What if large, messy objects could still act quantum? We welcome you, friends and fellow curious minds. Today we unpack a 2025 study that argues quantum rules can survive all the way up, even when our measurements get fuzzy and particles go missing. Stick with us to the end, and you’ll see why this changes how we think about the world you touch every day.
Can big systems stay quantum under real-world conditions?
We’ve long told a simple story. Microscopic things behave quantum. Macroscopic things look classical. Somewhere in between, “decoherence” washes out the weirdness. Case closed—right?
Not so fast. A new peer-reviewed analysis shows that under realistic coarse-grained measurements, large systems can keep the full quantum formalism. Not just math tricks. They can even violate Bell and Leggett–Garg inequalities in the macroscopic limit—despite noise, particle losses, and limited sensor resolution. That is a curveball to the classic narrative.
What actually changes when we scale up?
When we build bigger systems, two things kick in:
Coarse-graining: Our detectors blur fine details. They resolve only big intensity sums, not single particles. The practical resolution scales like the square root of the number of particles. We write that as .
Environment and losses: Particles decohere. Some never reach the apparatus. Yet the 2025 framework builds these effects into the model and still recovers a quantum description at scale.
And here’s the twist. Many “classical limit” arguments quietly assume IID data: independent and identically distributed microscopic pairs, repeated over and over. Real macroscopic samples aren’t like that. They’re strongly interacting, highly correlated, and far from IID. Drop that assumption, and macroscopic quantum correlations can survive. That’s the big aha.
What does the new 2025 study actually show?
The paper by Miguel Gallego and Borivoje Dakić (received 9 December 2024; accepted 3 June 2025) develops a unified “macroscopic limit” where:
- You perform only collective, coarse-grained measurements.
- The detector reads an intensity (a sum of single-particle outcomes).
- Resolution is limited to the √N scale.
- The system can suffer local decoherence and random particle loss.
- The measurement is modeled with Kraus operators acting in a limit Hilbert space (think of an effective quantum system emerging for the macroscopic data stream).
Crucially, they show the Born rule, superposition, and measurement incompatibility survive in the limit. Then they go device-independent: they build Bell and Leggett–Garg tests that still violate classical bounds in this macroscopic setting.
Plain-English snapshot (from our wheelchair-friendly chalkboard):
- We don’t need microscopic, single-particle resolution.
- It’s enough to measure fluctuations of big totals.
- If correlations aren’t IID at the micro-level, macroscopic quantum features remain visible.
- So “classicality by size alone” isn’t guaranteed.
How do decoherence and coarse-graining compare?
Here’s a quick map you can bookmark.
| Mechanism | What it is | Scale parameter | Standard expectation | What the 2025 framework finds |
|---|---|---|---|---|
| Decoherence (environment) | Loss of phase info via environment coupling | Interaction strength, time | Drives classicality (pointer states) | Still compatible with a macroscopic quantum limit under coarse-grained readout |
| Local decoherence | Independent channels on each particle | Per-particle noise rate | Destroys delicate many-body states | Robustness proven for one-step macroscopic measurements (MQB₁) |
| Coarse-graining | Detectors resolve only totals with blur ~√N | Should force classical statistics | Quantum formalism preserved; nonclassical correlations still testable |
What’s inside the macroscopic model?
- System: Many identical subsystems (qubits in examples). State lives in a big Hilbert space; losses map it into a Fock-like direct sum.
- Measurement: A single-particle observable is chosen, but the apparatus reads the sum over all particles (the “intensity”). The pointer’s position gets shifted by that total.
- Macroscopic limit: You rescale outcomes by an affine transformation so distributions converge, just like the central limit theorem tells us.
- Concrete example: Restrict to a small Dicke subspace of the N-qubit Hilbert space. In the limit, the data behave like a quantum harmonic oscillator measured by smooth Gaussian POVMs. This gives MQB (macroscopic quantum behavior), proven for one step robustly, and argued more broadly for multi-step sequences.
Can we still violate Bell and Leggett–Garg at scale?
Yes. The study derives macroscopic experiments that violate both families of inequalities.
Bell–CHSH (spatial): Split a macroscopic source into two parts. Each side performs coarse-grained measurements. The resulting correlations, in the macroscopic limit, do not admit a local hidden-variable model. That’s macroscopic nonlocality.
Leggett–Garg (temporal): Send the same macroscopic system through two coarse-grained measurements in time. Using a specific superposition of low-lying number states, the study reaches with a reported value around 2.42, beyond the macrorealist bound of 2. Settings are built from Pauli-plane directions.
A quick digest of the numbers you’ll want handy:
| Quantity | Reported value / detail | Why it matters |
|---|---|---|
| Coarse-graining scale | Quantum survives at this resolution | |
| Leggett–Garg CHSH | ≈ 2.42 > 2 | Rules out macrorealism under coarse-grained reads |
| Peer-review status | Accepted 3 June 2025 | Signals scientific vetting |
Where might we test this for real?
Two platforms look promising right now:
- Atomic memories in optical cavities. They already show entanglement-enhanced interferometry.
- Bose–Einstein condensates with collective spin readout. Coarse-grained detection is natural here.
We can aim for fluctuation measurements rather than single-particle resolution. That’s the actionable recipe for macroscopic quantum tests.
Does this kill the correspondence principle?
No. It refines it. Bohr’s idea—quantum must recover classical in the macroscopic limit—still guides us. But how you take that limit matters. If your detectors blur too much (well beyond the √N scale), classicality should reappear, as coarse-graining arguments by Kofler and Brukner suggest for finite-dimensional systems. The 2025 work shows a wide middle ground where quantum structure stays intact under realistic noise and resolution. The precise border is subtle and system-dependent. That honesty is good science.
Key terms you can use (and why they matter)
- Macroscopic quantum behavior (MQB): A property showing that limit data obey the Born rule and exhibit incompatible measurements, even at large N.
- Dicke states: Symmetric many-qubit states used to construct tractable macroscopic limits.
- Kraus operators: The mathematics of general quantum measurements; here they define the coarse-grained readout.
- Non-IID correlations: Realistic many-body correlations that don’t look like independent coin flips. Essential for macroscopic nonlocality.
Practical checklist for your mental toolkit
- Start from fluctuations, not microscopic clicks.
- Expect √N resolution for big detectors.
- Don’t assume IID when many particles interact.
- Use Bell and Leggett–Garg tests as sanity checks in the macroscopic limit.
- Remember: “large” doesn’t automatically mean “classical.”
Our FreeAstroScience take
We write this for commuters, caregivers, coders, and kids—anyone who wonders if reality hides more than meets the eye. From our shared desk, wheels locked and tea cooling, we felt an honest jolt reading this work. The aha was simple: you don’t need perfect, microscopic control to see quantum structure. You just need to look at the right collective variables in the right scaling limit. That’s empowering.
FreeAstroScience exists to make that feeling common. We translate tough physics into accessible steps. We also remind you—gently but firmly—never to switch off your mind. Because the sleep of reason breeds monsters.
Sources and further reading
- Quantum theory at the macroscopic scale — Gallego & Dakić, Proc. R. Soc. A 481 (2025). The technical backbone with proofs, models, and explicit inequality violations.
- Correlazioni su scala macroscopica in fisica quantistica — a clear Italian overview on why dropping IID matters and how fluctuations reveal macroscopic quantum behavior.
Conclusion
Big systems don’t have to play classical. With coarse-grained sensors and real-world noise, quantum structure can persist and even signal itself through Bell and Leggett–Garg violations. The lesson is practical and philosophical: size isn’t destiny; assumptions are. Keep watching your fluctuations; they might be whispering quantum truths. Come back to FreeAstroScience.com when you’re ready for the next layer—we’ll keep you thinking, awake, and fearless.
Scholar articles
Quantum theory at the macroscopic scale
M Gallego, B Dakić - Proceedings of the Royal Society A, 2025
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