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# Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement

## Abstract

In continuously monitored systems the standard quantum limit is given by the trade-off between shot noise and back-action noise. In gravitational-wave detectors, such as Advanced LIGO, both contributions can be simultaneously squeezed in a broad frequency band by injecting a spectrum of squeezed vacuum states with a frequency-dependent squeeze angle. This approach requires setting up an additional long baseline, low-loss filter cavity in a vacuum system at the detector’s site. Here, we show that the need for such a filter cavity can be eliminated, by exploiting Einstein–Podolsky–Rosen (EPR)-entangled signals and idler beams. By harnessing their mutual quantum correlations and the difference in the way each beam propagates in the interferometer, we can engineer the input signal beam to have the appropriate frequency-dependent conditional squeezing once the out-going idler beam is detected. Our proposal is appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit.

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## Acknowledgements

Research of Y.M., B.H.P. and Y.C. is supported by NSF grant PHY-1404569 and PHY-1506453, as well as the Institute for Quantum Information and Matter, a Physics Frontier Center. H.M. is supported by the Marie-Curie Fellowship and UK STFC Ernest Rutherford Fellowship. C.Z. would like to thank the support of Australian Research Council Discovery Project DP120104676 and DP120100898. R.S. is supported by DFG grant SCHN757/6 and by ERC grant 339897 (‘Mass Q’).

## Author information

Authors

### Contributions

Y.M., H.M. and Y.C. formulated the idea; Y.M. performed the analysis of the idea and wrote the initial draft, which was later revised by Y.C.; B.H.P. checked Y.M.’s calculation; M.E., J.H., R.S. and C.Z. provided important experimental parameters for doing theoretical analysis and gave valuable comments on Y.M.’s calculations and initial/revised draft.

### Corresponding author

Correspondence to Yiqiu Ma.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary information

Supplementary information (PDF 626 kb)

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Ma, Y., Miao, H., Pang, B. et al. Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement. Nature Phys 13, 776–780 (2017). https://doi.org/10.1038/nphys4118

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