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Room-temperature quantum devices: innovations in active pre-cooling protocol, spin-phonon coupling devices, and stable molecular spins

Kuan-Cheng Chen

Published Nov 4, 2024• 0 citations
Publisher

Abstract

The feasibility of active pre-cooling (APC) by temporarily over-coupling microwave cavity modes to a low-noise amplifier acting as an active cold load was demonstrated, offering an effective method to reduce thermal photon occupancy without cryogenic cooling. The potential of the organic radical BDPA as a tunable qubit medium with remarkably long spin-lattice and spin coherence relaxation times was assessed, comparing favorably with state-of-the-art molecular qubit media. A high-quality surface acoustic wave device integrated with pentacene-deposited molecular thin films was developed to achieve heterogeneous optically-detected spin acoustic resonance (HODSAR), enabling efficient zero-field spin control at room temperature via spin-phonon coupling. Through the construction of specialized experimental setups, the effectiveness of these techniques was demonstrated, facilitating pre-cooling of cavity modes, enhancing coherence times in qubit media, and achieving precise spin manipulation. While APC offers advantages over methods like masers and anti-masers, challenges such as the need for fast, low-loss microwave switches remain. The BDPA radicals showed significant promise for spin ensemble, and the SAW-based HODSAR device provides a scalable platform for spin manipulation in quantum technologies.

DOI
10.25560/116927

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The feasibility of active pre-cooling (APC) by temporarily over-coupling microwave cavity modes to a low-noise amplifier acting as an active cold load was demonstrated, offering an effective method to reduce thermal photon occupancy without cryogenic cooling. The potential of the organic radical BDPA as a tunable qubit medium with remarkably long spin-lattice and spin coherence relaxation times was assessed, comparing favorably with state-of-the-art molecular qubit media. A high-quality surface acoustic wave device integrated with pentacene-deposited molecular thin films was developed to achieve heterogeneous optically-detected spin acoustic resonance (HODSAR), enabling efficient zero-field spin control at room temperature via spin-phonon coupling. Through the construction of specialized experimental setups, the effectiveness of these techniques was demonstrated, facilitating pre-cooling of cavity modes, enhancing coherence times in qubit media, and achieving precise spin manipulation. While APC offers advantages over methods like masers and anti-masers, challenges such as the need for fast, low-loss microwave switches remain. The BDPA radicals showed significant promise for spin ensemble, and the SAW-based HODSAR device provides a scalable platform for spin manipulation in quantum technologies.
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Masers once represented the state of the art in low-noise microwave amplification technology but eventually became obsolete due to their need for cryogenic cooling. Masers based on solid-state spin systems perform most effectively as amplifiers, since they provide a large density of spins and can, therefore, operate at relatively high powers. While solid-state maser oscillators have been demonstrated at room temperature, continuous-wave amplification in these systems has only ever been realized at cryogenic temperatures. Here, we report on a continuous-wave solid-state maser amplifier operating at room temperature. We achieve this feat using a practical setup that includes an ensemble of nitrogen-vacancy center spins in a diamond crystal, a strong permanent magnet, and a simple laser diode. We describe important amplifier characteristics including gain, bandwidth, compression power, and noise temperature and discuss the prospects of realizing a room-temperature near-quantum-noise-limited amplifier with this system. Finally, we show that in a different mode of operation the spins can be used to reduce the microwave noise in an external circuit to cryogenic levels, all without the requirement for physical cooling. Published by the American Physical Society 2024
Surface acoustic wave (SAW) phonon coupling with nitrogen-vacancy (NV) center spins in diamond offers a promising platform for on-chip quantum phononic manipulations. Although an ensemble of NV centers coupled to a common SAW phonon mode enables superradiance and collective quantum control, achieving a tunable superradiant phase transition remains challenging. Here, we show that optically driving NV centers level transitions enhances the effective spin-phonon coupling, triggering a SAW phonon superradiant phase transition in the weak-coupling regime. We also demonstrate that above a critical threshold, the driving light rapidly switches on the phonon superradiance--a dynamic effect that persists in finite-number NV ensembles. Our results provide a controllable route to coherent phonon-NV spin manipulation in solid state quantum devices.
The first solid-state masers to operate at room-temperature and ambient air-pressure were recently demonstrated using optically pumped spin-triplet states as the gain medium. In this Perspective, we briefly review the previous state-of-the-art in cryogenic solid-state masers and then discuss the development of the room-temperature solid-state maser: from the organic pentacene pulsed maser to the diamond nitrogen-vacancy continuous-wave maser. We characterize the operation of these masers as coherent microwave sources and ultra-low noise amplifiers before outlining how they can be adapted to act as model systems in which to explore room-temperature cavity quantum electrodynamics. After discussing challenges facing current embodiments of the room-temperature solid-state maser, we explore how they might be addressed or by-passed altogether through the development of alternative materials and masing mechanisms. Finally, we speculate on how the advent of masers that can operate in ambient conditions might lead to novel applications in metrology and quantum technologies.
We report an implementation of spin-acoustic resonance in pentacene thin films integrated on a high-quality-factor (high-Q) surface acoustic wave (SAW) resonator on a lithium niobate substrate. Heterogeneous optically detected spin-acoustic resonance (HODSAR) is an optically detected spin-resonance measurement in which the resonant drive is delivered mechanically by a surface acoustic wave (SAW). By leveraging the photo-excited triplet state of pentacene at room temperature, we demonstrate coherent spin manipulation via acoustic driving under zero externally applied magnetic field. The heterogeneously integrated device, referred to as HODSAR, utilizes spin-phonon coupling to achieve mechanically driven, zero-field spin resonance, opening avenues for room-temperature mechanically addressable spin control and device integration. We show that the high-Q multimode response of the SAW resonator enables spectrally selective acoustic addressing of triplet transitions near 105 MHz. Coherent control is evidenced by Rabi oscillations, with a Rabi frequency that increases linearly with the square root of the applied RF input power over the measured drive range, consistent with driven two-level dynamics under acoustic excitation. These results establish spin-acoustic resonance in a heterogeneously integrated molecular thin-film platform and provide a quantitative basis for benchmarking mechanically mediated spin control.
A fundamental result of quantum mechanics is that the fluctuations of a bosonic field are given by its temperature TTT. An electromagnetic mode with frequency ωωω in the microwave band has a significant thermal photon occupation at room temperature according to the Bose-Einstein distribution nˉ=kBT/ℏω\bar{n} = k_BT / \hbarωnˉ=kB​T/ℏω. The room temperature thermal state of a 3 GHz mode, for example, is characterized by a mean photon number nˉ∼2000\bar{n} \sim 2000nˉ∼2000 and variance Δn2≈nˉ2Δn^2 \approx \bar{n}^2Δn2≈nˉ2. This thermal variance sets the measurement noise floor in applications ranging from wireless communications to positioning, navigation, and timing to magnetic resonance imaging. We overcome this barrier in continuously cooling a ∼3{\sim} 3∼3 GHz cavity mode by coupling it to an ensemble of optically spin-polarized nitrogen-vacancy (NV) centers in a room-temperature diamond. The NV spins are pumped into a low entropy state via a green laser and act as a heat sink to the microwave mode through their collective interaction with microwave photons. Using a simple detection circuit we report a peak noise reduction of −2.3±0.1 dB-2.3 \pm 0.1 \, \textrm{dB}−2.3±0.1dB and minimum cavity mode temperature of 150±5K150 \pm 5 \textrm{K}150±5K. We present also a linearized model to identify the important features of the cooling, and demonstrate its validity through magnetically tuned, spectrally resolved measurements. The realization of efficient mode cooling at ambient temperature opens the door to applications in precision measurement and communication, with the potential to scale towards fundamental quantum limits.