# About Quantum Information

From a practical point-of-view, quantum properties emerge as the size of traditional (classical) computing technology shrinks to the atomic scale. Researchers are interested in not only understanding the implications of this miniaturization, but they also want to harness quantum physics to enhance information processing.

JQI scientists are investigating methods to produce and control quantum effects that can be exploited to process information in new ways. Their work epitomizes the stark differences between the quantum and classical worlds.

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. That either/or digital condition is easily represented with electrical charges in chips, tiny holes on a CD surface, or microscopic magnetized spots on a disk. The longer the string of bits, the larger the number of values that can be represented. Thus, for example, it takes a string of three bits to represent any one of the eight numerical values from zero (000) to seven (111). So a total of 24 bits would be needed to represent all eight decimal possibilities.

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. Thus only three qubits are needed to represent all eight decimal digits simultaneously. This parallelism could make some mathematical operations exponentially faster.

One application is the task of factoring the extremely large numbers that serve as the "public keys" in current encryption and data-protection schemes. These keys are used to encode sensitive information in a myriad of applications from bank transfers to military intelligence. The public-key numbers - which typically require a string of more than 1,000 bits to represent - are the product of a set of prime numbers known only to the sender and receiver. Decoding the information requires the user to know those factors. The encoded data are safe because it would take even the best modern computers years to run through all the possible combinations of prime numbers which, when multiplied together, produce the giant public keys. But a quantum computer could potentially solve the problem in hours or even minutes.

Another possible use is in searching large, unstructured databases in which the entries are in no particular order. Locating the specific desired items could be done at vastly greater speed with quantum computation.

When quantum objects encounter their environments, or are measured or otherwise tampered with, they lose the condition of superposition and take on specific properties - a process called "decoherence." That is, they become part of the everyday classical (i.e., non-quantum) world in which everything has definable characteristics.

JQI scientists are observing and analyzing exactly what happens as decoherence occurs over small time intervals. This information will be important for devising and regulating quantum computers, among many other uses.

Currently, a full-scale quantum computer does not exist and classical computers often cannot solve quantum problems. But scientists can build quantum simulators that investigate complex systems. Originating in large part with Richard Feynman’s 1982 proposal, quantum simulation has evolved into a field where scientists use a controllable quantum system to study a second, less experimentally feasible quantum phenomenon.

One attractive platform for simulations is made from laser-cooled atoms. JQI researchers use either ultracold neutral gases or ion crystals to simulate condensed matter physics, for example.

An ultracold gas may not sound like a solid, being nearly 10 orders of magnitude less dense, but under certain conditions, this unusual quantum matter behaves just like a crystal found in nature. Neutral atoms trapped by laser light are analogous to electrons in a crystalline solid. Similarly, crystals of ions can be manipulated with lasers causing them to behave like quantum magnets.

Typically these experiments involve the isolation, capture, manipulation and measurement of atoms as they absorb and emit photons - the smallest units of light. Scientists use laser light sources of various frequencies along with magnetic and/or electrical fields, to slow and gradually immobilize the atomic samples.

JQI projects test quantum theories and observe condensed matter physics phenomena under a wide range of conditions. Lasers are often used to engineer novel types of atom-light interactions. Recent example simulations include quantum phase transitions, creating synthetic magnetic and electric fields, collisions, and disordered quantum systems.

In general, the target atoms are extremely cold, some of them only a fraction of a millionth of a degree above absolute zero in a high-vacuum chamber. That not only makes them controllable, but it also isolates them from contact with their surroundings, which would destroy the fragile quantum states.

Similar to quantum simulation, the Physics Frontier Center at JQI supports an experiment that focuses on developing atomtronics. Rings of atoms are used to generate analogous circuit elements, where atoms take the role of the electrons. This particular experiment will also be configured as a superfluid analog to a superconducting circuit.

Quantum information science has grown to encompass a variety of areas, much of which is the focus of JQI research. The JQI is also home to an NSF Physics Frontier Center, which focuses on quantum information at the interface of condensed matter physics and atomic, molecular, and optical physics.

### Quantum Architectures

JQI researchers work on devising different quantum computing architectures. Theoretical and experimental teams examine these systems, including methods to induce and control entanglement in various platforms. Proposed qubit and computation architectures include SQUIDs (Superconducting QUantum Interference Devices), ion traps, neutral atoms, photons, quantum dots, and topological materials. In some cases, scientists seek to combine these platforms into a hybrid quantum device.

## Subscribe to A Quantum Bit

Quantum physics began with revolutionary discoveries in the early twentieth century and continues to be central in today’s physics research. Learn about quantum physics, bit by bit. From definitions to the latest research, this is your portal. Subscribe to receive regular emails from the quantum world. Previous Issues...

## Sign Up Now

Sign up to receive A Quantum Bit in your email!

Have an idea for A Quantum Bit? Submit your suggestions to jqi-comm@umd.edu