A real quantum computer
The quantum physics of the hardware performs the computational steps.
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Understand quantum computing. Step by step.
Start with a qubit. Then explore shared states and the idea behind quantum search. Illustrations help you get started; we explain the limits of each analogy.
Your learning path
One step at a time.
Try an experiment. Continue when you are ready for the next idea.
Superposition · The coin analogy
From a light switch to a qubit
A qubit measurement gives you 0 or 1. The coin toss shows why equal probabilities can produce different counts.
The explanation in detail
A classical bit has the value 0 or 1. A qubit can be in a superposition of both possibilities. The coin is a starting point: a measurement gives one outcome, with a probability we can calculate. Amplitudes distinguish this from a coin toss. Like waves, they can reinforce or cancel each other.
Learn more: what does the picture really mean?
The real idea: before measurement, both amplitudes can contribute to the calculation. Measurement gives only one result. The program determines the probabilities.
The spinning is just a picture. A qubit does not have to physically rotate, and many qubits do not reveal every answer simultaneously. One measurement gives one bit string.
A covered coin cannot fully describe this difference. A qubit also has a relative phase. With suitable gates, it changes the probabilities of a later measurement. This interference makes superposition useful in quantum programs.
Heads
0 Heads0 Tails
Explore interference
Waves can reinforce or cancel.
Two water waves can reinforce or cancel each other. Contributions in the quantum model combine in a similar way. Shift the waves relative to each other and compare the probabilities of 0 and 1.
The circuit behind it
The qubit starts in 0. We apply H, change the relative phase, then apply H again. The probability of 0 is cos²(phase / 2), and of 1 is sin²(phase / 2). Your browser calculates these values; it does not draw measurement samples.
The moving lights and curves are an illustration. They do not show particle trajectories or measured computation speed.
Illustration · calculated in your browser
Outcome 0
Outcome 1
Another analogy: water in a maze
Interference · The wave analogy
How waves change a search
The animation compares a path-by-path search with a wave analogy. A quantum program can change amplitudes to suppress unwanted outcomes and amplify a target. This requires a suitable circuit. The preset animation timings are not measured computation times.
Classical computer: 0.0 sQuantum computer: 0.0 s
Learn more: the idea behind it
Amplitudes can reinforce or cancel each other. A search algorithm such as Grover uses interference to increase the probability of a match step by step. A measurement does not always succeed. Performance depends on the problem and the chosen gates.
The maze illustrates the idea but does not run Grover's algorithm. Try a calculated search in the combination-lock experiment.
Classical computer: one path at a timeQuantum analogy: a wave in all corridors
Heads
Berlin
Heads
New York
0 of 0 pairs matched0 of 0 correctly predicted
Entanglement · The twin coins
Two qubits, one joint state
Reveal the first coin and predict the second. Here, both show the same outcome. The explanation covers why entanglement means more than matching coins.
The explanation in detail
Two coins illustrate an entangled pair: one in Berlin and one in New York. In this example, measurements in the same basis give matching results. Each result on its own is random. The pair shares a quantum state; it sends no message. Matching coins illustrate correlation but do not, on their own, demonstrate entanglement.
Learn more: a pair of gloves
Imagine a left glove and a right glove sent in separate parcels to Berlin and New York. Finding the left glove in Berlin tells you that New York has the right one. That is ordinary correlation: the contents were fixed when packed.
The quantum difference emerges when comparing different measurement settings. Entangled states can show correlations that no local model with predetermined glove properties reproduces. Our coin analogy shows just one setting, not that full test.
Bell tests distinguish quantum correlations from local models with predetermined outcomes. Experiments with entangled photons and violations of Bell inequalities were honoured with the 2022 Nobel Prize in Physics. They do not allow messages to travel faster than light.
Learn more: what does the picture really mean?
The real idea: entangled systems can produce correlations over large distances. Experiments have repeatedly demonstrated this.
One coin does not send the other a message. The correlation becomes visible when the two result records are compared.
Bits, qubits and our simulator
The basics
What is a quantum computer?
A quantum computer processes information using quantum systems, such as individual atoms or specialised electrical circuits. Its units of computation are called qubits.
An ordinary computer works with bits. A quantum program instead changes a shared quantum state. Possibilities can reinforce or cancel one another like waves. At the end, a measurement gives a classical result.
Our simulator
Our ordinary i9 calculates a mathematical model of these steps. This lets us try and test programs without owning a quantum computer.
The lock introduced one such program. Now we explore its basic ideas in order: a qubit, waves and shared states. The analogies help explain them; they are not a complete description of the physics.
Applications and a quiz
What could this be useful for?
Quantum programs for molecules and materials
Colder than outer space.
Researchers operate superconducting qubits in refrigerators near absolute zero. The gold structure illustrates such a refrigerator. Other technologies use atoms or spins in diamond, for example. Our simulator lets you explore quantum programs without owning quantum hardware.
Medicine
Researchers calculate molecular energies and properties. These models can support drug research. Further studies are needed to establish a medicine's safety and effectiveness.
Climate and materials
Researchers use quantum models to study battery materials and catalysts, testing which methods improve on existing classical calculations.
Logistics
For routes and delivery plans, researchers compare quantum algorithms with classical optimisers. A practical advantage must be demonstrated on a specific task.
Application status: Many proposed applications require larger, error-corrected quantum computers. Researchers are testing which tasks justify the effort. Here you can explore the underlying programs in a simulator.
Quiz
Got the two ideas?
Five questions, instant explanations. Wrong guesses are welcome.
A classical bit in your phone has …
Distinguishing computation from illustrations
Computation and illustrations
Using our quantum lab
- The computation: Our i9 system with a graphics card simulates the circuits in the experiment cards and treasure hunt. We draw the displayed measurement samples from the calculated probabilities.
- The illustrations: Coins and the maze help explain randomness, correlations and interference. The animations do not depict particle trajectories. The calculations use complex amplitudes.
- The computational cost: Our PC simulates quantum programs using classical hardware. Each extra qubit doubles the memory required for a full state vector. Compact representations use less memory for suitable tasks. That is why we name the method and task alongside the qubit count; a specialised 120-qubit task is not a general-purpose 120-qubit simulator.
Now try it yourself.
Predict a result and let our computer simulate the quantum circuit.