Density Matrices, Entanglement and the Bell Test · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

When half a pair looks like a blur

Science · Quantum & Modern Physics · ages 23-24
Name ______________________   Date ____________

  1. You trace out one half of an entangled pair. What is left?

    • A mixed state
    • A pure state
    • Nothing at all

  2. Which state is a statistical blend of several possibilities?

    • A pure state
    • A state vector always
    • A mixed state

  3. Holding one half, entanglement looks just like ordinary ignorance.

    Circle one:   True   False

  4. Left halves only: can you tell entanglement from shuffled preparation?

    • Yes, easily, with one glance
    • Yes, by guessing loudly
    • No, one half alone cannot tell them apart

  5. Which density matrix belongs to a pure state?

    • The one blending several possibilities
    • The one for a single definite state
    • Any matrix with extra ignorance

  6. What does a Bell inequality assume?

    • A local explanation of the correlations
    • Faster-than-light messaging
    • Infinite measurement precision

  7. A violation is confirmed. A friend claims this names the true mechanism. What is wrong?

    • Violations explain everything
    • Local stories are all confirmed
    • It only closes the local door, it names no winner

  8. A Bell test is violated. What exactly is ruled out?

    • Only one local model, others survive
    • Every local explanation of the results
    • Quantum theory itself
LightMySky · lightmysky.comW1-mt_qwUJ2qV0l3-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

When half a pair looks like a blur W1-mt_qwUJ2qV0l3-s1

  1. A mixed state · The remaining half is mixed.
  2. A mixed state · Blend means mixed.
  3. True · One half alone cannot tell the difference.
  4. No, one half alone cannot tell them apart · The blur hides its own origin.
  5. The one for a single definite state · Pure means single and definite.
  6. A local explanation of the correlations · Bell bounds what local stories allow.
  7. It only closes the local door, it names no winner · Ruling out is not naming.
  8. Every local explanation of the results · Violation closes the whole local door.
Worksheet · LightMySky