Quantum Memories Gain Precision with 200-Picosecond Control Pulses

Scientists at the University of Washington in collaboration with Pacific Northwest National Laboratory, The University of New Mexico and Sandia National Laboratories have developed a new, low-cost platform for manipulating spin-defect quantum memories. Victor Marcenac and colleagues present a system achieving sub-nanosecond timing control using an inexpensive field-programmable gate array, extending the open-source QICK framework. The system overcomes limitations in timing resolution that typically hinder the accurate characterisation of quantum systems, enabling the precise measurement of hyperfine couplings within nitrogen-vacancy centres in diamond. The research demonstrates a scalable and flexible alternative to conventional, costly control hardware, paving the way for more accessible and detailed investigation of spin-based quantum technologies.

Sub-nanosecond control unlocks hyperfine coupling resolution in diamond quantum sensors

Timing resolution in dynamical decoupling spectroscopy of nitrogen-vacancy centres in diamond has improved from nanosecond to 200 picosecond granularity, representing a five-fold increase. This breakthrough surpasses a vital threshold for resolving hyperfine couplings, as narrow spectral features were previously undersampled, distorting extracted parameters and limiting the size of usable nuclear spin registers. Dr. Marcenac and colleagues achieved this sub-nanosecond control by extending the open-source QICK framework with a waveform-offset method on an FPGA-based platform, without modifying the underlying radiofrequency system-on-chip hardware. Dynamical decoupling is a crucial technique employed to mitigate the effects of environmental noise on delicate quantum states, and its effectiveness is directly linked to the precision of control pulses.

Nitrogen-vacancy (NV) centres in diamond are point defects exhibiting quantum mechanical properties, making them promising candidates for quantum information processing and sensing. These centres possess an electron spin that can be manipulated and read out using microwave radiation and optical techniques. However, the electron spin interacts with its surrounding nuclear spin environment, leading to complex hyperfine interactions. Accurately characterising these interactions is essential for both understanding the NV centre’s behaviour and utilising it as a quantum memory or sensor. Previous limitations in timing resolution hindered the ability to fully resolve these hyperfine couplings, effectively blurring the spectral features and introducing inaccuracies in parameter estimation. The 200 picosecond resolution represents a significant advancement, allowing for the observation of previously unresolved spectral components.

The enhanced resolution now enables precise extraction of hyperfine couplings from individual carbon-13 nuclear spins. This reveals previously hidden spectral details important for building larger, more stable quantum memories. Detailed analysis showed that this heightened precision allows accurate measurement of hyperfine couplings, the interactions between nuclear spins and the electron spin, of individual carbon-13 isotopes with sub-kilohertz accuracy. Carbon-13, a naturally occurring isotope of carbon, possesses a nuclear spin that interacts with the NV centre’s electron spin. By precisely measuring the strength of this interaction, researchers can gain insights into the local nuclear environment and potentially use these nuclear spins as additional qubits for quantum information storage. The ability to resolve individual hyperfine couplings is crucial for developing quantum registers based on nuclear spins, as it allows for selective addressing and manipulation of each nuclear spin qubit.

Previously obscured spectral features were also detected, including peaks inconsistent with established models of nuclear spin behaviour under applied magnetic fields. These discrepancies suggest that current theoretical models may need refinement to fully account for the complex interactions within the NV centre’s environment. Further investigation of these anomalous peaks could lead to a deeper understanding of the underlying physics and potentially reveal new quantum phenomena. An inexpensive, FPGA-based control system offers a scalable alternative to conventional, costly arbitrary waveform generators. Arbitrary waveform generators (AWGs) are traditionally used to generate the precise microwave pulses required for controlling NV centres, but they can be prohibitively expensive, limiting access to advanced quantum research. Field-programmable gate arrays (FPGAs) provide a cost-effective alternative, allowing researchers to generate complex waveforms with high precision and flexibility.

However, the current setup still requires careful calibration and does not yet address the challenges of controlling many qubits simultaneously. Calibration involves precisely aligning the timing and amplitude of the control pulses to ensure optimal performance. Scaling up the system to control multiple qubits presents significant challenges, as it requires coordinating the control signals for each qubit and minimising crosstalk between them. Accurately probing these systems demands precise control over experimental timing, a feat traditionally reliant on expensive, dedicated equipment like arbitrary waveform generators. This demonstration offers a compelling alternative, achieving sub-nanosecond control using readily available field-programmable gate arrays, yet Dr. Marcenac and their team acknowledge a key limitation. Field-programmable gate arrays, increasingly common in research labs, offer a cost-effective pathway to high-resolution experiments, although they are not a drop-in replacement for fully automated commercial systems. The QICK framework facilitates the development of custom control sequences and data acquisition routines, simplifying the process of implementing complex experiments on an FPGA platform.

The 200-picosecond timing resolution, achieved via a field-programmable gate array and waveform-offset method, establishes a new benchmark for characterising spin-based quantum memories. Consequently, precise extraction of hyperfine couplings, the subtle interactions between electron and nuclear spins, becomes possible, revealing previously hidden details within the quantum system. The waveform-offset method involves generating a series of short pulses with precisely controlled timing offsets, allowing for the creation of complex waveforms with high resolution. As a result, this work shifts the focus from hardware limitations to the complexities of interpreting the resulting data, and future investigations must now address how to fully use this increased resolution to build more durable and scalable quantum technologies. The ability to accurately measure and control hyperfine couplings is a critical step towards realising robust and scalable quantum memories and sensors based on NV centres in diamond, potentially impacting fields such as quantum computing, metrology, and biological imaging.

The research successfully demonstrated 200-picosecond timing resolution in controlling experiments on nitrogen-vacancy centres in diamond using an inexpensive, field-programmable gate array. This improved timing allows for the precise measurement of hyperfine couplings, subtle interactions within the quantum system that were previously difficult to observe. Consequently, researchers can now characterise spin-based quantum memories with greater accuracy, potentially aiding the development of more robust quantum technologies. The authors suggest that future work should focus on fully utilising this increased resolution to improve the performance of these systems.