Checkpoint: qubits · Qubits · Learn quantum computing

Question 1 of 8 · Bits and qubits

Eight questions, one from each idea in this level. Miss one and you can simply try again; your first-try score is shown at the end.

Your turn

A qubit can be set to endlessly many states. How much information do you get from measuring it once?

Question 2 of 8 · Superposition

Your turn

Which sentence describes the state |+⟩ best?

Question 3 of 8 · Odds from amplitudes

Your turn

A qubit is in the state 0.8|0⟩ + 0.6i|1⟩. What's the chance of measuring 1?

Question 4 of 8 · The Bloch sphere

Remember: chance of 0 = (1 + height) ÷ 2.

Tilt the arrow up or downheight −0.34

chance of 0 = (1 − 0.34) ÷ 2 = 0.33

0: 33%1: 67%

0.57|0⟩ + 0.82|1⟩

GoalTilt the arrow so the qubit gives 0 only a quarter of the time (25%).

Height runs from +1 at the North Pole to −1 at the South Pole. The chance of 0 is (1 + height) ÷ 2.

Question 5 of 8 · Phase

Your turn

|+⟩ and |−⟩ both give 0 half the time. What's actually different about them?

Question 6 of 8 · Measurement

Your turn

You measure |+⟩ and get 0. Then you apply an H gate and measure again. What do you get?

Question 7 of 8 · Kets

Your turn

Which column is the state |−⟩?

Question 8 of 8 · Gates as turns

Start at |0⟩ and steer the qubit to |−⟩. Two gates are enough.

|0⟩

0: 100%1: 0%

Start at |0⟩, the North Pole. Tap a gate.

|−⟩

GoalVisit every state above. You can go back to |0⟩ whenever you like.

Every gate on one qubit is a turn of the sphere. Drag the sphere to watch from a better angle.

Qubits: done

You now know what a qubit really is: an arrow on the Bloch sphere, set by amplitudes, whose height sets the odds and whose phase is hidden until a gate turns it into something you can measure. You know what measuring does, and you can read kets.

Finish the checkpoint to see your score. Level 2 is next: gates and circuits.