# Quantum Information Science – A Discovery ERA Exemplar

November 22, 2017

Quantum Information Science (QIS) sits at the intersections of quantum, material, computer, information, and engineering sciences, and is emerging as a field with the potential to revolutionize multiple technologies for the Army, DoD, and the broader population as a whole. At a fundamental level, QIS seeks to exploit the unique properties of nature which quantum physics provides, to take advantage of phenomena beyond those accessible utilizing only classical physics. More concretely, QIS leverages explicit quantum properties of matter such as its wave nature, the superposition principle, quantum statistics, and entanglement (the property Einstein dubbed as "spooky") to enable phenomena which break classical-physics-based theoretical limits. These beyond-classical functionalities extend from sensing and measurement precision, to computational power and computational complexity, to information density. Even novel modalities of sensing and novel security protocols are possible. These advanced functionalities, in turn, give us the power to create limit-breaking technologies in computing, sensing, communications, networking, and imaging, just to name a few.

Quantum mechanics emerged as a branch of physics in the early 1900s as a consequence of Max Plank's study of black-body radiation and Albert Einstein's work explaining the photoelectric effect. Its development continued through the twentieth century thanks to work by physicists such as Erwin Schrodinger, Werner Heisenberg, Max Born, Enrico Fermi, Wolfgang Pauli, and Niels Bohr among many others. These physicists laid the foundations of quantum theory and essentially gave us a fundamental theory of nature which describes behavior on the smallest of length scales, down to atoms, subatomic particles, and photons.

The application of quantum physics to information science, however, did not emerge until much later in the 20th century. Pieces of the topic started to develop in the 1970's but the concepts behind perhaps the most well-known potential application of QIS, quantum computing, didn't appear until the 1980s. While he was not the first to introduce the idea, in 1982 Richard Feynman gave a talk (later published [1]) in which he laid out a framework that explained that conventional, classical computers would reach a bottleneck in computational speed when tackling quantum phenomena. In contrast, a computer based on quantum phenomena employed to simulate quantum phenomena, would not. A more formal framework for what would constitute a universal "quantum computer" was laid out in 1985 by David Deutsch [2]. The topic garnered little overall interest, however, until 1994 when Peter Shor developed an algorithm which clearly demonstrated the potential power behind a quantum computer [3]. This paper, in short, showed that a quantum computer could factor large numbers efficiently, where "efficiently" means in a time that scales polynomially with the size of the problem instead of scaling exponentially with the size of the problem as the most efficient classical algorithms do. The relevance of this cannot be overstated as the security of nearly every transaction we conduct electronically relies on an RSA cryptosystem which derives its power from the fact that the factoring problem is "hard," meaning that it cannot be solved on a classical machine in a time which would ever impact the security of the transaction. In short, the security of near every electronic action we all take, and the security of all high-level encryption algorithms, hinge on the fact that we don't yet have a universal quantum computer.

In the year following the derivation of Shor's algorithm, the realization that it could be implemented in AMO systems led to an ARO-AMRDEC workshop, which, in turn, led to the creation of the ARO Quantum Computing program. The latter has since developed into the ARL-ARO Quantum Information Science program. The initial quantum computing program had a heavy customer investment component from its inception, and that investment has grown steadily over the past 22 years. This is, in part, due to the very significance of the factoring application, and in part because of the potential boon to complex computational problems which stretches far beyond the factoring application. Logistics and complex optimization and efficiency problems, for example, could also become enormously more tractable with a universal quantum computer. All of the investment and diligent work has led to significant progress over the past 22 years, but the challenge of creating and sustaining a quantum computer containing enough "qubits" (the quantum equivalent of a classical binary -- 1 or 0 -- bit), is great. As a result, quantum computing itself is still in its infancy with only small numbers of qubits (on the order of 20) and limited capabilities to tackle more than a small number of operations before the systems need to be reset. Several experimental physical platforms, ranging from superconductors, to trapped ions, to semiconducting systems, exist in quantum computing labs around the world, and more research at the 6.1 level is still needed to elucidate which platform will ultimately show the most promise to host a universal quantum computer. Increasingly, it looks like combinations of different qubit systems combined into hybrid architectures may be the best way forward, yet simultaneously there is also a large investment in looking at completely new qubit platforms which have yet to be substantially researched. Some images showing different components and types of qubit systems are included in figure [1]; and reference [4] is a review article discussing fundamentals and implementations of quantum computing.

Quantum computing, however, is only one facet of QIS and the full impact of the field stretches into all corners of C4ISR. Traditional limits on sensing, imaging, communication, networking, simulation and more, all have possible enhancements enabled by the exploitation of quantum mechanics. In imaging, not only can resolution be enhanced beyond classical limits, but new concepts and forms of imaging become possible. Some of these have no classical counterparts, such as interaction-free measurement. In communications, new secret-sharing, authentication/verification, and encryption protocols, as well as secure computing approaches are possible. In sensing, one can not only break classical limits on sensitivity, but one can devise a "sensor" that enables secure positioning, navigation, and timing (PNT) in the absence of GPS. This is a single design which incorporates a clock, gyro, accelerometer and gravimeter (i.e., all components of an IMU) into a single design based on trapped ultra-cold atoms.

A handful of these additional QIS fields are approaching somewhat reasonable levels of maturity now, and the Army is heavily supporting many of these efforts. One such area is in ultra-precise magnetometry. This can be used for detection of tanks, subs, etc. at one extreme, or in battlefield medicine and MEG in another extreme, or in research laboratories studying the most novel of quantum materials in yet another extreme. The required metrics are very different depending on the situation, and no longer is just the sensitivity relevant. Many other aspects including the measurement volume (which must be very small for medical applications while not small at all if you are, for instance, looking for a tank) come into play as various implementations are considered. QIS based magnetometers vary greatly in composition and range from atomic vapors, to nitrogen vacancy (NV) diamond color centers, to Bose Einstein condensates, among others. These all host remarkably different physics and very different potential realms of applications for the military and civilian populations.

As mentioned a bit above, another QIS area approaching a higher level of maturity is security in communication. QIS can provide provably unbreakable codes and protocols such as the ability to appropriately share secret information with only authenticated individuals. These capabilities are not possible through any other methodology since classical information can be readily copied, but the "no cloning" theorem of quantum mechanics guarantees that quantum information cannot be. On a different front, quantum simulation may provide a promising potential pathway to advanced material, drug, and chemical design capabilities.

In addition to the ARL-ARO extramural and customer supported efforts, ARL has supported in-house QIS related research for multiple years. Recently, with POM 15, ARL increased investment specifically in quantum networking. This plays best into communication needs and also increases the ability to coherently enhance sensing and time distribution. Small components of QIS have also reached a level of experimentation within AMRDEC and CERDEC. OSD is investing in a Tri-Service 6.2 program of which ARL is a key part. As suggested above, various pieces of QIS are at varying TRL levels from 0 to 6-7. Currently, however, key capabilities depend much more on 6.1 and 6.2 investment, as so much of the discipline is still in its infancy.

As the field of QIS has matured, it has only become increasingly obvious that for many applications there are no reasonable alternatives to pursing QIS based solutions. Quantum physics provides advantages that are manifestly impossible under the classical laws of physics, so to achieve these advantages one must exploit quantum processes. While we have ideas of what can be done, we by no means have the full picture of all the possibilities. Even those areas where we know what can be done, in most cases we don't yet know how to bring the implementations to reality. The QIS field is highly multifaceted. New discoveries and enhancements are still very much a part of current research, and, as different areas begin to reach higher levels of maturity, they stand to revolutionize many aspects of both Army and civilian life as we move forward.

[1] Richard Feynman, "Simulating Physics with Computers," International Journal of Theoretical Physics, Volume 21, Issue 6-7, pp. 467-488 (1982).

[2] David Deutsch, "Quantum theory, the Church-Turing principle and the universal quantum computer," Proceedings of the Royal Society of London Series A Mathematical and Physical Sciences, Volume 400, No. 1818, pp. 97-117 (1985).

[3] Original Paper: Peter W. Shor, "Algorithms for Quantum Computation: Discrete Logarithms and Factoring," Proceedings, 35th Annual Symposium on Foundations of Computer Science, Santa Fe, NM, Nov. 20-22 (1994). Revised Paper: Peter W. Shor, "Polynomial-Time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer," SIAM Journal on Computing, Volume 26, Issue 5, pp. 1484-1509.

[4] T. D. Ladd, et al., "Quantum Computers," Nature, Volume 464, pp. 45-53 (4 March 2010).