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Quantum computing flashcards

Qubits, gates and circuits you can run in Qiskit, up to error correction and the post-quantum shift.

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Quantum computingStart from zero · 1 of 12

What is a quantum computer, in one plain sentence?

A quantum computer is a new kind of computer that uses the odd rules of tiny particles to solve some special problems far faster than normal (classical) computers can.

Not a faster laptop, a different kind of machine.

All topics

Quantum computing at a glance

A quantum computer uses the odd rules of tiny particles to solve some special problems much faster.

Like waves on a pond: a quantum program makes wrong answers cancel out and the right one grow stronger.

Words you will meet

Qubit
The quantum version of a bit, able to be a blend of 0 and 1
Superposition
That blend of 0 and 1 before you look
Measurement
Reading a qubit, which always gives a plain 0 or 1
Amplitude
A qubit's weight for 0 or 1, which can be negative
Interference
Amplitudes adding up or cancelling out, like waves
Entanglement
Qubits linked so their results match up
Gate
One step that changes qubits, like flip or blend
Shot
One full run of a circuit

The steps

  1. 1. Start from zero (12 cards): What a quantum computer is, how a qubit differs from a bit, and the plain words behind every circuit.
  2. 2. Core basics (15 cards): Qubits, superposition, measurement, entanglement and your first circuits that print real results.
  3. 3. Next steps (16 cards): Two-qubit gates, primitives, transpiling, real hardware, noise and the famous algorithms.
  4. 4. Go deeper (16 cards): Error correction, hybrid algorithms, advantage claims, resource estimates and post-quantum migration.
All 12 cards in Start from zero
What is a quantum computer, in one plain sentence?
A quantum computer is a new kind of computer that uses the odd rules of tiny particles to solve some special problems far faster than normal (classical) computers can.
What problem does quantum computing try to solve?
Some problems, like how a new medicine molecule behaves, grow so fast that even the biggest normal computer would take impossibly long. Quantum computers may crack some of these.
Where do you already meet quantum computing in daily life?
Not on your desk yet. But your browser is already moving to new quantum-safe locks for logins and payments, because a large quantum computer could one day break today's locks.
What everyday thing is a quantum computer like?
Think of waves on a pond. A quantum program sends waves through all the paths of a maze; wrong paths cancel out and the right path grows stronger, so you are likely to find it.
What are a bit and a qubit?
A bit is the smallest piece of computer information: 0 or 1, like a switch off or on. A qubit is the quantum version: it can be in a blend of 0 and 1 until you look.
What are superposition and probability here?
Superposition is that blend of 0 and 1. When you check the qubit you get 0 or 1 at random, and the probability, the chance of each, is set by the blend, like 50 percent each.
What do measurement and collapse mean?
Measurement means reading the qubit. It always gives a plain 0 or 1, and then the blend is gone: the qubit collapses and stays at the answer you saw.
What are amplitude and interference?
An amplitude is a qubit's weight for 0 or for 1, and unlike a chance it can be negative. Interference is amplitudes adding up or cancelling out, like two water waves meeting.
What is entanglement?
Entangled qubits are linked: when you measure both, their results always match up, even far apart, though each one alone looks random.
What are a gate, a circuit and a shot?
A gate is one step that changes qubits, like X (flip 0 and 1) or H (make a 50/50 blend). A circuit is a list of gates, and a shot is one full run; you run many shots to see the pattern.
What do you need, and what is your first tiny win?
Python and the free Qiskit kit, which imitates qubits on your laptop. QuantumCircuit(1) makes one qubit, h gives it a 50/50 blend, and measure_all reads it as 0 or 1.
What will you be able to do after this topic?
Read and write small quantum circuits, predict what they print, run them on a simulator or real cloud hardware, and tell real quantum uses from hype.
Quantum computing cheatsheet

Gates

  • H |0> to |+>, makes 50/50
  • X / Y / Z Bit flip / both / phase flip
  • S, T Quarter and eighth phase turns
  • RX, RY, RZ(theta) Rotate by an angle
  • CX (CNOT) Flip target if control is 1
  • CZ Phase flip if both are 1
  • SWAP Exchange two qubits
  • CCX (Toffoli) Reversible AND

Qiskit 2.x

  • qc = QuantumCircuit(2) Two-qubit circuit
  • qc.h(0); qc.cx(0, 1) Bell pair
  • qc.measure_all() Measure into register meas
  • qc.draw("mpl") Draw the circuit
  • Statevector(qc).probabilities_dict() Exact probabilities
  • StatevectorSampler().run([qc], shots=1000) Local counts
  • res[0].data.meas.get_counts() Read counts
  • generate_preset_pass_manager(3, backend).run(qc) Transpile for a device

IBM Runtime

  • QiskitRuntimeService(channel="ibm_quantum_platform") Connect
  • service.least_busy(simulator=False) Pick a device
  • SamplerV2(mode=backend) Counts on hardware
  • EstimatorV2(mode=backend) Expectation values
  • options.resilience_level = 2 Error mitigation on
  • job.job_id() / service.job(id) Resume a job later

Algorithms

  • Grover Search, about (pi/4) sqrt(N) calls
  • Shor Factoring via period finding
  • QFT Quantum Fourier transform
  • QPE Phase estimation, eigenvalues
  • VQE Hybrid, lowest energy
  • QAOA Hybrid, combinatorial optimisation

Numbers worth knowing

  • 2^n amplitudes 30 qubits needs about 16 GB to simulate
  • 1/sqrt(shots) Sampling error
  • About 1 percent Surface code threshold
  • 0.1 to 1 percent Typical two-qubit gate error
  • ML-KEM, ML-DSA, SLH-DSA NIST PQC, FIPS 203 to 205
  • X25519MLKEM768 Hybrid TLS key exchange in browsers
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