Hallo, My name is Yuji Hasegawa and I am now working at the Atominstitut der Österreichischen Universitäten, Wien. Our group is engaged in quantum optical experiments with neutrons, namely with the use of the neutron interferometer and polarimeter to investigate fundamental phenomena of quantum mechanics.
Before I explain the detail of my research (interests), a little more about myself: I studied Applied Physics at the University of Tokyo, Japan. My first stay in Austria, namely in Vienna, was between 1991 and 1992 as an exchange student between TU Wien and the University of Tokyo. In this period, I joined Prof. Rauch's neutron interferometer group at the Atominstitut. It was really a great experience for my future life and research! After coming back to the University of Tokyo, I finished my Ph.D. thesis, which dealt with interference experiments using high-energy photons, x-rays from synchrotron radiation, and neutrons. One topic of my thesis was a series of interference experiments with nuclear excitation, where an incident beam of single photons that splits into two beam paths, is absorbed and re-emitted after a time-interval. Finally, interference was observed between the re-emitted beams [1]. In addition to the coherence properties, a phase information transfer through the absorbed photon and the emitted photon was confirmed (see Fig.1). I feel that experiments with coherent excitation of nuclei contain abundant of unknown physics still to be explored.
Fig.1 Phase information transfer through the absorption and re-emission.
After I finished my Ph.D. thesis, I became a Postdoc at the University of Tokyo and constructed a precise neutron optics (PNO) beam-line at the JRR-3M, Japan Atomic Energy Research Institute (JAERI), Tokai, Japan [2]. This beam-line is dedicated to neutron optical experiments with perfect crystals, e.g., neutron interferometry. Many quantum mechanical phenomena are being studied using photons. In contrast, the use of neutrons together with a perfect crystal interferometer is our special strategy for research on the foundations of quantum mechanics. Our neutron interferometer is physically equivalent to the Mach-Zehnder interferometer for visible light, i.e., an incident beam is split into two at the beam splitter, reflected at the mirror and recombined at the analyzer. It was first demonstrated at the Atominstitut and offers elegant opportunities to study the wave properties of neutrons. A variety of perfect crystal neutron interferometers fabricated at the Atominstitut is shown in Fig. 2. This device is used for thermal neutrons, i.e., neutrons with wavelength ~2Å, energy ~20meV, and velocity ~2000m/s. (It is rather "low" energy physics??)
Fig. 2 A variety of perfect crystal neutron interferometers at the Atominstitut.
One of the subjects I am interested in is the so-called geometric or alternatively called topological phase. A geometric phase is deeply connected to the curvature of underlying (state- or parameter-) space: A two-dimensional plane in three dimensional real space does not have an intrinsic curvature, but considering a sphere embedded in Euclidean real space we have to take the curvature of this manifold into account. In geometry this curvature is reflected for example in the angle difference of a vector transported around a loop along geodesics, e.g., great circles of a sphere. It was Berry who first addressed this issue in quantum mechanics. The geometric phase can be interpreted to be brought by global change, instead of a local change, of the system. The classical counterpart is the following: Consider a transport of a vector along geodesics on a sphere so that the angle between the vector along the geodesics is unchanged (no local change). After one cyclic excursion, the vector will turn to a different direction due to the curvature of the sphere (global change). This situation is depicted in Fig. 3. Note that a classical system in general does not remember its past whereas a quantum system can remember its history: it is stored in the geometric phase of the state of the quantum system.
Fig. 3 Geometric phase obtained in cyclic evolution on the sphere.
Such a geometric phase is known to be derived for spinor evolutions. We applied this formalism to situations in a split-beam experiment, where similar two-level system for 1/2-spinor is involved, and further theory was developed. Associated experiments were done with the use of specially fabricated "two-loop" neutron interferometer at the Hahn-Meitner Institut, Berlin, Germany [3].
Thinking the incident polarization and its changes as the superposition of the spin-up and spin-down states and the consequences of interference, it turned out that the polarization-dependent phase shift can be studied more accurately and in simpler configurations by neutron polarimetry than by interferometry. We have utilized this device to observe the non-commuting property of the Pauli matrices [4]. The experimental setup is shown in Fig. 4.
Fig. 4 Neutron polarimeter experiments to show the non-commuting property of Pauli matrices.
Another main interest of mine is quantum contextuality. For those to whom it may sound unfamiliar, I explain first the word "quantum non-contextuality": Non-contextuality implies that the result of a measurement on the observable 
is determined independently of the previous or simultaneous measurement of any set of observables mutually commuting with 
. One will easily accept this assumption, when taking, for instance, two-observables one for the colour of the jacket and the other for the colour of the pants of a skier: these two colours should be independently determined!? In neutron interferometer experiments, there are two commuting observables: one for the path and the other is for the spin, which belong to different Hilbert spaces, then commuting each other. Nevertheless, in our neutron interferometer experiments, it was shown that these two properties are not independent but correlated!! The experiment was accomplished at the Institute of Laue Langevin, Grenoble, France, and the setup is shown in Fig. 5. This experiment has close relation to the Einstein-Podolsky-Rosen (EPR) experiment, where quantum non-locality of an entangled system is argued. We should mention, quantum contextuality is physically a more general concept than non-locality, thus it might give more perspectives?
Fig.5 Neutron interferometer experiment to show quantum contextuality.
The strategy of using a neutron interferometer, which was first realized and intensively developed in Austria, is still providing us with new aspects of quantum phenomena. Those who are interested in this field can consult the book [7] for other accomplishments.
[1] See, a review, Yuji Hasegawa and Seishi Kikuta, in Nuclear Resonant Scattering of Synchrotron Radiation, Eds. U. Gerdau and H. de Waard (Balzer, Oxford, 1999).
[2] H. Tomimitsu, Y. Hasegawa, K. Aizawa, and S. Kikuta, Nucl. Instr. Meth. A420 (1998) 453-466.
[3] Y. Hasegawa, M. Zawisky, H. Rauch, and A.I. Ioffe, Phys. Rev. A53 (1996) 2486-2492.
[4] Y. Hasegawa, S. Menhart, R. Meixner, and G. Badurek, Phys. Lett. A234 (1997) 322-328: Y. Hasegawa and G. Badurek, Phys. Rev. A59 (1999) 4614-4622.
[5] Y. Hasegawa, R. Loidl, M. Baron, G. Badurek, and H. Rauch, Phys. Rev. Lett. 87 (2001) 070401: Y. Hasegawa, R. Loidl, G. Badurek, M. Baron, N. Manini, F. Pistolesi, and H. Rauch, quant-ph/0201054, Phys. Rev. A65 (2002) 052111.
[6] Yuji Hasegawa, Rudolf Loidl, Gerald Badurek, Matthias Baron, and Helmut Rauch, Nature 425 (2003) 45-48.
[7] H. Rauch, and S.A. Werner, Neutron interferometry (Clarendon Press, Oxford, 2000).
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