## What Is Quantum Physics?

If you're new to the field, we suggest you start here. Learn about the origins of quantum physics, also known as quantum mechanics, why mathematics is essential to the field, and how the act of observing the smallest objects can affect them.

## What Is Quantum Computing?

Find out how quantum computers work, the advances they might make possible, and why universities, technology companies, and government agencies are racing to develop them.

## What Is Entanglement and Why Is It Important?

Entanglement is at the heart of quantum physics and emerging quantum technologies. Read about how scientists proved its existence, and watch Caltech scientists take a stab at explaining this "spooky" phenomenon.

## What Is Superposition and Why Is It Important?

Go beyond Schrödinger's cat and learn more about superposition, a concept that might be difficult to visualize but could hold the key to advancing technology such as quantum computers.

## What Is the Uncertainty Principle and Why Is It Important?

Formulated by the German physicist and Nobel laureate Werner Heisenberg in 1927, the uncertainty principle states that we cannot know both the position and speed of a particle, such as a photon or electron, with perfect accuracy. Find out why.

## How Do Scientists Conduct Quantum Experiments?

Let this comic take you inside the labs where researchers probe the subatomic world of quantum physics.

## How Will Quantum Technologies Change Cryptography?

Quantum information science has the potential to change computer encryption. Researchers envision secure cryptography that can't be cracked by future quantum computers and are exploring how the properties of quantum mechanics could be used to send communications that are impervious to eavesdropping.

## How Are Quantum Phenomena Used in Technology Today?

Surprising but true: Quantum mechanics are at work inside of your toaster and other devices we encounter in everyday life.

**QUANTUM SCIENCE AND TECHNOLOGY**

## Ask a Caltech Expert

Quantum computers aren't the only application for quantum science. Caltech scientists are researching ways to revolutionize chemistry, materials, sensing, and more.

## Terms to Know

An atom is a tiny object, the basic unit of matter. It is composed of a nucleus, which is made up of particles called protons and neutrons, and one or more electrons, which are much lighter particles that surround the nucleus. Atoms play an important role in quantum science because quantum effects become evident at the atomic scale.

Entanglement is a form of correlation between quantum particles. When two particles, such as a pair of photons or electrons, become entangled, they remain connected even when separated by vast distances, and the state of one particle cannot, in general, be described without referring to the other.

NISQ stands for "Noisy Intermediate-Scale Quantum," a term coined by John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter. The term is often used to describe the current state of quantum computing, in which processors contain a fraction of the qubits necessary to tackle complex problems and still interact with the environment in ways that partially make them lose their fragile quantum properties. Finding interesting uses for NISQ devices and going beyond the NISQ era are important research goals.

At its most basic level, all matter consists of "fundamental particles" that do not seem to break down into smaller units. These particles include quarks (which make up the protons and neutrons in an atom's nucleus) and electrons. Light also consists of particles, called photons.

Quantum describes the rules and patterns of atomic and subatomic particles. Our experiences in daily life, which occur at much larger scales, follow the rules of classical mechanics. But when we zoom in to examine the tiniest particles that make up the universe, we see strikingly different dynamics. Quantum science explains these phenomena to give us a deeper understanding of nature. The word "quantum" originates from the idea that physical properties, such as energy, are not continuous but rather exist in specific—or quantized—amounts. A quantum is the smallest possible unit of a physical property.

The term "quantum supremacy" was coined in 2012 by John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter, to describe the point at which a quantum computer could perform tasks that would be impossible for a classical computer. More recently, many scientists and engineers have instead begun to use the term "quantum advantage" to describe the same benchmark.

Also known as quantum cryptography, quantum encryption proposes to use the laws of quantum physics, such as the uncertainty principle, to transmit private information in a way that makes undetected eavesdropping impossible.

A quantum internet would use the laws of quantum physics, such as entanglement and superposition, to send, receive, and store information among networks of quantum devices far more quickly and securely than is possible using the classical internet. A quantum internet would also enable the distribution of entanglement, which could be used to perform quantum encryption, quantum sensing, and other quantum mechanical tasks. Though experiments continue to yield advances, a functioning quantum internet remains a far-off goal.

Tiny particles are extremely sensitive to the environment. Scientists and engineers seek to harness this characteristic, along with quantum principles including entanglement, to develop instruments capable of performing measurements with far more precision than typical sensors such as seismometers, microscopes, or MRI machines. Many researchers regard quantum sensing as one of the nearest-term applications of quantum technology.

A quantum state is a mathematical representation of a quantum system that allows us to compute certain properties of the system that can be observed, such as its position or momentum. However, due to the uncertainty principle, these properties are not always determined, so the resulting calculation usually returns a probability distribution (for example, position X with a probability of 30 percent and position Y with a probability of 70 percent).

The basic unit of information in a classical computer is a bit. A bit is a binary digit that takes a value of either 0 or 1. In contrast, the basic unit of quantum information is a qubit, or quantum bit. A qubit can be in a superposition of 1 and 0 simultaneously until its state is measured. In addition, the states of multiple qubits can be entangled, meaning they are linked quantum mechanically to one other. Superposition and entanglement give quantum computers capabilities unavailable to their traditional counterparts.

Spin is an intrinsic property of tiny particles, such as electrons, that represents a quantum-mechanical form of angular momentum (rotation).

The thought experiment proposed by physicist Erwin Schrödinger helps illustrate the concept of superposition, or the ability of a particle to exist in two states at the same time until it is measured. In the thought experiment, Schrödinger imagined placing a cat in a sealed box along with a poisonous substance that has an equal chance of killing the cat—or not—within an hour. He proposed that at the end of the hour, the cat could be said to be both alive *and* dead, in a superposition of states, until the box is opened and the act of observation determines its state as alive *or* dead. Schrödinger intended the example to demonstrate what he saw as the absurdity of quantum theory.