantum network nodes.
Researchers at the University of Ulm in Germany recently exploited a germanium (GeV) center in diamond to achieve quantum memory. The resulting quantum memory, presented in a Physical Examination Letters paper, was found to exhibit a promising coherence time of over 20 ms.
“The main goal of our research group is the exploration of color centers in diamonds for quantum applications,” Katharina Senkalla, co-author of the paper, told Phys.org. “The most common defect in diamond so far has been nitrogen vacancies, but recently other color centers have also become a focus of research. This is an element of the column IV of the periodic table: Si, Ge, Sn or Pb, and a vacancy in the lattice (i.e. a missing neighboring carbon atom).”
Group IV color centers were found to exhibit much stronger emissions in the nonphonon lineage than the previously used nitrogen vacancy centers. Furthermore, the inversion symmetry of these centers makes them well suited for integration into nanophotonic devices, an important step for an efficient scalable quantum network based on solid-state single-photon sources.
“Our goal is to make significant contributions to the development of quantum networks facilitating long-distance quantum communication and distributed quantum computing,” Senkalla said. “In the field of quantum networks, a crucial aspect is the quantum network node, which requires an efficient spin-photon interface and extended memory times.”
The research group at the University of Ulm has been exploring the potential of group IV defects as candidates for quantum network nodes for some time now, recently focusing on the GeV center. These particular defects have an inherent efficiency at the spin-photon interface, characterized by a highly coherent photon flow.
Such a coherent flow of photons is a crucial element in enabling efficient quantum communication over long distances. Nevertheless, realizing quantum systems using Group IV diamond defects involves overcoming various challenges.
“These defects face obstacles related to prolonged memory times due to phonon-mediated relaxation, which impacts coherence and memory time,” Senkalla explained. “Our recent work aims to address this crucial challenge, advancing the development of robust quantum network nodes. Through our efforts, we aspire to overcome these obstacles and significantly contribute to the advancement of quantum technologies.”
Operating at ultra-low temperatures allows Ulm researchers to push GeV memory times to a record 20 ms. Credit: Katharina Senkalla.
The system developed by Senkalla and his colleagues uses a GeV as a quantum memory element. To overcome the challenges commonly associated with quantum systems based on Development Group IV defects, the researchers used a dual strategy.
The first part of this strategy aims to mitigate the negative impact of phonons on quantum information. In fact, group IV defects can easily couple with phonons, which can destroy quantum information.
“To overcome this challenge, we used a dilution refrigerator (DR), a sophisticated device widely used for sophisticated quantum computing experiments, for example in IBM’s quantum computing experiments. It can prepare temperatures on the order of a few hundred millikelvins,” Senkalla said.
“The second part of our approach, on the other hand, tackles decoupling spin noise and optimizing information storage. Working at such a low temperature range revealed that spin noise is the main decoherence factor. To extend memory times and protect quantum information, we implemented meticulous spin refocusing with microwave pulses and at strategically chosen time intervals during which computational operations can be performed .
Another aspect that Senkalla and his colleagues had to take into account when developing their quantum memory was managing the thermal charge introduced with each control pulse. In fact, dilution refrigerators have a limited cooling capacity, and exceeding this limited capacity could raise the temperature and thus facilitate the generation of phonons, which in turn could lead to decoherence.
“Developing an optimized pulse sequence involved using the Ornstein-Uhlenbeck process, a noise modeling technique capturing system dynamics,” Senkalla said.
“The Ornstein-Uhlenbeck simulations provided important insights into noise dynamics, enabling the discovery of sequences delicately balancing spin refocusing, computational intervals, and experimental thermal load management.”
The researchers tested the proposed quantum memory in experiments and simulations. Notably, the results obtained during the simulations were closely aligned with the experimental data.
“This is the first successful demonstration of effective control of germanium vacancy (GeV) rotation at millikelvin temperatures,” Senkalla said. “The comprehensive methodology we introduced, whose relevance extends beyond GeV, offers potential for advancing quantum memory performance under various experimental conditions and other Group IV defects.”
By operating at ultra-low temperatures and employing optimized sequences, the Ulm researchers achieved the record memory time of 20 ms for GeV, making it a promising candidate for a quantum network node. Credit: Highlight photo reprinted with permission from K. Senkalla, G. Genov, MH Metsch, P. Siyushev, and F. Jelezko, Germanium Vacancy in Diamond Quantum Memory Exceeding 20 ms. Phys. Reverend Lett. 132, 026901 (2024) Copyright 2024 by the American Physical Society.
The design behind the quantum memory proposed by the researchers is relatively simple and could be reproduced using other Group IV defects beyond GeV. This design was ultimately found to extend the coherence times of GeV-based memories by a factor of up to 45, achieving a record coherence time of 20 milliseconds.
The remarkable results presented in the paper highlight the potential of GeV defects for the development of systems based on quantum networks. In the future, this work may spur greater use of Group IV defects for quantum communication applications.
“Our study extends beyond the laboratory, providing valuable insights into the practical applications of GeV and other Group IV defects in quantum technologies,” Senkalla said.
“Our Ornstein-Uhlenbeck simulations pave the way for optimized control schemes for GeV and similar defects under various experimental conditions. The potential impact extends to industries like Amazon Web Services (AWS), exploring quantum networks based on group IV defects like SiV.”
The recent study by Senkalla and colleagues could potentially contribute to the advancement of quantum communication systems, as well as various industries that could benefit from high-performance quantum technologies. At the same time, the researchers plan to continue exploring the potential of GeV diamond defects as quantum network nodes.
“By expanding our exploration of GeV and its potential as a quantum network node, we are actively integrating GeV into a true quantum network,” Senkalla said.
“Our team in Ulm is building experimental facilities to serve as additional nodes in this quantum network, which fits with our vision for Ulm: to become the demonstration site for a quantum network focused on group defects IV in Germany.”
In their next studies, the researchers plan to incorporate GeV into nanophotonic cavities, while also tackling surrounding nuclear spins. These two steps are both crucial for scaling quantum networks.
“The first of these steps improves our photon rate and therefore the entanglement rate and the second allows the implementation of quantum error correction protocols, an important step towards achieving fault-tolerant quantum computing” , added Senkalla.
“We are on an exciting journey and look forward to taking our research further.”
More information:
Katharina Senkalla et al, Germanium vacancy in diamond quantum memory exceeding 20 ms, Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.132.026901. On arXiv: DOI: 10.48550/arxiv.2308.09666
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