Scientific Autobiography of Ronnie Kosloff - ACS Publications

Special Issue Preface pubs.acs.org/JPCA

Scientific Autobiography of Ronnie Kosloff

M

conclusion that quantum dynamics must be regular.[11] This result contradicted the common viewpoint at the time. Thus, my result was important in redirecting the community. But from my point of view, it closed that venue of research, forcing me to look for a new direction. In 1980, I returned to a faculty position in The Hebrew University of Jerusalem.

y research career was driven by curiosity and opportunity. For as long as I can remember, I have been curious about how things work. In my view, science is unique in this respect since it is based on objective criteria for answering such questions. Opportunity was facilitated by working in an engaging scientific environment that consisted of supportive mentors, talented students, and creative collaborators that I had the privilege of working with.

4. TIME-DEPENDENT METHODS FOR QUANTUM MOLECULAR DYNAMICS Quantum dynamics is a complex field. Developing intuition is difficult. Upon starting my independent research career, I felt that I needed to develop my own toolkit. In this respect, analytical mathematical methods are limited, since most interesting problems have no closed-form solutions. Moreover, collaboration with experimentalists required being able to simulate quantum dynamics for systems for which analytical solutions were not attainable. I was seeking an approach that would allow me to follow chemical events sequentially in time. As it turned out, at the same time, my brother Dan Kosloff was involved in developing simulation techniques for seismic wave geophysical exploration. Discussions at the Friday dinner table led to a joint research venture aimed at using Fourier grid methods for solving the time-dependent Schrödinger equation.[19, 20] What I liked about the Fourier method was that the algorithm reflected my quantum intuition. For example, the position−momentum commutation relation was incorporated as part of the representation. The method was also centered around the idea of a wave packet, which represented a semilocal description of a particle. The first application of the new approach was with Tamar Yinnon in 1983. We studied the scattering of helium from a stepped tungsten surface.[21] Benny Gerber, Tamar’s adviser, joined the fun, suggesting to simulate scattering of He from impurities.[26] Those early applications identified a few crucial problems. The first issue was the time propagation method. Originally we used second-order differencing. The accuracy was limited to polynomial convergence; therefore, the method became unbalanced with the exponential convergence of the Fourier method. In 1984, Hillel Tal-Ezer, at that time a graduate student in Tel Aviv University, proposed to use a Chebychev expansion of the evolution operator.[28] The method was simple to use and allowed for exponential convergence. The second issue was related to boundary conditions. In particular in scattering problems the wavepacket that leaves the simulation box must satisfy outgoing boundary conditions. Together with Charlie Cerjan, during his visit to Israel in 1985, we solved the boundary problem by introducing absorbing boundary conditions. With Dan Kosloff we also applied the method both to geophysical wave propagation[25] and for

1. EARLY YEARS I was born in Los Angles on July 26, 1948. My family was in the U.S. at the time for medical treatment for my father following a car accident involving a British army truck in Gaza. I came to Israel when I was one year old. I spent much of my childhood in Jerusalem. My family moved to Haifa in 1960, where I attended The Reali High School. It was during this time that I met Yaffa Freidheim, my companion for life. Following graduation in 1966, I was drafted to the Israeli Defense Forces, where I served in the armored corps. During my compulsory army service, I took part in the battles on the Sinai front during the 1967 Six Day War. I entered the Hebrew University of Jerusalem in 1969, graduating in 1972 with a joint B.Sc. in Chemistry and Physics. I got married in 1971, and my first son, Omri, was born in July of 1972. 2. GRADUATE SCHOOL IN THE HEBREW UNIVERSITY OF JERUSALEM I started graduate school in the Hebrew University of Jerusalem in 1972, with Professor Raphael D. Levine as my advisor. This was an inspiring period in Jerusalem. Any scientific topic was legitimate. The main topic was information theory and its utility in chemical dynamics. My M.Sc. thesis was on a statistical theory of nonadiabatic multiple transitions. My studies were interrupted by the 1973 war. I was drafted for six months and took part in the battles on the Golan Heights front. I started my Ph.D. in 1974. I participated in research on surprisal analysis, which was the main research focus of the Levine group at the time.[2, 5, 6] Nevertheless I am grateful to my advisor Raphael Levine for allowing me to explore other research interests. As a result, my thesis ended up focusing on the relationship between the second law of thermodynamics and quantum measurement theory.[10] I obtained my Ph.D. in 1979. My second son, Aviv, was born in April of 1977, and my third, Elad, in 1990. 3. POSTDOC AT THE UNIVERSITY OF CHICAGO Following graduate school, I moved with my family to U.S. in 1978, to start a postdoc under Professor Stuart Rice in the University of Chicago. Chicago at that time was the world center of physical chemistry, and my postdoc advisor, Stuart Rice, was the best of scientific leaders. The hot topic was chaos. Classical dynamics on typical molecular potentials was found to be chaotic. I was assigned with answering the question of whether the corresponding quantum dynamics was also chaotic. I studied the formal mathematical literature and reached the © 2016 Ronnie Kosloff

Special Issue: Ronnie Kosloff Festschrift Published: May 19, 2016 2943

DOI: 10.1021/acs.jpca.6b00104 J. Phys. Chem. A 2016, 120, 2943−2949

The Journal of Physical Chemistry A

Special Issue Preface

second problem was related to the time-ordering problem when the Hamiltonian operator is explicitly time-dependent. A joint effort with Michael Berman and Hillel Tal Ezer in 1991 led to a new type of propagator based on Newtonian interpolation.[80] Sampling points in the complex plane allowed to address complex eigenvalues. An improved procedure was developed with Hillel Tal Ezer and Guy Ashkenazi based on Lejat interpolation points.[118] A comprehensive review of propagation methods was published in 1994.[104] Propagating with a time-dependent Hamiltonian was more difficult to solve. Our first attempt with Hillel Tal Ezer and Charlie Cerjan in 1991 was based on a low-order polynomial approximation.[79] Our second attempt was based on the (t,t′) method developed by Uri Peskin and Nimrod Moiseyev in 1994. The method solved completely the time-ordering problem by embedding in a larger Hilbert space.[105] In 2010 I returned to the time propagation issue initiated by solving optimal control problems where the (t,t′) method was not applicable. With Mamadou Ndong, Hillel Tal-Ezer, and Christiane Koch we developed an iterative double-Chebychev method.[253] Generalization to nonlinear equations and other improvements were carried out by Ido Schaefer, a graduate student, in 2012.[273] The basic Fourier method is optimal for Cartesian coordinates and periodic boundary conditions. To enhance the sampling efficiency, Eyal Fattal and Roi Baer developed the mapped Fourier method based on a phase-space optimization. [119] An important improvement was suggested in 1998 by V. Kokoouline, O. Dulieu, and F. Masnou-Seeuws from Orsay. The idea was to base the mapping onto a semiclassical approach.[145] Reflecting on my experience in developing time-dependent quantum molecular dynamics the grand challenge remains addressing the exponential growth in computational complexity with dimension. In my view, the challenge is to find a method that will focus on a particular observable in a many-body calculation. A hint in this direction is the work of Michael Khasin,[256] which proposed adding noise to the calculation to limit the exponential growth of the computational complexity. Other random approaches based on quantum typicality for large quantum simulations could work.

general wave equations.[38] The third issue was to obtain a general method to generate initial conditions. It is natural to start from an eigenstate of the Hamiltonian. Then the dynamics is initiated by applying a perturbation, such as by a laser pulse. I found that by a slight modification of the Fourier method, namely, changing the Chebychev propagator from real time to imaginary time, we obtained a relaxation method that allowed us to generate the ground state.[42] My first graduate student, Rob Bisseling, joined my group in 1984. His first assignment was to extend the method to polar coordinates. Rob came up with a Fast Henkel transform.[31] Our main challenge in developing the method was how to extend it to many degrees of freedom. An obvious choice was to use a tensor product state representation. Rob Bisseling was able to show that the sampling efficiency degrades significantly with dimensionality. The optimal representation was found to be equivalent to packing balls in an N-dimensional box.[50] The first phase of the development of our time-dependent quantum dynamics methodology was summarized in a feature article published in 1988.[56] The key points were the realization that the simulation box can be defined in position−momentum and time−energy phase spaces and that trigonometric basis functions such as the Fourier basis and Chebychev lead to exponential convergence. Another cross disciplinary research was performed by Jose Carcione a graduate student of my brother Dan Kosloff. Together we pioneered the study of viscoelastic waves.[58−61] The numerical effort of a direct product basis scales exponentially with dimensionality. A possible solution was to use a Time-Dependent Self-Consistent Field approximation (TDSCF). We explored this possibility in 1987 together with Mark Ratner, Benny Gerber, Charlie Cerjan, and Rob Bisseling. [52] Mean field self-consistent methods supply a reasonable approximation, but for problems with bifurcation they miss essential physics. As a remedy we started to develop a multiconfiguration (MC) approach. With Zvi Kottler and Abraham Nizan we applied the idea to electronic nonadiabatic dynamics.[62] Audrey Dell Hammerich joined my group as a postdoc in 1987 and developed the idea for reactive scattering with Mark Ratner.[51, 75] The idea of MC-TDSCF spread, eventually becoming the core of the Multi Configuration TimeDependent-Hartree method (MCTDH). In 1994 Audrey Dell Hammerich and myself joined forces with the MCTDH teamUwe Manthe, Hans-Dieter Meyer, and Lorenz S. Cederbaumin applying the method for the photodissociation of CH3I in five dimensions.[106] The elegance of the Chebychev propagator led to other novel applications. It was used to calculate the Green’s function, [66] to solve the Liouville von Neumann equation[70] and the diffusion equation (with Noam Agmon),[47] as a Gaussian spectral filter[68] and with Roi Baer and Gil Katz for a flux calculation.[115] A CECAM workshop in Orsay, organized jointly with Claude Leforestier in 1989, initiated a comparative study of popular propagation methods for the time-dependent Schrödinger equation. A thorough error analysis was performed indicating the overwhelming superiority of the Chebychev propagator in both accuracy and efficiency compared to other popular methods.[71] Two disadvantages of the Chebychev method emerged. The first was the inability to deal with propagators that have complex eigenvalues. These appear in the Lindblad equation the generator of dynamics in an open quantum systems. The

5. DYNAMICS AT SURFACES Scattering represents a central class of problems in quantum dynamics. The Fourier method was ideal for calculating diffraction of He from crystal surfaces. With Tamar Yinnon and Benny Gerber we also explored scattering from surface impurities.[27] In 1983 I met Charlie Cerjan in Chicago while visiting Stuart Rice. We started a collaboration on the study of quantum mechanical scattering and desorption.[32] We next added inelastic scattering from phonons.[43, 65] In 1987 I attended a conference in Holland together with my colleague from Jerusalem, Micha Asscher. It required a trip to a conference to initiate an in-house experimental−theoretical collaboration. Micha’s primary interest was inhomogeneous surface catalysis of nitrogen on metal surfaces. I suggested a mechanism based on nonadiabatic tunneling of the N 2 molecule as a precursor for dissociation. Gadi Hasse, at the time a graduate student of Micha, confirmed the tunneling mechanism by finding a 25% difference between dissociation rate of the heavy and light isotopes 14N2 and 15N2.[57, 63] The nitrogen dissociation experiment in Micha Asscher’s lab was 2944




quantum dynamics to obtain the desorption yield and kinetic energy distribution. Dynamics on surfaces also had a profound influence on our research in the area of open quantum systems, for example, the development of the surrogate Hamiltonian by Roi Baer for modeling the ejection of subsurface hydrogen in a nickel surface.[133] The theory of surface science still lacks a reliable nonadiabatic theory able to tackle catalytic processes that involve charge transfer. Electronic structure theory cannot reliably calculate multiple potential surface. In addition the dynamical methods are inadequate in solving for the quantum dynamics in cases of close degeneracy. In my view, addressing these challenges is key for future breakthroughs.

advanced by introducing a supersonic nozzle. The incident kinetic energy was controlled by seeding a supersonic molecular beam. Ofra Citri, a post doc in my group, simulated the experiment with a quantum time-dependent approach.[81] Gil Katz, in 1995 a new M.Sc. student, performed a new set of simulations testing various aspects of the experiments.[117, 134] A bold and risky endeavor was conducted by Micha Asscher with Leonid Romm by employing the isotope 15N2 in a molecular beam dissociation experiment. The results were surprising. The isotope effect as expected almost vanished at high incident energy. But in the tunneling region of low energy the isotope effect exceeded Ofra Citri’s calculated estimates by an order of magnitude.[155] These results remain puzzling. At the time we were criticized for not using ab initio potentials. At that period, density functional theory (DFT) calculations started to appear, and the computational surface science community was obsessed with them. The DFT calculations showed very high barriers for nitrogen dissociation, which was in fact inconsistent with experiment. I was convinced that our general nonadibatic approach was correct. However, in 1996 there were no reliable electronic structure methods able to calculate multiple electronic potentials on surfaces. In 2001 Gil Katz started his Ph.D. in collaboration with Yehuda Zeiri. We decided to study the dissociation of oxygen on metal surfaces. The mechanism we suggested was based on charge transfer from the metal to generate O2. A second charge transfer event leads to dissociation of the molecule. With Ofra Citri and Roi Baer we used this model to study the dissociation of oxygen on silver.[120] The key experiment was the dissociation of oxygen from aluminum by Ertl. Our nonadiabatic model was able to explain the apparent activation energy of 0.5 eV as well as other experimental observations such as the appearance of lone adsorbed oxygen atoms on the surface at low incident energy. [194] Extensive DFT calculations showed no barrier for dissociation. Gil Katz performed 5D quantum time-dependent calculations that were summarized in a comprehensive paper[194] explaining all experimental findings. With Micha Asscher and Yehuda Zeiri we organized in 2004 a meeting on Non-Adiabatic Processes on Surfaces at Ein Gedi, Israel. Still, we could not convince the dominant DFT community. It was the work of Ester Livshits and Roi Baer that eventually solved the puzzle.[245] A basic flaw in many DFT functionals is the inability to account correctly for charge transfer. Once the problem was identified, new functionals could capture the correct physics underlying surface dissociation. Current experiments are consistent with the charge-transfer nonadiabatic mechanism. Peter Saalfrank came on a visit to Jerusalem with the purpose of employing our solvers of the Liouoville equation for simulating desorption of molecules from metal surfaces.[112, 123] The mechanism of photodesorption on oxide surfaces is also via an excited charge-transfer state. Together with T. Klüner and H.-J. Freund we conducted time-dependent calculation of the photodesorption of CO from Cr2O3.[168] Christiane Koch, at the time a graduate student in the Fritz Haber Institute in Berlin, had the ambitious assignment of pursuing a complete ab initio theory of the photodesorption of NO from a NiO surface.[186, 214] The approach included electronic structure calculations, constructing a surrogate Hamiltonian for the electronic quenching, and executing

6. COHERENT CONTROL I first met David Tannor in the summer of 1985, while I was visiting Stuart Rice in Chicago. David came with the intriguing proposal of using a pump−dump scheme to control chemical reactions. The idea was based on a semiclassical estimation of controlling the timing between pulses. With my newly developed solvers of the time-dependent Schrödinger equation we were able to test these assumptions.[46] It became evident that the scheme relied on a focusing potential in the excited state. We therefore looked for methods to compensate for the excited-state dispersion. As a result we developed the method of pulse shaping.[67] Pier Gaspard and Sam Tersigni consolidated the approach by incorporating optimal control theory. I was sitting with David listening to a talk by Keith Nelson in Jerusalem in 1991. Keith was suggesting to use a series of pulses to excite vibrational motion. The pulses were displaced by the vibrational period. Nevertheless the results were not convincing. At that point both of us immediately knew that the problem was in the phase. The outcome was the development of the method of local control with Audrey Dell Hammerich. [89] Borrowing ideas from quantum thermodynamics, with Allon Bartana and David Tannor, we employed the scheme of local control to study laser cooling of molecules.[94] We continued this study by developing optimal control schemes for open quantum systems.[131] Finally we were able to identify the cooling mechanism as Laser Cooling of Molecules by Dynamically Trapped States.[163] This scheme has been realized experimentally in Aiḿ e Cotton for cooling vibrations of ultracold Cs2 and in Northwestern in cooling rotations of a molecular ion. I was on sabbatical in Boulder in 1998−1999. My graduate student Jiri Vala came on a visit. Jiri was excited about the prospects of quantum computing. Discussions with my neighbor Zohar Amitay, at that time an experimental postdoc at the laboratory of Steve Leone, led to experimental implementation of the Deutsch−Jozsa algorithm for threequbit functions using pure coherent molecular superpositions. [182] I then became interested in quantum information processing. At that period Jośe Palao joined my group as a postdoc in 2001. Together we generalized optimal control theory for quantum computing. The idea was to set the control target as a unitary transformation.[181] The marriage of coherent control with ultracold matter is natural. This was realized by Christiane Koch who moved to Orsay as a postdoc with Francoise Masnou in 2003. Christiane initiated the use of coherent control for the stabilization of 2945




insight and accuracy. With Olivier Dulieu we published our first joint paper on the subject.[137] Vladislav Kokoouline, at the time a graduate student of Francoise, joined our efforts, introducing coordinate mapping. As a result, the efficiency of representation of cold molecules improved significantly.[145] Ultracold matter provides an ideal platform for coherent control. Jiri Vala suggested the use of chirped pulses for coldmolecule formation through photoassociation.[160] A more realistic study of the photoassociation of cold atoms with chirped laser pulses was performed by Luc-Koenig and Vatasescu, leading to an improved analysis of the adiabatic transfer within a two-state model.[199] A first experimental verification was performed by Gould and Wright.[207] A significant achievement was a first-principle calculation of the absolute number of molecules per pulse.[223] Solvejg Jorgensen joined my group in 2001. Together we proposed pulse shaping of coherent matter waves thereby directly controlling reaction dynamics.[198] Mathias Nest improved on pulse shaping of matter waves.[255] Returning to Denmark, Solvejg Jorgensen worked with Michael Drewsen. Together we studied intensity and wavelength control of a single ion−molecule reaction.[209] Vladislav Kokoouline, who joined my group as a postdoc, proposed together with Jiri Vala using light to modify the scattering length.[162] With Chistiane Koch we translated ideas from impulsive excitation of molecules to cold matter. [249] In 2008 we organized at Ein Gedi Israel, the Batsheva de Rothschild seminar on ultrafast−ultracold processes with the idea of linking the fields. The challenge in physical chemistry is to develop an approach able to cool and trap molecules of chemical interest unraveling their underlying quantum nature.

ultracold molecules using optimal control theory.[196] We discovered that experimental realization would require shaping pulses in the nanoscale range. Such a technology became available only recently by the work of Jenifer Carini and Phil Gould. We were able to observe the enhancement of ultracold molecule formation using shaped nanosecond frequency chirp pulses.[300] In 2005, Shimshon Kallush was on his way to a summer school to study feedback control. As an exercise we decided to study the prospect of a quantum governor: Automatic quantum control and reduction of the influence of noise without measuring.[218] We formulated the problem as an optimal control task of generating a whole vocabulary of unitary transformations for an open quantum system. In 2006 Gil Katz was a postdoc with Mark Ratner at Northwestern University. We worked out a way to protect a system from external noise by using a noiseless reference system. By using local control with respect to a scalar product between the reference state and the noisy state we were able to reduce the noise.[227] Our analysis indicated that the mechanism was based on a quantum refrigerator, where the noise could be transferred to an ancilla and dissipated. Noise in the environment can be suppressed, for example, by error correction. But what happens to the noise generated by the controller? Such a device must be fast on the time scale of the controlled system, therefore bringing with it an unavoidable Markovian noise. Does such noise limit the controllability? Discussions with Lorenza Viola and Michael Khasin in a workshop in Santa Barbara initiated a study to address those questions. We addressed the influence of Markovian noise on state-to-state controllability. We found that in large quantum systems complete controllability is lost.[260] With Shimshon Kallush we were able to verify that noisy fields destroy in particular superpositions of classical like generalized coherent states.[286] This study has implications on the construction of large quantum computers. Control of binary chemical reactions is one of the outstanding goals of coherent control. Our approach was to split the task into a first step of photoassociation and a second step of controlling the outcome. To realize this goal we constructed an international team headed by Chistiane Koch in Kassel, Zohar Amitay at the Technion, Robert Moszynski in Warsaw, and myself in Jerusalem. Sai Amaran, a postdoc from India, laid the foundation for the quantum simulation. Our idea was to employ a two-photon photoassociation step[285] followed by coherent control.[298] In 2011 I became chairman of the Gordon conference on Quantum Control of Light and Matter at Mt. Holyoke, USA. In 2012 with David Tannor I organized the Batsheva de Rothschild Seminar on Laser Control of Chemical Reactions at Safed Israel. Coherent control of binary chemical reactions with distinct products is still lacking. The challenge in theory is to develop a comprehensive approach for control of open quantum systems. Ideas of feedback control are still absent in molecular dynamics.

8. STRONG-FIELD DYNAMICS In 1985 Charlie Cerjan moved to Lorenz Livermore National Laboratory. He introduced me to the emerging research topic of atoms in high intense laser fields. On the basis of the Fourier method we developed a nonperturbative treatment of abovethreshold ionization.[49] We then teamed with Nir Ben-Tal and Nimrod Moiseyev in 1992 to study high-harmonic generation.[93] I came back to high-harmonic generation employing tools of optimal control theory with Ido Schaefer in 2012.[279] We formulated the problem of optimizing emission and limiting the driving frequency. The study of strong-field dynamics influenced the development of time-dependent computational methods. A still lingering computational challenge is to overcome the singularity and long-range aspects of Coulombic forces. 9. OPEN QUANTUM SYSTEMS I first encountered the paper of Lindblad on the generators of the dynamics of open quantum systems when I was in graduate school. I realized that those equations provide a phenomenological approach to the dynamics of molecules in the condensed phase. With Stuart Rice I published my first paper on the subject.[8] Discussions with Mark Ratner in 1981 led to the suggestion to use quantum relaxation effects as a molecular switch.[13] Independently I found a method to derive the semigroup equation as a limit of a more general stochastic process.[14]

7. ULTRACOLD MATTER When temperature is reduced, quantum effects become prominent. Francoise Masnou-Seeuws visited us in 1996 with the mission of generating and studying ultracold molecules. At that period time-dependent quantum computational methods were not used in the field. I was able to convince Francoise that the Fourier grid method had significant advantages in both 2946




10. QUANTUM THERMODYNAMICS Thermodynamics and quantum mechanics deal with the same subject matter. The deep connections between the two subjects fascinated me throughout my career. I devoted my Ph.D. thesis[10] to this issue and kept an active research effort in quantum thermodynamics ever since. Thermodynamics tradition is to study by example. When I came back to Jerusalem in 1981, one of my first projects was the study of a quantum dynamical model of a heat engine. I based the theory on a dynamical definition of the first law of thermodynamics formulated for open quantum systems. I composed the engine model from two harmonic oscillators, each coupled to a different (hot and cold) bath. Power was extracted by an external periodic field driven by excitation exchange between the oscillators. The quantum equations were found to be consistent with the first and second laws of thermodynamics.[24] Moreover the engine could operate far from equilibrium delivering finite power. This result allowed us to make contact with the emerging empirical theory of finitetime thermodynamics. For example, at high temperature, the engine efficiency at maximum power coincided with the endoreversible result. In 1990, Eitan Geva joined my group and became my first student to work in the area of quantum thermodynamics. Employing the framework of the Lindblad equations, we constructed quantum models of four-stroke engines based on the Carnot and Otto cycles.[85, 87] While these early models were empirical in nature, Eitan’s ambition was to base the analysis on a first-principle approach that started from a Hamiltonian model that includes the system and hot and cold baths. To this end, we chose to study the three-level quantum amplifier. The maximal efficiency of this quantum engine was known to be given by the Carnot efficiency. We set out to derive and solve the equation of motion of the three-level quantum amplifier under finite power conditions. The power output mechanism was modeled by coupling to a classical light field.[102, 122] We found two limits where we could obtain equations consistent with thermodynamics. The first was the adiabatic limit where the driving frequency is slow relative to the bath time scale.[102] We could then use an adiabatic description of instantaneous equations of motion for the engine. The opposite limit was found when the driving frequency was fast relative to the bath time scale.[122] In this case, we had to develop a dressed-state framework where the dissipative dynamics became dependent on the driving field. We found that accounting for this field dependence was crucial for consistency with the second law of thermodynamics. Recently Amikam Levy, a current graduate student, generalized this result to quantum transport finding that a local approach to quantum transport may violate the second law of thermodynamics.[290] The study of the discrete Otto engine continued in 1996 with Tova Feldmann, Eitan Geva, and Peter Salamon.[116] A comprehensive model of the performance both of an engine and a refrigerator was carried out by Tova Feldmann.[152] Friction was introduced in a phenomenological way. Amazingly in these early studies, all quantum engine models resembled closely their macroscopic counterparts. The only quantum feature was the discrete energy levels of the working medium. The quantum models shared almost every feature with actual engines operating out of equilibrium conditions. This

In 1983, with Mark Ratner, we proposed a line shape formalism beyond linear response. The idea was based on a direct calculation of the dissipated power employing the time derivative of the first law of thermodynamics.[17] These ideas were later also used in my studies of quantum thermodynamics. I returned to the problem in 1992 with Michael Berman. We developed a numerical scheme for solving the Lindblad equation.[80] Together with Allon Bartana, who joined my group as a graduate student in 1992, we used this approach to simulate the pump−probe spectra of the impulsive excitation of I3 in solution measured by Sandy Ruhman and Uri Banin. Our approach, which was based on Lindblad’s equation for impulsive excitations, went beyond the traditional perturbative approach.[108] Another graduate student in my group, Guy Ashkenazi, later developed a comprehensive open system dynamical description of the pump−probe spectroscopy of the photochemical reaction decomposition of I3. In 1999 with Mark Ratner, Guy found a general turnover phenomena for photoexcited electron transfer.[146] In the process, we also developed computer-aided visualization tools to teach quantum mechanics. Jiri Vala, a graduate student from the Czech Republic, continued the study of impulsive excitation of I− in collaborations with Erez Gershgoren and Sandy Ruhman who performed the experiments. The result was a theory for phase-space measurements by chirped pulses. In addition we found experimental evidence for a non-Gaussian dephasing model for the vibration of I−.[167] Challenged by dynamics at surfaces, Roi Baer, who was a graduate student in 1996, developed a new approach for describing the quantum dynamics of open systems, which was based on a Surrogate Hamiltonian.[135] The idea was to replace the surrounding bath by a set of representative spins. The method was used to study the diffusion of hydrogen on a Ni crystal,[138] locating the Wigner transition temperature from an activated to a tunneling mechanism. The method was further developed by Christiane Koch and Thorsten Klüner in 2001. The method is non-Markovian and includes initial system bath correlation. With these new methods we were able to construct a complete quantum description of an ultrafast pump−probe charge transfer event in the condensed phase.[173] An additional development was led by David Gelman who joined my group in 2003, who used random-phase wave functions to describe thermal states.[192] In 2008, with Gil Katz, David Gelman, and Mark A. Ratner, we added a stochastic outer layer of spins, which allowed us to extend the Surrogate Hamiltonian method to time scales leading to equilibrium.[238] We used this method to explore the optimization of solar energy capture.[293] The Lindblad equation exhibits nonhermitian properties. Morag Am Shalem, a graduate student in my group, visited me while I was on sabbatical at ITAMP in 2012. We teamed with my friend and colleague Nimrod Moiseyev, an expert on nonhermitian dynamics, to study exceptional points nonhermitian degeneracies. Surprisingly we found a line of such points in the well-studied Bloch equation.[301] In my view, the grand challenge facing the Surrogate Hamiltonian method, as well as other methods for simulating quantum open system dynamics, is to base them on a a fully quantum ab intio platform. 2947




observation raised the question: What is quantum in a quantum heat engine? Together with Tova Feldmann, in 2002 we constructed an Otto cycle model where the Hamiltonian is split into a controlled part and an interaction part. What made the model unique was that the two parts of the Hamiltonian did not commute. Speeding up the engine cycle led to quantum nonadiabatic phenomena resulting in increased dissipation. We therefore found a quantum origin of microscopic friction and an equivalence between quantum and thermodynamic adiabatic dynamics.[176, 190] In 2004, Yair Rezek joined my group. As a starter I asked him to study a quantum Otto engine based on the harmonic oscillator.[221] This model, which is analytically solvable, led to many surprises. On the one hand it behaved like a standard Otto cycle in finite time where the performance is restricted by friction and finite heat transport. Planck constant miraculously disappeared from the results. Two time scales emerged: The cycle period and the oscillator period. We were surprised when we found quantized frictionless solutions. In parallel Muga Gonzales found similar phenomena and termed it shortcut to adiabaticity. Frictionless solutions mean that the engine is restricted only by heat transport. Ignoring the details such engines are equivalent to macroscopic models. Resetting the question what is quantum in a quantum heat engine? A quantum heat engine can be reversed, thus becoming a refrigerator. I first realized this possibility by inverting the threelevel amplifier.[122] This raised the question: What is the optimal rate of cooling when the absolute temperature is approached?[156] This idea initiated a quest to find a dynamical version of the third law of thermodynamics[281]. In 2001 Jośe Palao from Spain joined my group as a postdoc. His first assignment was to add noise to the three-level refrigerator model. We expected noise to interfere with the refrigerator’s operation. To our surprise, we found that thermal noise can drive the refrigerator without external power. Together with Jefferey Gordon we analyzed the device as an autonomous refrigerator, a quantum analogue of an absorption refrigerator.[169] It was only natural to invert the harmonic engine studied by Yair Rezek. The first step was to find the optimal cooling rate. Peter Salamon and Karl Heinz Hoffmann suggested to use optimal control theory for this task.[242] The optimal cooling was obtained when the oscillator frequency was proportional to the cold bath temperature. Cooling could continue to the absolute zero temperature. Addition of noise did not change this result.[283] Tova Feldmann inverted her two-spin Otto cycle model. The unique feature of the model was an uncontrolled gap in the spectrum of the working substance. Once noise was added we found that the refrigerator could only cool to minimum temperature on the order of the gap energy. [258, 274] These studies set the stage to tackle directly the dynamical version of third law of thermodynamics. In 2011 Amikam Levy joined my group as a graduate student. He first established the absorption refrigerator model.[272] In collaboration with Robert Alicki who was on sabbatical in Israel, we found the connection between the two versions of the third law: The untenability principle and the vanishing of entropy production when the absolute zero is approached. The dynamical equivalent was the vanishing of the entropy production on the cold bath and the vanishing of the rate of temperature decrease.[275]

Quantum thermodynamics is undergoing rapid development. When Eitan Geva joined my group in 1990 we were quite isolated. With David Tannor and Tal Mor we organized a conference at Safed Israel in 2007 on Cooling and Thermodynamics of Quantum Systems. In 2012 Cost action quantum thermodynamics consolidated the field in Europe. With this auspice we organized a small workshop in Ma’agn Israel in 2013. Two topics were highlighted. The first was to demonstrate an experimental realization of a quantum device. The second topic addressed again the question: what is quantum in a quantum heat engine? At that period Raam Uzdin joined my group and became fascinated with quantum thermodynamics. Raam found a way to unify the discrete and continuous models of quantum heat engines. Moreover he was able to show that in continuous engines the power extraction mechanism requires coherence. [299] Studying quantum thermodynamics was always a source of joy for me. After 40 years, the field is thriving with many young scientists joining the fun. One of the remaining challenges is the experimental realization of a single quantum device. I am proud that I could take part in this development.

11. PUBLIC SAFETY In 2002 Yehuda Zeiri approached me with a proposal to study a mysterious molecule by the name of TATP. I first declined, claiming the study is not in my line of research. But Yehuda was very persuasive, claiming that we should contribute to the ability to detect the molecule that has been used as an improvised explosive by terrorist organizations. Faina Dubnikova joined the effort, which led to the first calculation of the structure of TATP and its metal complexes.[174] In 2005 jointly with the experimental groups of Ehud Keinan and Joseph Almog, we published a comprehensive paper on the properties of the molecule, its spectra, and the explosion mechanism.[204] We then formed a scientific study group, financed by NATO, on improvised explosives, with Jimmie Oxley from URI as our NATO integrator. Our goals were to explore remote detection methods, decomposition mechanisms, and safe disposal once the material was located.[239, 240] To expand our theoretical tools for modeling the chemical reactions during detonation, we joined forces with the group of William Goddard and Edri van Duin in Caltech. Our first joint paper was on TATP using the ReaxFF reactive force field.[208] In 2006 Elhanan Wurzberg asked me to work on the decomposition of TNT, a military explosive. I was surprised since the material has been known for more than a century. Revital Cohen joined my group in 2007, and together we calculated the unimolecular gas-phase decomposition pathway of TNT.[230] However, the calculated gas-phase unimolecular activation energy was 20 kcal higher than experiment. We then assembled a team to study the bulk decomposition mechanism of TNT. Our hypothesis was that the gas-phase unimolecular mechanism is replaced by a series of bimolecular reactions in bulk. The team consisted of Naomi Rom and Barak Hirshberg, who concentrated on liquid-phase decomposition, and David Furman, who concentrated on the crystal phase. Sergey Zabin and William Goddard from the Caltech group contributed to the ReaxFF reactive force fields used in the bulk simulations. Faina Dubnikova performed electronic structure calculations. Yehuda Zeiri shared with me the management of the project. Our efforts resulted in a comprehensive paper identifying the 2948




details of the bulk decomposition of TNT, which were found to be consistent with experiment.[288] Our current efforts in this area focus on developing innovative detection methods of explosives. The scientific challenge is to construct a reliable method able to detect explosives remotely. With Gil Katz we proposed nonlinear THz spectroscopy,[287] and with David Furman and Yehuda Zeiri we are studying the mechanism of laser ablation.

12. OUTLOOK In my view, the grand challenge currently facing molecular science is the integration of different disciplines. My research career was always on the borderline between physics and chemistry. I realized early on that physicists are obsessed with the idea of reducing reality into a small and compact equation, while chemists tend to be obsessed with complexity. In my view, a multicultural approach that benefits from both worlds works the best. For example, if a quantum computer is to be developed, it must be protected from noise. Cold matter can provide such noise-free environment. This means that a quantum refrigerator must be part of the design. Coherent control can then be used to solve the problem of the quantum compiler. In principle any molecule can serve as an element in quantum technology. But what is the optimal one? I thank my teachers for introducing me to the scope of science. Even more I thank my students for each day teaching me something new.

■

REFERENCES The numbering is according to the publication list. Ronnie Kosloff

2949


Scientific Autobiography of Ronnie Kosloff - ACS Publications

Recommend Documents