Due to recent advances in several fields of solid state physics, there is a growing interest in performing quantum optics, quantum opto-mechanics and other measurements in the sub-100 mK regime. For example, since a few years, it has become possible to prepare macroscopic mechanical oscillators in their groundstate, which opens the door to sophisticated experiments which couple such quantum resonators to quantum dots, single spins in diamond, or high-finesse optical cavities. Applications range from single-molecule mass detection to magnetic resonance force detection of single electron spins [Aspelmeyer & Schwab, New J. Phys. 10, 095001 (2008)]. Other experiments utilize resonant fluoresence to directly measure the power spectrum of a single quantum dot, photo conductivity, Kondo excitons, spin cooling or nuclei polarization. Also, optically detected magnetic resonance for magnetometry can benefit from ultra-low temperatures (quantum spin hall effect, topological insulators). At the same time, realizing such experimental setups within closed-cycle dilution refrigerators even for free-beam confocal optics is now possible. Since years, attocube maintains close ties to many leading scientists in these fields in order to deliver new platforms for groundbreaking experiments in quantum optics.
Our integrated dry mK-SPM solutions are based on state-of-the-art dry dilution refrigerators with a cooling power of up to 1500 µW, fully automated gas handling systems and wiring that is perfectly matched to both the operation of the microscope as well as to the additional requirements of the customer. We are working very closely with the leading suppliers of dry dilution refrigerators, and have achieved outstanding results both in terms of base temperature and noise specifications.
The biggest advantage of a one-supplier-solution for the end customer is that there is a clear responsibility for the overall performance of the mK-SPM. Over the last few years, we have put all of our experience into optimizing the critical parameters to achieving the one thing that counts the most: a reliably working mK-SPM for the user right after installation.
In close collaboration with Leiden Cryogenics and the Quantum Sensing group of Prof. Patrick Maletinsky (Basel, CH), we have developed a complete mK AFM/CFM based on a closed-cycle toploading dilution refrigerator. The system will be used for quantum sensing and imaging at mK temperatures in Basel. A toploading probe is a crucial feature for SPM, in order to minimize the turnaround time upon tip or sample exchange. Thanks to the sophisticated design of the cryostat with a toploading probe, this takes only about 8 hours instead of 24-48 hours for bottom-loading systems, especially with heavy magnets. This can quickly become a prohibitive shortcoming for efficient SPM measurements.
The combined AFM/CFM features a free-beam confocal microscope, with the attoCFM I external optics head sitting on top of the toploading insert. Despite the long distance to the magnetic field center where the sample is mounted, this allows for the full range of confocal applications with all the flexibility of having several optical channels with easy alignment and very high long-term stability.
The microscope module itself has been completely redesigned for the mK environment, and is carefully thermalized and wired. In this configuration, a base temperature of 38 mK at the sample location was reached.
PRODUCT KEY FEATURES
- dry dilution refrigerator with toploading insert
- base temperature (DR): 8 mK
- cooling power (DR): 1 mW@100 mK
- cooling power (toploading probe): 100 μW @120 mK
- cooldown time (toploading probe): ~8 hours
- 5-1-1 vector magnet
- mK-compatible attoAFM/CFM combined atomic force and confocal microscope with free-beam optical access and external optics head
- independent tip and sample positioning
- 38 mK base temperature at sample in microscope
attoAFM III Mounted on Mixing Chamber
This AFM-topography test-measurement of an 20 nm high calibration grating was performed in a pulse-tube based dilution fridge from Leiden Cryogenics. Even though the sample was scanned with 3 μm/s, the temperature did not rise above 80 mK, while the base temperature of the (not yet optimized) braid cooled sample was at around 62 mK. Geophone measurements verified the low vibrations of the platform and showed that it is a suitable approach for high resolution, ultra-low temperature AFM-type experiments.
|(attocube applications in collaboration with Leiden Cryogenics, 2011)|
attoAFM/CFM in Toploading Insert
This data was taken with a mk-compatible version of the attoAFM/CFM mounted on a toploading insert of a Leiden Cryogenics closed-cycle dilution refrigerator (see images and project description on the left page). The sample temperature was 60 mK during an AFM scan with a speed of 400nm/s. The images nicely demonstrates that the delicate microscope works very well even under these extreme conditions.