Fundamental Quantum Mechanics Experiments

Why This Matters

These experiments aren't just historical curiosities—they're the foundation of everything you'll encounter in quantum mechanics. Each one forced physicists to abandon classical intuitions and accept that nature operates by fundamentally different rules at the atomic scale. You're being tested on your ability to connect experimental observations to core quantum principles: wave-particle duality, quantization, superposition, and the measurement problem. Understanding why these experiments were revolutionary tells you what classical physics couldn't explain.

Don't just memorize what happened in each experiment—know what concept each one demonstrates. When an exam question asks you to "provide experimental evidence for wave-particle duality," you need to immediately connect that to specific experiments and explain how the results support the concept. The experiments below are grouped by the quantum principle they reveal, which is exactly how you should organize them in your mind for exams.

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Experiments Demonstrating Wave-Particle Duality

The central mystery of quantum mechanics is that entities like electrons and photons behave as waves in some experiments and particles in others. The behavior you observe depends on the experimental setup—not on some hidden property of the object itself.

Double-Slit Experiment

• Interference pattern with both slits open—particles arriving one at a time still build up wave-like fringes, proving individual particles interfere with themselves
• Measurement destroys interference—observing which slit a particle passes through collapses the wave function and produces two bands instead of an interference pattern
• Central demonstration of complementarity—you cannot simultaneously observe wave behavior (interference) and particle behavior (which-path information)

Davisson-Germer Experiment

• Electron diffraction from crystal surfaces—electrons scattered off nickel crystals produce interference patterns identical to X-ray diffraction
• Validates de Broglie's hypothesis—confirmed that particles have wavelength $\lambda = h/p$, where $h$ is Planck's constant and $p$ is momentum
• Bridge between matter and waves—extended wave-particle duality from photons to massive particles, fundamentally changing our understanding of matter

Compton Scattering

• Wavelength shift in scattered X-rays—when X-rays scatter off electrons, the outgoing wavelength increases by $\Delta\lambda = \frac{h}{m_e c}(1 - \cos\theta)$
• Photons carry momentum—the scattering follows conservation of energy and momentum as if photons were billiard balls colliding with electrons
• Definitive particle evidence for light—wave theory predicts no wavelength change; only treating photons as particles with momentum $p = h/\lambda$ explains the results

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Experiments Revealing Quantization

Classical physics assumed energy and angular momentum could take any continuous value. These experiments proved that certain quantities come only in discrete chunks—quantization is built into nature at the fundamental level.

Photoelectric Effect

• Threshold frequency required for emission—no electrons are ejected below a critical frequency, regardless of light intensity
• Electron energy depends on frequency, not intensity—maximum kinetic energy follows $KE_{max} = hf - \phi$, where $\phi$ is the work function
• Einstein's photon hypothesis—light energy is quantized in packets $E = hf$, explaining why dim blue light ejects electrons but bright red light cannot

Stern-Gerlach Experiment

• Discrete beam splitting in magnetic field—silver atoms passing through an inhomogeneous magnetic field split into exactly two spots, not a continuous smear
• Discovery of spin quantization—reveals intrinsic angular momentum (spin) that takes only values $\pm\hbar/2$ for electrons
• Foundation for quantum measurement theory—demonstrates that measuring one spin component randomizes others, illustrating incompatible observables

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Experiments Probing Superposition and Measurement

Quantum systems exist in superpositions of states until measured. These experiments explore what happens during measurement and why quantum mechanics forces us to rethink causality and reality.

Schrödinger's Cat Thought Experiment

• Superposition at macroscopic scales—a cat entangled with a radioactive atom would be simultaneously alive and dead until observed, illustrating the measurement problem
• Highlights the observer's role—raises the question of what constitutes a "measurement" and when wave function collapse occurs
• Not a real experiment—this is a conceptual tool designed to expose the absurdity of applying quantum superposition to everyday objects

Delayed Choice Quantum Eraser

• Retroactive determination of behavior—erasing which-path information after detection can restore interference patterns in correlated data
• Challenges classical causality—the decision to measure or erase information seems to affect what the particle "did" in the past
• Demonstrates nonlocal correlations—shows that quantum correlations don't respect our intuitions about time ordering, though no information travels backward

Quantum Interference Experiments

• Superposition made visible—particles in superposition states produce interference fringes when probability amplitudes add coherently
• Coherence is fragile—any interaction that reveals which-path information destroys interference, a process called decoherence
• Foundation for quantum technologies—interference effects underlie quantum computing, where qubits in superposition enable parallel processing

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Experiments Demonstrating Nonclassical Phenomena

Some quantum effects have no classical analog whatsoever. These experiments reveal behaviors that simply cannot occur in a classical universe.

Quantum Tunneling Experiments

• Barrier penetration without sufficient energy—particles have nonzero probability of appearing on the other side of classically forbidden potential barriers
• Probability decays exponentially with barrier width—transmission probability depends on $e^{-2\kappa d}$, where $\kappa$ relates to barrier height and $d$ is thickness
• Real-world applications—explains alpha decay, scanning tunneling microscopes, and tunnel diodes; essential for understanding nuclear fusion in stars

Quantum Entanglement Experiments

• Instantaneous correlations across distance—measuring one entangled particle immediately determines the state of its partner, regardless of separation
• Violates Bell inequalities—experimental tests confirm correlations stronger than any local hidden variable theory allows, ruling out classical explanations
• No faster-than-light communication—correlations are random until compared, so entanglement cannot transmit information, preserving causality

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Quick Reference Table

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Self-Check Questions

1. Which two experiments both demonstrate wave-particle duality but approach it from opposite directions—one showing particles behaving as waves, the other showing waves behaving as particles?

2. If asked to provide experimental evidence that angular momentum is quantized, which experiment would you cite, and what specific observation supports this conclusion?

3. Compare and contrast the photoelectric effect and Compton scattering: what do they have in common, and what distinct aspect of photon behavior does each reveal?

4. An FRQ asks you to explain how measurement affects quantum systems. Which experiments would you use to illustrate (a) wave function collapse and (b) the role of which-path information?

5. What distinguishes quantum tunneling from quantum entanglement as nonclassical phenomena, and why can neither be explained by classical physics?