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Executive Summary:
Concept of the Microkelvin Collaboration

European Microkelvin Collaboration ― MICROKELVIN ― was an EU-funded Integrating Activity project of the FP7 Capacities Specific Programme "Research Infrastructures". Its task was to promote sub-millikelvin temperatures as a new frontier of research, by providing access to and developing applications in the ultra-low temperature regime. The grant was divided in four Networking Activities (NAs), four Joint Research Activities (JRAs) and four Transnational Access Activities (TAs). The collaboration included 12 leading European partner organizations, of which three core institutions offered expertize and facilities to external users, and one was a cryogenic business developing and manufacturing very low temperature research equipment.

The quest to study materials and new physical systems at ever lower temperatures has for two centuries led to the discovery of a multitude of new phenomena and concepts in physics and beyond. Today we find that many of the technical resources have been developed how to reach and measure microkelvin temperatures or to perform studies in this regime. However, in practice often enough the expertise is missing and the use of the ultra-low temperatures is prevented by a high starting threshold. The goal of the Microkelvin Collaboration is to facilitate this step, by creating “a European laboratory without walls” which offers education, counselling, facilities and instrumentation to European researchers. For this end, further technical development of practical user-friendly refrigeration and measurement methods is of importance.

Research in nanosciences, materials physics, particle physics, cosmology, or instrumentation are examples which could draw immediate benefit from extending experimental work to the low millikelvin and microkelvin regimes. For instance, in nanoscience a central aim is to reach the regime where quantum phenomena govern the behaviour of the system. Experiments in this quantum engineering regime make it possible to discover new phenomena, new materials properties, and allow us to develop novel quantum devices with much improved sensitivity and resolution. We are in urgent need of such innovations, as conventional microcircuits are running up against the physical limits of further miniaturization. While quantum behaviour can be observed in very small samples at relatively high temperatures, it becomes much more apparent as the temperature is lowered. The lack of expertise in microkelvin techniques has hitherto been a deterrent against performing nanoscience experiments at the very lowest temperatures.

Project Context and Objectives:
Objectives of the Microkelvin Collaboration

The Microkelvin Collaboration was a bottom-up approach of leading European low-temperature laboratories to create and manage an integrated research infrastructure and to make better use of the combined European expertise which we believe to represent the leading edge of ultra-low-temperature physics on the global scale. Among its objectives were

•To integrate and upgrade the leading microkelvin facilities in Europe.
•To assemble a critical mass for effective work on large scale issues and provide access to a wider range of European users.
•To create new capability by exploiting the combined microkelvin capacity of these facilities for research in new areas of physics, especially nanophysics.
•To enhance the capacities of the access-giving facilities.
•To network the members of the low temperature and related research communities, scientists with cryo-engineers and the end-users with access providers, to facilitate cross-disciplinary sharing of knowledge.
•To disseminate the expertise of the core institutes to the wider community by the development of compact user-friendly refrigerators for microkelvin research in efficient infrastructure environments.
•To foster the development of the next generation of refrigerators and instrumentation for ultralow temperature measurements and to help in their commercialization.
•To develop strategies and tools for the long-term build-up of a virtual European Ultralow Temperature Laboratory without walls.
Project Results:
Organization and Results of the Microkelvin Collaboration

Microkelvin promoted new alternatives for physics research through the use of ultra-low temperatures. To achieve this goal, three main routes were followed:

1) Microkelvin disseminated information to the low temperature community at large through its Networking Activities NA3 (“knowledge and technology transfer”) and NA4 (“strengthening European low temperature research”). The former activity maintained a data base on microkelvin physics and techniques which was available in the public domain on the internet. A second important task was to organize meetings and workshops. Most important have been three one-week workshops which have included reports of all Microkelvin research activities and the periodic review of the grant programme at 18-month intervals. The NA4 activities involved the founding and running of a “European Cryogenic Society” which was accomplished in the form of a Low Temperature Section created within the Condensed Matter Division of the European Physical Society. A second example was the effort to enhance connections to high-level research in third countries outside the EU regime, by sponsoring invitations of distinguished speakers to European physics workshops and schools.

2) Microkelvin provided access to its three core laboratories through the Transnational Access Activity packages TA1 – TA3. These packages carried the provisions for researchers to perform experiments and for students to learn working procedures in the three access institutions. For one individual visitor, the total visiting time could amount up to three months. The visits were carefully discussed and planned in advance, and finally approved by the Selection Committee, to provide the visitor the maximum gain from his/her stay. In total, access was provided to 72 users for 81 months. The users came from 14 different EU countries or associated countries. In spite of a slow start, the access activity achieved the goals foreseen in the Microkelvin plan outlined in Annex I.

3) Microkelvin also involved new research to develop the experimental tools needed at the very lowest temperature. This effort was contained in four work packages, the Joint Research Activities JRA1 – JRA4. The four work packages contained the following tasks:

JRA1 Opening the low temperature regime to nanoscience with ex-chip techniques
JRA2 Development of low temperature on-chip nanorefrigerators and thermometry
JRA3 Fundamental physics questions with microkelvin condensed-matter experiments
JRA4 Novel methods and devices for ultra-low-temperature measurements

JRA1: Microkelvin was both upgrading existing refrigeration capacity in its collaborating laboratories as well as developing new concepts for efficient refrigeration and thermometry. For instance, the construction of a new large-scale low-heat leak nuclear refrigeration installation was in progress during the grant period in Lancaster University. This apparatus has ambitious technical specifications and is designed to provide an efficient environment for the refrigeration of nano-structured samples and devices to sub-millikelvin temperatures by means of the most advanced filtering and thermalization techniques.

The cooling of electronic sensors and devices has proven difficult, even if we only consider low dilution refrigerator temperatures of order 10 mK, while the sub-mK regime has been entirely out of reach. A new scheme has been devised in the University of Basel where the electrical leads to the sensor are individually carefully filtered and thermalized to the different cooling stages of the dilution refrigerator. With this approach the group has cooled Coulomb blockade thermometers consisting of a metallic superconducting tunnel junction array to about 5 mK, probably the lowest verifiable on-chip electron temperature so far. By including active cooling of the sensor leads, each with its own nuclear coolant, it is expected that much lower temperatures become realistic for conduction electrons in planar on-chip micro-fabricated devices in the nearest future.

The development of adiabatic nuclear demagnetization cooling in a pulse-tube-cooler precooled 3He-4He dilution refrigerator has been a central goal during the last few years. Currently the Microkelvin industrial partner Bluefors is selling approximately 30 units annually of fully automated dry dilution refrigerators. This is perhaps about one third of the world production. The combination of a cryogen-free fully automated refrigeration apparatus with nuclear cooling, all behind “push-button operation”, will make this approach practical for in-house sub-millikelvin operation in nanoscience laboratories. The measuring devices and measurement procedures performed can conveniently be operated remotely over the internet, which increases control and reliability of all procedures. Several of such refrigeration installations have been commissioned from BlueFors and are now coming into operation. Of central interest are measurements of their cooling properties and heat leaks to the nuclear cooling stage from the mechanical vibrations transmitted by the pulse tube cooler. Presumably in the next few years we will see more and more deliveries of such refrigeration systems, providing sub-mK temperatures with a residual heat leak of order 1 nW to the nuclear cooling stage, at a cost of less than 0.5 MEuros.

JRA2: The goal of this work package was to use nanofabrication to develop on-chip refrigeration and thermometry. Both superconducting tunnel junction and quantum dot structures have been developed for cooling. For instance, in tunnel junction cooling in a superconcuctor –insulator – normal metal – insulator – superconductor tunnel structure (S-I-N-I-S structure) a thermal current appears while an electrical current is directed through the tunnel barrier. Two approaches have been pursued to improve the cooling effect at lower temperatures, since the optimum efficiency is achieved at about 0.4 Tc: either by reducing the superconducting gap with a small applied magnetic field or by selecting a superconductor with a lower Tc and smaller gap value. Both techniques have now been demonstrated to work and the low-temperature limit of these devices has dropped down to 30 mK. Further reduction of temperature is expected, for instance with multistage cooling schemes, so that ultimately the long-term goal of microcooler operation in the 10 mK range might become achievable.

JRA3: The ultra-low temperature regime is a frontier where selected fundamental physics questions can be attacked with low-noise high-sensitivity measurements. Simultaneously the goal of this work package is to test the measuring techniques developed in JRA4. A number of first-time discoveries have been made in the course of this work. The study of the dynamics of quantized vortex lines in superfluid 3He-B in the T ⟶ 0 limit has provided new understanding about the interplay of laminar and turbulent flow and their respective dissipation mechanisms. The phenomenon of Bose-Einstein condensation of magnons or spin-wave excitations has been clarified and developed to a practical research method for measuring the order parameter texture in 3He-B. Spin relaxation in these resonance modes, both in the homogeneous mode at intermediate temperatures and the inhomogeneous magnetically trapped very-low-temperature mode, has proven particularly sensitive to the presence and properties of vortex cores. Another new development is the identification of BCS pairing states of superfluid 3He in a nano-fabricated restricted planar geometry between two smooth parallel plates with different separations using SQUID-based NMR techniques.

Owing to their inherent nature some of the fundamental questions which were planned to be investigated within JRA3 have turned out less feasible than originally anticipated and have been replaced by other more promising tasks. An example is a dark-matter detector where the target material is superfluid 3He-B at 100 μK. Owing to difficulties with financing and collapsing roofs in the underground laboratory, this effort was postponed and some of the preliminary studies were expanded, namely the development of micro-fabricated mechanical resonators and measurements of the excitation spectrum of the 3He Fermi liquid. The latter effort lead to a remarkable discovery of a long-lived roton-like plasmon mode, a collective mode in the density of two-dimensional 3He liquid.

JRA4: Of vital importance for research in the sub-mK range is the development of novel techniques for thermometry and sample characterization, particularly in the case of nano-size samples. Low noise and high sensitivity dictate the use of SQUID amplifiers which need to be coupled to micron-size sensors, often in a contactless measuring setup. The viability of this approach has been demonstrated in measurements of dielectric polarization echoes, thermal conductivity, and heat capacity of glassy materials down to 7 mK. Another demonstration of high sensitivity was the measurement of resistive current noise in a piece of copper, using inductive readout and calibration against a 195Pt NMR thermometer down to 200 μK. These measurements would not have been possible without the development work invested in SQUID-based preamplifiers. The development of high-frequency SQUID amplifiers – vital for quantum engineering experiments at the quantum limit of sensitivity – was a further important Microkelvin effort.

Potential Impact:
Future of the Microkelvin Collaboration

Traditionally physics at ultralow temperatures has required elaborate large-scale infrastructure which cannot be bought, but must be home-built, and is therefore difficult both to acquire and to maintain by a single academic research unit of usual size. Nevertheless, over the past few decades several groups in Europe have managed to establish large-scale cryogenic facilities that are unique on the worldwide scale. Today these laboratories are leading much of the research in quantum fluids and solids, as well as in materials and nanosciences at ultra-low temperatures. Their collaboration within the Microkelvin programme has been working on two frontiers: (1) to upgrade the existing large-scale infrastructure which caters for the most demanding special research tasks and (2) to further the development and usage of more specialized equipment.

If we accept the preferences expressed by nanophysics researchers, for instance, then the evolution in microkelvin technology ought to develop the same way as we have seen computing changing during the past fifty years: from large main-frame equipment to diversified small-scale almost table-top apparatus which can be dedicated to specific tasks. Materials and nanophysics labs prefer to collect small-scale apparatus, ranging from sample fabrication and characterization to measurement and analysis, with affordable unit cost. In this environment the role of the European Microkelvin Collaboration becomes ever more important: it is to provide expertise, education, services, and new research in the development of refrigeration and measurement techniques. This development can be viewed as partial outsourcing of a research discipline, whereby European materials and nanophysics laboratories obtain research expertise and resources from a continent-wide Microkelvin Collaboration.

A further major mission of the Microkelvin grant programme is to extend the working regime in nanophysics towards lower temperatures. Lower temperatures can be predicted to lead to easily certifiable benefits. So far experiments in nanoelectronics or nanomechanics have not been cooled to sub-millikelvin temperatures. Microkelvin has helped to fix attention to this goal and today many different approaches are being experimented with to make this possible some day.

To consolidate this activity after the finish of the Microkelvin grant, the Collaboration has formally established a European Ultra-Low-Temperature Laboratory, a distributed infra-structure with complementary instrumentation and the following goals: to give European re-search open access to its facilities and to operate as a flexible coordinated superstructure with the aim to help European research to make use of low temperatures. To provide continued support for this activity, the Collaboration will be submitting a new grant application within the 2014 EU call for research infrastructure programmes.

List of Websites:
Microkelvin maintained a website

Its management office in the Lounasmaa Laboratory of the Aalto University included the following personnel at the time of termination of the project:
Project Coordinator: Matti Krusius (email: or
Project Manager: Katariina Toivonen (email:
Project WEB-officer: Jonne Koski (email:
Project Secretary: Mari Kaarni (email: