Single-Electron and Single-Photon Sensitivity with a Silicon Skipper CCD

We have developed ultralow-noise electronics in combination with repetitive, nondestructive readout of a thick, fully depleted charge-coupled device (CCD) to achieve an unprecedented noise level of $0.068e^{-}\mathrm{rms}/\mathrm{pixel}$. This is the first time that discrete subelectron readout noise has been achieved reproducible over millions of pixels on a stable, large-area detector. This enables the contemporaneous, discrete, and quantized measurement of charge in pixels, irrespective of whether they contain zero electrons or thousands of electrons. Thus, the resulting CCD detector is an ultra-sensitive calorimeter. It is also capable of counting single photons in the optical and near-infrared regime. Implementing this innovative non-destructive readout system has a negligible impact on CCD design and fabrication, and there are nearly immediate scientific applications. As a particle detector, this CCD will have unprecedented sensitivity to low-mass dark matter particles and coherent neutrino-nucleus scattering, while future astronomical applications may include direct imaging and spectroscopy of exoplanets.

Figure 1

Single-electron charge resolution using a Skipper CCD with 4000 samples per pixel (bin width of $0.03e^{-}$). The measured charge per pixel is shown for low (main) and high (inset) illumination levels. Integer electron peaks can be distinctly resolved in both regimes contemporaneously. The $0e^{-}$ peak has rms noise of $0.068e^{-}\mathrm{rms}/\mathrm{pixel}$ while the $777e^{-}$ peak has $0.086e^{-}\mathrm{rms}/\mathrm{pixel}$, demonstrating single-electron sensitivity over a large dynamical range. The Gaussian fits have ${\chi }^2=22.6/22$ and ${\chi }^2=19.5/21$, respectively.

Figure 2

Schematic of the Skipper CCD output stage. H1, H2, and H3 are the horizontal register clock phases. MR is a switch to reset the sense node to $V_{\mathrm{ref}}$. M1 is a MOSFET in a source follower configuration. Because of its floating gate, the Skipper CCD readout performs a nondestructive measurement of the charge at the SN.

Figure 3

Readout noise as a function of the number of nondestructive readout samples per pixel for the Skipper CCD. Black points show the rms of the empty-pixel distribution as a function of the number of averaged samples. The red line is the theoretical expectation assuming independent, uncorrelated samples [Eq. (1)].