A report on the XX international quantum electronics conference Sidney, Australia 14–19 July, 1996

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EUROPEAN PHYSICAL SOCIETY

NEWSLETTER

4/96

Quantum Electronics and Optics Division of the European Physical Society Responsible Editor:.

R. Corbal~tn, Deparmment de Ffsica, Universitat Autbnoma de Barcelona E-08193 Bellaterm. Spain. Tel.: +34 (3) 581 16 53, Fax: +34 (3) 581 21 55

The current year issues of the Newsletter are available via anonymous - Ftp to Ftp.uab.es at the folder/pub/docs/

A Report on The

XX International Quantum Electronics Conference Sidney, Australia 14- 19 July, 1996 The International Quantum Electronics Conference (IQEC'96) was held this year at the Sydney Convention and Exhibition Center, Sydney, Ausl~alia from 14--19 July. This conference usually takes place every secGdd year since 1959, and is an international forum for discussion of all aspects of quantum electronics. In this occasion it was organized by the Auslralasian Council on Quantum Electronics under the aegis of the International Council on Quantum Electronics. This is the first time IQEC has been held in the Southern Hemisphere. In IQEC'96 600 papers have been presented, which included 3 plenary and 5 tutorial lectures, 90 invited papers, 472 regular contributed papers and 30 postdeadline contributed papers. Of the total 600 papers, 253 were presented in poster sessions and 347 in oral sessions, which were held in 5 different rooms (there were 5 parallel sessions each day). They covered most of the aspects of quantum electronics. In particular, topics of IQEC'96 included the physics of coherent light sources, laser cooling and spectroscopy, atom and quantum optics, optical materials, nonlinear optics, optical interactions with condensed matter, ultrafast

phenomena, advanced photonics, and optical communications. There were also three more specialized satellite meetings, also held in Al~str~.lia, dealing with guided--wave propagation and devices, quantum optics, and atom optics. The plenary lectures summarized last advances in three outstanding areas of quantum electronics. Prof. Cornell (University of Colorado) explained the processes that take place when an atomic gas is cooled from room temperature to nano Kelvins, emphasizing the counter intuitive behaviours that emerge at very low temperatures, as well as the occurrence of Bose--Einstein condensation. Prof. Polany (University of Toronto) reported on recent experimental advances in the field of photochemistry of adsorbates and clusters. Prof. Miller (Bell Laboratories) showed how recent advances allow quantum wells to be regarded as an ideal "laboratory" to perform fundamental experiments of quantum mechanics, as well as to construct useful devices. The five tutorials covered rapidly developed areas in quantum electronics. Prof. Hanna (University of Southampton) devoted his tutorial to planar waveguide lasers in dielectric materials, explaining what may be their role in future applications. Prof. Keller (ETH, Zurich) explained the new developments in ultrafast solid--state laser that have given us the possibility to create pulses as short as 8 fs, which has applications in other areas apart from Physics. Prof. Ekert (Oxford University) gave a tutorial on quantum computation. He showed how a quantum computer would perform certain operations much faster than the computers we have today, and explained the present obstacles to build one. Prof. Weisbuch (Ecole Polytechnique, Palaiseau) showed how the control of optical modes through cavity effects can be used to

improve light emission properties, as well as to perform some of the quantum mechanical experiements that so far have been carried out only in the context of quantum optics. Prof. Lugiato (Univetsita degli Studi di Milano) explained the phenomena of spontaneous formation of two-dimensional patterns in the radiation field, when it interacts with a nonlinear medium. An special symposium was devoted to commemorate the 80th birthday of Prof. A.M. Prokhorov, who was one of the founders of Quantum Electronics. He, Prof. Basov, and Prof. Townes are the three key scientists who proposed the generation of radiation based on stimulated emission of atoms. He received the Nobel Prize in Physics in 1964. There were four invited papers in this symposium, given by outstanding contributors in the field of quantum electronics: Profs. Manenkov (Russian Academy of Sciences), Krupke (Lawrence Livermore National Laboratory), Byer (Standford University) and Walhh,er (Max--Plank--Institut). In the following I will give a brief summary of some of the results presented at the conference. Since there were 5 parallel session each day, I have only included the oral sessions that I attended, which were those related to laser cooling, Bose.-Einstein condensation, atom and quantum optics, and quantum measurement and information. Parenthetical names credit the speakers who presented the corresponding taR. I apologize to the participants in other oral sessions and in the poster sessions, whose work I have not mentioned. Fortunately, a summary of their contributions is available in the IQEC'96 Technical Digest.

frequencies of the condensate, and how the corresponding oscillations damp out. They have also measured the specific heat of the condensate. The excitation spectrum has been also measured by the MIT group (W. Ketterle). This group has "'seen" the condensate using off--resonant laser light, without perturbing the condensate. In addition, they have been able to "'cut" the condensate in two parts using laser light and, after dropping it, they have observed interferences. These expedmental results coincide remarkably well with the theoretical predictions of K. Burnett's group based on the non--linear SchrOdinger equation. This equation seems to contain most of the physics of a Bose--Einstein condensate (K. Burnett). For example, it predicts the possibility of a stable condensate for gases with negative scattering length, provided the number of atoms is small enough. One can also understand theoretically the growth dynamics of a condensate by studying how it is affected if one adds particles to it (R. Ballagh). Other theoretical studies of the dynamics of a condensate show that the macroscopic wavefunction that describe the condensate can undergo collapses and revivals (D. Walls). This prediction could be observed in an interference experiment. After having observed Bose--Einstein condensation, the possibility of building a coherent source of atoms (the boser or atom laser) is one of the present goals in this field. So far, there are two approaches to this problem: the first one is to use evaporative cooling, whereas the other one relies on laser cooling. Theoretical models describing these two situations were analyzed (H.M. Wiseman and U. Janicke, respectively).

Bose--Einstein Condensation Laser cooling and atom optics Bose--Einstein condensation has been one of the most important achievements in atomic physics during the last years. It was predicted 70 years ago that if a gas of weakly interacting bosonic atoms is cooled below a critical temperature, the atoms tend to occupy the same quantum state. This prediction was verified experimentally in 1995 by three groups, at the University of Colorado, MIT, and Rice University. Once the condensate is formed, one can perform a great variety of experiments with it. Some of these remarkable experiments were presented at IQEC by the ftrst two groups. The JILA group (E. Cornell) reported on the measurements of the collective excitation

The progress made during the last ten years in atom trapping and cooling can be now applied in many areas. For example, it has permitted to observe Bose--Einstein condensation, to build atomic interferometers, and to reach an extraordinary precision in spectroscopy. Several experimental groups presented new results in this rapidly progressing field. Atomic interferometers have reached a point where they can be used not only for demonstration purposes, but also to measure with more precision than with any other technique. Using one of such devices the quotient hiM (M is the atomic mass) has

been measured with a relative error of 10"8 (S. Chu). It is expected that in the near future this will be improved by more than a factor 10. This will allow to measure the fme structure constant a with a higher precision. High precision measurements can be also achieved with atomic intefferometers based on the Moire effect (J. Schmiedmayer). On the other hand, two--atom correlations (i.e. quantum statistical effects) have been observed for the first time with bosonic atoms by detecting the arrival time (F. Shimizu). This is the analogue of the Brown--Twiss interferometer for photons. The basic components of atomic interferometers and other atom--optical experiments are improving very rapidly. At this conference very efficient atomic mirrors were presented. They were based either on evanescent waves (J.H. Eschner and A. Aspect) or on magnetic forces coming from permanent magnets (P. Hannarofd and D. Meschede). Other atom-optical tools are well suited to write spatial patterns with high spatial resolution. At the University of Konstanz, they have written submicron structures using Cr beams focused by laser standing waves (J. Mlynek). They have also written more sophisticated patterns using He. Physicists at the University of Tokyo have managed to paint a picture with Ne atoms by passing the~Yi through a computer generated hologram (F. Shimizu). On the other hand, Rb atoms have been successfully guided through a hollow fiber filled with an evanescent wave (H. Ito). The role of the van der Waals interaction between the atoms and the dielectric surroundings have played an important role in these experiments (A. Aspect and H. Ito). The developments in laser cooling and trapping have also permitted to carry out a series of experiments that so far were considered as "'gedanken". Using laser standing waves one can entangle the internal and external degrees of freedom of an atom (G. Rempe). The diffraction pattern of the atoms in a screen can be modified or erased by measuring and manipulating the internal atomic state. Entanglement between atoms and photons in an atom--optical experiment has been also demonstrated (C. Kurtsiefer). Furthermore, Bloch oscillations of ultracold Cs atoms in a periodic optical potential have been observed for the first time at the ENS in Paris (E. Peik). On the other hand, a single Ba ion has been cooled near the zero point energy via "'stochastic cooling" (P.E. Toschek). This will permit to generate non--classical states of motion of a single Ba ion.

Other precision measurements have been carried out with atoms in optical traps. Information about the scattering length, dipole moments, and van der Waals interaction has been obtained (DJ. Heinzen). This information is very important to achieve and manipulate Bose--Einstein condensates. Dipole traps are being constructed by means of holographic produced light fields (W. Ertmer). Other optical traps using blue far--off resonance 3D optical lattices have been constructed in Konstanz (J. Mlynek). Combined with laser cooling, they will allow to observe quantum statistical effects of atomic gases. Other theoretical efforts to see these effects in optical lattices were presented (I.H. Deutsch). Cavity QED Cavity QED provides one of the most important scenarios to observe some of the most intriguing properties of Quantum Mechanics. Recent experi~,',ental advances allow today to have single atoms crossing high--Q cavities in the strong coupling regime, whereby the cavity mode--atom interaction is stronger than all other dissipative processes. This has allowed the observation of several quantum phenomena, without classical analogy (H. Walther). The possibility of using an atom interacting with a cavity mode to perform a simple quantum logical operation has been predicted theoretically some time ago. Steps towards this goal have been taken by several experimental groups. In particular, at the ENS in Paris they have been able to measure the collapse and revival of the population inversion of an atom that crosses a cavity (S. Haroche). This gives evidence of the granularity of the electromagnetic field in the cavity. In a very beautiful experiment, they have also produced a Schr'odinger cat state of the electromagnetic field, which paves the way for observing "'live" the process of decoherence, i.e. the transition between the microscopic world ruled by quantum mechanics to the macroscopic world ruled by classical mechanics. At Caltech, in another remarkable experiment, they have been able to observe conditional phase shifts of two weak laser beams that enter a cavity (H. Mabuchi). The dynamics of this and other cavity QED processes is described by the Jaynes--Cummings Model, in which dissipation in both the cavity and the atoms can be included. Numerical predictions

obtained using quantum trajectories were reported at the conference, and a new semiclassical theory to describe these phenomena was presented (H. Carmichael). Using quantum trajectories the possibility of entangling two cavity modes by sending an atom through them was analyzed (H.W. Giesen). It was also shown that direct evidence of completely quantum effects in cavity QED can be obtained by performing two--photon coincidence measurements on the outgoing cavity field (B. Sanders). The possibility of Irapping atoms inside the cavity by continuously monitoring the outgoing cavity field was also proposed (A.S. Parkins).

system to be measured. However, this back action can be manipulated in such a way that it does not disturb the quantity that is measured. Individual and sequential quantum non--demolition experiments with a squeezed light beamsplitter was achieved (S. Schiller). An adaptative measurement based on a feedback loop was proposed to improve the uncertainty in the phase of an electromagnetic field (HJ. Wiseman). On the other hand, an experimental demonstration of a high--efficiency interaction free detection of an opaque object was presented (P. Kwia0. Quantum computation and cryptography

Quantum Measurement The quantum theory of measurement is one of the open problems in quantum mechanics. Different interpretations for the way a collapse of the wavefunction takes place (if it does) are still under dispute. From the more practical point of view, however, there are several situations in which quantum mechanics has to be taken into account in the measurement process. Quantum non--demolition measurements, quantum tomography, and interaction--free measurements are examples that have been already demonstrated by several experimental groups. Some of the new advances in this field were presented at IQEC'96. The problem of determining completely the quantum state of a system (quantum tomography) is a delicate one. The key point is that the measuring apparatus (the quantum ruler) also behaves quantum mechanically, and this has to be taken into account in order to reconstruct the original state of the system (P. Knight). The tomography of an electromagnetic field generated by an optical parametric amplifier has been carried out experimentally (S. Schiller). The reconstructed field corresponds to a squeezed vacuum state that presents oscillations in the photon number distribution. It was also shown how the quantum tomography of a trapped ion moving in a time--dependent potential could be carried out (W. Schleich). The possibility of detecting quantum chaos in this system was proposed as well. On the other hand, it is well known that in a single shot measurement there is always back action in the

Quantum cryptography and computation are two of the most striking applications of interference phenomena in quantum mechanics. If some day they are going to be useful, we first have to understand how are they affected by the presence of decoherence, and how to avoid this last process. It is not yet clear whether error correction schemes will be sufficiently efficient to circumvent this problem (A. Ekert), at least in the field of quantum computation. In quantum cryptography, however, after the latest experiments, it seems that may be of practical use in the foreseeable future (S. Barnett). Unfortunately, existing quantum cryptographic schemes are not secure in the presence of decoherence. An entanglement--based quantum cryptographic scheme that is probably secure over a noisy channel was presented (C. Machiavello). There are other possible cryptographic systems that are easier to operate than the present ones (J.D. Franson and N. Imoto). Quantum communication could be also benefited from quantum dense coding, in which acting only on one two--level system, one can produce four (orthogonal) bits of information. This has been demonstrated for the first time with polarization--entangled photons (P. Kwiat). In summary, IQEC'96 presented some of the most important contributions of the last few years in the field of quantum electronics. J.L Cirac Departamento Fisica, Universidad Castilla--La Mancha 13071 Ciudad Real, SPAIN