Can sound waves carry entangled quantum states across a chip?
Single quanta of sound travelling along a tiny silicon waveguide were shown to carry entanglement, passing a Bell test with a partner photon.
Source
On-chip distribution of quantum information using traveling phonons
Study at a glance
- Design
- Other — Single silicon optomechanical cavity coupled to a phononic waveguide, driven by write/read laser pulses at 10 mK with single-photon detection.
- N
- One device; more than 500 coincidence events for the entanglement witness, about 30 events per hour for the Bell test.
- Population
- A silicon optomechanical cavity attached to a roughly 100-micrometre phononic waveguide
- Outcome
- Photon cross-correlations, entanglement witness R, and CHSH parameter S
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What they did
The team built a silicon nanobeam cavity that couples light and mechanical vibration, attached to a waveguide whose free end reflects phonons back. Cooled to 10 mK, the device was hit with weak blue-detuned write pulses that create photon-phonon pairs and red-detuned read pulses that convert phonons back to photons. Two write pulses half a round trip apart plus an interferometer made an early/late (time-bin) phonon qubit, whose correlations with photons were measured on single-photon detectors.
What they found
The phonon round-trip time in the waveguide was 126 ns and packets lasted about 30 ns, so early and late phonons were distinguishable. An entanglement witness gave R = 0.72 ± 0.06, below the classical threshold of 1. A CHSH Bell test between the photon and the travelling phonon qubit gave S = 2.32 ± 0.08, exceeding the classical limit of 2 by about 4 standard deviations.
The limits
What it doesn't show
The mechanical lifetime is short, so in this device phonons could only travel a few millimetres; longer distances are projected, not shown. Event rates were very low, so the Bell dataset took 56 hours, and heating from optical absorption limited fidelity. Only two time bins in one device were demonstrated, and entanglement was not distributed between two separate points or to other qubit types.
Key terms
- Phonon
- A quantum of mechanical vibration, here a GHz-frequency sound wave confined in silicon.
- Optomechanical cavity
- A structure where trapped light and a mechanical vibration interact, letting laser pulses create or read out phonons.
- Time-bin qubit
- A qubit encoded in whether a single excitation is in an early or a late time slot, or a superposition of both.
- CHSH inequality
- A Bell inequality: any local classical model gives S at most 2, so a larger value shows quantum correlations.
- Entanglement swapping
- Creating entanglement between two systems by a joint measurement on partners they were each correlated with.
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Quiz yourself
What classical bound does the CHSH parameter S have?
Common questions
Why use phonons instead of photons to move quantum information on a chip?
Phonons travel orders of magnitude slower and occupy far smaller volumes than GHz photons, so they can be stored, delayed and routed compactly, and they couple to many kinds of quantum systems.
How do the authors know the state is quantum and not classical noise?
They measure a witness below the classical bound and violate a CHSH inequality with S above 2, which no classical correlation can do.
Why is the device cooled to millikelvin temperatures?
The GHz mechanical mode must start in its quantum ground state; any thermal phonons would wash out the entanglement.
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