IAPS SCHOOL DAY 2025

Quantum Science Experiments

IAPS SCHOOL DAY 2025

QUANTUM SCIENCE AND TECHNOLOGY

EXPERIMENTS GUIDE

Single Slit Experiment

Goal

To observe and measure the diffraction pattern created when light passes through a single narrow slit, demonstrating the wave nature of light.

Materials

  • Low-power laser pointer (<5 mW, red or green)
  • Single slit slide or mask
  • Screen (white card or wall)
  • Ruler or meter stick
  • Mounting board or stable surface
  • Black cardboard to reduce stray light
  • Safety goggles

Safety First!

Never point the laser at anyone’s eyes. Keep the beam directed only at the screen. Wear safety goggles if required by your institution.

Procedure

  1. Set up the laser on a stable surface so the beam is horizontal
  2. Place the single slit slide a few centimeters in front of the laser, perpendicular to the beam
  3. Position the screen 1-2 meters away from the slit
  4. Dim the room lights and turn on the laser
  5. Observe the diffraction pattern on the screen – a central bright band with dimmer side bands
  6. Measure the distance between the first minima on either side of the central maximum
  7. Record the slit-to-screen distance (D) and the slit width (a) if known

Theory Behind Single Slit Diffraction

When light passes through a narrow slit, it spreads out due to diffraction. The pattern consists of a central bright maximum with progressively dimmer maxima on either side.

Central maximum width ≈ 2λD/a

Where λ is the wavelength, D is the slit-to-screen distance, and a is the slit width.

Double Slit Experiment

Goal

To observe and measure the interference pattern created when light passes through two closely spaced slits, demonstrating the wave nature of light and the principle of superposition.

Materials

  • Low-power laser pointer (<5 mW, red or green)
  • Double slit slide or mask
  • Screen (white card or wall)
  • Ruler or meter stick
  • Mounting board or stable surface
  • Black cardboard to reduce stray light
  • Safety goggles
  • Camera or smartphone (optional, for measurements)

Procedure

  1. Set up the laser on a stable surface so the beam is horizontal
  2. Place the double slit slide a few centimeters in front of the laser, perpendicular to the beam
  3. Position the screen 1-2 meters away from the slits
  4. Dim the room lights and turn on the laser
  5. Observe the interference pattern on the screen – multiple evenly spaced bright and dark fringes
  6. Measure the distance between several adjacent bright fringes (e.g., measure across 10 fringes)
  7. Calculate the average fringe spacing (Δy)
  8. Record the slit-to-screen distance (D) and the slit separation (d) if known

Theory Behind Double Slit Interference

When light passes through two closely spaced slits, the waves from each slit interfere with each other, creating a pattern of bright and dark fringes.

Fringe spacing Δy = λD/d

Where λ is the wavelength, D is the slit-to-screen distance, and d is the slit separation.

You can use this to calculate the wavelength: λ = Δy·d/D

Sample Calculation

If your laser has a wavelength of 650 nm (red), slit separation d = 0.25 mm, and screen distance D = 1.5 m:

Δy = (650×10⁻⁹ × 1.5) / (0.25×10⁻³) = 0.0039 m = 3.9 mm

You should measure fringe spacing of about 3.9 mm.

Discussion Questions

  • Why does the double-slit pattern show many evenly spaced fringes while the single-slit pattern shows a wide central maximum?
  • What would happen to the fringe spacing if you used a green laser instead of red?
  • How would the pattern change if the slit separation doubled?
  • What does this experiment tell us about the nature of light?

Interactive Wave Interference Simulation

Experiment with different parameters to see how they affect the diffraction and interference patterns.

650
100
250
1.5

Calculated Results

Fringe Spacing (Δy): 3.90 mm
Central Max Width: 19.50 mm
Number of Visible Fringes: 7

Beyond the Slits: The Quantum Eraser

The Mystery of Measurement

The double-slit experiment reveals that particles like photons or electrons can behave as waves, creating an interference pattern. But what happens if we try to “peek” and see which slit the particle goes through? When we do, the interference pattern vanishes! The very act of measuring which path the particle takes forces it to behave like a particle, not a wave.

The Quantum Eraser is a clever variation of this experiment that takes it a step further. It shows that we can “erase” the which-path information and recover the interference pattern, even *after* the particle has hit the screen. This has profound implications for our understanding of time and causality in the quantum world.

Real-World Applications

Quantum Computing

The principle of superposition, demonstrated in the double-slit experiment, is a cornerstone of quantum computing. Qubits can exist in multiple states at once, allowing for massively parallel computations.

Cryptography

Quantum mechanics provides new, ultra-secure methods of communication. Quantum Key Distribution (QKD) uses the principles of quantum measurement to ensure that any attempt to eavesdrop on a communication is immediately detected.

High-Precision Measurements

Interferometry, based on the principles of the double-slit experiment, is used in a wide range of scientific and engineering applications, from detecting gravitational waves (LIGO) to testing the quality of optical components.