How does a false vacuum decay on a quantum chip?
When a chain of quantum spins is pushed out of a metastable state, bubbles of the stable state form only at specific quantized sizes, and they then grow by trading spins with neighbouring bubbles rather than on their own.
Source
Stirring the false vacuum via interacting quantized bubbles on a 5,564-qubit quantum annealer
Study at a glance
- Design
- Other — Analogue quantum simulation on a D-Wave annealer (ferromagnetic Ising ring in transverse and longitudinal fields), compared with Bloch-Redfield, matrix-product-state and effective-Hamiltonian calculations
- N
- No participant N; the simulator is a ring of 5,564 superconducting flux qubits, and one closed-system MPS simulation used 100 spins.
- Population
- Superconducting flux qubits on a D-Wave Advantage quantum annealer arranged as a 1D ring
- Outcome
- Magnetization and density of n-spin bubbles over time after the longitudinal field is flipped
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What they did
The team set up a ring of 5,564 superconducting qubits on a quantum annealer to behave like a quantum Ising chain. They prepared all spins up, then flipped the sign of the longitudinal field so 'all up' became a metastable false vacuum, let the system evolve for up to a few microseconds, and read out every spin. They varied the fields, counted bubbles of flipped spins of each size, and compared with numerical simulations and effective models.
What they found
Bubbles of flipped spins appeared only when the field matched the resonance condition where the surface cost of a bubble of n spins balances its volume gain; the team saw 1- to 6-bubble resonances and bubbles as large as about 300 spins. Magnetization curves collapsed onto single curves when time was rescaled as the theory predicts, and at the one-spin resonance neighbouring flipped spins were strongly suppressed, an emergent blockade. Large bubbles could not spread alone; they grew or shrank only by exchanging spins with a neighbouring bubble.
The limits
What it doesn't show
The annealer is an open, noisy system: the longitudinal field wobbles strongly after the flip and thermalization mixes with bubble dynamics, so the authors say they cannot cleanly separate thermal effects from bubble interactions. It is a one-dimensional lattice analogue in the weak transverse-field regime, not a test of cosmological false vacuum decay itself. Later-time behaviour is shaped by the device's slow measurement ramp.
Key terms
- False vacuum
- A metastable state that is a local but not global energy minimum; it can decay to the true ground state by tunnelling.
- Quantum annealer
- A device of coupled superconducting qubits whose fields are swept in time; here used as an analogue simulator of a spin model.
- Transverse-field Ising model
- A chain of spins with nearest-neighbour coupling plus fields along and across the spin axis; the transverse field makes spins quantum-fluctuate.
- Quantized bubble
- A domain of n flipped spins whose creation is resonant only when the longitudinal field is tuned so bubble surface and volume energies exactly balance.
- Kinetic constraint
- A rule, emerging here from energy conservation, that forbids certain moves such as two neighbouring flipped spins.
Flashcards
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Quiz yourself
At what condition can a weak transverse field create an n-spin bubble?
Common questions
Why do bubbles only form at particular field values?
Flipping a block of n spins costs a fixed amount of domain-wall energy but gains energy in proportion to n; only when the longitudinal field makes these equal is the transition resonant enough for a weak transverse field to drive it.
What does this have to do with cosmology?
The idea that the universe might sit in a metastable false vacuum decaying through bubbles comes from cosmology; this chip lets physicists watch an analogous bubble process in a controllable lattice system.
Why are bigger bubbles slower to form?
Creating an n-spin bubble is an n-th order process in the small transverse field, so its rate falls rapidly as n grows.
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