Measurement: from quantum to classical

What does measurement do to a qubit, why are quantum programs run thousands of times, and why can’t quantum information be copied?

Measurement changes the state

Reading a classical bit does not change it. A qubit is different: measurement forces a qubit in superposition to “choose” 0 or 1. After the measurement the qubit is in the state of that result. Measure it again right away and you get the same result.

So if you measured after an H gate and saw 1, the mixture is gone. The information about the qubit’s amplitudes before the measurement is lost.

Why run thousands of times?

A single measurement tells you only one result. To see the probabilities themselves we rebuild the same circuit and run it many times, counting the results. Each run is called a shot. On real quantum computers an experiment usually involves thousands of shots.

The playground’s “1000 kez ölç” button does exactly that. The probability bars are the exact values computed by the simulator; on a real device you can only estimate them with many shots.

No copying allowed

Making an exact copy of an unknown qubit state is impossible by the laws of nature. If we could copy, we could measure thousands of copies and learn the amplitudes of a single qubit; but we can’t.

This may look like a shortcoming, but it is the foundation of quantum cryptography. In protocols such as BB84 an eavesdropper has to measure the qubits, the measurement disturbs the state, and the eavesdropping is detected. You can simulate this with the store’s BB84 quantum key distribution package.

Try it yourself

Open “Durum vektörünü göster” (show the state vector): after H both amplitudes are 0.707 and the probabilities 50%. Then switch to the “Klasik bit gibi: NOT” example (like a classical bit): a single amplitude is 1 and the result is certain; you know it without even measuring.

Open the circuit in your browser

The circuit playground is in Turkish. Button guide: “1000 kez ölç” = measure 1000 times, “Durum vektörünü göster” = show the state vector, “Bu devrenin Qiskit kodu” = this circuit’s Qiskit code, “Geri al” = undo, “Temizle” = clear. Click a gate in the circuit to remove it.

Key points

Measurement ends superposition; the qubit stays in the state of the measured result.
To learn the probabilities a circuit is run many times; each run is a shot.
An unknown quantum state cannot be copied.
Measurement disturbing the state is used in quantum cryptography to catch eavesdroppers.

Test yourself

Why is a quantum program usually run thousands of times?

To warm up the qubits
Each measurement gives a single random result, so we need many to see the probabilities
Because compiling the program takes a long time

Show the answer

Each measurement gives a single random result, so we need many to see the probabilities. A single measurement gives only one result. To see the probability distribution the circuit is run again and again and the results are counted.

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