Computer systems that perform tasks normally associated with human intelligence, for example, recognizing images, understanding language, making decisions, and generating content.

Computing based on bits, each holding a 0 or 1, implemented with transistors. Every laptop, phone, and supercomputer today is a classical computer. It follows the rules of classical physics and deterministic logic.

Traditional sensing devices like radar or LiDAR that uses classical sources and detectors to measure objects. The sensitivity of this measurement is limited to the “classical limit,” the best measurement resolution achievable using classical devices and techniques.

In physics, a system well described by classical (non-quantum) physics, for example, Newtonian mechanics and electromagnetism, where quantities have definite values, and do not show quantum mechanical properties such as superposition or entanglement. The everyday, macroscopic (big) world behaves classically.

When a quantum system is measured, its superposition resolves instantly into a single definite outcome - this is called collapse. Before measurement, the system exists across multiple possible states simultaneously; after, only one remains. Collapse is not a physical process that can be watched or slowed - it is the boundary between the quantum world and the classical one.

The capacity of a system to do work or cause change. It comes in two basic forms: kinetic energy (energy of motion) and potential energy (stored energy due to position or configuration, like a stretched spring or a ball held above the ground). Classically, energy can take any continuous value, and the total energy of an isolated system is conserved. It transforms between forms but is never created nor destroyed.

In quantum systems, energy often comes in discrete chunks rather than a continuous range. The system can only occupy certain specific energy levels, like rungs on a ladder, with nothing in between. This is the origin of the word "quantum": a quantum of energy is the smallest allowed unit a system can absorb or emit. The lowest possible level is the ground state; higher ones are excited states. Same as with classical energy, quantum energy (& matter) transforms between forms but is never created nor destroyed.

The allowed energy levels of a system are determined by its physical structure and interactions — only specific values are permitted, with nothing in between. For light, each discrete unit of energy is a photon, carrying energy proportional to its frequency. 

A unique quantum link between two or more particles, where their states are correlated so strongly that you cannot fully describe one without the others, even when they are far apart. Measuring one particle’s state immediately tells you what the others’ states might be. Not because a signal traveled between them, but because they were never fully independent to begin with.

A technique to entangle two particles that have never interacted, by entangling each with an intermediary and then performing a joint measurement on the intermediaries. It is a key building block for quantum repeaters and the quantum internet.

With the anticipation of Q-Day, malicious actors are using the HNDL strategy which involves intercepting encrypted data and storing it, waiting until they have a powerful enough computer (classical or quantum) to hack it.

A subfield of AI where systems learn patterns directly from data instead of being programmed with explicit rules. Most modern AI is built on ML.

It is fundamentally not allowed in quantum physics to make a perfect copy of a quantum state using a unitary gate.

A photon is an indivisible particle of light (energy) that cannot be split in half. Photons can be used to store, transmit and compute quantum information; 0s and 1s are encoded into properties of the light, like polarization, frequency, time-of-arrival, etc.

Machine learning that uses quantum computers or quantum algorithms, either running learning tasks on quantum hardware, or processing data that is itself quantum. Whether it offers a real advantage over classical ML for practical problems is still an open research question.

A quantum (pl. quanta) is the smallest indivisible packet of energy a system can absorb or emit. This granularity is a universal feature of nature at small scales, governing atoms, ions, photons, and the forces between them. Quantum also refers to Quantum Physics or, also called Quantum Mechanics, the branch of physics dedicated to understanding this small-scale world and the rules that govern it (math). It is the study of the smallest particles & energy that make up our universe (atoms, ions, photons, etc.).

The point at which a quantum computer solves a problem faster or more accurately than the best-known classical computer and algorithm. It has been demonstrated for narrow, purpose-built problems; whether it can be achieved for practically useful problems remains an active and contested research question.

A computer that processes information in the form of qubits and exploits quantum phenomena like superposition, entanglement, and interference, to perform certain computations.

A term for the hypothetical future moment when a quantum computer becomes powerful enough to break public-key encryption which forms the foundation of most secure communication today, including banking, email, and government systems. No confirmed timeline exists, but the prospect is taken seriously enough that governments and standards bodies are actively preparing post-quantum cryptography.

Information encoded in quantum states such as qubits. It obeys quantum rules — superposition, entanglement, and the no-cloning theorem (you cannot make a perfect copy of an unknown quantum state) — which is what makes it behave differently from classical information.

Global quantum network connecting quantum-enabled devices, such as Quantum Sensors, Quantum Computers, Quantum Communication Systems, etc.

QKD is the real-time generation of an encryption key between two parties, usually called Alice (sender) and Bob (receiver), using particles of light (single photons).

Materials whose useful properties arise from quantum effects that have no classical explanation, often from the collective behaviour of many particles. Examples include superconductors, topological insulators, and quantum sensors.

The act of extracting information from a quantum system. Quantum measurement is an active interaction and consequently forces a system to resolve into one of its allowed states. The outcome is inherently probabilistic: quantum mechanics predicts the likelihood of each possible result, but not which one will occur.

A device that exploits quantum properties, such as superposition or entanglement, to achieve measurement precision beyond what classical physics permits.

At the most fundamental level of our universe, there exists an energy field that is everywhere all the time called the quantum vacuum field. Particles and their anti-particles can pop in and out of existence due to random energy fluctuations in the vacuum. We can measure the purely random noise of the vacuum to create quantum random number generators.

The quantum version of a bit and the basic unit of quantum information. Unlike a classical bit, which is either 0 or 1, a qubit can be in a superposition of both until it is measured. Qubits can be built from photons, atoms, ions, electron spins, or superconducting circuits, ... the list goes on!

An intrinsic quantum property of particles, a kind of internal angular momentum with no everyday classical equivalent. Despite the name, quantum particles do not literally spin like tops. For an electron, it takes one of two values, often called "up" and "down", which is one common way to encode a qubit.

The ability of a quantum system to exist in multiple states simultaneously until it is measured. A qubit in superposition is not just 0 or 1, it is a combination of both! All superposed states have associated probabilities that only resolve into a definite outcome upon measurement. Superposition is one of the core properties that distinguishes quantum from classical computing.

A quantum logic gate that performs an operation on 1 or more qubits (i.e. Pauli gates, Hadamard gates, CNOT, SWAP, etc.).