Quantum-Biology Experiment Shortlist

Superseded by the redesigned protocol in A19, which starts from flavin fluorescence; kept here for the electrode-based variant.

Goal. Test whether weak magnetic fields, and radio-frequency (RF) fields tuned to disrupt radical pairs, change a fungus's response to blue light.

Setup.

• Organisms: Neurospora crassa as the photobiology model (well-mapped blue-light genes and a cryptochrome-family protein), plus Pleurotus ostreatus or Ganoderma for electrophysiology, where fungal electrode methods are established.
• Field control: triaxial Helmholtz coils inside a mu-metal enclosure to null Earth's field and apply static fields from 0 to ~1 mT; a separate RF coil inside a Faraday cage.
• RF: a narrowband tone at the electron Larmor frequency for the applied static field (~1.4 MHz at 50 µT), plus a broadband 0.1–10 MHz noise condition, the type of field shown to disrupt bird orientation.
• Light: blue LED (~450 nm), calibrated intensity, pulsed on a fixed schedule.
• Readouts: differential needle electrodes sampled at ≥1 Hz (fungal spiking); qPCR of fast light-response genes in Neurospora; carotenoid accumulation as a slower readout.

Procedure.

1. Grow replicate cultures under identical temperature and humidity; dark-adapt 24–48 h.
2. Randomly assign each culture to a field condition: Earth-field, null field, elevated static field, Larmor-tuned RF, broadband RF, sham.
3. Deliver identical light pulses; record electrical activity and sample tissue for gene expression at fixed times.
4. Repeat across independent batches and days.

Controls.

• Sham coils: double-wound (bifilar) coils carrying opposing currents, so heat and vibration match while the net field is zero.
• Experimenters blinded to condition; condition codes held by a third party.
• Dark controls under every field condition, to separate light-dependent from field-only effects.
• Off-resonance RF control, to test frequency specificity.

Analysis and decision rule. Preregister the primary endpoint (for example, light-induced gene-expression fold change). A real radical-pair effect should be light-dependent, sensitive to field strength, and abolished or altered specifically by Larmor-tuned RF. An effect that appears without light, or equally under off-resonance RF, points to an artifact.

Cost and timeline. Low; 6–12 months to a first answer.

Why it matters.

• If positive: a cheap, genetically tractable model organism for quantum biology. Labs could knock out candidate genes to identify which protein hosts the radical pair (a question still contested in birds), and it would suggest radical-pair chemistry is widespread across eukaryotes.
• If null: a clean bound that narrows where radical-pair sensing evolved, and a reusable rig for testing other organisms.

Goal. Determine, under field conditions, whether fungal networks linking trees carry resources or warning signals, the claim at the center of the Karst vs. Simard dispute.

Setup.

• Sites: at least three forest sites with established ectomycorrhizal trees (for example Douglas-fir stands, as in Simard's work), to separate site effects from network effects.
• Tree pairs: a mature or sapling "donor" and a seedling "receiver" 0.5–2 m apart.
• Barriers around each receiver's root zone, three types: fine mesh (~0.5 µm) blocking hyphae and roots; coarse mesh (~35 µm) passing hyphae but not roots; coarse mesh in cores rotated every few weeks, severing hyphae while keeping soil chemistry the same.
• Instruments: enclosure bags for volatile sampling; tissue sampling kits; isotope labeling chambers; trunk and soil electrodes.

Procedure.

1. Install barriers; allow a full season for networks to re-form through coarse mesh.
2. Confirm connection by genotyping fungi on donor and receiver roots.
3. Pulse-label donors with ¹³CO₂ to trace carbon movement.
4. Stress donors with a standardized trigger: mechanical defoliation, a methyl jasmonate application, or caged herbivores.
5. Sample receivers at fixed times (hours to weeks): defense enzymes (peroxidase, polyphenol oxidase), defense-gene expression, ¹³C content, volatile emissions; log electrical potentials continuously.

Controls.

• Unstressed donors under every barrier type.
• Volatile sampling and airflow barriers, so airborne signals aren't mistaken for underground ones.
• Root-only transfer accounted for by the rotated-core treatment.
• Analysts blinded to treatment; at least ~20 pairs per treatment per site, set by a power analysis.

Analysis and decision rule. Preregister primary endpoints. Network-mediated signaling is supported only if receivers behind hyphae-passing mesh respond more than those behind blocking or rotated mesh, with airborne routes ruled out. Ideally run as an adversarial collaboration, with researchers from both camps co-designing the protocol and agreeing in advance what result would change their minds.

Cost and timeline. Medium; 3–4 years including network establishment.

Why it matters.

• If positive: the first strong field evidence for underground warning signals, with direct consequences for forestry: retaining mother trees, thinning practices, replanting design.
• If null: corrects a popular claim that has spread through textbooks and media, and redirects funding toward mechanisms that do drive forest resilience.
• Either way: settles a public dispute with a protocol both sides accepted in advance, valuable for scientific credibility.

Goal. Record how a fungal network electrically responds to a parasitic fungus attacking one end, and whether any signal reaches distant tissue before the attacker or its chemicals do.

Setup.

• Host: Pleurotus ostreatus grown as a continuous strip (~30 cm) on agar or hemp substrate in a sealed tray.
• Attacker: a Trichoderma species known to parasitize oyster mushrooms (the cause of "green mold" on mushroom farms).
• Electrodes: differential needle pairs every 3–5 cm along the strip, logged continuously at ≥1 Hz.
• Chemistry: small volatile samplers (solid-phase microextraction fibers analyzed by GC-MS) at each electrode station; periodic tissue sampling for stress markers such as reactive oxygen species.
• Imaging: time-lapse camera to track the visible infection front.

Procedure.

1. Grow the host strip to full colonization; record a 48 h baseline.
2. Inoculate one end with a Trichoderma agar plug.
3. Record electrical activity, volatiles and front position until the attacker reaches the far end or the host is consumed.
4. Replicate across many strips and batches.

Controls.

• Sterile agar plug (inoculation handling only).
• Heat-killed Trichoderma (chemical cues without active attack).
• A non-parasitic fungus (competition without parasitism).
• Mechanical wounding at the same spot (damage without an organism).

Analysis and decision rule. Map spike rate, amplitude and baseline shifts against distance from the infection front and time to contact. An "early warning" signal requires distant stations to change before the visible front or detectable volatiles arrive, and to differ from the wounding and non-parasite controls.

Cost and timeline. Low; 3–6 months.

Why it matters.

• Basic biology: fungi have no nervous system, yet their electrical activity is real and poorly understood. A defined attack-and-response system gives the field a reproducible benchmark, like the stimulus experiments that built plant electrophysiology.
• Applied: green mold causes serious losses in mushroom farming. An electrical early-warning signature could become a cheap sensor for spotting infection before it is visible.
• If an early signal exists: strong evidence that fungal networks transmit information faster than chemical diffusion, a real step toward understanding "fungal communication" claims that currently outrun the data.

Goal. Find out whether a fungus's rejection of genetically incompatible "non-self" hyphae produces a distinct electrical signature, and how it relates in time to the programmed cell death that follows.

Setup.

• Organisms: Neurospora crassa or Podospora anserina strains with known compatibility genes (well-characterized model systems); optionally Pleurotus pairings that form visible rejection "barrage" lines.
• Plates: thin agar on electrode arrays (printed circuit boards with embedded electrode pairs, or microelectrode arrays) so the fusion zone sits between electrodes.
• Staining: a membrane-impermeable dye such as propidium iodide to mark dying compartments; fluorescence time-lapse microscopy.

Procedure.

1. Inoculate two colonies to meet over the electrode region: compatible pairs (same compatibility genes) and incompatible pairs (differing genes).
2. Record electrical activity continuously from first contact through fusion and, for incompatible pairs, compartment death.
3. Image fusion events and dye uptake on the same timeline.

Controls.

• Self-pairings (a colony meeting itself).
• Mutants with incompatibility genes deleted: rejection should vanish, and so should any rejection-specific signature.
• Mechanical cutting at the contact zone, to separate injury signals from recognition signals.

Analysis and decision rule. Align electrical events with fusion and cell death. A recognition signature must appear in incompatible pairings, be absent in compatible and deletion-mutant pairings, and differ from the injury control. Most informative is whether it appears before visible cell death; that would make it a candidate recognition signal rather than a side effect of dying cells.

Cost and timeline. Low; 3–6 months, faster in a lab already working with these fungi.

Why it matters.

• Self/non-self recognition is fundamental across life: immune systems, plant defense, fungal individuality. Fungi offer a genetically simple version with known genes.
• If positive: an electrical readout for a decision a fungus makes, linkable to specific genes, something fungal electrophysiology has lacked.
• Applied: compatibility systems limit the spread of fungal viruses between strains; a fast electrical readout could help screen strains in agriculture and biocontrol.

Goal. Test whether a photon emitted by a living organism is quantum-mechanically identical to a photon from a non-living source with the same properties, using Hong-Ou-Mandel (HOM) interference: two photons meeting at a beam splitter leave together only if nothing distinguishes them.

Setup.

• Biological source: a dense, oxygenated culture of luminous bacteria (Aliivibrio fischeri or Photobacterium, emitting ~490 nm) in a flow cell with continuous nutrient and air supply, so brightness stays steady for days.
• Collection: high-numerical-aperture optics into a single-mode fiber (one spatial mode); polarizer.
• Spectral filter: interference filter plus a temperature-tuned etalon narrowing to ~1 GHz, matching the reference source, the approach used in the 2019 sunlight–quantum-dot experiment.
• Reference sources: stage 1, a filtered thermal lamp (or sunlight) through an identical filter chain; stage 2, true single photons at the same wavelength (a quantum dot, or frequency-converted photons, since common quantum dots emit near 900 nm).
• Interference: fiber 50:50 beam splitter, two single-photon detectors (superconducting nanowire detectors preferred for low noise), time-tagging electronics with tens-of-picoseconds resolution.

Procedure.

1. Characterize the bacterial light: spectrum, brightness after filtering, stability over time.
2. Stage 1: interfere filtered bioluminescence with filtered lamp light; scan relative delay and polarization; record coincidences.
3. Repeat with lamp vs. lamp through the identical chain as the benchmark.
4. Stage 2: interfere filtered bioluminescence with single photons; measure HOM visibility.

Controls.

• Orthogonal polarizations (fully distinguishable), giving the zero-interference baseline.
• Detector dark counts and accidental coincidences subtracted.
• Same filter chain for every source, so any difference can't come from filtering.

Analysis and decision rule. Quantum mechanics predicts bioluminescent photons reach the same HOM visibility as thermal light under identical filtering (stage 1), and above the 50% classical limit against single photons (stage 2), within the limit set by multi-photon contamination. A visibility shortfall not explained by measured spectral, timing or polarization mismatch would indicate a hidden distinguishing property.

Cost and timeline. Medium; 6–12 months in a quantum-optics lab. The main difficulty is brightness: filtering to ~1 GHz discards most of the light, so expect runs of days.

Why it matters.

• Expected (null): the first quantum-optical characterization of biological light, and a decisive test of decades of "biophoton" coherence claims; it gives the field a hard benchmark.
• If a hidden tag existed: it would overturn the indistinguishability of identical particles, a pillar of quantum mechanics. The prior is extremely low, but the test is cheap relative to the stakes.
• Spin-off: methods to treat biological emitters as quantum-optics sources, relevant to biosensing with genetically encoded light sources.

Goal. Measure how biological light's photons cluster in time: thermal ("bunched"), laser-like (random), or non-classical ("antibunched"), with enough time resolution to give a definitive answer.

Setup.

• Source: the same luminous-bacteria flow cell as A5, which can share hardware.
• Filtering: narrowband filter to ~1 GHz. Bunching shows up only within the light's coherence time (~1 ns at this bandwidth), so wider filtering hides it, the likely reason earlier biophoton studies saw apparently random light.
• Measurement: Hanbury Brown–Twiss arrangement: light split 50:50 onto two single-photon detectors with low timing jitter (superconducting nanowire detectors at tens of picoseconds), and a time-tagger building the second-order correlation g²(τ) versus time delay τ.

Procedure.

1. Collect filtered bioluminescence into a single-mode fiber.
2. Record coincidences between detectors over hours to days, building g²(τ).
3. Repeat with a filtered thermal lamp and an attenuated laser through the identical chain.
4. Vary the filter bandwidth to show the bunching peak narrowing and shrinking as expected.

Controls.

• Laser reference: g²(0) = 1, flat; checks for detector artifacts.
• Lamp reference: the thermal benchmark.
• Detector afterpulsing and crosstalk measured and excluded (blocking one arm, swapping detectors).

Analysis and decision rule. Thermal light gives a peak at zero delay approaching g²(0) = 2 for a single spectral and spatial mode, reduced by multimode collection in a calculable way. Bioluminescence matching the lamp's curve after correction is the expected result. Any g²(0) below 1 (antibunching) or a laser-like flat curve would be a genuine anomaly requiring replication.

Cost and timeline. Low to medium; 3–6 months, mostly sharing equipment with A5.

Why it matters.

• Settles, with modern detectors, the decades-old claim that biological light is coherent or non-classical. A 2015 critical review found no reliable evidence either way, because earlier experiments lacked the time resolution to tell.
• Provides a reference measurement for anyone using bioluminescence in quantum optics or biosensing.
• If an anomaly appeared: it would point to collective emission effects in living cells, a major discovery, but one requiring extraordinary evidence.

Goal. Show with light that two kinds of fringe loss that look identical, reversible phase scrambling (dephasing) and genuine path recording (decoherence), can be told apart, using the correlation method proposed by Batelaan's group for electrons.

Setup.

• Light: a stable, narrow-linewidth laser (helium-neon or single-frequency diode).
• Slits: a precision double slit; fringe spacing at the camera of a few hundred micrometers.
• Chamber between slits and camera, with two interchangeable conditions: (a) heat shimmer, a heated wire or hot plate beneath the beam with controlled power; (b) smoke, fine aerosol from a calibrated generator at controlled density.
• Camera: fast scientific camera able to take thousands of frames with exposures short enough (~0.1–1 ms) to freeze the shimmer.

Procedure.

1. Record clean fringes (empty chamber).
2. Tune heater power until long-exposure fringe visibility drops to a target (say 30%).
3. Tune smoke density until long-exposure visibility matches the same target.
4. For each condition, capture thousands of short-exposure frames.
5. Compute the average image and the spatial correlation of intensity fluctuations across frames.

Controls.

• Static random phase plate: frozen dephasing: distorted fringes, fully recoverable.
• Empty-chamber frames processed identically, to check the analysis adds no artificial structure.
• Laser stability logged throughout.

Analysis and decision rule. The prediction: for heat shimmer, the correlation analysis restores the double-slit structure that the average washed out; for smoke, the fraction of light scattered out cannot be restored, and the remaining unscattered light shows full fringes on a background. A clean separation at matched visibility confirms the method optically.

Cost and timeline. Low; 2–4 months; suitable for an undergraduate lab or student project.

Why it matters.

• Gives experimentalists a practical test for whether lost contrast is truly gone or merely scrambled, directly relevant to matter-wave, electron and atom interferometers, where telling dephasing from decoherence is a recurring problem.
• Makes the central distinction of this whole series visible: "looks observed" versus "is observed."
• Check first whether an optical demonstration has already been published; none turned up in this review.

Goal. Rebuild the "asking photons where they have been" method (Danan, Vaidman et al. 2013) in the cascaded slit geometry from this series (slit wall, angled mirror, second slit wall, screen) and record which elements leave a trace in the detected light.

Setup.

• Light: laser; continuous light works (Danan used it), with an optional single-photon version using a heralded source.
• Elements: slit plate 1, angled mirror, slit plate 2, each on a piezoelectric mount that tilts it by a tiny amount at its own frequency (for example 280, 316 and 355 Hz, chosen to avoid harmonics overlapping).
• Detector: a quad-cell photodiode or fast camera at the final wall, so tilts show up as tiny beam displacements.
• Analysis electronics: lock-in amplifiers or digitized signals for Fourier analysis.

Procedure.

1. Align for clean fringes on the final wall; confirm vibrations are small enough not to wash out interference.
2. Record the detector signal; compute its frequency spectrum and identify which tag frequencies appear.
3. Repeat across configurations: each slot blocked in turn, and settings tuned so destructive interference blocks one route to the detector, the analogue of Danan's "inner interferometer" condition.
4. Compare each configuration's spectrum with quantum-mechanical predictions and with the two-state-vector account Vaidman uses.

Controls.

• Piezos driven with all slits open but beam blocked (electrical pickup check).
• Each piezo driven alone, to calibrate its signature.
• Frequencies shuffled between elements, to rule out frequency-dependent artifacts.

Analysis and decision rule. Standard quantum mechanics predicts which frequency peaks appear in each configuration; the experiment checks those predictions in a new geometry. The interpretive question, whether peaks from elements a photon "couldn't have passed" mean it was there, is what the data feed, not settle.

Cost and timeline. Low to medium; 4–8 months.

Why it matters.

• The 2013 result remains disputed; critics argue its traces have simpler explanations. Data in a different geometry, with a different nesting structure, tests whether the pattern of traces generalizes as each camp predicts.
• Sharpens the broader debate over what can be said about a quantum particle's past, central to interpreting weak measurements, which are increasingly used in precision sensing.
• Expected impact is moderate and interpretive, not new physics; it suits a group already working with weak measurements.

Goal. Measure whether a living cell acting as a partial which-path marker destroys interference any differently from a dead cell or a non-living particle with matched optical properties: a precise bound on any "life" effect in quantum measurement.

Setup.

• Interferometer: a stable Mach-Zehnder (two-path) interferometer, with attenuated laser or heralded single photons; both arms brought to a tight focus at one point.
• Sample stage: a microfluidic channel at the focus in one arm, with an optical trap holding a single object in the beam.
• Objects: live E. coli; the same strain heat- or UV-killed; beads matched in size (~1 µm) and refractive index as closely as possible (silica or index-matched hydrogel; polystyrene's index is too high).
• Detection: output-port detectors for fringe visibility; a side detector collecting scattered light to measure path information directly.

Procedure.

1. Calibrate visibility with an empty channel.
2. Trap one object; scan the interferometer phase; record visibility and scattered-light counts.
3. Replace the object; cycle through live, dead and bead in randomized, blinded order.
4. Confirm cell viability before and after each run (motility, membrane-integrity dye).
5. Collect hundreds of objects per class.

Controls.

• Size and index characterization of every trapped object (imaging plus calibrated scattering), so differences can be corrected for physical properties.
• Laser-heating checks; light dose kept below levels that harm the cells.
• Run order randomized against slow drifts.

Analysis and decision rule. For each class, fit visibility against measured scattering strength. Quantum mechanics predicts all three classes fall on the same curve. Report the bound on any life-dependent offset; the target is better than one part in a thousand.

Cost and timeline. Medium; 6–12 months; needs both optics and microbiology skills.

Why it matters.

• Turns a long-running speculation (that consciousness or life plays a special role in quantum measurement) into a measured number, done to a far higher standard than the consciousness double-slit studies that failed replication.
• If null (expected): a quotable, rigorous bound that closes off one family of claims.
• If not null: it would reshape physics and biology at once, so the design deliberately makes that result hard to obtain by accident.

Goal. Continuously tune how strongly two photons are entangled and show interference migrating from each photon's own pattern into the correlations between the pair: the trade-off behind the two-sided-box idea in this series.

Setup.

• Source: spontaneous parametric down-conversion (SPDC): a violet pump laser (~405 nm) through a nonlinear crystal (BBO or periodically poled KTP), producing photon pairs near 810 nm with correlated directions (momentum entanglement).
• Entanglement control: pump beam profile. A wide, collimated pump gives strong direction anticorrelation (strong entanglement); a tightly focused pump weakens it. Crystal length is a second dial.
• Two double slits, one in each photon's path, imaged or placed in the far field.
• Detection: two scanning fiber-coupled single-photon detectors, or single-photon cameras, plus a coincidence counter.

Procedure.

1. Characterize entanglement at several pump settings (measure the joint direction correlations directly).
2. At each setting, record single-photon patterns behind each double slit and two-photon coincidence patterns.
3. Add a which-slit detector on one side (for example, a polarization marker on one slit) and repeat.

Controls.

• Single-slit runs for the diffraction envelope.
• Accidental-coincidence subtraction.
• A separable-photon source (two independent attenuated lasers) as the zero-entanglement endpoint.

Analysis and decision rule. Plot single-photon fringe visibility against two-photon fringe visibility as entanglement varies. Theory predicts a trade-off: as one rises, the other falls. Confirmation within error across the full range is the expected result.

Cost and timeline. Low to medium; 3–6 months in any quantum-optics teaching or research lab.

Why it matters.

• A clear, continuous demonstration of how entanglement redistributes interference, the most intuitive way to show students and the public that "watching one particle changes the other" only when they're entangled.
• Directly answers the two-sided-box questions: monitoring one particle affects the other in proportion to the entanglement, and not at all when they're independent.
• Modest research novelty; strong teaching and outreach value.

Goal. Find out whether quantum superposition breaks down on its own above some mass, as collapse theories (CSL, Diósi–Penrose) predict, or keeps holding as standard quantum mechanics says.

Setup: two complementary routes.

• Route 1, interferometric: a long-baseline matter-wave interferometer (the Vienna Talbot–Lau design) sending ever more massive particles (large molecules, then metal clusters of 10⁵–10⁶ atomic mass units) through gratings, with detection by mass spectrometry or fluorescence.
• Route 2, non-interferometric: a silica nanoparticle (~100 nm) levitated in an optical, electrical or magnetic trap in cryogenic ultra-high vacuum, its motion cooled toward the quantum ground state. Collapse would add a tiny random jitter, detected as excess heating or force noise.

Procedure.

1. Route 1: measure fringe visibility as particle mass rises, with residual gas pressure, particle temperature and vibration measured and modeled.
2. Route 2: measure the particle's heating rate as gas pressure, trap frequency and temperature are varied.
3. Subtract every known decoherence source: gas collisions, thermal radiation, laser noise; collapse is whatever remains.
4. Translate any residual, or its absence, into bounds on collapse-model parameters.

Controls.

• Pressure scans (gas collisions scale with pressure; collapse doesn't).
• Particle size and material scans (collapse has a characteristic mass and size dependence).
• Independent noise characterization of lasers, traps and detectors.

Analysis and decision rule. Collapse models predict distinctive signatures: visibility loss scaling with mass squared, saturation with superposition size, a heating rate independent of gas pressure. A residual with those fingerprints that survives pressure and material scans would be evidence for collapse; otherwise the result is a tighter bound.

Cost and timeline. High; multi-year programs at specialized labs (Vienna, Southampton, Zurich and others). Each order-of-magnitude step in mass takes years.

Why it matters.

• Addresses the deepest open question in quantum foundations: why the everyday world looks classical. Collapse models are the main proposals that answer it with new, testable physics.
• If collapse is found: quantum mechanics would need modifying, among the most important physics discoveries possible, with consequences for quantum computing at scale.
• If not: each null shrinks the space of viable collapse models and extends quantum mechanics' verified reach. Several collapse parameter ranges, including ones proposed specifically to explain measurement, are already being excluded.

Goal. Put a biological object (first a protein, eventually a virus) into a superposition of positions and show interference, the closest realistic step toward "Schrödinger's microbe."

Setup: staged.

• Stage 1, proteins: a Talbot–Lau or optical-grating matter-wave interferometer. Proteins must be launched as a slow, neutral beam in high vacuum, the hardest step, since standard methods (electrospray, laser desorption) produce charged or fast molecules. Detection by mass spectrometry or post-ionization.
• Stage 2, viruses or small bacteria: proposals use optical levitation of the organism in a high-finesse cavity in vacuum (Romero-Isart et al.), or placing a cryopreserved bacterium on a superconducting mechanical oscillator cooled to its quantum ground state (Li and Yin).

Procedure (stage 1).

1. Develop a neutral, slow, mass-selected protein beam, the main engineering milestone.
2. Pass it through the interferometer; scan the grating to map fringes.
3. Measure visibility against velocity, internal temperature and gas pressure, and compare with decoherence models.

Controls.

• Velocity-resolved data to separate quantum interference from classical shadow (moiré) effects, the standard proof in these interferometers.
• Pressure and laser-heating scans to quantify decoherence.
• Structure checks (does the protein stay folded after launch?), relevant to claims about a "biological" superposition.

Analysis and decision rule. Interference is shown when fringe visibility exceeds the classical shadow prediction and varies with velocity as quantum theory predicts. For organisms, the proposals' success condition is ground-state cooling plus a verified superposition of the center-of-mass motion.

Cost and timeline. High; proteins plausibly within several years at leading labs; viruses and bacteria likely a decade or more. Heating from light absorption is the main obstacle for optical levitation.

Why it matters.

• Moves the quantum–classical boundary into biology, giving the question "can a living thing be in two places?" an experimental answer instead of a thought experiment.
• Raises sharper questions, such as whether a biomolecule's internal structure (folding state) acts as a which-path marker, directly testable here.
• Feeds collapse tests (A11) with a new class of massive, complex objects.
• Public and cultural impact would be very large; the first superposition of a living organism would be a landmark result.

Goal. Extend the 2025 single-atom version of Einstein's recoiling-slit experiment to heavier movable elements, mapping how interference fades as the recoiling object grows from one atom toward something visible.

Setup: a ladder of recoiling elements.

• Rung 1: a single atom in an optical tweezer, cooled to its motional ground state (done by Pan's group, PRL 2025), the calibration point.
• Rung 2: a small chain of trapped ions, or a heavier atom, giving a smaller recoil relative to its momentum uncertainty.
• Rung 3: a levitated nanoparticle or nanomembrane mirror cooled to near its ground state, using the optomechanics of A11.
• Interferometer: single photons (or weak pulses) whose two paths differ by which way they kick the element; detectors at the outputs measure visibility.

Procedure.

1. For each rung, characterize the element's momentum uncertainty (trap frequency, temperature).
2. Send single photons through; measure visibility while tuning trap stiffness, which changes the momentum uncertainty.
3. Record visibility as a function of the ratio between the photon's recoil and the element's momentum spread.

Controls.

• Visibility with the element removed or replaced by a fixed mirror.
• Thermal-occupation scans (warmer element, larger momentum spread, expected higher visibility).
• Classical vibration noise measured separately so it isn't mistaken for recoil-induced loss.

Analysis and decision rule. Quantum mechanics predicts visibility equals the overlap between the element's two recoiled states: a single curve against the recoil-to-spread ratio, the same for every rung. Data collapsing onto that curve confirms Bohr's resolution at larger scale; systematic deviation that grows with mass would be a collapse-model signal (A11).

Cost and timeline. Medium for rung 2, high for rung 3; several years.

Why it matters.

• Turns a famous 1927 debate into a quantitative curve spanning many orders of magnitude in mass, exactly where the line between "quantum object" and "classical recorder" should show if it exists.
• Provides a calibrated, tunable which-path detector useful for foundational tests and quantum sensing.
• Links directly to collapse tests: a mirror large enough to matter for gravity-related collapse (Diósi–Penrose) is the long-range goal.

Goal. Mark an optical photon's path coherently in a superconducting qubit, show interference disappears, then erase the record by measuring the qubit differently and recover the interference, linking light and superconducting quantum hardware in one foundational test.

Setup.

• Optical side: an interferometer for single photons at a telecom or near-infrared wavelength suited to the transducer.
• Transducer: a device converting optical photons to microwave photons while preserving quantum states: electro-optic (lithium niobate), piezo-optomechanical, or electro-optomechanical. This is the bottleneck: current devices lose most photons and add noise.
• Superconducting side: a transmon qubit in a dilution refrigerator (~10 mK), coupled so the photon's presence in one interferometer path flips or phase-shifts the qubit.
• Detection: superconducting nanowire detectors on the optical outputs; standard dispersive readout of the qubit.

Procedure.

1. Benchmark transducer efficiency and added noise; set a minimum threshold before attempting the experiment.
2. Run the interferometer with the qubit coupled to one path; measure the qubit in its "which-path" basis; confirm fringes vanish.
3. Measure the qubit in the complementary basis instead; sort photon detections by the qubit result; confirm fringes reappear in each sorted subset.
4. Optionally delay the qubit-basis choice until after photon detection (delayed-choice version).

Controls.

• Qubit decoupled (fringes present).
• Overall unsorted pattern (must show no fringes even with erasure; a key teaching point).
• Transducer noise characterized with the optical input blocked.

Analysis and decision rule. Visibility with the qubit coupled should follow the entanglement–visibility relation demonstrated in superconducting circuits (Huang et al. 2023), with sorted subsets recovering fringes. Success means visibility recovery above the noise floor set by the transducer.

Cost and timeline. High; 3–5 years, gated on transducer progress. Groups building quantum transducers are natural hosts.

Why it matters.

• Every step needed here (converting a photon's quantum state into a superconducting qubit and back) is the same capability needed to network superconducting quantum computers over optical fiber. A clean eraser is a vivid benchmark of that capability.
• Foundationally predictable, but the first eraser spanning these two platforms would be a notable demonstration.
• Shows directly that a superconducting "recorder" can hold path information reversibly, the opposite of the self-communicating-system intuition behind the superconductor idea in this series.

Goal. Run extended Wigner's-friend experiments, where an inner "friend" makes a measurement that an outer observer later reverses, with friends of increasing complexity, testing the Local Friendliness inequalities (Bong et al. 2020) as the observer grows from a single photon toward a computational agent.

Setup: a ladder of friends.

• Rung 1 (done): photonic experiments where a photon's path plays the friend.
• Rung 2: a quantum processor (superconducting or trapped-ion) where the friend is a register of qubits that records a measurement outcome and can be coherently reversed by the outer "superobserver."
• Rung 3: larger registers running a simple program that processes the outcome (stores, copies, computes on it) before reversal.
• Long range: the proposal by Wiseman, Cavalcanti and Rieffel of a human-level AI running on a large quantum computer as the friend.
• Two separated labs (or two isolated processor partitions) with random measurement-setting choices, as in a Bell test.

Procedure.

1. Prepare an entangled pair shared between the two labs.
2. In each lab, the friend register measures its half and records the result.
3. Each superobserver randomly either asks the friend's result or reverses the friend's memory and measures in another basis.
4. Collect statistics and evaluate the Local Friendliness inequalities.
5. Repeat with larger and more complex friend registers.

Controls.

• Gate-fidelity and reversal-fidelity benchmarks (imperfect reversal mimics a violation's absence).
• Randomness of setting choices verified; timing analyzed for locality loopholes.
• Simulations of each circuit including the processor's noise model.

Analysis and decision rule. Quantum mechanics predicts violation of the Local Friendliness inequalities whenever reversal is good enough. The quantity of interest is whether violation persists as the friend grows, and how the required reversal fidelity scales with friend complexity.

Cost and timeline. Rung 2 is feasible now on existing quantum processors (medium). Rung 3 needs years; the AI-friend version is decades away.

Why it matters.

• Local Friendliness assumptions are weaker than Bell's; their violation rules out more interpretations of quantum mechanics, including some popular "quantum-causal" ones. Each larger friend tightens the constraint.
• It is the rigorous version of the observer-chain question raised in this series (the rupture-and-loudspeaker idea): what counts as an observer, and whether reversible observations count as facts.
• Practical spin-off: reversal-fidelity benchmarks for quantum processors.

Goal. Build an interference experiment where every component is biological (the light source, the grating and the detector) as a museum or classroom piece that makes wave optics and biology visible together.

Setup.

• Source: a sealed flask of luminous bacteria (Aliivibrio fischeri), shaken or aerated to stay bright, behind a pinhole so the light is spatially coherent enough to diffract.
• Grating: a cleaned diatom shell (Coscinodiscus valve), whose regular pore array acts as a natural diffraction grating, mounted on a microscope slide. A color filter narrows the bacterial light's spectrum for sharper features.
• Detector (choose one): (a) a dense film of photosynthetic algae, with the diffraction pattern read as a pattern of photosynthetic activity using a chlorophyll-fluorescence imager; (b) an ethical retinal preparation read by electroretinogram, for a research version; (c) a sensitive camera alongside, to verify the pattern for viewers.

Procedure.

1. Validate the optics first with a laser at the bacteria's wavelength (~490 nm), confirming the diatom produces a clear pattern at the chosen distance.
2. Swap in the bacterial source; integrate long enough (minutes to hours) for the dim light to register.
3. For the algae detector, expose the film to the pattern, then image fluorescence to read the "biological photograph."

Controls.

• Pattern with the diatom removed (no grating structure).
• Uniform illumination of the algae film to calibrate its response.

Analysis and decision rule. Success is a recognizable diffraction pattern recorded in living tissue matching the camera's image of the same pattern.

Cost and timeline. Low; a few months, suitable for a science museum, art-science collaboration or student project.

Why it matters.

• No new physics, but high communication value: shows that life runs on, and can display, the same wave optics as lab equipment, an intuitive answer to "does life change quantum behavior?"
• Diatom shells are an active area in photonics; a public piece can draw attention to bio-inspired optical materials.
• The algae-detector readout is a genuinely unusual demonstration that could become a teaching module.

Goal. Demonstrate that interference patterns are written by amplification events (a metastable medium "popping" where a particle lands) and read only through sound, as in the rupture-chain idea.

A feasibility problem to flag first. A true single-particle version is very hard. Particles that nucleate bubbles (neutrons, charged particles) have wavelengths so short that double-slit fringes are micrometers apart, while bubble positions are located to about a millimeter. The fringes would be far finer than the detector can resolve. Two practical versions follow.

Version 1: neutron research version (hard).

• Cold-neutron beam at a research reactor; neutron double slits have been demonstrated before.
• Long flight path and magnifying neutron optics to stretch the fringe spacing.
• Superheated-droplet detector gel (the material used in neutron bubble dosimeters) as the screen; piezoelectric sensors localize each pop, cross-checked by camera.
• Realistic only as a proposal to a neutron-optics group; resolution remains the limiting factor.

Version 2: acoustic-readout teaching version (feasible).

• Pulsed laser through a double slit onto a thin absorbing film in water or gel.
• Each pulse's absorbed pattern makes a tiny pressure wave (the photoacoustic effect); an array of acoustic sensors reconstructs where the light was absorbed.
• Accumulate many pulses; reconstruct the fringe pattern from sound alone, then compare with a camera image.
• Uses classical light, so it demonstrates amplification-and-acoustic readout, not single-quantum detection.

Controls. Single-slit and blocked-slit runs; camera ground truth; acoustic reconstruction tested on a known pattern first.

Analysis and decision rule. Success is an acoustically reconstructed pattern that matches the optical one, including the switch from fringes to no fringes when one slit is blocked or marked.

Cost and timeline. Version 2: low to medium, 3–6 months. Version 1: high, and resolution-limited.

Why it matters.

• Makes concrete a point the series kept returning to: every detector is an amplifier, and the record forms at the first irreversible event, not when someone looks or listens.
• Photoacoustic imaging is a growing medical-imaging technique, so version 2 doubles as an accessible introduction to it.
• Pairs naturally with A7 in a teaching sequence: one shows when information is lost, the other when it is recorded.

Goal. Decide whether reported differences between lithium-6 and lithium-7 in the brain come from nuclear spin (a quantum effect) or from mass (ordinary chemistry), by testing whether the isotope difference depends on magnetic field.

Background. Lithium-6 and lithium-7 differ in mass and nuclear spin. Rat studies report behavioral differences, and a 2025 preprint reports large, opposite isotope effects on hippocampal synaptic activity. A 2026 computational study found a spin-dependent flavin–vitamin C radical-pair mechanism could produce effects of the right size, but untested. Isotope effects can also arise from mass alone, so the open question is mechanism.

Setup.

• Tissue: acute rat hippocampal slices (CA1 region), the preparation used in the 2025 preprint.
• Recording: multi-electrode array measuring field excitatory postsynaptic potentials and long-term potentiation.
• Drugs: isotopically pure ⁶LiCl and ⁷LiCl (≥95% enrichment), natural LiCl, and NaCl vehicle, at matched concentrations; solutions coded by a third party.
• Magnetic control: the recording chamber inside Helmholtz coils (or between adjustable permanent magnets) giving fields from near-zero (shielded) through Earth-strength to ~100 mT, plus a sham-coil condition.
• Optional chemistry arm: an in-vitro flavin–ascorbate radical-pair assay with each isotope, read by fluorescence or electron spin resonance, to test the computational prediction directly.

Procedure.

1. Replicate first: reproduce the isotope difference in synaptic activity at ambient field, in a lab independent of the original authors.
2. If it replicates, repeat at several field strengths, randomized across slices and days.
3. Run the in-vitro radical-pair assay across the same field range.
4. As a mass control, test whether a different isotope pair with no difference in nuclear spin but a similar relative mass difference produces any effect (choose an available pair with suitable chemistry).

Controls. Blinded drug coding; vehicle and natural-lithium arms; slice-health criteria fixed in advance; coil heating and vibration matched by sham coils.

Analysis and decision rule. A spin mechanism predicts an isotope difference that changes with field strength and typically saturates at high field. A mass mechanism predicts a field-independent difference. The preregistered primary test is the isotope × field interaction.

Cost and timeline. Low to medium; 9–15 months including independent replication.

Why it matters.

• If spin-dependent and replicated: the first demonstrated nuclear-spin quantum effect on mammalian brain function, moving quantum neurobiology from theory to measurement.
• Clinical: a mechanistic clue to lithium, used for bipolar disorder for over 70 years without a settled mechanism, and a possible new lever: tuning a drug's action by isotope composition.
• If mass-dependent or not replicated: closes a high-profile hypothesis cleanly and redirects the field.

Goal. Test, step by step, whether light-driven flavin chemistry in a fungus responds to magnetic fields, and whether that response reaches the fungus's physiology. This supersedes the electrode-first design in A1 with a cleaner readout chain.

Background. A 1974 study found no magnetic-field effect on Neurospora growth or daily rhythm, but used no light-specific or RF readouts. Since then, Woodward's group showed flavin autofluorescence in living cells drops under a magnetic field, and fly work showed raised FAD alone makes neurons respond to blue light with magnetic potentiation. Fungal blue-light receptors are FAD-based, so the chemistry should be present.

Setup.

• Strains: wild-type Neurospora crassa; a mutant lacking the main blue-light receptor (WC-1); a mutant lacking the cryptochrome-family gene; optionally a strain with raised cellular FAD.
• Stage 1 microscope: a fluorescence microscope with blue excitation and an electromagnet that switches the field on and off every few seconds (Woodward-style), so field effects appear as a modulation locked to the switching.
• Stage 2 field chamber: triaxial coils in a mu-metal enclosure for Earth-strength and null fields, plus an RF coil (Larmor-tuned and broadband), as in A1.
• Readouts: autofluorescence (stage 1); fast light-induced gene expression by qPCR, and carotenoid accumulation (stage 2).

Procedure.

1. Stage 1 (chemistry in living cells): image hyphae under blue light while switching a ~10–100 mT field; measure field-locked fluorescence change in each strain.
2. Map the effect against field strength and light intensity.
3. Stage 2 (physiology): if stage 1 is positive, expose dark-adapted cultures to blue light under null, Earth-strength, elevated and RF conditions; measure gene expression and pigment.
4. Compare wild-type with mutants at every stage.

Controls. Field-off epochs interleaved with field-on; dark controls; sham coils; blinded condition coding; temperature logged at the sample.

Analysis and decision rule. Stage 1 succeeds if a field-locked fluorescence change appears, scales with field as radical-pair models predict, and depends on light. Stage 2 succeeds if light responses shift with field and with Larmor-tuned RF, and the shift disappears in the relevant mutant.

Cost and timeline. Low; stage 1 in 3–6 months, stage 2 in a further 6–9.

Why it matters.

• Stage 1 positive (likely): extends direct evidence of radical-pair chemistry in living cells to fungi.
• Stage 2 positive (the real discovery): a genetically tractable model organism for magnetobiology. Knockouts could identify the sensing molecule (still contested in birds) and show radical-pair sensing is widespread in multicellular life.
• Stage 2 null: a clean bound showing the chemistry exists but isn't used, which itself constrains evolutionary stories of magnetoreception.

Goal. Determine whether general anesthesia acts through a quantum optical property of microtubules, or only through ordinary effects of microtubule binding, by testing whether collective light emission tracks unconsciousness better than classical markers do.

Background. Wiest's group (2024) found a microtubule-stabilizing drug, epothilone B, delayed isoflurane-induced unconsciousness in rats by about 69 s on average, citing work that anesthetics dampen microtubule quantum optical effects. Babcock et al. (2024) predicted collective "superradiant" emission from large tryptophan networks in microtubules and measured fluorescence efficiency rising with network size. Microtubule involvement does not by itself show a quantum mechanism.

Setup.

• Arm 1 (replication): rats, epothilone B or vehicle, isoflurane at controlled concentration; loss of righting reflex timed by blinded observers; group sizes from a power analysis on the 2024 effect.
• Arm 2 (optics): tubulin, assembled microtubules, cultured neurons and brain slices; ultraviolet excitation of tryptophan; time-resolved fluorescence (lifetime and quantum yield) to quantify superradiant signatures.
• Arm 3 (isotope check, optional): if the reported xenon nuclear-spin result is confirmed in the literature first, compare spin-carrying and spin-free xenon isotopes, with magnetic-field control (same logic as A18).

Procedure.

1. Replicate the epothilone B delay in an independent lab.
2. In arm 2, measure superradiant signatures across a range of anesthetic concentrations, with and without microtubule stabilizers, and with non-anesthetic control gases matched for solubility.
3. Test whether the optical signature's concentration curve matches the behavioral anesthesia curve.
4. Run classical comparators in the same samples (microtubule dynamics, membrane and ion-channel markers) to see which tracks unconsciousness best.

Controls. Blinding and randomization in animal work; non-anesthetic gases (anesthetic-like solubility, no anesthetic action); tryptophan-free or depolymerized controls for optics; UV-dose limits to avoid photodamage.

Analysis and decision rule. Supported only if the optical signature changes at anesthetic concentrations, is unaffected by non-anesthetic gases, is rescued by microtubule stabilizers in parallel with behavior, and predicts unconsciousness better than classical markers. Any one of these alone is insufficient.

Cost and timeline. Medium; 1–2 years across arms.

Why it matters.

• If supported and replicated: reshapes understanding of how general anesthesia works (used in millions of operations, mechanism still unsettled) and gives quantum theories of consciousness their first physical foothold.
• Clinical: better prediction of anesthesia resistance, for example in patients on microtubule-targeting chemotherapy, which the 2024 paper flags.
• If not supported: separates the microtubule role from the quantum claim, tightening a debate that has run for decades on thin data.

Goal. Make a living animal the detector at the slits, then take away its awareness while the physical record stays in place, to ask whether it matters that path information merely exists or is actually known.

Background. In physics a rock counts as a detector: a photon leaves a physical trace in it, even though the rock knows nothing. On that view, only the existence of path information removes the stripes. Von Neumann and Wigner suggested instead that consciousness might play a role in measurement. An animal whose awareness can be dimmed while its eye keeps working puts a physical record and a known result as far apart as an experiment can.

Hypotheses.

• H1, existing is enough (what physics predicts). The final wall looks the same whether the path information reaches an awake animal, a sleeping one, an anesthetized one, a dead retina or a rock.
• H2, knowing matters. The wall pattern or its hidden stripes change with awareness, in a way the physical record can't explain. A change in the raw wall pattern could allow faster-than-light signaling, so this would mean breaking quantum mechanics as used today and needs the strongest evidence.
• H3, the record fades in stages (an open question in biology). As awareness falls, the signal from a single photon stops at some point between retina and cortex, at different points for sleep, anesthesia and low oxygen. This needs no new physics, and any answer teaches something.

Setup.

• Photons: entangled pairs. One photon passes through the double slit to a single-photon camera, the final wall. Its twin carries the path information to the detector. An eye absorbs the photon it detects, so the photon reaching the wall can't be the one the eye sees.
• Living detector: a small animal, head stabilized, with the twin's path aligned to its open eye. Rod cells already respond to single photons, but most photons are lost before reaching them, so start with unmodified animals and add a genetically engineered, more sensitive retina if it becomes available.
• Neural recording: electrodes at the retina, visual thalamus and visual cortex, time-stamped against each photon, with brain state confirmed by EEG. Single-photon signals are tiny and will need averaging over many events.
• Two ways of delivering the twin: path-revealing, where each path lands on a different spot on the retina; and path-scrambling, where a beam splitter mixes the paths first, so the eye's response can't reveal which slit was used.
• Isolation: the animal sits on a separate platform and receives the twin by optical fiber, so breathing, heartbeat and movement can't shake the optics.
• Stand-ins for the animal: a lab single-photon detector, a rock, an isolated living retina and a dead retina.

Procedure.

1. Establish the baseline with the lab detector and the rock.
2. Take the same animal through a ladder of states in randomized blocks: awake, natural sleep, light anesthesia, deep anesthesia, recovery.
3. In the path-revealing arrangement, measure the raw wall pattern in each state.
4. In the path-scrambling arrangement, sort the wall dots by the animal's response at each recording level, then measure the hidden stripes in each state.
5. Add milder stressors: brief, reversible low oxygen, and naturally parasite-infected animals compared with matched uninfected ones.
6. Answer the dying question with tissue instead of a dying animal: an isolated retina as its oxygen is withdrawn until it stops responding, then a dead retina, then a rock.

Controls.

• Reversal: any effect of a state must disappear on recovery.
• Blinding and randomization: analysts don't know the state, and animal and rock sessions are mixed at random. Analysis plans are preregistered.
• Motion replay: recorded breathing and heartbeat motion played back mechanically, with no animal present.
• Readout calibration: test pulses with a known path show how far the hidden stripes should fade simply because the neural signal gets noisier.
• Adversarial collaboration: proponents and skeptics agree on the design and the decision rule before any data are taken.
• Animal welfare: ethics review for every stage, no slow deaths and no unrelieved pain. Stages that use tissue come first, and live-animal work goes ahead only if they justify it.

Analysis and decision rule.

• H1 is supported if the raw wall pattern stays the same across all states within a preregistered bound, and the hidden stripes fade only as far as the readout calibration predicts.
• H2 is supported only if a change follows awareness and not the quality of the readout, reverses on recovery, is absent with the rock, survives blinding and replicates in an independent lab.
• H3 is answered either way, by reporting where along the chain the photon-linked signal stops in each state.
• What a flat result can't show: that a mind plays no role at all. If a mind completes a measurement in a way that leaves the statistics unchanged, this experiment wouldn't see it. It would show that any such role leaves no trace here.

Cost and timeline. High; multi-year. Rock and retina stages come first, and whole-animal work only after they succeed.

Why it matters.

• If H1 holds: turns "does the observer have to be conscious?" into a measured bound using a living detector, and gives experimental weight to the answer that a rock and a mind are alike at the wall.
• If H2 holds: one of the largest results in physics, since it would mean quantum mechanics needs revising. An experimental error would have to be ruled out first.
• For H3, whatever the physics shows: a new way to study how signals fail as awareness fades, relevant to anesthesia, sleep, coma and cardiac arrest, and possibly better light-sensing proteins for imaging.