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Research at the Joint Quantum Institute

Credit J. Consoli

Introduction

Quantum mechanics, which describes the behavior of matter and energy on the smallest physical scales, is the most accurate theory of nature ever devised. Many of its predictions can be confirmed to 10 decimal places or more. Its principles explain a host of phenomena from the grand sweep of the Periodic Table of Elements to the everyday workings of lasers, microchips, light-emitting diodes and MRI medical scans.

The ability to understand and manipulate objects at the quantum level is among the most urgent goals of 21st century science. It is essential to progress in physics, chemistry, electronic engineering and nanotechnology, to creating new computers of unprecedented power, and to determining how atoms behave in large arrays, such as the crystals of semiconductors or the metal-oxide layers of superconductors. But researchers at JQI and elsewhere have only begun to control and exploit quantum processes, which are inherently difficult to manage for several reasons.

One is that, at quantum dimensions, objects (for example, an electron orbiting an atom's nucleus) do not have specific, fixed properties such as location or momentum. Instead, they exist in a peculiar condition called "superposition," in which each object simultaneously embodies all the properties that are possible for that object - until the act of measurement forces it to take on specific values. It is impossible to predict those values prior to measurement, although the probability of any particular value occurring can be calculated.

However, that same indeterminacy gives quantum systems an enormous potential for information processing because they can perform operations on all the superposed values at once instead of one value at a time. And even though a quantum state is unknowable prior to measurement, the states of two separate objects, such as individual atoms, can be "entangled" in ways such that the state of each is inextricably correlated with the state of the other. That phenomenon, which is the subject of intensive research at JQI, makes it possible to move quantum information from one place to another.

The intrinsically probabilistic nature of quantum behavior, the sub-microscopic dimensions and extremely short time periods over which quantum events take place, and the exotic experimental conditions required to study them, all pose substantial problems for researchers. As a result, much important work is still at a stage equivalent to the demonstration of the first transistor in 1947: the principles are generally understood, but the capacity to control specific phenomena, ensure desired outcomes and link quantum systems together reliably is largely lacking.

Yet the need is great. Major advances in a dozen areas - as well as probable economic benefits in the form of advanced synthetic materials, ultra-precise sensors, nanotechnology, new forms of data encryption and the next generation of information processors and computers - await developments in quantum science. The Joint Quantum Institute was created to hasten the pace of progress. Its investigators, including theorists, experimentalists and applied physics researchers, work on dozens of problems in numerous areas. Some of these focus areas are listed here. 

Quantum Computing and Information Processing

In conventional electronic computers, information is stored and processed in the form of strings of "bits" (binary digits). Each individual bit can have only one of two values: 0 or 1. In a quantum computer, information would be stored in quantum bits, or qubits, each of which, thanks to the nature of superposition, can be 0, 1 or both at once. This parallelism could make some mathematical operations exponentially faster compared to conventional computing speeds for the same problem. One important future application of quantum computers is the task of factoring the extremely large numbers that serve as the "public keys" in current encryption and data-protection schemes. JQI physicists are investigating promising quantum computing architectures as well as developing methods to control quantum effects that can be exploited to process information in new ways.

Quantum Many-Body Physics

Physicists use theoretical and experimental techniques to develop explanations of the goings-on in nature. Somewhat surprisingly, many phenomena such as electrical conduction can be explained through relatively simplified mathematical pictures— models that were constructed well before the advent of modern computation. And then there are things in nature that push even the limits of high performance computing and sophisticated experimental tools. Computers particularly struggle at simulating systems made of numerous particlesor many-bodiesinteracting with each other through multiple competing pathways (e.g. charge and motion). Yet, some of the most intriguing physics happens when the individual particle behaviors give way to emergent collective properties. In the quest to better explain and even harness the strange and amazing behaviors of interacting quantum systems, JQI physicists use experimental and theoretical tools to study the complexities of many-body physics, with an emphasis on topics such as entanglement and topology.

Quantum Control, Measurement and Sensing

Creating quantum states on demand and controlling them is a critical component to developing practical quantum-based devices. Subsequent measurement of such states is also a challenge, because by definition, quantum superpositions collapse upon interaction, whether through intentional measurement or due to outside disturbances. Notably, the instability of a quantum state can also be used advantageously to create sensors. Quantum systems can be calibrated such that exposure to certain changing environmental conditions will force a switch from one quantum state to another. In some cases, a quantum phase of matter can abruptly change to a non-quantum phase of matter. Alterations to a quantum system can be monitored and detected, giving physicists information on the environment itself. JQI physicists are researching the many facets of quantum measurement and control, which has applications in areas such as precision spectroscopy and sensing.