Dry dilution refrigerator with anti-vibration system for mK scanning probe microscopy

NPL is seeking a solution for a new dry dilution refrigerator (“the system”) in which scanning probe microscopy experiments will be carried out at mK temperatures on live superconducting quantum circuits. NPL wishes to use the system for atomic scale scanning tunnelling microscopy and atomic force microscopy.

As such, the system must provide a low vibration and high coherence environment (light tight mixing chamber volume). Significant emphasis will be placed on the lowest possible vibration levels that can be achieved.

It is anticipated that a solution provides both a mK vibration isolation system and various vibration mitigation measures at room temperature. The scope includes the dilution refrigerator having a low-vibration experimental platform at mK temperature, the experimental wiring tree to the mK experimental platform, and any room-temperature or low-temperature vibration reducing measures, any gas handling system, vacuum pumps or electronics required to operate the system.

The system does not include the scanning probe microscopy unit itself, this will be provided and integrated by NPL. In the below we denote “the mixing chamber plate” as the standard plate of the dilution refrigerator that reaches the specified temperature and cryogenic performance.

We further specify an “experimental plate” which may be the same as the “mixing chamber plate” in some implementations, but it may for some vibration isolating solutions be a separate plate on which the experimental scanning probe system and apparatus is mounted. It is in that case assumed that the experimental plate is thermally anchored to the mixing chamber plate and reaches the same base temperature, and sits within the enclosed mixing chamber shielded volume.

Description • A “dry” pulse tube cooled dilution refrigerator that does not use liquid nitrogen or liquid helium to achieve cooling to 4 Kelvin. • The dilution refrigerator shall reach a base temperature of 20 mK or less at the vibration-isolated experimental platform, and the dilution refrigerator shall have a cooling power at 100 mK of 400 microWatts or more. • The experimental space below the experimental plate (if not the same as the mixing chamber plate, if e.g. a mK vibration isolation solution is used) shall be at least of diameter 360 mm, with a vertical height available for custom experiments of at least 300 mm. • The experimental plate shall be able to hold an experimental load of up to at least 1.5 kg, for custom experimental apparatus NPL wishes to install. Higher load will score higher. • Of particular importance is the achieved vibration levels at the experimental plate at 20mK or less, which shall have a demonstrated vibration level both horizontally and vertically during continuous operation of the dilution refrigerator as follows: o Any individual peaks in the vibration spectrum (horizontal and vertical) measured from 10 Hz up to at least 1000 Hz not exceeding a peak-to-peak displacement of 0.5 nm/rtHz. o An integrated noise power spectral density in the frequency range 1-10 Hz shall not exceed a root mean square amplitude of 10 nmrms. o The scoring in the evaluation of supplier bids will be heavily weighted against rewarding bids that significantly exceed the above minimum requirement specifications for vibrations. o NPL recognises that laboratory vibration levels may differ and as such will set the requirement that the above vibration levels shall be achieved in factory testing, with NPL staff present during final testing to witness compliance. o Any cold vibration isolation solution must remain well-thermalised and reach a base temperature of 50 mK or less and capable of conducting the active heat loads from piezoelectric nanopositioners.

In order to achieve these specifications the supplier may chose to include a range of different solutions, and may work with other suppliers to provide external (room temperature) or internal (mK) vibration isolation solutions. No such solution shall be regarded as optional and must be part of the total quoted price of the base system and the above vibration specifications are to be measured and evidenced with all the solution(s) in place.

Furthermore, all these vibration reducing measures must be achievable and demonstrated with all the subsequent requirements listed below in place. • There shall be at least 12 coaxial input lines (high attenuation) for microwave signals up to 18 GHz down to the experimental plate. • There shall be at least 2 coaxial output lines (low attenuation) for microwave signals up to 18 GHz down to the experimental plate. • The system should be upgradeable in the future to at least a total of 24 coaxial lines. • There shall be at least 6 different DC wiring twisted-pair looms down to the experimental plate, each of 24 twisted pair wires going from room temperature to the mixing chamber plate operational up to +-220V for use with piezo positioners, cryogenic slip stick positioner motion, nanopositioner capacitive position readouts and experimental current and voltage biasing of experiments. Each loom shall have an interruptable connection at the 4K stage where custom filters can be installed. • Have an entirely sealed mixing chamber volume with a mixing chamber plate shield that is anchored to the mixing chamber plate in a sealed manner.

The mixing chamber plate and wiring through it shall also be entirely shielded such as to prevent light from passing through. • Operation of the dilution refrigerator, shall be fully automatable and the automated scripts user configurable. • The total space taken up by the entire system shall not exceed 4.0m (w) x 4.0m (l) x 3.5m (h), excluding compressors for the pulse tube cooler(s). It will be expected that the entire system can fit in a single laboratory with this dimension (but any PT compressors can be located elsewhere). • The total weight of the system shall not exceed 2000 kg. • The system shall be delivered and installed before March 2028. This is not an evaluation exercise and nothing in this activity will limit the opportunity to engage in any follow-on ITT