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Sandbox Physics

Particles · Probability · Measurement

Quantum & fields

Build a picture of the invisible, one measurement at a time.

129 experiments

Start here

Start from a single measurement

Predict an outcome, then count what the measurements actually give. The path starts with one qubit and moves on to wave packets, bound states and two-photon interference.

6 Labs · about 120 min

  1. 01

    Observe

    Quantum State & Born Measurement

    Predict a probability, turn the analyzer, then compare your prediction with the measured frequency over many shots.

    15 min
  2. 02

    Compare

    Stern–Gerlach Sequence

    Turn the middle magnet to a crossed axis and compare it with the same-axis case. What does the middle measurement change?

    15 min
  3. 03

    Model

    Wave Packet & Uncertainty

    Release a narrow packet and a wide one. Which spreads faster, and how can a focused packet first get narrower?

    20 min
  4. 04

    Model

    Quantum Harmonic Oscillator

    Start from the ground state, then mix in a second level or displace the state. When does the probability cloud move, and what do sampled positions show?

    20 min
  5. 05

    Compare

    Which Path & Quantum Eraser

    Tag the two paths, then erase the which-path information. Do the fringes come back in all the data, or only after the detections are sorted into groups?

    25 min
  6. 06

    Transfer

    Hong–Ou–Mandel Two-Photon Interference

    Scan the arrival delay of two photons and watch coincidences drop when they become indistinguishable.

    25 min

Browse the collection

129 experiments

  1. Quantum & fields

    Decay Chain Counter

    Why can daughter counts rise while the parent source is decaying?

    15 min

  2. Quantum & fields

    Proton Form Factors

    How can scattered electrons separate a proton’s electric and magnetic responses?

    15 min

  3. Quantum & fields

    Nuclear Shell Filling

    How does spin–orbit splitting open a shell gap at twenty-eight neutrons?

    15 min

  4. Quantum & fields

    SQUID: Flux into Voltage

    How can a magnetic flux change a superconducting circuit’s voltage?

    15 min

  5. Quantum & fields

    Josephson Switching Statistics

    Why does a biased Josephson junction switch before its critical current?

    15 min

  6. Quantum & fields

    Superconductors in a Magnetic Field

    Why can a magnetic field enter one superconductor before it becomes normal?

    15 min

  7. Quantum & fields

    How Cooper Pairs Condense

    How do loose Cooper pairs turn smoothly into tightly bound molecules that condense?

    16 min

  8. Quantum & fields

    Weak Decay and the Mirror Test

    Why does reversing the nuclear spin reverse the preferred electron direction?

    15 min

  9. Quantum & fields

    Neutral Kaons in Flight

    How can a neutral kaon change flavour while its probability of survival falls?

    20 min

  10. Quantum & fields

    Inside the Proton

    What do a scattered electron’s energy and direction reveal about the proton?

    20 min

  11. Quantum & fields

    Quantum Phase Estimation

    In quantum phase estimation, how much more evolution does each extra bit of precision need?

    25 min

  12. Quantum & fields

    Grover Amplitude Amplification

    Can one extra round make the answer harder to find?

    25 min

  13. Quantum & fields

    Resonant Tunneling Transport

    Can two barriers transmit better than one?

    20 min

  14. Quantum & fields

    Gravitational Quantum Bouncer

    How tall must a slit be for a neutron to get through?

    20 min

  15. Quantum & fields

    Thouless Transport Pump

    How can slowly cycling a chain's parameters back to their start still move particles across it?

    20 min

  16. Quantum & fields

    Spin Squeezing Metrology

    Does squeezing the noise of a spin ensemble always give a more precise measurement?

    20 min

  17. Quantum & fields

    Quantum Spin Chain Quench

    If you flip one spin in a chain, where does its influence go?

    20 min

  18. Quantum & fields

    Quantum State & Process Tomography

    Can measuring along one direction reveal a whole quantum state?

    20 min

  19. Quantum & fields

    Quantum Error Correction

    Can you find an error without reading the encoded message?

    20 min

  20. Quantum & fields

    Identical Atom Exchange

    When two atoms in neighboring traps tunnel at the same time, do they end up together or apart?

    18 min

  21. Quantum & fields

    Quantum Gas Occupation

    When the same trap is cooled, which particles crowd into the lowest energy level?

    18 min

  22. Quantum & fields

    Hydrogen Orbitals & Spectra

    Why can a hydrogen spectral line be missing even when the energy gap matches?

    20 min

  23. Quantum & fields

    Partial-Wave Scattering

    Why does scattering off a single round target show so many minima?

    20 min

  24. Quantum & fields

    Franck–Hertz Excitation

    Why can a higher voltage make the current drop?

    20 min

  25. Quantum & fields

    Laser Cooling & MOT

    Can light slow an atom down and also hold it in place?

    20 min

  26. Quantum & fields

    Homodyne Quantum Tomography

    Can you draw a quantum state from detector noise?

    20 min

  27. Quantum & fields

    Quantum Phase Metrology

    Do sharper interference fringes always mean a better phase measurement?

    20 min

  28. Quantum & fields

    Coupled Spin Spectroscopy

    Why can a spectral line be missing even though both levels exist?

    18 min

  29. Quantum & fields

    Transmon Artificial Atom

    What do you give up when you make an artificial atom less sensitive to stray charge?

    18 min

  30. Quantum & fields

    Quantum Zeno Monitoring

    Can repeated checks slow a quantum transition?

    18 min

  31. Quantum & fields

    Two-Level Quantum Otto Engine

    Can one spin turn heat into work?

    18 min

  32. Quantum & fields

    Aharonov–Bohm Flux Interferometer

    Can a magnetic field the electrons never pass through change which exit they take?

    18 min

  33. Quantum & fields

    Atom Interferometer Gravimeter

    How can three laser pulses measure gravity with falling atoms?

    18 min

  34. Quantum & fields

    Anderson Localization

    Can a wave get stuck without a wall?

    20 min

  35. Quantum & fields

    Bloch Oscillations

    Why does a steady push make a particle in a lattice swing back and forth instead of speeding up?

    20 min

  36. Quantum & fields

    Trapped-Ion Sideband Spectroscopy

    Why does the red sideband disappear when a trapped ion reaches its motional ground state?

    25 min

  37. Quantum & fields

    Cavity Quantum Excitation Exchange

    Can a cavity give a photon back to the atom that emitted it?

    25 min

  38. Quantum & fields

    STIRAP Dark-State Transfer

    How can two laser pulses transfer an atom while it almost never sits in the lossy middle state?

    25 min

  39. Quantum & fields

    Rydberg Atom Blockade

    Why are two nearby atoms rarely excited to Rydberg states at the same time?

    25 min

  40. Quantum & fields

    Fluorescence & Quantum Jumps

    When the detector goes quiet, has the atom really gone dark?

    25 min

  41. Quantum & fields

    HBT Photon Correlation

    How can you tell equally bright light sources apart by the way their photons arrive?

    25 min

  42. Quantum & fields

    Quantum Well Spectroscopy

    How much do the energy levels rise when a quantum well is made narrower?

    25 min

  43. Quantum & fields

    Coherent Double Well Transfer

    How can a particle get into the other well without climbing over the barrier?

    25 min

  44. Quantum & fields

    Wave Packet & Uncertainty

    Does a narrower wave packet end up spreading wider?

    20 min

  45. Quantum & fields

    Quantum Harmonic Oscillator

    If the probability density of a quantum oscillator never changes, is its energy zero?

    20 min

  46. Quantum & fields

    Landau–Zener Passage

    Does a slow sweep through an avoided crossing switch a two-level system into its other state?

    25 min

  47. Quantum & fields

    Berry Phase Interferometer

    How far does an interference fringe shift when a spin is carried around a loop?

    25 min

  48. Quantum & fields

    Quantum Teleportation & Swapping

    If the classical message is blocked, does a teleported quantum state still arrive?

    25 min

  49. Quantum & fields

    BB84 Key Distribution

    In BB84, which bits are kept for the key after the two sides compare notes in public?

    25 min

  50. Quantum & fields

    Which Path & Quantum Eraser

    Can sorting the hits by their which-path tags bring the interference fringes back?

    25 min

  51. Quantum & fields

    Weak Measurement & Conditional Pointer

    To make the pointer shift bigger in a weak measurement, how many events must you give up?

    25 min

  52. Quantum & fields

    Hong–Ou–Mandel Two-Photon Interference

    When two photons enter a beam splitter from opposite sides, why do the two counters stop clicking together?

    25 min

  53. Quantum & fields

    Bell Correlations & CHSH

    Can two stations whose results each look random still beat the limit of any local theory?

    25 min

  54. Quantum & fields

    Spin Echo & Noise Spectroscopy

    Which lost coherence can a pulse bring back?

    25 min

  55. Quantum & fields

    Decoherence Channels & Information Loss

    Does losing phase also mean losing energy?

    25 min

  56. Quantum & fields

    Quantum State & Born Measurement

    Can two states look identical in one basis and different in another?

    15 min

  57. Quantum & fields

    Stern–Gerlach Sequence

    Why does a middle analyzer change a repeated measurement?

    15 min

  58. Quantum & fields

    Rabi Pulse Calibration

    How long must a pulse last to flip a state?

    15 min

  59. Quantum & fields

    Ramsey Frequency Measurement

    Can a longer wait reveal a smaller frequency difference?

    15 min

  60. Quantum & fields

    Can a Particle Cross an Impossible Wall?

    Why can a particle sometimes be detected beyond a barrier it classically cannot cross?

    10 min

  61. Quantum & fields

    Can One Particle Interfere?

    How do unpredictable hits accumulate into stable fringes?

    12 min

  62. Quantum & fields

    How a Field Becomes Quantized

    How can a continuous field produce discrete particle excitations?

    15 min

  63. Quantum & fields

    SSH Chain & Edge States

    Is a particle trapped at the edge of an SSH chain always in a topological state?

    20 min

  64. Quantum & fields

    Chern Insulator · Follow the Edge

    When the edge of a Chern insulator lights up, does the wave really travel along it?

    25 min

  65. Quantum & fields

    Haldane Lattice · Turn the Phase

    If you reverse the hopping phase in a Haldane lattice, does the edge carry waves the other way?

    25 min

  66. Quantum & fields

    Cavity Photon Counting & Feedback

    When can atoms passing through a cavity reveal its photon number, and when can delayed feedback keep that number steady?

    25 min

  67. Quantum & fields

    Zeeman Atomic-beam Slower

    Which atoms does a Zeeman slower actually slow down, and what can the glow of atoms at its exit prove?

    22 min

  68. Quantum & fields

    Two-Ion Quantum Logic Spectroscopy

    How can you take the spectrum of one ion by watching the light from another ion beside it?

    25 min

  69. Quantum & fields

    Three-Photon GHZ Correlation Bench

    How can three photons whose individual results are random still beat every local explanation?

    25 min

  70. Quantum & fields

    Single-Electron Penning Trap & Spin Readout

    How can the spin of a single trapped electron be read from the frequency of its motion?

    25 min

  71. Quantum & fields

    Molecular-Beam Proton Magnetic Moment

    How can a beam of hydrogen molecules measure the proton's magnetic moment when the molecules' rotation is magnetic too?

    25 min

  72. Quantum & fields

    Hydrogen–Deuterium Isotope Spectrometer

    Why do hydrogen and deuterium emit their spectral lines at slightly different wavelengths?

    20 min

  73. Quantum & fields

    Two-Isotope Doppler Thermometer

    How can two hydrogen isotopes give a gas temperature when the spectrometer also blurs the lines?

    20 min

  74. Quantum & fields

    Compton Angular Spectrometer

    How does a scattered photon's energy change with angle, and what does that reveal?

    20 min

  75. Quantum & fields

    Compton Coincidence Energy Analyzer

    Do the scattered photon and the recoiling electron together carry all of the incoming photon's energy?

    20 min

  76. Quantum & fields

    Continuous-wave NMR Spectrometer

    How do you find the center and width of a nuclear resonance with a continuous radio-frequency scan?

    20 min

  77. Quantum & fields

    NMR Power Saturation Analyzer

    Why does the absorption signal of a nuclear resonance shrink when the radio-frequency drive gets too strong?

    20 min

  78. Quantum & fields

    NMR Free Induction Spectrometer

    What can the fading voltage after a radio-frequency pulse tell you about the nuclear spins?

    20 min

  79. Quantum & fields

    NMR Inversion Recovery Relaxometer

    How do you measure how fast nuclear magnetization recovers after it is flipped?

    20 min

  80. Quantum & fields

    Optical Pumping and Relaxation

    How can light measure how fast atoms relax in the dark?

    20 min

  81. Quantum & fields

    Hanle Lifetime Spectrometer

    How can a magnetic field measure how long an excited atom lives?

    20 min

  82. Quantum & fields

    Oil-Drop Charge Measurement

    How can the way an oil drop moves reveal its electric charge?

    20 min

  83. Quantum & fields

    Atomic Magnetic-Moment Comparator

    How can you measure the electron's spin factor without knowing the magnetic field?

    20 min

  84. Quantum & fields

    Zeeman Polarization Spectrometer

    What does a spectral line look like in a magnetic field, viewed across and along the field?

    20 min

  85. Quantum & fields

    Canal-Ray Doppler Spectrometer

    How does the light from fast-moving hydrogen atoms reveal their speed?

    20 min

  86. Quantum & fields

    Stark Splitting Spectrometer

    How can a split hydrogen line measure an electric field?

    20 min

  87. Quantum & fields

    Lamb Shift Microwave Bench

    How can microwaves find the Lamb shift in hydrogen?

    20 min

  88. Quantum & fields

    Mössbauer Doppler Observatory

    Why can a source moving a few millimeters per second resolve tiny shifts in nuclear energy levels?

    20 min

  89. Quantum & fields

    Blackbody Radiation Observatory

    Can one temperature and one emissivity explain the measured spectrum?

    18 min

  90. Quantum & fields

    The Matter-Wave Antilaser

    How does the relative phase of two matter waves decide whether a lossy site swallows them?

    16 min

  91. Quantum & fields

    A Cut That Moves Every Mode

    Why does cutting one link in a ring where hopping favors one direction push every mode to one edge?

    16 min

  92. Quantum & fields

    When a Molecular Bond Breaks

    Why can a kicked molecular bond break when an ideal spring of the same stiffness always pulls back?

    18 min

  93. Quantum & fields

    Through a Conical Intersection

    What happens to a molecular wavepacket when it passes the point where two energy surfaces touch?

    18 min

  94. Quantum & fields

    A Quantum Newton’s Cradle

    Why do two colliding atom clouds keep returning to their starting shape in a harmonic trap but not in a distorted one?

    18 min

  95. Quantum & fields

    When Matter-Wave Solitons Collide

    What decides whether two colliding matter-wave solitons make a bright peak or a dark gap where they meet?

    18 min

  96. Quantum & fields

    Build an Atomic Corral

    Why does a ring of atoms on a metal surface make the surface electrons form rings of standing waves?

    18 min

  97. Quantum & fields

    A Quantum Mirage at an Empty Focus

    Can an atom at one focus of an elliptical corral change the electrons at the other, empty focus?

    18 min

  98. Quantum & fields

    A Giant Atom Hears Its Echo

    What happens when an atom coupled to a waveguide at two distant points meets its own emitted wave again?

    18 min

  99. Quantum & fields

    A Photon Cloud That Cannot Escape

    Why does an excited atom tuned into a waveguide's band gap keep part of its excitation instead of emitting it all?

    18 min

  100. Quantum & fields

    Dynamical Casimir Photon Pairs

    How can you tell that a modulated boundary emits photons in pairs, when each mode alone looks like thermal noise?

    18 min

  101. Quantum & fields

    Single-Atom Photon Blockade

    How can one atom in a cavity keep a second photon out once the first is inside?

    18 min

  102. Quantum & fields

    Andreev Reflection

    What comes back when an electron hits a superconductor with an energy inside its gap?

    18 min

  103. Quantum & fields

    Majorana End Modes

    Why does a superconducting wire with a gapped interior show zero-energy states at its ends?

    18 min

  104. Quantum & fields

    Airy Matter Waves

    How can the brightest crest of a free matter wave curve when no force acts on it?

    16 min

  105. Quantum & fields

    Matter-Wave Lens

    How can one short pulse slow or refocus a spreading matter wave?

    16 min

  106. Quantum & fields

    Quantum Billiards

    How does the shape of a quantum billiard change the pattern a reflected wave makes?

    18 min

  107. Quantum & fields

    Quantum Time Mirror

    Can a scattered wave be gathered back into exactly its starting state?

    18 min

  108. Quantum & fields

    Perfect Quantum State Transfer

    How can a chain with fixed couplings deliver an excitation to the far end with certainty?

    18 min

  109. Quantum & fields

    Rydberg Many-Body Scars

    Does an interacting chain of atoms always forget the pattern it started in?

    18 min

  110. Quantum & fields

    Aharonov–Bohm Quantum Cage

    How can a magnetic flux trap a particle in a diamond chain without walls?

    18 min

  111. Quantum & fields

    Repulsively Bound Atom Pair

    How can two atoms that repel each other move together as a pair?

    18 min

  112. Quantum & fields

    Dicke Superradiant Burst

    Why can a group of excited atoms release its light in one bright burst?

    18 min

  113. Quantum & fields

    Subradiant Atomic Array

    How can neighboring atoms keep an excitation from radiating away so quickly?

    18 min

  114. Quantum & fields

    Dirac Trembling Motion

    Why does a free Dirac wave packet tremble when no force acts on it?

    18 min

  115. Quantum & fields

    Klein Wave-Packet Tunnelling

    Why can a higher barrier let a Dirac wave packet through when a lower one reflects it?

    18 min

  116. Quantum & fields

    Kerr Cats & Wigner Interference

    How can one wave packet of light end up in several places at once?

    18 min

  117. Quantum & fields

    Quantum Backflow on a Ring

    Can the probability current on a ring flow backward when every momentum points forward?

    18 min

  118. Quantum & fields

    Quantum Walk & Phase Traps

    How does a quantum walk differ from an ordinary random walk?

    18 min

  119. Quantum & fields

    Quantum Kicks & Dynamical Localization

    If you keep kicking a quantum rotor, can its momentum stop spreading?

    18 min

  120. Quantum & fields

    Quantum Carpet & Revivals

    Why does a wave spreading in a box come back to its starting shape?

    18 min

  121. Quantum & fields

    Molecular Alignment Revivals

    Why do molecules line up again long after the laser pulse has passed?

    18 min

  122. Quantum & fields

    Matter-Wave Collision

    What happens when two clouds of condensed atoms overlap?

    20 min

  123. Quantum & fields

    Quantum Vortex Studio

    How can you tell that a hole in a condensate is a quantized vortex?

    20 min

  124. Quantum & fields

    Bose–Hubbard · Count the Fluctuations

    What happens to atoms in a lattice as their repulsion grows?

    25 min

  125. Quantum & fields

    Fermi–Hubbard · Read the Spin Neighbors

    Why do neighbors prefer opposite spins?

    25 min

  126. Quantum & fields

    Hofstadter Lattice · Split a Band

    How much flux does it take to turn one band into several?

    25 min

  127. Quantum & fields

    Magnetic Edge · Find the Boundary State

    Which states move along the boundary instead of filling the sample?

    25 min

  128. Quantum & fields

    How to Read a Particle Collision

    How do physicists reconstruct a collision they cannot see from what the detector records?

    18 min

  129. Quantum & fields

    How a Knot Becomes a Quantum-Field Observable

    Why does the expectation value of a Wilson loop depend only on the knot type, not on how the knot is drawn?

    18 min

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