REVIEW 2 major objections 3 minor 62 references
Deterministic multi-phonon entanglement between two mechanical resonators on separate substrates
T0 review · 2 major / 3 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Two mechanical resonators on separate chips are deterministically entangled, with measured Bell and two-phonon N00N state fidelities of 0.872 and 0.748.
desk verdict Real step forward: first two-phonon N00N state between separate-substrate mechanical resonators, with a disclosed but untested tomography truncation that should be checked in revision. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central mechanism is the qubit-resonator coherent swap, in which each tunable transmon is tuned into resonance with its surface acoustic wave mode and its coupling strength is set by a variable coupler, so that an excitation deterministically moves between the qubit and the resonator. A two-qubit Bell state is generated by a controlled swap and then transferred in parallel to the two resonators. For the N00N state, each qubit is first raised to its $|f\rangle$ state and the two qubits are entangled into $(|fg\rangle+|gf\rangle)/\sqrt{2}$; the $f\leftrightarrow e$ swap transfers one phonon per side, and the subsequent $e\leftrightarrow g$ swap transfers a second phonon, ideally leaving $(|20\rangle+|02\rangle)/\sqrt{2}$. Tomography is performed by applying coherent displacement pulses $\hat D_j(-\alpha_j)$ to each resonator and measuring the qubits after a fixed interaction time, then reconstructing the two-mode density matrix via convex optimization with bootstrap error bars.
What would settle it
Re-run the N00N generation protocol but reconstruct the joint resonator density matrix allowing up to three or more phonons per resonator in the convex optimization; alternatively, directly measure the population in the $|30\rangle$ and $|03\rangle$ states by extending the swap-time traces to longer interactions. If these populations are non-negligible, the fidelity to $(|20\rangle+|02\rangle)/\sqrt{2}$ is inflated relative to the true state.
Extended reading notes
Core claim
The central claim is that a modular platform, in which two surface acoustic wave resonators are fabricated on separate substrates and inductively coupled to two superconducting qubits on a third chip, can deterministically produce and fully characterize entangled phonon states spanning both mechanical modes. By first entangling the two qubits and then performing simultaneous resonant swaps, the system yields the mechanical Bell state $(|10\rangle+|01\rangle)/\sqrt{2}$ with fidelity $0.872\pm0.002$. By promoting the qubits to their second excited state and using sequential $f\leftrightarrow e$ and $e\leftrightarrow g$ swaps, the system yields the two-phonon N00N state $(|20\rangle+|02\rangle)/\sqrt{2}$ with fidelity $0.748\pm0.008$. The states are reconstructed from joint Wigner tomography using coherent displacement pulses and convex optimization. If correct, this demonstrates that the bosonic nature of phonons can be exploited across distant mechanical resonators, going beyond single-phonon entanglement.
Load-bearing premise
The fidelity of the N00N state is computed from a density matrix that was reconstructed under the assumption that neither resonator ever holds more than two phonons, so any leaked population in higher Fock states would be invisible to the tomography and the reported fidelity could be overstated.
Editorial extensions
If this is right
- The same swap ladder can be iterated to produce N00N states with $N>2$, limited by decoherence and unwanted non-resonant phonon emission during the $f\leftrightarrow e$ swap.
- Because the resonators are on separate dies with no direct mechanical contact, the architecture supports distributing entanglement across larger multi-chip networks, e.g., GHZ and W states.
- Joint Wigner tomography with convex optimization provides a general readout for multi-mode phonon states, applicable to other circuit quantum acoustics platforms.
- The measured fidelities are limited by short resonator lifetimes (roughly 0.27–0.38 µs) and qubit $T_1$ during swaps; improving these lifetimes would directly increase the achievable state fidelities.
Reading between the lines
- If the two-phonon subspace truncation in the tomography were relaxed, the N00N state fidelity could fall; an independent measurement of higher Fock state populations would tell whether the reported 0.748 is robust.
- The modular flip-chip design could be adapted to link more than two mechanical nodes, and the same qubit-mediated swap technique could entangle mechanical resonators at different frequencies without needing direct mechanical coupling.
- Narrowing the IDT bandwidth to keep the $e\leftrightarrow g$ transition outside the transducer emission window should suppress the unwanted phonon emission during the $f\leftrightarrow e$ swap, potentially raising N00N fidelity and enabling efficient generation of higher-$N$ states.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports deterministic generation of entanglement between two surface acoustic wave (SAW) resonators fabricated on separate lithium niobate substrates, each coupled to a superconducting transmon qubit. A two-qubit Bell state is generated and swapped into the resonators, yielding a mechanical Bell state (|10>+|01>)/√2 with reported fidelity F = 0.872±0.002/0.003. Extending the protocol to qubit f-states, the authors generate a two-phonon N00N state (|20>+|02>)/√2 and report fidelity F = 0.748±0.008 from joint Wigner tomography with convex optimization. The paper also presents device characterization, simulations based on independently measured parameters, and a discussion of scalability to multi-mode and multi-chip configurations.
Significance. If the reported fidelities hold, this work demonstrates multi-phonon (N=2) entanglement between mechanical resonators on separate substrates, going beyond previous single-phonon entanglement demonstrations and strengthening the case for modular SAW-based quantum information processing. The protocol is deterministic and produces entangled bosonic states that can be analyzed with direct qubit-resonator swaps. Notable strengths include the use of independently measured T1, T2, and coupling parameters in simulations, the consistency of experimental signatures (e.g., the √2 faster oscillations for the N00N state), and the transparent disclosure of the truncation assumption in the N00N tomography. However, the central N00N fidelity claim rests on a subspace assumption that is not yet verified; this must be addressed before the multi-phonon claim is fully supported.
major comments (2)
- [Supplementary, 'Joint state tomography of two mechanical resonators'] The N00N state reconstruction imposes a maximum of two excitations per resonator and zero-pads higher phonon indices, as stated explicitly in the supplementary. Yet the time-trace fits used for the tomography (Extended Data Fig. 3) include up to six resonator levels per resonator, and the generation protocol involves two swap steps in which imperfect swaps and non-resonant phonon emission could populate higher Fock states. If the actual joint state has any population in |3> or above, the convex-optimization inversion will force it to zero, making the reported fidelity to (|20>+|02>)/√2 an upper bound rather than a measured value. This is load-bearing for the multi-phonon claim. Please either reconstruct ρm without the two-phonon truncation (e.g., allowing up to six phonons per resonator, matching the fit space) and report the fidelity and n>2 populations, or provide direct simulation evidence that n>2 populations are negligible. The existing simulation (F=0.745 vs 0.748 measured) can settle this, so the check appears straightforward.
- [Main text, Fig. 3 and abstract] The Bell-state fidelity is quoted as 0.872±0.002 in the abstract and in the Fig. 3e caption, but as 0.872±0.003 in the main text (paragraph after Fig. 3). The authors should correct this inconsistency and state the exact method for error propagation.
minor comments (3)
- [Supplementary, 'Joint state tomography of two mechanical resonators'] The bootstrap procedure uses only 10 resamples. This is small for 225–261 pulse combinations; please report the distribution and consider more resamples or a separate uncertainty estimate.
- [Main text, Fig. 3e and Fig. 4e] The fidelity expression F = sqrt(Tr(ρ_target|ρ|)) is unusual; please clarify whether |ρ| denotes the absolute value of the density matrix and how the square root is applied. Standard definitions are F = ⟨ψ|ρ|ψ⟩ or the square root of that; please define explicitly.
- [Supplementary, Extended Data Fig. 3] The figure caption says 'For Fig. 3 in the main text, we only show three energy levels...' while the text says five levels are used in the fits; please clarify what is displayed versus what is used in the reconstruction.
Circularity Check
No significant circularity: experimental fidelities are measured via independently calibrated tomography, with a disclosed truncation caveat in the N00N reconstruction.
full rationale
The paper's central claims are measurements, not derivations. The Bell-state fidelity F=0.872±0.002 is obtained from joint Wigner tomography using displacement pulses calibrated independently (Extended Data Fig. 2), and the density matrix is reconstructed by convex optimization constrained only by Hermiticity, positive semidefiniteness, and unit trace. The N00N-state fidelity F=0.748±0.008 uses the same independent displacement calibration and reconstruction procedure. The only potentially circular-looking element is the tomography truncation stated in the Supplementary: 'we assume a maximum of two excitations in each resonator, so we zero-pad ρm for phonon indices larger than 2.' This is an explicit, disclosed modeling constraint on the inversion, not a parameter fitted to force the target state. It could bias the fidelity upward if the physical state had significant population above two phonons per resonator, but it does not by itself determine the |20⟩⟨02| coherence, and the reported agreement with an independent Lindblad master-equation simulation (F=0.748 measured vs F=0.745 simulated) using independently measured lifetimes, coherence times, and couplings provides a cross-check. The simulations use independently measured system parameters given in Extended Data Table 1, including T1, T2, gge, gef, and gq. Self-citations in the reference list concern device integration (e.g., Ref. [47]) and phonon routing (e.g., Ref. [19]); they are not load-bearing for the entanglement-generation or tomography claims. There is no equation in the paper that reduces a predicted fidelity to its input, and no empirical pattern is merely renamed as organization. The disclosed truncation is a fidelity-estimation caveat, not circular reasoning; hence the score is 1 rather than 0.
Assumptions & free parameters
free parameters (3)
- Qubit |e> lifetime during |e0> <-> |g1> swap (node A) =
784 ns
- Qubit |e> lifetime during |e0> <-> |g1> swap (node B) =
350 ns
- Mechanical resonator T1 and T2 (nodes A and B) =
380/270 ns (T1), 709/527 ns (T2)
assumptions (4)
- domain assumption The qubit-resonator system is described by the Lindblad master equation with independently measured decay rates.
- domain assumption The SAW resonator is a single-mode Fabry-Perot cavity; the COM model captures its linear response.
- domain assumption The displacement pulses implement ideal coherent displacements D(-alpha) on each resonator, with calibrated amplitudes and phases.
- ad hoc to paper For the N00N tomography, the resonator state has at most two excitations per resonator.
Cite this review
Pith. "Pith review of Deterministic multi-phonon entanglement between two mechanical resonators on separate substrates." pith.science (2026). https://pith.science/paper/CQUKU2VM
@misc{pith2026241115726,
author = {Pith},
title = {Pith review of: Deterministic multi-phonon entanglement between two mechanical resonators on separate substrates},
year = {2026},
howpublished = {\url{https://pith.science/paper/CQUKU2VM}},
note = {Machine review of arXiv:2411.15726}
}
abstract
Mechanical systems have emerged as a compelling platform for applications in quantum information, leveraging recent advances in the control of phonons, the quanta of mechanical vibrations. Several experiments have demonstrated control and measurement of phonon states in mechanical resonators integrated with superconducting qubits, and while entanglement of two mechanical resonators has been demonstrated in some approaches, a full exploitation of the bosonic nature of phonons, such as multi-phonon entanglement, remains a challenge. Here, we describe a modular platform capable of rapid multi-phonon entanglement generation and subsequent tomographic analysis, using two surface acoustic wave resonators on separate substrates, each connected to a superconducting qubit. We generate a mechanical Bell state between the two mechanical resonators, achieving a fidelity of $\mathcal{F} = 0.872\pm 0.002$, and further demonstrate the creation of a multi-phonon entangled state (N=2 N00N state), shared between the two resonators, with fidelity $\mathcal{F} = 0.748\pm 0.008$. This approach promises the generation and manipulation of more complex phonon states, with potential future applications in bosonic quantum computing in mechanical systems. The compactness, modularity, and scalability of our platform further promises advances in both fundamental science and advanced quantum protocols, including quantum random access memory and quantum error correction.
Figures
Figures from the paper (1 more)
Reference graph
Works this paper leans on
-
[1]
O’Connell, A. D. et al. Quantum ground state and single-phonon control of a mechanical resonator. Nature 464, 697–703 (2010)
work page 2010
-
[2]
We then bring each qubit into resonance with its corresponding mechanical resonator, and turn on the variable couplers to perform full qubit-resonator swaps, ideally resulting in a dual-resonator Bell state ( |10⟩ + |01⟩)/ √
-
[3]
( ) ( ) QB QA RB RA b Figure 3: Deterministic mechanical Bell state generation and tomography
Note this method is not negatively 6 a e d |ρ| 0.5 0.0 Pgg Peg Pge Pee (ns) P c Pn 0.5 0.0 0.5 0.0 Bell generation Wigner tomo. ( ) ( ) QB QA RB RA b Figure 3: Deterministic mechanical Bell state generation and tomography. a, Principle: A two-qubit Bell state ( |eg⟩ + |ge⟩)/ √ 2 is generated in the qubits, then swapped coherently into the resonators, gene...
-
[4]
This protocol can be extended to (|N 0⟩+|0N ⟩)/ √ 2 states by iterating the first two steps. Alternatively, we can generate ( |N 0⟩ + |0M ⟩)/ √ 2 N00M states, if one qubit is initially excited to its |f ⟩ state while the other qubit remains in |e⟩. We analyze the final resonator state using Wigner tomography, similar to the Bell state 8 a b 1 2 3 4 c e |ρ...
-
[5]
Chu, Y. et al. Quantum acoustics with superconducting qubits. Science 358, 199–202 (2017)
work page 2017
-
[6]
Satzinger, K. J. et al. Quantum control of surface acoustic-wave phonons.Nature 563, 661–665 (2018)
work page 2018
-
[7]
Arrangoiz-Arriola, P. et al. Resolving the energy levels of a nanomechanical oscillator. Nature 571, 537–540 (2019)
work page 2019
-
[8]
Riedinger, R. et al. Remote quantum entanglement between two micromechanical oscillators. Nature 556, 473–477 (2018)
work page 2018
Show all 62 references
-
[9]
Ockeloen-Korppi, C. F. et al. Stabilized entanglement of massive mechanical oscillators.Nature 556, 478–482 (2018)
2018
-
[10]
Kotler, S. et al. Direct observation of deterministic macroscopic entanglement. Science 372, 622–625 (2021)
2021
-
[11]
Wollack, E. A. et al. Quantum state preparation and tomography of entangled mechanical resonators. Nature 604, 463–467 (2022)
2022
-
[12]
C., Yang, Y., Fadel, M
von L¨ upke, U., Rodrigues, I. C., Yang, Y., Fadel, M. & Chu, Y. Engineering multimode interactions in circuit quantum acoustodynamics. Nature Physics 1–7 (2024)
2024
-
[13]
Hann, C. T. et al. Hardware-efficient quantum random access memory with hybrid quantum acoustic systems. Physical Review Letters 123, 250501 (2019)
2019
-
[14]
Wang, Z., Qiao, H., Cleland, A. N. & Jiang, L. Quantum random access memory with transmon-controlled phonon routing. Preprint at https://arxiv.org/abs/2411.00719 (2024)
2024 arXiv
-
[15]
Chamberland, C. et al. Building a fault-tolerant quantum computer using concatenated cat codes. PRX Quantum 3, 010329 (2022)
2022
-
[16]
MacCabe, G. S. et al. Nano-acoustic resonator with ultralong phonon lifetime. Science 370, 840–843 (2020)
2020
-
[17]
A., Sletten, L
Moores, B. A., Sletten, L. R., Viennot, J. J. & Lehnert, K. Cavity quantum acoustic device in the multimode strong coupling regime. Physical Review Letters 120, 227701 (2018)
2018
-
[18]
& Lehnert, K
Sletten, L., Moores, B., Viennot, J. & Lehnert, K. Resolving phonon Fock states in a mul- 11 timode cavity with a double-slit qubit. Physical Review X 9, 021056 (2019)
2019
-
[19]
Chu, Y. et al. Creation and control of multi-phonon Fock states in a bulk acoustic-wave resonator. Nature 563, 666–670 (2018)
2018
-
[20]
Chan, J. et al. Laser cooling of a nanomechanical oscillator into its quantum ground state. Nature 478, 89–92 (2011)
2011
-
[21]
& Gr¨ oblacher, S
Wallucks, A., Marinkovi´ c, I., Hensen, B., Stockill, R. & Gr¨ oblacher, S. A quantum memory at telecom wavelengths. Nature Physics 16, 772–777 (2020)
2020
-
[22]
Qiao, H. et al. Splitting phonons: Building a platform for linear mechanical quantum com- puting. Science 380, 1030–1033 (2023)
2023
-
[23]
Wollman, E. E. et al. Quantum squeezing of motion in a mechanical resonator. Science 349, 952–955 (2015)
2015
-
[24]
& Schliesser, A
Mason, D., Chen, J., Rossi, M., Tsaturyan, Y. & Schliesser, A. Continuous force and displace- ment measurement below the standard quantum limit. Nature Physics 15, 745–749 (2019)
2019
-
[25]
Huang, G., Beccari, A., Engelsen, N. J. & Kippenberg, T. J. Room-temperature quantum optomechanics using an ultralow noise cavity. Nature 626, 512–516 (2024)
2024
-
[26]
A., Teufel, J
Palomaki, T. A., Teufel, J. D., Simmonds, R. W. & Lehnert, K. W. Entangling mechanical motion with microwave fields. Science 342, 710–713 (2013)
2013
-
[27]
Gustafsson, M. V. et al. Propagating phonons coupled to an artificial atom. Science 346, 207–211 (2014)
2014
-
[28]
Manenti, R. et al. Circuit quantum acoustodynamics with surface acoustic waves. Nature Communications 8, 975 (2017)
2017
-
[29]
& Nakamura, Y
Noguchi, A., Yamazaki, R., Tabuchi, Y. & Nakamura, Y. Qubit-assisted transduction for a detection of surface acoustic waves near the quantum limit. Physical Review Letters 119, 180505 (2017)
2017
-
[30]
Bolgar, A. N. et al. Quantum regime of a two-dimensional phonon cavity. Physical Review Letters 120, 223603 (2018)
2018
-
[31]
Bienfait, A. et al. Phonon-mediated quantum state transfer and remote qubit entanglement. Science 364, 368–371 (2019)
2019
-
[32]
Bienfait, A. et al. Quantum erasure using entangled surface acoustic phonons. Physical Review X 10, 021055 (2020)
2020
-
[33]
Dumur, ´E. et al. Quantum communication with itinerant surface acoustic wave phonons. npj 12 Quantum Information 7, 173 (2021)
2021
-
[34]
& Gr¨ oblacher, S
Zivari, A., Stockill, R., Fiaschi, N. & Gr¨ oblacher, S. Non-classical mechanical states guided in a phononic waveguide. Nature Physics 18, 789–793 (2022)
2022
-
[35]
Zivari, A. et al. On-chip distribution of quantum information using traveling phonons. Science Advances 8, eadd2811 (2022)
2022
-
[36]
Cooling of a levitated nanoparticle to the motional quantum ground state
Deli´ c, U.et al. Cooling of a levitated nanoparticle to the motional quantum ground state. Science 367, 892–895 (2020)
2020
-
[37]
Shao, L. et al. Electrical control of surface acoustic waves. Nature Electronics 5, 348–355 (2022)
2022
-
[38]
Zhang, J. et al. NOON states of nine quantized vibrations in two radial modes of a trapped ion. Physical Review Letters 121, 160502 (2018)
2018
-
[39]
Bochmann, J., Vainsencher, A., Awschalom, D. D. & Cleland, A. N. Nanomechanical coupling between microwave and optical photons. Nature Physics 9, 712–716 (2013)
2013
-
[40]
Andrews, R. W. et al. Bidirectional and efficient conversion between microwave and optical light. Nature Physics 10, 321–326 (2014)
2014
-
[41]
& Cleland, A
Vainsencher, A., Satzinger, K., Peairs, G. & Cleland, A. Bi-directional conversion between microwave and optical frequencies in a piezoelectric optomechanical device. Applied Physics Letters 109, 033107 (2016)
2016
-
[42]
Peairs, G. et al. Continuous and time-domain coherent signal conversion between optical and microwave frequencies. Physical Review Applied 14, 061001 (2020)
2020
-
[43]
& Painter, O
Mirhosseini, M., Sipahigil, A., Kalaee, M. & Painter, O. Superconducting qubit to optical photon transduction. Nature 588, 599–603 (2020)
2020
-
[44]
Whiteley, S. J. et al. Spin–phonon interactions in silicon carbide addressed by gaussian acous- tics. Nature Physics 15, 490–495 (2019)
2019
-
[45]
L., Reinhard, F
Degen, C. L., Reinhard, F. & Cappellaro, P. Quantum sensing. Reviews of Modern Physics 89, 035002 (2017)
2017
-
[46]
Carney, D., Hook, A., Liu, Z., Taylor, J. M. & Zhao, Y. Ultralight dark matter detection with mechanical quantum sensors. New Journal of Physics 23, 023041 (2021)
2021
-
[47]
Goryachev, M. et al. Rare events detected with a bulk acoustic wave high frequency gravita- tional wave antenna. Physical Review Letters 127, 071102 (2021)
2021
-
[48]
Schrinski, B. et al. Macroscopic quantum test with bulk acoustic wave resonators. Physical 13 Review Letters 130, 133604 (2023)
2023
-
[49]
Linehan, R. et al. Listening for new physics with quantum acoustics. Preprint at https: //arxiv.org/abs/2410.17308 (2024)
2024 arXiv
-
[50]
Satzinger, K. J. et al. Simple non-galvanic flip-chip integration method for hybrid quantum systems. Applied Physics Letters 114, 173501 (2019)
2019
-
[51]
Koch, J. et al. Charge-insensitive qubit design derived from the Cooper pair box. Physical Review A 76, 042319 (2007)
2007
-
[52]
Barends, R. et al. Coherent Josephson qubit suitable for scalable quantum integrated circuits. Physical Review Letters 111, 080502 (2013)
2013
-
[53]
Surface acoustic wave filters: With applications to electronic communications and signal processing (Academic Press, 2010)
Morgan, D. Surface acoustic wave filters: With applications to electronic communications and signal processing (Academic Press, 2010)
2010
-
[54]
Wang, H. et al. Deterministic entanglement of photons in two superconducting microwave resonators. Physical Review Letters 106, 060401 (2011)
2011
-
[55]
Hofheinz, M. et al. Synthesizing arbitrary quantum states in a superconducting resonator. Nature 459, 546–549 (2009)
2009
-
[56]
Wang, C. et al. A Schr¨ odinger cat living in two boxes.Science 352, 1087–1091 (2016)
2016
-
[57]
Bild, M. et al. Schr¨ odinger cat states of a 16-microgram mechanical oscillator. Science 380, 274–278 (2023)
2023
-
[58]
Wollack, E. A. et al. Loss channels affecting lithium niobate phononic crystal resonators at cryogenic temperature. Applied Physics Letters 118, 123501 (2021)
2021
-
[59]
Lee, N. R. et al. Strong dispersive coupling between a mechanical resonator and a fluxonium superconducting qubit. PRX Quantum 4, 040342 (2023)
2023
-
[60]
An introduction to the bootstrap (Chapman & Hall, 1993)
Efron, B. An introduction to the bootstrap (Chapman & Hall, 1993)
1993
-
[61]
& Nori, F
Johansson, J., Nation, P. & Nori, F. QuTiP: An open-source python framework for the dynamics of open quantum systems. Computer Physics Communications 183, 1760–1772 (2012)
2012
-
[62]
Bialczak, R. C. et al. Quantum process tomography of a universal entangling gate implemented with Josephson phase qubits. Nature Physics 6, 409–413 (2010). 14 Supplementary Device fabrication. Each acoustic device is fabricated on a LiNbO 3 substrate, which is first cleaned us...
2010
Reviewed August 12, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.