Quantum Computing Glossary | The Millikelvin

Plain-English definitions for the quantum computing terms used across this site. Qubits, dilution refrigerators, error correction, decoherence, NISQ, and twenty more.

Dilution Refrigerator: A cryogenic device that cools quantum processors to near absolute zero (around 15 millikelvin) using a mixture of helium isotopes.
Millikelvin: One thousandth of a Kelvin. The operating temperature of superconducting quantum computers and the name of this publication.
Microkelvin: One millionth of a Kelvin. The temperature regime reached by laser cooling, where neutral atoms and trapped ions are held still enough to...
Qubit: The basic unit of quantum information, analogous to a classical bit but able to exist in superposition, a combination of 0 and 1 simultan...
Neutral Atom Qubits: Qubits built from individual neutral atoms, laser-cooled and trapped in optical tweezers or lattices. A leading qubit modality with stron...
Trapped Ion Qubits: Qubits built from electrically charged atoms (ions) held in electromagnetic traps and manipulated with lasers. Currently the highest gate...
Superconducting Qubits: Qubits built from superconducting circuits that must operate near absolute zero. Used by IBM, Google, and others; the most widely deploye...
Quantum Gate Fidelity: A measure of how accurately a quantum operation (gate) is executed. Higher is better; errors accumulate rapidly, so fidelity is one of th...
Quantum Error Correction: Techniques for detecting and correcting errors in quantum computations without directly measuring (and thereby collapsing) the quantum st...
Quantum Decoherence: The process by which a quantum system loses its quantum properties through interaction with its environment. The primary enemy of quantum...
Coherence Time (T1, T2): How long a qubit holds its quantum state before decoherence destroys it. T1 is the energy relaxation time; T2 is the dephasing time. Long...
Quantum Volume: A single-number benchmark for quantum computer capability that accounts for qubit count, gate fidelity, connectivity, and other factors....
Quantum Entanglement: A quantum phenomenon where two or more particles become correlated such that the state of one instantly determines the state of the other...
Quantum Superposition: The ability of a quantum system to exist in multiple states simultaneously until it is measured, at which point it collapses to a single...
Laser Cooling: A technique using laser light to slow atoms down, cooling them to temperatures within millionths of a degree above absolute zero. Essenti...
Optical Tweezers: Tightly focused laser beams used to trap and hold individual atoms or particles in precise positions. A key tool in neutral atom quantum...
Atomic Clock: A clock that uses the precise frequency of atomic transitions to keep time. The most accurate timekeeping devices ever built, with quantu...
Quantum Sensing: Using quantum systems to make measurements with precision beyond what classical sensors can achieve. One of the most commercially advance...
Bose-Einstein Condensate: A state of matter formed when certain atoms are cooled to near absolute zero and collapse into the same quantum state, behaving as a sing...
Quantum Advantage: The point at which a quantum computer solves a problem faster or more efficiently than any classical computer can. Also called quantum su...
NISQ (Noisy Intermediate-Scale Quantum): The current era of quantum computing: systems with 50-1000+ qubits that are too error-prone for fault-tolerant computation but large enou...
Quantum Key Distribution (QKD): A method of distributing encryption keys using quantum mechanics, where any eavesdropping attempt physically disturbs the quantum states...
Cryostat: An insulated, vacuum-jacketed apparatus that keeps quantum hardware near absolute zero. For superconducting processors, it is the buildin...
Josephson Junction: Two superconductors separated by a whisper-thin insulating barrier: the nonlinear, non-dissipative circuit element at the heart of every...
Transmon: The most widely deployed superconducting qubit: a Josephson junction shunted by a large capacitor to blunt the stray charge noise that pl...
Paul Trap: A device that confines charged atoms in mid-air using rapidly oscillating electric fields. The foundation of trapped-ion quantum computers.
Magneto-Optical Trap (MOT): The standard first step for cooling neutral atoms: six laser beams and a magnetic field gradient that chill millions of atoms to microkel...
Ultra-High Vacuum (UHV): Pressures below roughly 10⁻⁹ torr, emptier than low Earth orbit, so stray gas molecules can't knock atoms out of a trapped-ion or neutral...
Microwave Control Electronics: The room-temperature racks that generate the shaped pulses running a quantum processor, and one of the field's quietest scaling bottlenecks.
Optical Lattice: A standing wave of laser light that traps atoms in a perfectly periodic artificial crystal, used for the world's best atomic clocks and q...
Logical Qubit: An error-corrected qubit: information spread across many imperfect physical qubits so errors can be caught faster than they pile up. The...
Surface Code: The leading quantum error-correction recipe: qubits on a flat grid whose neighbors check for errors. Practical, but it costs hundreds of...
Two-Qubit Gate: An operation that makes one qubit's state depend on another, the step that creates entanglement and where most of a quantum computer's er...
T1 and T2 Coherence Times: The two standard clocks for qubit decay: T1 measures energy relaxation, T2 measures how long a qubit holds a definite phase. T2 usually l...
Rydberg Blockade: The effect that gives neutral atoms their two-qubit gates: excite one to a giant orbit and within a blockade radius only a single atom ca...
Mid-Circuit Measurement: Reading out some qubits partway through a computation while their neighbors stay coherent, then acting on the result. The capability that...
Qubit Readout: How a quantum computer reports its answer, by dispersive microwave shifts or atomic fluorescence. It carries its own error rate, often wo...
Randomized Benchmarking: The standard protocol behind nearly every gate-fidelity number: run long random gate sequences that should do nothing, and watch how fast...
Quantum Circuit: The standard way to write a quantum program: qubits as wires, gates as boxes applied left to right, measurements at the end. Sheet music...
Variational Quantum Eigensolver (VQE): A hybrid algorithm for noisy hardware: a short quantum circuit prepares a trial state and a classical optimizer tunes it toward a molecul...
Quantum Approximate Optimization Algorithm (QAOA): The optimization sibling of VQE: encode a combinatorial problem as an energy landscape and use quantum interference to hunt for low-energ...
Quantum Simulation: Using one controllable quantum system to imitate another. The field's most credible near-term payoff, and where hardware is closest to us...
Error Mitigation: Squeezing better answers out of noisy hardware without fixing the errors, using extra runs and statistics. The budget alternative to full...
Shor's Algorithm: The 1994 algorithm that factors large numbers exponentially faster than classical methods, threatening RSA. The reason quantum computing...
Grover's Algorithm: A 1996 algorithm that searches an unstructured haystack quadratically faster than brute force. A real quantum speedup, but a modest one.
Quantum Random Access Memory (QRAM): The hypothetical device that would let a quantum computer query a classical database in superposition. Many famous quantum algorithms qui...
Post-Quantum Cryptography (PQC): Classical cryptography chosen to survive quantum attack: ordinary math no known classical or quantum computer can crack efficiently. No q...
Quantum Repeater: The unbuilt device long-distance quantum networks are waiting for, stitching short entangled hops into long ones because quantum signals...
Quantum Network: A network that distributes entanglement between distant places. Not faster internet, but secure key exchange, linked quantum computers, a...
Quantum Memory: A device that catches a quantum state, holds it without measuring it, and releases it intact on demand. The quantum analog of RAM, with n...
Photonic Qubits: Qubits encoded in single particles of light: room-temperature, barely decohering, and the only qubits that travel. Their catch is that ph...
NV Center (Nitrogen-Vacancy): A defect in diamond that behaves like a trapped atom and stays quantum-coherent at room temperature. The workhorse of quantum sensing, no...
Atom Interferometry: Using matter waves to measure gravity, acceleration, and rotation with extraordinary sensitivity. The quantum sensor behind GPS-free iner...
Optical Frequency Comb: A laser whose spectrum is millions of perfectly spaced lines, a ruler for light. The Nobel-winning Boulder invention that lets optical at...