Two Frontiers of Quantum Simulation
Two 2024 papers, reproduced & extended — the analog face (mimic the physics with a stand-in physical system) and the digital face (run it as a quantum circuit) of quantum simulation, both on one core engine: our differentiable spectral solver.
False-vacuum decay, on a tabletop
Quantum nucleation of true-vacuum bubbles from a metastable "false vacuum" — the early-universe process behind eternal inflation, electroweak baryogenesis, Higgs metastability and gravitational-wave backgrounds — realized as an analog in a two-component ⁴¹K Bose–Einstein condensate.
- The condensate's relative phase φ obeys a relativistic Klein–Gordon equation (a relativistic wave equation) in a double-well potential.
- The "speed of light" is the sound speed c = √(gn/m) ≈ 6 mm/s — relativity, at walking pace.
- A metastable well at φ = π appears only when the modulation λ > 1; below that it vanishes.
- φ — the field: the relative phase between the two BEC species (the ball's position on the landscape).
- φ₀ — the yardstick: a fixed scale (√(ℏ²n/2m)); we plot φ/φ₀, so vacua land at 0, π, 2π…
- λ — the knob: the modulation strength that shapes the landscape; λ>1 creates the false-vacuum dip.
Spectral split-step + the correct quantum vacuum
We evolve the condensate with our own spectral split-step solver (the same engine as our FDM code — no new machinery). The hard part isn't evolving the field; it's how you start it.
- You must seed the correct quantum jitter. Even a "resting" vacuum trembles; the simulation must start from that exact tremor pattern — strong at large scales, flat at tiny scales.
- The lazy shortcut fails. Seeding plain random noise makes the vacuum decay too fast (it's secretly an excited state) — a fake, inflated result. The paper flags this as the easy-to-botch step.
- This chart is the receipt. It checks our starting jitter is right — and it is.
Bubbles nucleate; the rate obeys the theory
Jenkins et al. demonstrate quantum-regime false-vacuum decay in a two-component ⁴¹K condensate — an Editors' Suggestion in Phys. Rev. D. The headline: the decay rate is set by the fluctuation amplitude, exactly as the instanton theory predicts.

- Sweeps amplitude n̄ = 10–50, 1024 simulations per point.
- Left: survival P(t) ~ exp(−Γt). Right: decay rate Γ/V falls steeply as n̄ grows.
- Vacuum (blue) decays slower than white-noise (red) — white noise is an excited state that over-nucleates.
- The rate is set by the fluctuation amplitude, exactly as instanton theory predicts (Eq. 22).

- The same measurement: Γ vs n̄, vacuum (blue) vs white-noise (orange).
- Both trends reproduced: Γ falls with n̄, and white-noise decays faster.
- Honest differences: n̄ = 3–8 (a cheaper laptop slice), fewer samples, different normalization — trends match, not an exact overlay.
- The full n̄ = 10–50 GPU reproduction is queued.
A bubble nucleating, on our own engine
Blind, check-by-check reproduction (G0–G4) on our differentiable spectral solver — the money shot: a true-vacuum bubble whose wall expands at 45°, i.e. at the phonon "speed of light."

- A space-time diagram: time runs up, position across, colour = which vacuum each point sits in.
- A true-vacuum bubble (red) nucleates out of the fluctuating false vacuum (blue).
- Its walls expand at 45° — the phonon "speed of light" — exactly as vacuum-decay theory predicts.
- Run on their cold-atom analog model (2048-site lattice).

- Blind, check-by-check (G0–G4) on our differentiable spectral solver.
- A bubble nucleates (~ct 450) and its wall races out at the identical 45°.
- Engine checks: conserved quantities hold to 3×10⁻¹²; the correct quantum-jitter start to 0.1%.
- Honest: reduced-statistics laptop run (N=1024) — same phenomenon, our own run beside the paper's.
Massive neutrinos as hot dark matter
Neutrinos have tiny mass but huge velocities — hot dark matter that shapes cosmic large-scale structure differently from cold dark matter. Observing that structure constrains the neutrino mass.
- Because they fly freely (free-streaming), they wash out structure below a characteristic size.
- To predict that, you must track their full spread over both position and velocity — the 6-D distribution f(x, v).
- The neutrino mass is one of the last unmeasured Standard-Model numbers — cosmology is the sharpest probe.
From Vlasov to a quantum circuit
The Vlasov equation isn't quantum — but a few honest approximations turn it into exactly the kind of problem a quantum computer is built to solve. Follow the chain:
Linearize the physics
Neutrinos are <1% of all matter, so ignore their own gravity and treat the dark-matter pull as a fixed external force. With that, the Vlasov equation becomes linear.
∂f/∂t + v·∂f/∂x + FCDM·∂f/∂v = 0 (Eq. 4)
Rewrite it as a Schrödinger equation
Put the distribution f on a grid and the equation becomes df/dt = A·f with A antisymmetric. Multiply by i and it is literally a Schrödinger equation — a cosmology equation in disguise as quantum mechanics.
i ∂t|f⟩ = H|f⟩, H = iA (Hermitian)
Load f into qubits
Store the grid of numbers in the amplitudes of qubits. n qubits hold 2ⁿ grid points — so the 6-D phase-space explosion becomes "a few more qubits," not exponentially more memory. This is where the quantum advantage comes from.
n qubits → 2ⁿ grid points
Evolve with Hamiltonian simulationthe missing bridge
The answer is |f(T)⟩ = e−iHt|f(0)⟩. A quantum computer can't apply that giant operator in one shot — so split it into tiny alternating steps: a little streaming, a little gravity, repeated. That's Trotterization, and each tiny step is just a small set of standard quantum gates.
|f(T)⟩ = e−iHt|f(0)⟩ ≈ ( e−iHxτ · e−iHvτ )N
Run & measure
Those gates form a circuit. Run it, measure the qubits many times, and the measurement statistics reconstruct f(T) → the neutrino power spectrum P(k).
circuit → shots → f(T) → P(k)
A quantum algorithm, plus a small demo
This is a quantum-algorithm paper — designed for a future fault-tolerant machine — accompanied by a small classical demonstration of the physics.
- Small classical demo (Sec. IV): one dimension in position and velocity, a 64-point grid, a single gravity ripple F = A·sin(Kx).
- That one gravity ripple (wavenumber K) drives only the matching neutrino ripple (k=K) — nothing else (Figs. 3–4).
- The quantum algorithm scales as Õ(ngr+nt) vs classical O(ngr⁶) — the potential speedup.
- Reading out the power spectrum (structure vs. size) needs quantum amplitude estimation and a quantum memory (QRAM).
We ran it on real quantum hardware
Beyond reproducing the paper, we did what the authors never did: ran the core step on a real quantum computer — IonQ's trapped-ion Forte-1.
- Reproduced the classical demo (one gravity ripple drives one neutrino ripple; total probability held to 1e-15).
- The step-by-step (Trotter) version, run on an exact simulator (4–6 qubits), converges to the true answer (top figure).
- Our extension: ran it on IonQ Forte-1 — 4 real qubits, circuits of 40/78/116 two-qubit gates, 300 runs each.
- Checked classical → cloud simulator (0.996 match) → real machine; when well-calibrated it reproduces the answer to ~90–95%.
Why was the shallowest circuit the worst?
Counterintuitively, the fewest-gate circuit was reproducibly the worst (fidelity ~0.6). A root-cause analysis cleanly separated two error regimes.
- We varied the number of steps nt at the same total evolution time and compared to a random-noise model — that separates the two error types.
- Random gate noise alone would make the fewest-gate circuit best (model: 0.84) — but on hardware it's the worst.
- ⇒ the culprit is a systematic (coherent) error in the shallow, large-rotation-angle case — not random depth noise.
- From nt ≥ 2 the results follow the random-noise trend; the sweet spot is nt=2.
The Quantum-Simulation Map
Two axes — the method (how) and the phenomenon (what). Most phenomena can be done several ways; each method spans several phenomena. Our two papers sit here ↓; the next slide is the full catalog.
The Quantum Simulation Landscape
Where our two papers sit in the quantum-simulation landscape — 29 papers, analog and digital, from cosmology-in-the-lab to Rydberg arrays, and digital quantum processors running chemistry, high-energy, nuclear & scrambling problems. Our two reproductions are highlighted. Click any chart to enlarge.




























