2

Physics Division

The Physics Division supports research to discover and understand exotic quantum and extreme optical physics where new regimes are expected to create revolutionary capabilities for the future warfighter.¹ The division’s core budget of $9.3 million was leveraged against a $16.2 million investment by the Defense Advanced Research Projects Agency (DARPA) and other Department of Defense (DoD) programs and agencies in the physics domain. During fiscal year (FY) 2018, a total of 66 single investigator (SI) awards were funded along with nine Short-Term Innovative Research (STIR) awards focused on jump-starting high-risk projects. Four programs were reviewed: Atomic and Molecular Physics, Condensed Matter Physics, Quantum Information Science, and Optical Physics and Fields.

In general, the division’s metrics are strong, with 745 peer-reviewed publications in the FY 2016 to 2018 period, and funding for 279 graduate students and 113 postdoctoral researchers during the FY 2017 to 2018 period. There were 23 transitions reported for the 3-year period from FY 2016 to 2018, including the transition of fundamental physics research funded by ARO to applications developed in the ARL intramural laboratories, which is another good indicator of the success of this program.

ATOMIC AND MOLECULAR PHYSICS PROGRAM

The Atomic and Molecular Physics (AMP) Program seeks to discover and exploit quantum properties of atoms and molecules to support Army functional concepts of fires, intelligence, maneuver support, and mission command. Its research focuses on discoveries that will enable the development of new quantum sensors and computational platforms, with three main objectives, which will aid the development of light, low-power devices suitable for warfighter use on the battlefield: (1) metrology—to ensure that quantum systems measure the desired quantity; (2) connectivity—to determine the role played by connectivity in interacting many-body systems; and (3) optimization—to create efficient classical and quantum optimization platforms.

Specifically, the Atomic and Molecular Physics Program efforts are focused on the exploitation of the quantum properties of atoms and molecules with the long-term goal of achieving significant scientific breakthroughs. Major long-term technological opportunities likely to be enabled by those scientific breakthroughs include (1) the development and use of many-body quantum states for robust precision metrology; (2) the development of precision position, navigation, and timing in global positioning system (GPS)-denied environments; (3) the development of distributed sensor platforms and networks beyond the classical limit; and (4) the development of quantum neural networks—although whether they have the theoretical potential to present advantages over classical systems remains an open question at this time.

___________________

¹ Army Research Laboratory, Army Research Office, https://www.arl.army.mil/www/default.cfm?page=217, accessed October 1, 2019.

Overall Scientific Quality and Degree of Innovation

Considering the limited budget of ARO, the decision to stay away from the actively funded effort that is being made to develop quantum computers and to focus instead on quantum sensors seems well-advised, especially in view of the importance of position, navigation, and timing (PNT) for the Army. Nevertheless, it needs to be kept in mind that experimental work of this sort will still require very substantial support. It cannot be expected, therefore, that principal investigators (PIs) will be able to work at the forefront of this field if they are supported only by ARO funds. The funding that ARO provides will not necessarily be duplicative. It needs to function synergistically instead.

From the information provided, it appears that many, if not most, of the key players in this field are supported at some level by ARO and that the program manager (PM) has developed an excellent working relation with these groups. The PMs involvement in other programs, such as Defense Advanced Research Projects Agency (DARPA) programs, is also proving helpful. The leverage that this provides seems crucial for persuading the leading groups to consider the ARO-sponsored component of their work a key part of their activities, rather than a side program.

The risk/payoff balance is difficult to assess, because progress in the field is exceedingly rapid. Results that seemed impossible just a few years ago are now almost routine in the very best laboratories. These include, for instance, the realization and use of atomic microscopes, the achievement of molecular cooling, progress in atomic clocks, the realization of the first noisy intermediate-scale quantum (NISQ) computers, and almost routine measurements below the standard quantum limit. Nevertheless, the PM also indicated that in many cases, the results achieved by the PIs include discoveries that were not initially expected at the writing of the proposal. This is what happens when PIs pursue the best science, but also reflects the ability of creative and motivated PIs to turn problems into opportunities.

Scientific Opportunity

Major issues in many-body quantum sensing include the following:

1. Decoherence and sensitivity to noise. For this reason, significant work needs to be done to ensure that sensors measure the desired quantities. Among the kinds of projects that need to be pursued are the theoretical study and experimental realization of stable quantum states—for instance, entangled states—in the presence of noise, including topological protection.
2. Lack of theoretical understanding of many-body systems. Hence, the challenge of realizing controllable evolution of many-body-based sensors—here, the future use of NISQ computers may prove particularly useful.
3. Experimental challenges having to do with the likely need to work at low temperatures. This eliminates at this point the use of superconducting devices.

Some of the foundational research directions fundamental to AMP’s objective are not currently well represented in the program. These include identifying simple systems with the longest possible coherence times and the best quantum control. One recent example of such development is the optical tweezer for single atoms, which was first demonstrated over a decade ago in France, but only recently started to be explored in depth. It is now used in the Harvard multitweezer array quantum simulator. Because the quantum information science program is focused on development of systems based on already-demonstrated qubits, it would make sense for the AMP program to put more focus on the basic physics of clean quantum systems. Identifying such systems would allow improvements in both PNT and quantum information applications. At the same time, studying many-body interactions and some of the first demonstrations of quantum algorithms might fit better in the quantum information area.

Significant Accomplishments

This program is supporting an impressive array of some of the very best groups in the field. Their general productivity is remarkable. The progress witnessed in recent years in AMP and related quantum information science is extraordinary. It can be traced to three major developments: (1) the development of the super-radiant laser; (2) improvement in the understanding of the mechanical effects of light on atoms; and (3) the realization of the profound implications of Bell inequalities. These were disruptive advances. An amazingly rich harvest of new developments has resulted—starting from the isolating, cooling, and control of atoms, ions, and photons, and their manipulation at the quantum level, to the realization of quantum degenerate gases, extraordinary advances in clocks, molecular physics, and the development of new experimental tools aimed at the understanding and control of many-body quantum effects.

Relevance and Transitions

The ARO program builds on these developments, and its success reflects the ingenuity and creativity of the PIs it has supported and of the PM as well. Some of the best ARO success stories have resulted from its funding of fairly risky and relatively long-term projects, such as development of the super-radiant laser and the identification of the nuclear transition in thorium. ARO is better positioned to support programs like these than many other science agencies.

Several projects funded by ARO have clear potential for transition—in particular, the super-radiant laser, where many-body correlations allow one to achieve a remarkably narrow linewidth laser. This will have a dramatic impact on precision time keeping, similar to that produced by the development of the hydrogen maser. The search for and recent likely identification of the laser-accessible nuclear transition in a thorium isomer will also likely result in an ultra-stable and portable nuclear clock with remarkable stability. For other projects, such as demonstrations of collective quantum effects in various quantum systems, the potential for PNT applications is more speculative at this point.

This program competes effectively for ARO corporate resources such as Multidisciplinary University Research Initiative (MURI) and Defense University Research Instrumentation (DURIP). It strongly cooperates with DARPA and the other service agencies and is notable for its support of early- and mid-career researchers. It also gives appropriate support to conferences and international collaboration.

The usual path for transitions for successful ARO projects is through DARPA, which has the capability to bring research results closer to applications. At the same time, ARO has more flexibility and granularity in funding smaller scale projects that have high potential for eventual transition to practical applications, but that are not yet ready for Small Business Innovation Research/Small Business Technology Transfer (SBIR/STTR)-type funding.

CONDENSED MATTER PHYSICS PROGRAM

Overall Scientific Quality and Degree of Innovation

The Condensed Matter Physics (CMP) Program presented a clear and cogent strategy for ensuring that the major objectives of the program managers will result in CMP supporting projects that are likely to advance the scientific frontiers of their discipline. The vision of the CMP program is to discover and explain new electronic phenomena in the solid state that, for example, will make it possible to develop electronic devices that are unusually energy efficient. More specifically, advances in condensed matter physics can lead to the development of sensors with higher magnetic field noise rejection and advanced small platform computation capabilities. To this end, the CMP program is focused on four areas: (1) understanding the interactions between topological and magnetic states; (2) realizing and controlling anyons; (3) discovering new nonequilibrium states of solids; and (4) exploring strong correlations in

oxide heterostructures. These areas are at the forefront of modern CMP, and the CMP program has done a commendable job of including in its portfolio a good mix of projects being pursued by both well-established and new investigators. For each of these areas, both the scientific objectives and the challenges were described, and the results that have been obtained were presented. Although there was good synergy between the ARO-funded projects, information was not provided about the other sponsors of related research of the ARO-funded PIs or their competitors.

Scientific Opportunity

The scientific goals of the CMP program are well defined, and it is clear that much thought has gone into the project selection process. For the first topic, understanding topological-magnetic state interactions, the focus is on exploring the limits of (local) disorder on global (topological) properties. Since the CMP program is “materials agnostic,” this concept is being investigated in multiple systems. The accomplishments to date include demonstrations of the anomalous quantum Hall effect at topological-magnetic interfaces (at the Pennsylvania State University) and in hBN-graphene-hBN (at the University of California, Santa Barbara). The second topic is concerned with realizing and controlling anyons. Here, the accomplishments include (1) observations consistent with Majorana quasi-particles at graphene-superconductor interfaces (at Duke University); (2) density functional theory predictions of the influence of superconducting contacts on graphene band structure (at the University of Texas, Dallas); and (3) electrostatic gating of hBN-WTe₂-hBN to achieve the quantum spin Hall effect (at the Massachusetts Institute of Technology and Harvard University). For the third topic, discovering nonequilibrium states of solids, there are significant efforts in coherent electromagnetic excitation (at the California Institute of Technology), and nano-infrared spectroscopy has been developed and used to demonstrate thermally and optically induced phase transitions (at Columbia University). For the fourth topic, exploring strong correlations in oxide heterostructures, detailed plans and accomplishments were not discussed, but it was mentioned that the few remaining projects will be deemphasized.

A good case was made for each subgroup of topics on how the research being pursued would advance the interests of the Army—for example, sensors work in the field, real-time computation, and PNT. There is a good balance between well-established and new researchers in this program, but it could be better. There was good synergy between the ARO-funded projects.

Significant Accomplishments

Most of the research proposed and accomplished fits nicely with the advances that have been achieved by the major groups that make this an exciting field. The PIs have especially focused on some fundamental aspects that have not been studied before—for example, work at Pennsylvania State University has revealed the magnetic interactions at a topological/magnetic interface. These accomplishments map directly onto the stated program goals. It is clear that much thought has gone into the decision-making process.

The search for the elusive Majorana quasi-particle continues worldwide with significantly more funding than ARO’s entire budget. Nonetheless, the project being undertaken by the Duke University and Appalachian State University groups presents an interesting approach to using graphene edge contacted by superconductor and gated at the open ends. Preliminary experiments on graphene in a magnetic field show topologically quantized conductance.

Magic angle twisted bilayer graphene (tBLG) has been under intensive study over the past year since the discovery of Moiré-induced flat bands, superconductivity, and Mott insulators as a function of doping. With ARO support, the University of California, Santa Barbara, team has now added the possibility of high-temperature quantized resistance. They sandwich the tBLG between flakes of boron nitride and voltage gate the system into an orbital-polarized state. The sample then exhibits a quantum anomalous

Hall effect with plateaus at ±h/e² switchable by applying a magnetic field, H, and then returning to H = 0. The Hall resistance remains quantized at h/e² up to 4 K.

Theoretical support for understanding the graphene-superconductor interface is provided by density functional theory calculations by the University of Texas, Dallas, group, which provides the depth-of-band structure modulation from the contact superconductors.

Beyond graphene, the community has been investigating other two-dimensional (2D) van der Waals solids such as WTe₂. With ARO support, a Harvard University/Massachusetts Institute of Technology team has obtained evidence for more topological phases and new physics in this material, including quantum spin Hall effect in a sample with superconducting edge contacts. Such a configuration is a step toward anyon physics with implications for quantum computing.

Relevance and Transitions

The research supported by the CMP program has high relevance for long-term Army applications. Examples include enhanced navigation capabilities, energy-efficient electronics and sensors, and ultra-lightweight optical elements for increased warfighter awareness. Several examples of the transfer of basic research to applied research were presented, including follow-on work on topological Josephson junctions, energy-efficient electronic devices, frequency-selective limiters, and circulators in the GHz range, supported by ARL and DARPA.

The CMP program has had at least 16 years of leadership, which has enabled its manager to strategically build the program through leveraging various Department of Defense (DoD) cooperative opportunities. At the same time, the CMP program needs to beneficially continue its cooperative activities with the program managers in materials science and electronics.

QUANTUM INFORMATION SCIENCE PROGRAM

Overall Scientific Quality and Degree of Innovation

Within ARO, the locus of quantum information science is in the Physics Division, which has a dedicated Quantum Information Science program manager. An additional program manager specializing in the subject, who is an employee of another federal agency, is embedded in the division. The division’s Atomic and Molecular Physics and Condensed Matter Physics Programs engage in topics that cross-fertilize with quantum information, such as implementation of quantum logic in ultracold atoms and ions, and identification of quasi-particles in condensed matter potentially useful for future qubits.

Scientific Opportunity

The Quantum Information Science program manager articulated a clear focus on Army-specific objectives—information dominance on the battlefield, bolstered by sensing and secure communication, with low power and footprint requirements suitable for warfighters in the field.

These goals are pursued with research on multiqubit systems and protocols, the limits of quantum versus classical sensors, and noncryptographic quantum algorithms. The research performers include theorists and experimentalists, ranging from early-career scientists to the most eminent senior figures in the field. There is appropriate support for conferences and international collaboration.

The Quantum Information Science Program’s research is complementary to that of the Atomic and Molecular Physics Program, which addresses similar long-term objectives using different techniques. The interactivity and collegiality between the division’s PMs is noteworthy.

An interesting anecdote was told by the Quantum Information Science program manager. A quantum algorithm had been found to be superior to the public Netflix recommendation algorithm. A new classical algorithm was found later that performed comparably to the quantum algorithm—both exponentially more powerful than the original classical performance. Of particular note, the new classical algorithm was inspired by discoveries within quantum information. This is an important example of how classical and quantum approaches can strengthen each other and of the value of communication between physics and information sciences.

Significant Accomplishments

The overall scientific quality and degree of innovation of this program is high. It supports some of the best performers in the field, in both experiment and theory, while avoiding duplication of much larger programs that are focused on building scalable quantum computers or quantum key distribution systems. This shows good judgment and effective use of ARO resources.

The program aggressively pursues scientific opportunities, as demonstrated by recent successes in demonstration of spin textures in nitrogen-vacancy (NV) diamond systems, circuit quantum electrodynamics in synthetic hyperbolic spaces, and the theory of generative adversarial networks. These rank among the top recent achievements in this highly competitive field of science.

Relevance and Transitions

The transitions associated with this program are associated with commercialization of software, venture capital investment in an ion-trap start-up company, initiation of a DARPA program, and the placement of trained personnel in DoD science and technology positions. These are good outcomes for a basic research program in this field.

OPTICAL PHYSICS AND FIELDS PROGRAM

The PM of this program recently retired, and the report of the program’s progress was presented by two PMs who are involved in other programs in the Physics Division. This seems a good opportunity for the Physics Division to combine the search for a new PM with a discussion about possible new directions. The Optical Physics and Fields area has long been highly relevant to the Army, and there is compelling argument to continue this program, with potentially new directions and focus areas.

Overall Scientific Quality and Degree of Innovation

The quality of the program is high, and the projects it has supported have substantially advanced the science. Particularly noteworthy is the successful transition of a number of its key projects to larger programs.

The major research programs in this program are field-leading. They involve both well-established and early-career investigators, and the balance between perceived high success rate and high-risk/high-return projects is excellent.

Parts of the research portfolio seem to be uniquely ARO-funded or ARO-heavy, such as the light filament program and the thorium nuclear optical transition program. Others include niche areas that ARO has identified as being of particular interest, including novel symmetries in optics and epsilon near zero (ENZ) materials for optics.

No duplications of funded research were observed, and there is synergy between this program and other programs within the Physics Division, as well as with other divisions in Engineering and Materials Science. As an example, the thorium optical transition project may be scientifically related to the AMP program.

Scientific Opportunity

The Optical Physics and Fields Program’s scientific opportunities are in transition. Recent accomplishments in demonstrating light filaments and attosecond physics have resulted in successful transitions for follow-on funding. Continuing present opportunities include exploiting novel symmetries in optics and exciting opportunities for discoveries with the supersymmetric (SUSY) optics and ENZ materials. The program intends to initiate a new initiative to explore alternative solutions to Maxwell’s equations based on recent reported advances.

Significant Accomplishments

The project to accurately localize the ²²⁹Th isomer nuclear transition for doped thorium crystal was successful, culminating a long-standing, international effort to search for this transition energy. This accomplishment offers promise as the enabling technology for a compact, portable, precision optical clock with greater stability and accuracy than present-day clocks as a key element of PNT. Further work is planned to confirm the exact transition, to verify the transition quality, and to develop a clock.

The objective to extend the physics of extreme forms of light beyond established knowledge includes exploring optical materials with ENZ and supersymmetry. Adiabatic wavelength conversion was demonstrated with time varying change of the index of refraction to modify the frequency with approximately 100 times greater frequency shift over 100 times less distance than with indium tin oxide. This was a significant departure from known perturbative nonlinear optics. The result offers the opportunity to provide very compact optical isolator or optical protection components. The program intent is to continue to address the theoretical and experimental challenges of the ENZ regime.

The project applying and exploiting novel supersymmetry in optics has been successfully validated, which opens the door to many opportunities in the near future. This approach, based on passive elements to manipulate optical modes, is attractive not only for high-efficiency lasers but also for many other forms of optoelectronics. There is a parallel between optical and electronic properties when it comes to novel symmetries. One may expect synergy between condensed matter physics projects on symmetry and topological states and optics on novel symmetry.

In line with the objective to extend the physics of extreme forms of light beyond established knowledge, the project to understand and control the mechanisms of light filamentation and light filament-matter interactions was successful. The project involved use of a short-pulse laser to induce a plasma filament over significant distances to deliver potential effects such as microwave through submillimeter wavelength radiation to enable electromagnetic interference, sensing, or communications reception at a targeted object. Based on successful testing, the project was reported to have been transitioned to other parts of the DoD for potential applications.

Relevance and Transitions

The four highlighted projects have all been successfully transitioned to larger programs—light filaments to ARL and other DoD units, optical generation of MeV X-rays to ARL, demonstration of SUSY to DARPA-Microsystems Technology Office (MTO), and thorium isomer transition to DARPA-

MTO and U.S. Air Force. This is an excellent indicator of the magnitude of the impact the optics program is having.

Despite the limited resources, no notable program weaknesses were identified. The PMs are doing an excellent job in focusing efforts in selected areas. The PMs are clearly aware of the dynamic nature of the research frontier. The search for a new PM needs to be seen as an opportunity to identify research priorities at the boundaries of traditional areas. The new PM may further foster the interactions among the different programs within the Physics Division.

OVERALL ASSESSMENT OF THE PHYSICS DIVISION

Four programs were reviewed: Atomic and Molecular Physics, Condensed Matter Physics, Quantum Information Science, and Optical Physics and Fields. The overall scientific quality of the work presented was excellent and in many cases was significantly innovative, being at or near the forefront of the relevant fields. From a management perspective, the research funding strategy appeared to be coherent and was clearly enunciated. The objectives were designed to promote critical advances in the fields of concern. The quality of research carried out under the auspices of the ARO-funded programs was excellent. However, it was difficult to evaluate the level of risk versus payoff, because only a few examples of failures were given. Nonetheless, all of the presentations described results that were excellent, and in some cases outstanding. ARO is by no means the largest supporter of the work being done in the scientific areas described, but it has managed to benefit significantly from piggybacking on larger programs.

Many of the research activities supported by ARO are in “hot” fields in which many other researchers are working. In the four fields mentioned above, there were two accomplishments cited that represent significant advances. These are the work done on super-radiant laser and materials-agnostic demonstrations of the quantum anomalous Hall effect. It is likely that four other accomplishments will achieve breakthroughs: computing with neuromorphic dissipative quantum phase transitions, physical phenomena on topological surfaces, scaling up of trapped ion multiqubit systems, and exploitation of super-symmetries in optics.

The basic research that ARO supports is expected to provide knowledge that will ultimately form the basis of applications for the Army. The time scale for that transition is expected to be anywhere from 5 to 25 years (or longer). It is very commendable, therefore, that some of the research now being supported by the Physics Division of ARO is already being transitioned to other agencies and to potential end users.

Some cross-disciplinary opportunities were noticed, and these are listed below along with associated recommendations.

Condensed matter physics depends on the discovery of new phenomena in existing materials, and on the observation and exploitation of known phenomena in new materials. The interaction between the condensed matter activities and the materials science activities that ARO sponsors is not close enough, although assurance was provided that the two relevant ARO PMs are in contact.

Large advances in quantum information science are unlikely to occur unless there are correspondingly large advances in the development and analysis of algorithms. Little evidence was presented of any active engagement on algorithms between the Physics Division Quantum Information Science Program and its classical counterpart in the Information Sciences Division. This is a generic issue in modern information

science. The expertise and collegial environment evident at ARO suggest that ARO could become a nexus for breakthroughs in understanding the quantum/classical algorithmic frontier.

Collaborations between researchers in condensed matter and materials science and between investigators working on quantum information and information science have led to significant accomplishments, but such collaborations appear to be the exception rather than the rule. There is a growing recognition in the scientific community that the breakthroughs of the future are likely to occur in the boundaries between disciplines.

A substantial number of the single investigator (SI) grants that ARO makes are to individuals in “hot” fields who are at the peaks of their careers, and consequently are also supported by other organizations. This approach to research support makes the exploration of the frontier move faster, as well as making the programmatic activities of these PMs more successful. However, this strategy shortchanges early-career investigators.