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Cryogenically-Stabilized Frozen-State Quantum Memory with Barocaloric Thermal Management, NV-Center-Referenced AI Feedback, and Enhanced Electro-Optic Modulation in Exotic Diamond Cavities

Francis Procaccia

Published Apr 25, 2026• 0 citations
Publisher

Abstract

A cryogenically stabilized quantum memory architecture using synthetic diamond cavities with Möbius-toroidal and dodecahedral-photonic geometries, integrated with an ALD-deposited SrTiO₃ electro-optic shell (r_eff ≈ 345 pm/V at cryogenic temperatures) and barocaloric thermal management based on KPF₆. The system employs NV-center spin states as the primary reference for AI-driven feedback control (GPR + RL on cryo-FPGA), combined with redundant laser phase-locking for extended coherence. Designed for high-density wafer-scale integration and quantum repeater applications, the platform targets coherence times on the order of seconds at 4–77 K. Provisional Patent Application No. 63/806,469 (filed May 15, 2025).

DOI
10.5281/zenodo.19768426

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Chip integration of quantum emitters is a crucial milestone for scalable quantum photonic information processing. Among optically active defect centers for quantum photonics, diamond color centers are promising because of their long spin coherence times and high photon emission rates. However, for a coherent-photon emission, they typically require a cryogenic environment to protect optical coherence from thermal phonons, which makes chip integration challenging. In this paper, we develop a chip-integrated diamond photonic crystal cavity embedding an ensemble of nitrogen-vacancy (NV) centers. We confirm cryogenic operation by observing Purcell enhancement of NV-center emission via an edge-coupled optical fiber. This result demonstrates successful integration of diamond color centers, a photonic crystal cavity, and an optical waveguide-fiber package, representing a key step toward scalable diamond-based quantum communication platforms.
A growing variety of optically accessible spin qubits have emerged in recent years as key components for quantum sensors, qubits, and quantum memories. However, the scalability of conventional spin-based quantum architectures remains limited by direct microwave delivery, which introduces thermal noise, electromagnetic cross-talk, and design constraints for cryogenic, high-field, and distributed systems. In this work, we present a unified framework for RF-over-fiber (RFoF) control of optically accessible spins through RFoF optically detected magnetic resonance (ODMR) spectroscopy of nitrogen-vacancy (NV) centers in diamond. The RFoF platform relies on an electro-optically modulated telecom-band laser that transmits microwave signals over fiber and a high-speed photodiode that recovers the RF waveform to drive NV center spin transitions. We obtain an RFoF efficiency of 1.81\% at 2.90~GHz, corresponding to PRF,out=−0.7P_{\mathrm{RF,out}}=-0.7PRF,out​=−0.7~dBm. The RFoF architecture provides a path toward low-noise, thermally isolated, and cryo-compatible ODMR systems bridging conventional spin-based quantum sensing protocols with emerging distributed quantum technologies.
The implementation of a classical control infrastructure for large-scale quantum computers is challenging due to the need for integration and processing time, which is constrained by coherence time. We propose a cryogenic reconfigurable platform as the heart of the control infrastructure implementing the digital error-correction control loop. The platform is implemented on a field-programmable gate array (FPGA) that supports the functionality required by several qubit technologies and that can operate close to the physical qubits over a temperature range from 4 K to 300 K. This work focuses on the extensive characterization of the electronic platform over this temperature range. All major FPGA building blocks (such as look-up tables (LUTs), carry chains (CARRY4), mixed-mode clock manager (MMCM), phase-locked loop (PLL), block random access memory, and IDELAY2 (programmable delay element)) operate correctly and the logic speed is very stable. The logic speed of LUTs and CARRY4 changes less then 5%, whereas the jitter of MMCM and PLL clock managers is reduced by 20%. The stability is finally demonstrated by operating an integrated 1.2 GSa/s analog-to-digital converter (ADC) with a relatively stable performance over temperature. The ADCs effective number of bits drops from 6 to 4.5 bits when operating at 15 K.
For cavity quantum electrodynamics systems (cavity-QED) to play a role in quantum information processing applications and in quantum networks, they must be robust and scalable in addition to having a suitable method for the generation, processing and storage of quantum bits. One solution is to develop a composite system that couples a nitrogen vacancy (NV) center in diamond to a whispering gallery mode supported by a fused silica microsphere. Such a system is motivated by the optical and electron-spin properties of the NV center. The NV center is the leading spin-qubit and exhibits atomic like linewidths at cryogenic temperatures and has spin coherence times greater than milliseconds at room temperature. These long coherence times, coupled with nanosecond scale spin readout and manipulation times, allow for millions of quantum operations to be processed. Silica whispering gallery resonators are the only class of microresonators with quality factor high enough to reach the strong coupling regime, which is necessary for some quantum information processing applications. \n \nIntegrating these two components into a system that could position a diamond nanopillar near the surface of a deformed-double stemmed microsphere system, with nanometer precision, at 10 K was a major achievement of this research. Cavity resonances in deformed microspheres can be excited with a free-space coupling technique which simplifies their integration into cryogenic environments. In these intentionally deformed resonators, an enhanced evanescent field decay length was observed at specific locations along the ray orbit. The double-stem arrangement enables the cavity resonance to be tuned over 450 GHz, with sub-10 MHz resolution, at 10 K. These two features, the enhanced decay length and broad range tuning with high resolution, are indispensible tools for cavity-QED studies with silica microspheres. \n \nDiamond nanopillars were fabricated from single crystal diamond with diameters as small as 140 nm in order to maintain a high quality factor. Studies were conducted on NV centers in nanopillars and bulk diamond to determine their suitability for cavity-QED applications. In an attempt to increase the light-matter interaction between NV centers and whispering gallery modes, diamond substrates were optically characterized that were irradiated with nitrogen ions.
Efficient quantum devices across various physical systems have been rapidly developed for entanglement-based quantum repeaters and spin-photon conversion; however, far less attention has been paid to standardizing platforms through quantum memory optical interfaces. We present a modular interface for color centers in diamond that is structurally isolated from device-and temperature-related variation. Despite a more than 100-fold reduction in confocal volume, we achieve highly efficient photon collection through single-mode optical fibers, including zero-phonon line spectroscopy, at both room and cryogenic temperatures. These results establish a standardized minimal NV-center-based platform and pave the way for construction of scalable quantum infrastructure.
Connecting cryogenic solid-state quantum processors with room-temperature optical links is a key step toward a scalable quantum computer. This thesis presents two technological advancements toward the realization of a spin-ensemble-based quantum transducer using impurity spins in diamond. First, a low-loss dielectric microwave resonator based on rutile (TiO2) was developed, achieving an internal quality factor of Qint ∼ 104 despite incorporating large optical apertures. Second, an optical Fabry–Pérot cavity was stabilized at millikelvin temperatures with sub-0.1 nm root-mean-square cavity length fluctuations, enabling stable cavity locking to the laser in a dilution refrigerator. These components were integrated into a hybrid resonator enabling spatial mode overlap within a diamond sample containing an ensemble of nitrogen-vacancy centers. Measurements at 10 mK revealed spin coherence times of T1 = 1930 s and T2 = 470 µs, and characterized the inhomogeneous broadening of the optical transition. We estimate the current microwave-to-optical conversion efficiency and discuss potential improvements through device and cavity optimization. This work establishes a platform for spin ensemble-based transducers operating at millikelvin temperatures.