About

Kent Irwin is the Director of the Hansen Experimental Physics Laboratory (HEPL) and a Professor of Physics, specializing in Particle Physics, Astrophysics, and Photon Science at Stanford University. He holds a Ph.D. and an M.S.. from Stanford University, earned in 1995, and a B.S. from the California Institute of Technology obtained in 1988. His research interests encompass experimental and observational astrophysics and cosmology, as well as experimental particle physics. As a faculty member, he contributes to the advancement of physics through his leadership in experimental physics research and education at Stanford.

Research topics

• Physics
• Computer Science
• Astrophysics
• Astronomy
• Optics
• Quantum mechanics
• Engineering
• Mathematics
• Particle physics
• Statistics
• Systems engineering
• Algorithm
• Materials science
• Theoretical physics
• Aerospace engineering
• Telecommunications

Selected publications

• The Atacama Cosmology Telescope: DR6 Constraints on Extended Cosmological Models

  ArXiv.org · 2025-03-18 · 13 citations

  articleOpen access

  We use new cosmic microwave background (CMB) primary temperature and polarization anisotropy measurements from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) to test foundational assumptions of the standard cosmological model and set constraints on extensions to it. We derive constraints from the ACT DR6 power spectra alone, as well as in combination with legacy data from Planck. To break geometric degeneracies, we include ACT and Planck CMB lensing data and baryon acoustic oscillation data from DESI Year-1, and further add supernovae measurements from Pantheon+ for models that affect the late-time expansion history. We verify the near-scale-invariance (running of the spectral index $d n_s/d\ln k = 0.0062 \pm 0.0052$) and adiabaticity of the primordial perturbations. Neutrino properties are consistent with Standard Model predictions: we find no evidence for new light, relativistic species that are free-streaming ($N_{\rm eff} = 2.86 \pm 0.13$, which combined with external BBN data becomes $N_{\rm eff} = 2.89 \pm 0.11$), for non-zero neutrino masses ($\sum m_ν< 0.082$ eV at 95% CL), or for neutrino self-interactions. We also find no evidence for self-interacting dark radiation ($N_{\rm idr} < 0.134$), early-universe variation of fundamental constants, early dark energy, primordial magnetic fields, or modified recombination. Our data are consistent with standard BBN, the FIRAS-inferred CMB temperature, a dark matter component that is collisionless and with only a small fraction allowed as axion-like particles, a cosmological constant, and the late-time growth rate predicted by general relativity. We find no statistically significant preference for a departure from the baseline $Λ$CDM model. In general, models introduced to increase the Hubble constant or to decrease the amplitude of density fluctuations inferred from the primary CMB are not favored by our data.

  PublisherOA PDFDOI

• The Atacama Cosmology Telescope: DR6 Power Spectra, Likelihoods and $Λ$CDM Parameters

  ArXiv.org · 2025-03-18 · 11 citations

  preprintOpen access

  We present power spectra of the cosmic microwave background (CMB) anisotropy in temperature and polarization, measured from the Data Release 6 maps made from Atacama Cosmology Telescope (ACT) data. These cover 19,000 deg$^2$ of sky in bands centered at 98, 150 and 220 GHz, with white noise levels three times lower than Planck in polarization. We find that the ACT angular power spectra estimated over 10,000 deg$^2$, and measured to arcminute scales in TT, TE and EE, are well fit by the sum of CMB and foregrounds, where the CMB spectra are described by the $Λ$CDM model. Combining ACT with larger-scale Planck data, the joint P-ACT dataset provides tight limits on the ingredients, expansion rate, and initial conditions of the universe. We find similar constraining power, and consistent results, from either the Planck power spectra or from ACT combined with WMAP data, as well as from either temperature or polarization in the joint P-ACT dataset. When combined with CMB lensing from ACT and Planck, and baryon acoustic oscillation data from DESI DR1, we measure a baryon density of $Ω_b h^2=0.0226\pm0.0001$, a cold dark matter density of $Ω_c h^2=0.118\pm0.001$, a Hubble constant of $H_0=68.22\pm0.36$ km/s/Mpc, a spectral index of $n_s=0.974\pm0.003$, and an amplitude of density fluctuations of $σ_8=0.813\pm0.005$. Including the DESI DR2 data tightens the Hubble constant to $H_0=68.43\pm0.27$ km/s/Mpc; $Λ$CDM parameters agree between the P-ACT and DESI DR2 data at the $1.6σ$ level. We find no evidence for excess lensing in the power spectrum, and no departure from spatial flatness. The contribution from Sunyaev-Zel'dovich (SZ) anisotropy is detected at high significance; we find evidence for a tilt with suppressed small-scale power compared to our baseline SZ template spectrum, consistent with hydrodynamical simulations with feedback.

  PublisherOA PDFDOI

• Noise limits for dc SQUID readout of high-<i>Q</i> resonators below 300 MHz

  Journal of Applied Physics · 2025-09-04

  articleOpen access

  We present the limits on noise for the readout of cryogenic high-Q resonators using dc Superconducting Quantum Interference Devices (SQUIDs) below 300 MHz. This analysis uses realized first-stage SQUIDs (previously published), whose performance is well described by Tesche–Clarke (TC) theory, coupled directly to the resonators. We also present data from a prototype second-stage dc SQUID array designed to couple to this first-stage SQUID as a follow-on amplifier with high system bandwidth. This analysis is the first full consideration of dc SQUID noise performance referred to a high-Q resonator over this frequency range and is presented relative to the standard quantum limit. We include imprecision, backaction, and backaction–imprecision noise correlations from TC theory, the noise contributed by the second-stage SQUIDs, wiring, and preamplifiers, and optimizations for both on-resonance measurements and off-resonance scan sensitivity. This architecture has modern relevance due to the increased interest in axion searches and the requirements of the DMRadio-m3 axion search, which uses dc SQUIDs in this frequency range.

  PublisherDOI

• Magic Diamond: Covalent Bond Formation of Melamine and other Amines on Nanodiamond Surfaces

  ChemRxiv · 2025-12-12

  article

  High-temperature high-pressure (HPHT) nanoscale diamond (ND) is a host of the nitrogen vacancy (NV) center, a quantum bit that allows for a wide range of quantum sensing capabilities including magnetometry, electrometry and thermometry using all-optical techniques at room temperature. Yet challenges remain in diversifying the chemical modalities for covalent bond formation on diamond surfaces, which is typically limited to carboxylate-based chemistry but has been recently expanded. Amine termination of diamond is attractive due to theoretical studies that described the removal of mid-gap states, which results in extended electron-spin coherence times. Chemical activation of alcohol terminated NDs into alkyl-bromides (ND-Br) using SOBr2 has been shown to be a successful strategy. The alkyl bromide is a good leaving group that forms a carbocation intermediate to generate new bonds and has previously been used to form simple amine termination. Here we test the ability of larger amine molecules to generate covalent diamond-nitrogen bonds on the sterically hindered surface. In this update, we react ND-Br with branched, linear and cyclic amines including polyethyleneimine, diethylenetriamine and melamine, a triazine derivative used in “magic eraser” products and antibacterial products. A suite of X-ray spectroscopies was employed to verify the successful amination and probe the diamond-amine electronic structure. These results expand the routes for researchers who wish to chemically tune diamond and modify the surface dipole moment and electron affinity for quantum sensing applications with atomic defects.

  PublisherDOI

• The Atacama Cosmology Telescope: DR6 Maps

  ORCA Online Research @Cardiff (Cardiff University) · 2025-03-18 · 2 citations

  articleOpen access

  We present Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) maps of the Cosmic Microwave Background temperature and polarization anisotropy at arcminute resolution over three frequency bands centered on 98, 150 and 220 GHz. The maps are based on data collected with the AdvancedACT camera over the period 2017--2022 and cover 19,000 square degrees with a median combined depth of 10 uK arcmin. We describe the instrument, mapmaking and map properties and illustrate them with a number of figures and tables. The ACT DR6 maps and derived products are available on LAMBDA at https://lambda.gsfc.nasa.gov/product/act/actadv_prod_table.html. We also provide an interactive web atlas at https://phy-act1.princeton.edu/public/snaess/actpol/dr6/atlas and HiPS data sets in Aladin (e.g. https://alasky.cds.unistra.fr/ACT/DR4DR6/color_CMB).

  PublisherOA PDFDOI

• Description of the Electronic Structure of Oxyhemoglobin Using Fe L-Edge X-ray Absorption Spectroscopy

  Journal of the American Chemical Society · 2025-06-11 · 2 citations

  articleOpen access

  The electronic structure of oxyhemoglobin has been controversial since the discovery of the compound’s diamagnetism in 1936. This study uses partial fluorescence yield Fe L-edge X-ray absorption spectroscopy (XAS) in the 3s→2p fluorescence on oxyhemoglobin solutions, measured using a transition-edge sensor detector, to obtain a quantitative experimental description of the electronic structure of the O2-bound iron site. The spectrum is very different from typical low-spin FeII and FeIII heme spectra, and multiplet simulations indicate a mixed ground configuration with ∼57% low-spin FeIII and ∼43% low-spin FeII character. This is also very different from the FeII character found for the picket-fence porphyrin model complex. The oxyhemoglobin L-edge XAS data further show that the O2 ligand engages in a weak σ- but strong π-bond with the iron ion, leading to the overall strong Fe–O2 bond required for O2 transport.

  PublisherOA PDFDOI

• The Atacama Cosmology Telescope: DR6 power spectra, likelihoods and ΛCDM parameters

  Journal of Cosmology and Astroparticle Physics · 2025-11-01 · 117 citations

  articleOpen access

  Abstract We present power spectra of the cosmic microwave background (CMB) anisotropy in temperature and polarization, measured from the Data Release 6 maps made from Atacama Cosmology Telescope (ACT) data. These cover 19,000 deg 2 of sky in bands centered at 98, 150 and 220 GHz, with white noise levels three times lower than Planck in polarization. We find that the ACT angular power spectra estimated over 10,000 deg 2 , and measured to arcminute scales in TT, TE and EE, are well fit by the sum of CMB and foregrounds, where the CMB spectra are described by the ΛCDM model. Combining ACT with larger-scale Planck data, the joint P-ACT dataset provides tight limits on the ingredients, expansion rate, and initial conditions of the universe. We find similar constraining power, and consistent results, from either the Planck power spectra or from ACT combined with WMAP data, as well as from either temperature or polarization in the joint P-ACT dataset. When combined with CMB lensing from ACT and Planck , and baryon acoustic oscillation data from the Dark Energy Spectroscopic Instrument (DESI DR1), we measure a baryon density of Ω b h 2 = 0.0226 ± 0.0001, a cold dark matter density of Ω c h 2 = 0.118 ± 0.001, a Hubble constant of H 0 = 68.22 ± 0.36 km/s/Mpc, a spectral index of n s = 0.974 ± 0.003, and an amplitude of density fluctuations of σ 8 = 0.813 ± 0.005. Including the DESI DR2 data tightens the Hubble constant to H 0 = 68.43 ± 0.27 km/s/Mpc; ΛCDM parameters agree between the P-ACT and DESI DR2 data at the 1.6 σ level. We find no evidence for excess lensing in the power spectrum, and no departure from spatial flatness. The contribution from Sunyaev-Zel'dovich (SZ) anisotropy is detected at high significance; we find evidence for a tilt with suppressed small-scale power compared to our baseline SZ template spectrum, consistent with hydrodynamical simulations with feedback.

  PublisherDOI

• Analysis of Polarized Dust Emission Using Data from the First Flight of SPIDER

  The Astrophysical Journal · 2025-01-01 · 9 citations

  articleOpen access

  Abstract Using data from the first flight of Spider and from the Planck High Frequency Instrument, we probe the properties of polarized emission from interstellar dust in the Spider observing region. Component-separation algorithms operating in both the spatial and harmonic domains are applied to probe their consistency and to quantify modeling errors associated with their assumptions. Analyses of diffuse Galactic dust emission spanning the full Spider region demonstrate (i) a spectral energy distribution that is broadly consistent with a modified-blackbody (MBB) model with a spectral index of β d = 1.45 ± 0.05 (1.47 ± 0.06) for E ( B )-mode polarization, slightly lower than that reported by Planck for the full sky; (ii) an angular power spectrum broadly consistent with a power law; and (iii) no significant detection of line-of-sight polarization decorrelation. Tests of several modeling uncertainties find only a modest impact (∼10% in σ r ) on Spider ’s sensitivity to the cosmological tensor-to-scalar ratio. The size of the Spider region further allows for a statistically meaningful analysis of the variation in foreground properties within it. Assuming a fixed dust temperature T d = 19.6 K, an analysis of two independent subregions of that field results in inferred values of β d = 1.52 ± 0.06 and β d = 1.09 ± 0.09, which are inconsistent at the 3.9 σ level. Furthermore, a joint analysis of Spider and Planck 217 and 353 GHz data within one subregion is inconsistent with a simple MBB at more than 3 σ , assuming a common morphology of polarized dust emission over the full range of frequencies. This evidence of variation may inform the component-separation approaches of future cosmic microwave background polarization experiments.

  PublisherDOI

• Measurements of the Temperature and E-mode Polarization of the Cosmic Microwave Background from the Full 500-square-degree SPTpol Dataset

  arXiv (Cornell University) · 2025-01-12

  articleOpen access

  Using the full four-year SPTpol 500 deg$^2$ dataset in both the 95 GHz and 150 GHz frequency bands, we present measurements of the temperature and $E$-mode polarization of the cosmic microwave background (CMB), as well as the $E$-mode polarization auto-power spectrum ($EE$) and temperature-$E$-mode cross-power spectrum ($TE$) in the angular multipole range $50<\ell<8000$. We find the SPTpol dataset to be self-consistent, passing several internal consistency tests based on maps, frequency bands, bandpowers, and cosmological parameters. The full SPTpol dataset is well-fit by the $ΛCDM$ model, for which we find $H_0=70.48\pm2.16$ km s$^{-1}$ Mpc$^{-1}$ and $Ω_m=0.271\pm0.026$, when using only the SPTpol data and a Planck-based prior on the optical depth to reionization. The $ΛCDM$ parameter constraints are consistent across the 95 GHz-only, 150 GHz-only, $TE$-only, and $EE$-only data splits. Between the $\ell<1000$ and $\ell>1000$ data splits, the $ΛCDM$ parameter constraints are borderline consistent at the $\sim2σ$ level. This consistency improves when including a parameter $A_L$, the degree of lensing of the CMB inferred from the smearing of acoustic peaks. When marginalized over $A_L$, the $ΛCDM$ parameter constraints from SPTpol are consistent with those from Planck. The power spectra presented here are the most sensitive measurements of the lensed CMB damping tail to date for roughly $\ell > 1700$ in $TE$ and $\ell > 2000$ in $EE$.

  PublisherOA PDFDOI

• Quantum metrology of low-frequency electromagnetic modes with frequency upconverters

  Physical Review Research · 2025-03-17 · 6 citations

  articleOpen accessSenior author

  We present the RF Quantum Upconverter (RQU) and describe its application to quantum metrology of electromagnetic modes between dc and the very high frequency band (VHF) (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>≲</a:mo><a:mn>300</a:mn></a:mrow></a:math> MHz). The RQU uses a Josephson interferometer made up of superconducting loops and Josephson junctions to implement a parametric interaction between a low-frequency electromagnetic mode (between dc and VHF) and a mode in the microwave C Band (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mo>∼</b:mo><b:mn>5</b:mn></b:mrow></b:math> GHz), analogous to the radiation pressure interaction between electromagnetic and mechanical modes in cavity optomechanics. We analyze RQU performance with quantum amplifier theory and show that the RQU can operate as a quantum-limited op-amp in this frequency range. It can also use nonclassical measurement protocols equivalent to those used in cavity optomechanics, including back-action evading (BAE) measurements, sideband cooling, and two-mode squeezing. These protocols enable experiments using dc VHF electromagnetic modes as quantum sensors with sensitivity better than the standard quantum limit (SQL). We demonstrate signal upconversion from low frequencies to the microwave C band using an RQU and show a phase-sensitive gain (extinction ratio) of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mn>46.9</c:mn><c:mspace width="0.28em"/><c:mi>dB</c:mi></c:mrow></c:math>, which is a necessary step towards the realization of full BAE.

  PublisherDOI

Recent grants

• Collaborative Research: Polarization Sensitive Multi-Chroic MKIDs

  NSF · $321k · 2015–2017

Frequent coauthors

• G. C. Hilton

  National Institute of Standards and Technology

  799 shared

• P. A. R. Ade

  Cardiff University

  549 shared

• C. L. Kuo

  534 shared

• Zeeshan Ahmed

  529 shared

• C. Tucker

  Cardiff University

  467 shared

• K. W. Yoon

  Stanford University

  452 shared

• C. D. Reintsema

  401 shared

• R. W. Ogburn

  SLAC National Accelerator Laboratory

  380 shared

Labs

• Hansen Experimental Physics Laboratory (HEPL)PI