Quantum Sensing: Necessity for low temperature and low vibrations-Most quantum states are only visible and controllable when the thermal energy KBT is comparable or smaller than the energy difference ΔE between the quantum states. Therefore, to see the quantum properties of materials or devices, they often need to be cooled to and maintained at millikelvin temperatures. In the last ten years, we have seen great progress in improving the accessibility to millikelvin environments via advanced cryogenic platforms offered by industry. On the basis of these platforms, the application can be developed that would allow probing quantum states of materials or devices, for example by means of Scanning Probe Microscopy (SPM) techniques. However, for low temperature SPM techniques, besides the low temperature, low vibrations are also essential. First, the interaction distance between probe and quantum state needs to be maintained and second, in case of force-sensors like M(R)FM, the sensor needs to be decoupled from force-noise due to accelerations. It is to this end that quantum sensing will require progression towards low temperature and low vibration environments to use quantum-enhanced probe sensors for opening a new paradigm of Scanning Probe Microscopy: qSPM. 

With the growing adoption of cryogen-free dilution refrigerators, the pulse tubes used for cooldown create quite a challenge for obtaining low vibration levels at milli-kelvin temperatures. Ideally, experiments require a connection to the refrigerator that transfers heat, but not vibrations. Practically, it results in a trade-off between vibration levels and available cooling power due to the suboptimal thermalization of vibration isolating connections. Having half of the available cooling power left at a vibration isolated platform compared to the millikelvin plate is currently recognized as an extraordinary result. The quantum SPM requires, therefore, overall performance regarding its stiffness and heat dissipation that respects this trade-off between available cooling power and vibration levels as such that the complete quantum microscope setup can be operated at millikelvin temperatures and pm/√Hz vibration levels. 

Cryo-walking Technology-One of the essential components of the quantum microscope is the cryogenic nanopositioner to position probe and sample with sub-nm accuracy over hundreds of microns travel distance. It is required to do this positioning then with heat dissipation well below the decreased cooling power available at the vibration isolation platform, and offer reasonable stiffness to maintain the interaction distance between sample and probe while experiencing leftover vibrations coming through the vibration isolation due to the trade-off with the cooling power. 

Since 2022, Onnes Technologies has launched a novel cryogenic nanopositioning technique called cryo-walking. It offers three major benefits, collectively opening the path towards qSPM applications. First, low-heat dissipation enables operation at millikelvin temperatures even with decreased cooling power. Secondly, the high stiffness lowers the impact of remaining vibrations and suppresses drift effects. Finally, cryo-walking eliminates the need for separate mechanically amplified piezo scanner components to do large area scanning. 

Figure 1a shows the result as published in the white paper of Onnes Technologies demonstrating the heat dissipating characteristics at millikelvin temperatures. The heat dissipation was determined to be below 10 μW via a calorimetric method that verifies the main heat dissipating source is the piezo-based loss factor, estimated to be ~0.6%. Such a loss factor is a typical value for low temperature piezo dynamics operated in its lowest loss factor regime. However, it is quite remarkable that the cryo-walking technology allows for minimized operation in terms of driving voltage and frequency – and as such gives rise to operation in the lowest loss factor regime. This clearly demonstrates that cryo-walking is a very efficient nano-positioning technique. 

The underlying modus operandi can be explained as follows. Instead of making use of the static and dynamic friction differences, cryo-walking is based on making and breaking mechanical contact with multiple actuators in parallel with specific phase differences. In this way, the piezo material is allowed to move the slider while connected, break connection at the end of its stroke, and reset while moving freely. Besides the thermal efficiency benefits, it also allows the slider to be mechanically rigidly connected to the outer environment because the amount of force between piezo and slider is not limited due to facilitating slipping behavior as is the case with alternative slip-stick techniques. The high stiffness is demonstrated in Figure 1b, where atomic resolution is obtained on a HOPG sample. 

The well-controlled way of positioning, in combination with low heat dissipation, gives rise to the third benefit: scanning capabilities over long travel distances without the need for probe retraction. Cryo-walking allows sub-nm level accuracy over the complete mm’s travel distance. Where traditionally SPM is strongly focused on material sciences at the lower temperatures and at atomic scale, we recognize a growing interest in researching quantum-based devices by means of SPM techniques at larger scales – above 100 μm. In Figure 1c, an HOPG sample is being translated a few hundreds of nm’s while at all times being scanned by the STM probe. This demonstrates stable scanning capabilities without using mechanically amplified piezo scanners. 

Outlook-Quantum sensing requires the progression towards low temperature and low vibration environments in order to leverage the quantum-enhanced sensitivity of novel quantum systems as a means of probing materials and devices. Yet, the available cooling power trades off against the vibration isolation attenuation. Cryo-walking technology opens a novel path for qSPM application development that is compatible with a platform of cryogen-free dilution refrigerators and passive vibration isolation operable at millikelvin temperatures and pm/√Hz vibration levels. www.onnestechnologies.com

Figures 1a, 1b, and 1c: Figure 1a shows the result as published in the white paper of Onnes Technologies, demonstrating the heat dissipating characteristics at milliKelvin temperatures. High stiffness is demonstrated in Figure 1b, where atomic resolution is obtained on an HOPG sample. In Figure 1c, an HOPG sample is being translated a few hundreds of nm’s while at all times being scanned by the STM probe. Credit: Max Kouwenhoven, CEO, Onnes Technologies