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A quantum enhanced search for dark matter axions


The manipulation of quantum states of light1 holds the potential to enhance searches for fundamental physics. Only recently has the maturation of quantum squeezing technology coincided with the emergence of fundamental physics searches that are limited by quantum uncertainty2,3. In particular, the quantum chromodynamics axion provides a possible solution to two of the greatest outstanding problems in fundamental physics: the strong-CP (charge–parity) problem of quantum chromodynamics4 and the unknown nature of dark matter5,6,7. In dark matter axion searches, quantum uncertainty manifests as a fundamental noise source, limiting the measurement of the quadrature observables used for detection. Few dark matter searches have approached this limit3,8, and until now none has exceeded it. Here we use vacuum squeezing to circumvent the quantum limit in a search for dark matter. By preparing a microwave-frequency electromagnetic field in a squeezed state and near-noiselessly reading out only the squeezed quadrature9, we double the search rate for axions over a mass range favoured by some recent theoretical projections10,11. We find no evidence of dark matter within the axion rest energy windows of 16.96–17.12 and 17.14–17.28 microelectronvolts. Breaking through the quantum limit invites an era of fundamental physics searches in which noise reduction techniques yield unbounded benefit compared with the diminishing returns of approaching the quantum limit.

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Fig. 1: Illustration of the SSR-equipped haloscope, showing the transformation of the vacuum state in quadrature space.
Fig. 2: Advantage conferred by squeezing.
Fig. 3: Axion exclusion from this work.

Data availability

The data central to the results of this manuscript are available from the corresponding author upon reasonable request.

Code availability

The custom codes used to produce the results presented in this manuscript are available from the corresponding author upon reasonable request.


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We acknowledge support from the National Science Foundation under grant numbers PHY-1701396, PHY-1607223, PHY-1734006, PHY-1914199 and PHY-2011357 and the Heising-Simons Foundation under grants 2016-044 and 2016-044. We thank K. Thatcher and C. Schwadron for work on the design and fabrication of the SSR mechanical components, F. Vietmeyer for work on the room-temperature electronics and S. Burrows for graphical design work. We thank V. Bernardo and the J. W. Gibbs Professional Shop as well as C. Miller and D. Johnson for assistance with fabricating the system’s mechanical components. We thank M. Buehler of Low-T Solutions for cryogenics advice. Finally, we thank the Wright laboratory for housing the experiment and providing computing and facilities support.

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Authors and Affiliations



K.M.B. ran the experiment with support from D.A.P., D.H.S., S.G., H.W., S.B.C., R.H.M. and S.K.L. Data were analysed by D.A.P. and K.M.B. with D.H.S, E.C.v.A. and H.W. contributing. K.M.B., M.M., D.A.P., S.M.L., N.M.R., A.D., D.H.S., E.C.v.A., R.H.M and S.K.L. designed and assembled the experiment. D.A.P., M.M., B.M.B. and K.W.L. developed the squeezing concept. K.M.B. developed and implemented squeezing and other operational procedures with support from D.A.P., D.H.S, E.C.v.A. and H.W. The JPAs were designed by M.M., D.A.P. and K.W.L. and fabricated by L.R.V. and G.C.H. The cavity was designed and tested by N.M.R., S.M.L., S.A.K., H.J., A.F.L., M.S., A.D., I.U. and K.v.B. All authors, led by K.M.B. and D.A.P., contributed to the manuscript with figures created by S.G. and H.W.

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Correspondence to K. M. Backes.

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The authors declare no competing interests.

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Peer review information Nature thanks Igor Irastorza, David Marsh and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Simplified HAYSTAC experimental diagram.

A single signal generator provides the local oscillator (LO) tone, as well as the tones for pumping both JPAs (SQ, AMP). Each JPA has two ports: one for the input of pump tones and one for input/output signals. The LO is set at half the pump frequencies via a frequency divider, and the relative phase and amplitude of the pump tones are set using a variable phase shifter and attenuator on the SQ pump line. Switches in the SQ and AMP pump lines (not shown) are used to toggle the JPAs on and off. Microwave circulators route signals nonreciprocally in order to realize the time sequence of operations illustrated in Fig. 1. Circulators with a 50-Ω termination on one port act as isolators, shielding upstream circuit elements from unwanted noise coming from further down the measurement chain. During data acquisition and calibration measurements, signal and noise emitted from and reflected off the cavity are amplified by a HEMT amplifier at 4 K, fed into the RF port of an IQ mixer and mixed down to an intermediate frequency, digitized (ADC) and read into the computer (PC) where the power spectral density is calculated. The cavity’s Lorentzian profile is monitored with reflection and transmission measurements taken using a VNA, for which a portion of the output is split off before the mixer. A switch that toggles between hot (333 mK) and cold (61 mK) 50-Ω loads is used for the calibration measurements described in the text.

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Backes, K.M., Palken, D.A., Kenany, S.A. et al. A quantum enhanced search for dark matter axions. Nature 590, 238–242 (2021).

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