Quantum Computing - a very brief introduction
A whitepaper about what quantum computing is and how it works.
Quantum Computing
A new type of computing using principles of quantum mechanics to solve problems too complex for classical computers.
Credit: @Brendan_In_Byte
Classical vs. Quantum
| Classical | Quantum | |
|---|---|---|
| Storage | Stores and processes information in binary bits | Stores data in qubits — a weighted combination of zero and one at the same time |
| Information density | Each bit = one unit of information. N bits = 2N units of information | Entangled qubits exponentially increase storable information. N bits = 2^N units of information |
| Processing | Processes data sequentially, one step at a time | Processes data simultaneously — probability amplitudes overlap to provide efficient paths to a solution |
Quantum computing is less efficient for simple problems but significantly better for complex ones (e.g. optimisation, factorisation) — it finds efficient paths to a solution while scaling qubit information density rapidly.
Three Key Principles
Superposition — A quantum particle or system represents a combination of multiple possibilities, fluctuating until observed. When measured, this superposition collapses into a single state.
Entanglement — Multiple quantum particles can correlate their measurement results beyond regular probability. Measurements from one qubit can inform conclusions about the others.
Interference — The intrinsic behaviour of a qubit to influence the probability of it collapsing one way or another.
What Exactly Is a Qubit?
A qubit ("quantum bit") is not a single physical object — rather, a general term referring to any quantum system that can produce the desired characteristics.
Examples of qubits:
Trapped ions — Electromagnetic fields are used to trap entangled ions and provide signals to change qubit states. (Companies: Quantinuum, Universal Quantum, Oxford Ionics, IonQ, AQT)
Neutral atoms — Arrays of neutral atoms (e.g. Rb) are entangled by exciting them to high-energy Rydberg states, held in place and excited between energy levels using lasers. (Companies: QuEra, Pasqal, Infleqtion, Atom Computing)
Photons — Two approaches: (1) encode polarisation states as "0" or "1"; (2) use two distinct paths ("modes") where arrival times dictate which path a photon took, and hence qubit state. (Companies: Xanadu, ORCA Computing, Quandela, PsiQuantum, QUIX Quantum, Nu Quantum)
How Does a Quantum Computer Work?
Just as there are many ways to make a qubit, there are multiple ways to build a quantum computer.
Circuit model — The most common approach. Analogous to classical computing circuits: represents a computation as a sequence of quantum gates (mathematical operations) applied to qubits. EM waves replicate the mathematical operators physically.
Quantum annealing — Uses specialised, purpose-built hardware that excels at solving optimisation problems. Already commercially available (e.g. D-Wave). The optimisation problem is mapped to an energy landscape and the system is guided to find the lowest energy state, which represents the solution.
Measurement-based quantum computation (one-way quantum computation) — Uses a cluster state as a computational resource. Instead of actively controlling qubits, the answer is reached by measuring them. Computation is driven by single-qubit measurements, adapting based on prior outcomes.
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