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Bit
The basic unit of information in classical
computing. A bit can hold exactly one of two values: 0 or 1.
All classical computers — from laptops to servers — process information as sequences of bits.
Classical Bit Notation: A classical
bit is represented simply as a scalar value, 0 or 1. It can hold only one
definite value at a time.
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Classical Computing
The traditional model of computation that processes
information using bits (0s and 1s). Classical computers execute instructions sequentially or in parallel
using transistors and logic gates. Most everyday devices — smartphones, PCs, servers — are classical
computers.
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Qubit (Quantum Bit)
The fundamental unit of information in quantum computing.
Unlike a classical bit that is strictly 0 or 1, a qubit can exist as 0, 1, or any combination of
both simultaneously (superposition). Qubits are physically realised using particles like
electrons, photons, or superconducting circuits.
Quantum Qubit Notation: A quantum
qubit is represented using Dirac (bra-ket) notation as the basis states
|0⟩ and |1⟩.
A qubit can also exist in a superposition of these states, written as
|ψ⟩ = α|0⟩ + β|1⟩
Here, α and β are probability amplitudes
that determine the probability of measuring 0 or 1.
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Quantum Superposition
A qubit in superposition is in a combination of the 0
and 1 states at the same time, described by probability amplitudes. Only when the qubit is
measured does it collapse to a definite 0 or 1. This allows quantum computers to explore many
possible solutions simultaneously.
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Quantum Entanglement
A phenomenon where two or more qubits become
correlated such that the state of one instantly influences the other, regardless of
physical distance. Entanglement is a key resource that enables quantum computers to process information in
ways impossible for classical machines.
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Quantum Interference
Quantum algorithms use interference to amplify paths
leading to correct answers and cancel out paths leading to wrong ones — similar to how waves
reinforce or cancel each other. This is how quantum computers can efficiently solve certain problems.
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Quantum Coherence
Quantum coherence is the ability of a qubit to
maintain its fragile quantum state and the phase relationships between superposed states.
To function effectively, qubits must remain sufficiently isolated from their environment. This coherence
allows quantum effects such as superposition and interference, which are essential for quantum computing.
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Quantum Decoherence
Decoherence occurs when a qubit interacts with its
surrounding environment, such as through temperature changes, electromagnetic noise, or other
disturbances. These interactions disrupt the qubit's coherence, causing it to lose its quantum properties
and introducing errors into quantum information. Maintaining coherence is therefore a major challenge in
quantum computing.
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Quantum Measurement
Quantum measurement is the process of obtaining information
from a quantum system. A qubit can exist in a superposition of 0 and 1 before measurement. When it is
measured, the superposition collapses into a definite classical state, either 0 or 1,
according to the probabilities described by its quantum state. Because measurement can disturb the quantum
state, quantum algorithms are carefully designed to manipulate and process qubits before measurement to
obtain the desired result.
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Quantum Computing
A model of computation that harnesses the principles of
quantum mechanics — superposition, entanglement, and interference — to process information.
Quantum computers excel at specific tasks such as cryptography, optimisation, drug discovery, and simulating
molecules, where classical computers struggle.
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Quantum Gate
The quantum equivalent of logic gates in classical computing.
Quantum gates are unitary operations that manipulate qubits — rotating their state,
creating superpositions, or entangling them with other qubits. Common examples include the Hadamard gate
(H), CNOT gate, and Pauli-X gate.
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Quantum Algorithm
A step-by-step procedure designed to run on a quantum
computer, exploiting superposition, entanglement, and interference to solve problems faster
than any known classical algorithm. Key examples include:
- ▸Shor's
Algorithm — factorises large integers exponentially faster than classical methods,
threatening RSA encryption.
- ▸Grover's Algorithm — searches an
unsorted database of N items in ~√N steps, versus N steps classically.
- ▸Quantum Fourier Transform (QFT) —
the quantum analogue of the classical Fourier transform; a key subroutine in many quantum
algorithms.
- ▸VQE
(Variational Quantum Eigensolver) — estimates molecular ground-state energies, with
applications in drug discovery and materials science.
