Abstract
It is not easy to assess, or even to describe correctly a long and distinguished career that started about the time when I was born. In 1964 Stig Stenholm got both an engineering degree at the Helsinki University of Technology (HUT), and an MSc degree (in Mathematics) at the University of Helsinki. The two degrees demonstrate Stig's ability to understand both complex mathematics and experimental physics. Statistical physics or rather, quantum liquids, was the field in which Stig got his DPhil at Oxford in 1967, under the guidance of Dirk ter Haar. It is interesting that together they worked on studying fermions in a bosonic background [1]; at the time this meant, of course, 3He atoms as impurities in 4He liquid, but nowadays one would immediately connect such systems to the physics of cold atomic gases.
The postdoctoral period in 1967–1968 at Yale University brought Stig in contact with Willis Lamb and laser physics [2]. Back in Finland, Stig's career in the 1970s was dominated by theoretical studies of gas lasers, especially pressure and collision effects on spectral lines and saturation spectroscopy, together with his first PhD student, Rainer Salomaa. A professorship at the University of Helsinki came in 1974, and in 1980 an important era started as Stig became the scientific director of the Research Institute for Theoretical Physics (TFT). At that time he also developed the semiclassical theory of laser cooling especially with Juha Javanainen. The laser spectroscopy work led to a textbook in 1984 [3], and the semiclassical laser cooling theory was summarized in a review article in 1986 [4]. These were not, of course, his only interests, as he also worked on free-electron lasers, ring-laser gyroscopes, multiphoton processes and quantum amplifiers.
In an article written in 1990 in honour of Olli Lounasmaa [5], the founder of the famous Low Temperature Laboratory at HUT, Stig mentions that one of his most memorable achievements was acting as a bridge between the Western and Soviet laser cooling communities. In neutral Finland, accessible to both parties, he organized informal workshops that crucially sped the development of laser cooling. The importance of these meetings is highlighted in Bill Phillips' Nobel lecture in 1997 [6]. However, as the emphasis in laser cooling moved from a semiclassical description to quantum theory, Stig started to look for new avenues of research.
My contact with Stig came when I went to see him for an MSc thesis topic. This involved polarization spectroscopy and modelling of an experiment performed by the laser spectroscopy group (Birger Ståhlberg) at the University of Helsinki. Without knowing anything about quantum optics I then found myself immediately in one of the Finnish–Soviet workshops on quantum electronics in the small town of Porvoo slightly east of Helsinki. There I met people like Juha Javanainen, Peter Knight, Axel Schenzle and Vladimir Chebotayev, and my own career in quantum optics began, working on the mathematics of the Landau–Zener model and how to apply it and other such models in molecular excitation by femtosecond pulses for my PhD thesis. This work was done together with Barry Garraway, who was a postdoc in Helsinki in the early 1990s. The Finnish–Soviet meetings continued for a while, and in 1990 we even got to travel to Novosibirsk (and back, too).
During its existence (1964–1996), the Research Institute of Theoretical Physics (TFT) had a tremendous impact on physics in Finland. Short and long stays by visitors provided a unique and high-level environment for local researchers, the research fields covered a wide range of physics, and for many Finns returning from abroad it provided a place to stay until something more permanent turned up. Thus many researchers who later became professors had, at some point in their career, a connection with TFT. As a director Stig was very broad-minded and without this the happy atmosphere of TFT could not have existed. In the 1980s young researchers such as Marc-Andre Dupertuis and Steve Barnett worked with Stig at TFT, and in the 1990s it was the turn of Barry Garraway, Ilkka Tittonen and Nikolay Vitanov among others.
For his work in 1992–1997 Stig Stenholm received the prestigious Academy of Finland Professorship which provided him with valuable research funds. My graduation in 1992 was followed by Mackillo Kira in 1995 and Päivi Törmä in 1996. Following developments in the field, in the mid-1990s Stig started to work on Bose-Einstein condensation and quantum information. Later he had a Humboldt fellowship, with stays in Germany shared by the Universities at Konstanz (Jürgen Mlynek) and Ulm (Wolfgang Schleich).
Unfortunately, the University of Helsinki decided to replace TFT and a corresponding experimental particle physics institute with a new institute, which was mostly seen as the Finnish front-end for CERN collaboration; in 1997 the Helsinki Institute of Physics (HIP) was started. Although the activities of TFT still existed in the theory section of HIP, many things and especially the atmosphere were changed and in 1997 this partly led Stig to accept a position at the Royal Institute of Technology in Stockholm, Sweden, where two of his Finnish students, Patrik Öhberg and Erika Andersson graduated shortly after the move. I moved to HIP at the time, but left for a position in Turku in 2000, and the quantum optics project was finally switched off at HIP in 2003—only later it was found that quantum optics had provided the Institute with its most cited papers, propelling it into the top 5 percent most cited institutes in its field (especially thanks to Norbert Lütkenhaus and his work on quantum cryptography).
The time in Stockholm was fruitful for Stig scientifically, but he also became a member of the Royal Swedish Academy of Sciences, and although Nobel committee papers remain secret for 50 years, it is likely that he had a hand in its activities. In 2005 we jointly published a textbook on quantum information [7] and in Stockholm he had, again, very successful postdocs such as Ulf Leonhardt. Finally, in 2005, Stig Stenholm retired, although he is still active, writing papers, taking part in conferences and making research visits. We honoured his 70th birthday at the CEWQO2009 conference, and hope that the future provides us with further opportunities for such events.
Looking at the obituary of Dirk ter Haar, I see that his style with students reminds me of Stig's approach. In my opinion, Stig expects independence and initiative from a student, giving perhaps a broad topic in which the student is expected to find his or her own way, whilst working perhaps with a postdoc. Juha Javanainen has talked about the 'sink or swim' style (not referring to Stig, though). There is a famous series of children's books about Moomin trolls by Tove Jansson (another Swedish-speaking Finn like Stig). In one of them, the Moomin find in early spring a small flower in a patch of land uncovered by snow, pushing its way up. One of them wants to cover it against frost during the night, but another says 'Don't, it'll fare better later if it has some difficulties at first'.
At CEWQO2009 Stig gave the full list of his finished PhD students: Rainer Salomaa (1973), Temba Dlodlo (1980), Juha Javanainen (1980), Markus Lindberg (1985), Matti Kaivola (1985), Birger Ståhlberg (1985), Kalle-Antti Suominen (1992), Mackillo Kira (1995), Päivi Törmä (1996), Asta Paloviita (1997), Patrik Öhberg (1998), Martti Havukainen (1999), Erika Andersson (2000), Pawel Piwnicki (2001), Åsa Larson (2001), Markku Jääskeläinen (2003), and Jonas Larson (2005). One should also mention Erkki Kyrölä, who eventually graduated at Rochester and Olli Serimaa, who never graduated but published some important early-stage laser cooling work.
As a final note I must mention a passion that Stig and I share, namely books. I have nearly 400 professional physics and mathematics books, but I am certain that the size of Stig's library exceeds this by far. And this passion is not solely limited to professional books, as one can see, for example, from the references in Stig's article for these proceedings. His interests cover philosophy, history, science fiction and many other topics, and more importantly, the books in his library also get read. In modern times of narrow specialization, high efficiency and h-index, his approach to science and culture sets an example that is worth following.
References
[1] Stenholm S and ter Haar D 1968 Dilute mixtures of 3He in superfluid 4He Physica 38 133 [2] Stenholm S and Lamb W E 1969 Semiclassical theory of a high-intensity laser Phys. Rev. 181 618 [3] Stenholm S 1984 The Foundations of Laser Spectroscopy (New York: Wiley) [4] Stenholm S 1986 The semiclassical theory of laser cooling Rev. Mod. Phys. 58 699 [5] Stenholm S 1998 Physics that matters Arkhimedes 42 197 [6] Phillips W D 1990 Nobel lecture: laser cooling and trapping of neutral atoms Rev. Mod. Phys. 70 721 [7] Stenholm S and Suominen K-A 2005 Quantum Approach to Informatics (New York: Wiley)