Orientation
Theory gives us clean stories; experiments decide which stories survive. In quantum physics, the decisive tests are not only the famous ones you hear in class. Beyond the double slit and Schrödinger’s cat are quieter experiments that closed loopholes, measured phases you cannot see, and showed how information itself changes outcomes. Read slowly. Each experiment below is written for a careful first pass and then extends into the harder questions researchers argue about. Try to explain each result in one sentence to your future self: what was measured, what changed, and what that change tells us about nature.
1) Double-slit and single-particle interference
Fire one electron at a time toward two narrow slits. On the screen you still get a fringe pattern, not two simple piles. The pattern is set by the de Broglie wavelength \( \lambda = h/p \) and by geometry: \[ I(\theta) = I_0 \cos^2\!\left(\frac{\pi d \sin\theta}{\lambda}\right), \] where \( d \) is slit separation. The lesson is not “electrons are waves.” It is sharper: amplitudes add; probabilities appear only after you square the total amplitude. If a reliable record of the path exists, amplitudes from different paths stop adding coherently and fringes wash out.
A practical lab version uses photons with neutral-density filters so only one photon is in the apparatus at a time. A long exposure builds the same fringes dot by dot. This is the cleanest way to separate “what travels” (amplitudes) from “what we finally register” (clicks). Add a tiny piece of glass to one slit to shift phase by \( \phi \). The bright fringes slide by the angle that makes \( \frac{\pi d \sin\theta}{\lambda} \to \frac{\pi d \sin\theta}{\lambda} + \phi/2 \). Phase is invisible by itself; interference makes it visible.
Lab sketch
Use a diode laser, spatial filter, and adjustable slits. With a CCD at distance \(L\), measure fringe spacing \( \Delta y \approx \lambda L/d \). Insert a polarizer in front of one slit to mark the path; fringes vanish. Place a second polarizer at \(45^\circ\) after the slits to erase which-path marks; fringes return. The “eraser” does not go back in time—it erases distinguishability.
2) Mach–Zehnder interferometer: phase control without slits
Two beam splitters replace the slits and screen. A single photon meets the first splitter, takes a superposition of paths, and recombines at the second. Tuning a phase plate by \( \phi \) in one arm controls which output port clicks: \[ P_{\text{bright}} = \cos^2\!\left(\frac{\phi}{2}\right),\quad P_{\text{dark}} = \sin^2\!\left(\frac{\phi}{2}\right). \] Remove the second splitter and the outputs act like which-path detectors; interference disappears. This makes the role of the measurement context explicit and removes geometry clutter from the double slit.
What it teaches
Interference is about relative phase, not about slits. A balanced interferometer is the backbone of precision metrology (LIGO, atom interferometers) and of many quantum information experiments (phase encoding, feed-forward control).
3) Photoelectric effect: light as quanta
Classical waves predict that higher intensity light should eject more energetic electrons. Instead the kinetic energy depended on frequency, not intensity: \[ E_k = h\nu - \phi. \] No electron is ejected below a threshold frequency \( \nu_0 = \phi/h \), however bright the light. This forced the photon picture and gave a practical way to measure Planck’s constant from the slope of stopping potential vs frequency.
4) Compton scattering: photons carry momentum
Scatter X-rays from carbon and measure the wavelength shift: \[ \Delta \lambda = \lambda' - \lambda = \frac{h}{m_e c}(1-\cos\theta). \] The electron recoil demands that light carries particle-like momentum \( p=h/\lambda \). Together, the photoelectric and Compton effects closed the door on a purely wave picture of light.
5) Franck–Hertz: discrete atomic levels
Accelerate electrons through mercury vapor and record current vs accelerating voltage. The current rises smoothly then dips sharply at specific voltages. Those dips are electrons losing a fixed chunk of energy to excite atoms. The spacing gives excitation energies and thus the first direct evidence for quantized levels in matter.
6) Stern–Gerlach and sequential measurement
Silver atoms crossing a magnetic gradient split into two discrete beams, \( m_s=\pm \tfrac{1}{2} \). Place a second analyzer aligned with the first: one beam passes. Rotate the second by \(90^\circ\): the beam splits again equally. Now reinsert a third analyzer aligned with the first after the \(90^\circ\) stage: you do not recover a single beam—you get two again. This is the cleanest tabletop demo that non-commuting measurements change the state and that “having values for all directions at once” is not a sensible classical idea.
7) Bell tests and the fall of local realism
John Bell derived inequalities that any local hidden-variable model must satisfy. The CHSH form bounds a correlation parameter by \( S\le 2 \). Quantum mechanics predicts \( S\le 2\sqrt{2} \) and achieves this with entangled pairs measured at angle settings \( (a,a',b,b') \). Experiments beginning with Aspect and culminating in loophole-free tests (space-like separation, high efficiency) consistently find \( S \gt 2 \). The conclusion is not “instantaneous signals”—it is stronger: the joint outcomes cannot be reduced to pre-existing local values.
Reading a Bell data run
What matters are time-tagged coincident detections and random, independent setting choices. Plot the four correlations \(E(a,b)\) etc., form \(S\), and compare to statistical bounds including finite-sample effects. A clean analysis also reports no-signaling checks (marginals independent of remote settings).
8) Kochen–Specker contextuality: values depend on the question set
Contextuality tests go beyond locality. They show that assigning definite values to all observables at once, independent of which compatible set you choose to measure, conflicts with quantum predictions. Modern photonic and trapped-ion experiments implement Peres–Mermin-type tests and violate non-contextual inequalities. The moral: outcomes depend not only on the system but on the measurement context.
9) Leggett–Garg: macrorealism under pressure
Instead of spacelike-separated pairs, test correlations of a single system at different times. Macrorealism says the system has definite properties at all times and can be measured without disturbance. Inequalities built from temporal correlations \(K \le 1\) are violated in superconducting circuits, nuclear spins, and photons with invasive-measurement controls. This pushes quantum strangeness from “microscopic pairs” into everyday-scale devices.
10) Quantum Zeno and anti-Zeno effects
For short times the survival probability of a state is quadratic: \[ P(t) \approx 1 - \frac{(\Delta H)^2}{\hbar^2} t^2. \] If you measure repeatedly at intervals \( \tau \), after \(N=t/\tau\) checks the survival probability becomes \[ P_N(t)\approx \left(1 - \frac{(\Delta H)^2}{\hbar^2}\tau^2\right)^N \!\to 1 \] as \( \tau \to 0 \). Real experiments with trapped ions and superconducting qubits freeze or accelerate decay by tuning the measurement rate, demonstrating control over evolution via information flow.
11) Delayed-choice quantum eraser
Create entangled photons; send one through a double-path interferometer to a screen and the partner to a device that either keeps or erases which-path information. Sort the screen hits by the partner’s later outcome. When which-path is erased, fringes reappear in the subset. Nothing travels backward in time—the full, unsorted screen still looks classical. What is delayed is the inference you are allowed to make from joint data.
12) Hong–Ou–Mandel dip: two-photon indistinguishability
Send two identical photons into the two inputs of a 50/50 beam splitter. If they are truly indistinguishable, they “bunch” and exit the same port, giving a dip in coincidences as the relative delay \( \tau \) is scanned: \[ C(\tau) \propto 1 - V\, e^{-(\tau/\tau_c)^2}. \] This is interference of probability amplitudes for two-photon processes, not of classical waves. The HOM dip is a workhorse check in photonic quantum computing.
13) Aharonov–Bohm phase: potentials matter
Electrons travel around a region with magnetic field confined to a solenoid so \( \vec B=0 \) along their paths. The interference fringes shift by a phase \[ \Delta \phi = \frac{q}{\hbar}\oint \vec A\!\cdot d\vec \ell = \frac{q\Phi_B}{\hbar}, \] where \( \Phi_B \) is magnetic flux. The shift happens even though no magnetic force acts on the electrons. Potentials are not mere conveniences; they encode physical phase.
14) Neutron interferometry and gravity (COW experiment)
Perfect-crystal interferometers split and recombine neutron matter waves. Rotate the device in Earth’s gravity and the phase shifts by approximately \[ \Delta \phi \approx \frac{m g A}{\hbar v}, \] where \(A\) is the interferometer area and \(v\) the neutron speed. This puts gravitational potential directly into a quantum phase in a tabletop experiment.
15) Tunneling and the scanning tunneling microscope
Quantum tunneling probability through a barrier of width \(L\) and height \(V\) (for energy \(E\lt V\)) scales like \[ T \sim e^{-2\kappa L},\quad \kappa=\frac{\sqrt{2m(V-E)}}{\hbar}. \] The STM turns that exponential sensitivity into a nanoscale height probe: the tunneling current \( I \propto e^{-2\kappa s} \) depends on tip–sample separation \( s \) by fractions of a nanometer. You are literally “feeling” atomic orbitals through a quantum leak.
16) Josephson junctions and SQUID interferometry
Two superconductors separated by a thin barrier support a supercurrent \[ I_s = I_c \sin\phi,\qquad V=\frac{\hbar}{2e}\frac{d\phi}{dt}. \] A SQUID uses two junctions in a loop; its critical current oscillates with magnetic flux by \( \Phi_0 = h/2e \). These are macroscopic phase devices—quantum interference on a chip—and the backbone of many qubits and ultrasensitive magnetometers.
17) Cavity QED and photon number without destruction
In cavity QED, atoms cross a high-Q microwave cavity and pick up phase shifts that depend on photon number \(n\). By reading out the atom you can infer \(n\) without absorbing the photon (a quantum nondemolition measurement). Watching the cavity perform quantum jumps of light—\(n\to n\pm1\)—made “photon as a countable entity” more than a slogan.
18) Bose–Einstein condensates and matter-wave interference
Laser cooling and evaporative cooling bring dilute gases to nanokelvin temperatures. Many atoms occupy a single ground state described by a macroscopic wavefunction. Release two condensates and let them overlap: you see fringes just like light. The Gross–Pitaevskii equation governs the dynamics and reveals superfluid behavior, vortices, and sound modes in quantum fluids.
19) Rabi oscillations, Ramsey fringes, and coherence time
In trapped ions and superconducting qubits, a resonant drive rotates a two-level system on the Bloch sphere with Rabi frequency \( \Omega \). Two short pulses separated by time \(T\) create Ramsey fringes whose contrast decays with dephasing time \(T_2\). These simple curves are the workhorse evidence of controlled quantum coherence in the lab.
20) Rydberg blockade and controlled interactions
Highly excited atoms interact so strongly that exciting one prevents its neighbor from being excited—the blockade. Arrays of such atoms implement fast two-qubit gates and analog quantum simulators. Measuring the blockade radius and correlated excitation patterns turns abstract interactions into precise, programmable quantum dynamics.
21) Interference of large molecules (C\(_{60}\) and beyond)
Send fullerene molecules through a grating and record fringes after a flight path and oven control. These objects are thousands of times heavier than electrons and still show wave behavior. Results place strong limits on collapse models that try to force macroscopic classicality.
22) Quantum teleportation (lab reality, not sci-fi)
Prepare an entangled pair \(AB\). Interfere an unknown state \(X\) with \(A\) and perform a joint Bell-state measurement. Send the two classical bits to the holder of \(B\), who applies a conditional unitary. The state of \(X\) appears on \(B\) without moving the particle. Experiments now perform this over kilometers of fiber and satellite links. Teleportation is the operational proof that quantum information is physical but not a copyable commodity.
23) Weak measurement and “weak values”
Couple a system gently to a pointer so the back-action is small; pre-select \(|i\rangle\) and post-select \(|f\rangle\). The pointer shift is proportional to the “weak value” \[ A_w=\frac{\langle f|\hat A|i\rangle}{\langle f|i\rangle}. \] In the lab these allow ultra-sensitive phase estimation and offer a controlled window on measurement disturbance. They also spark debate: are weak values properties or just calibrated signal processing? Knowing the argument is part of being fluent in modern experiments.
Glossary (experiment-side terms)
Coincidence counter — Electronics that only records events when two detectors click within a short time window.
Visibility — Contrast of an interference pattern \( V=(I_{\max}-I_{\min})/(I_{\max}+I_{\min}) \).
QND measurement — Quantum nondemolition: read an observable without absorbing the system (e.g., photon number in cavity QED).
Contextuality — Dependence of outcomes on which compatible set of observables is measured together.
Dephasing vs relaxation — \(T_2\) describes loss of phase coherence; \(T_1\) energy relaxation to equilibrium.