Ultrastable Silicon Cavity in a Continuously Operating Closed-Cycle Cryostat at 4 K

We report on a laser locked to a silicon cavity operating continuously at 4 K with $1\times {10}^{-16}$ instability and a median linewidth of 17 mHz at 1542 nm. This is a tenfold improvement in short-term instability, and a $10^4$ improvement in linewidth, over previous sub-10-K systems. Operating at low temperatures reduces the thermal noise floor and, thus, is advantageous toward reaching an instability of ${10}^{-18}$, a long-sought goal of the optical clock community. The performance of this system demonstrates the technical readiness for the development of the next generation of ultrastable lasers that operate with an ultranarrow linewidth and long-term stability without user intervention.

Figure 1

(a) Schematic of the chamber. The second stage of the cryocooler (not shown) is coupled to the cold finger. The thermal model is equivalent to an electrical second-order low-pass filter. The passive shield heat capacity is $C_p=3.5\mathrm{J}/\mathrm{K}$, and the thermal resistivity between the active and passive shields is $R_{a\text{−}p}=395\mathrm{K}/\mathrm{W}$. The thermal resistivity between the passive shield and the cavity is $R_{p\text{−}\mathrm{Si}}=2.3\times {10}^5\mathrm{K}/\mathrm{W}$, and the cavity heat capacity is $C_{\mathrm{Si}}=2.6\mathrm{mJ}/\mathrm{K}$. A section of the cavity’s support structure is also shown. (b) Measured temperature fluctuations of the passive shield (triangles), which are used to estimate temperature fluctuations on the cavity (circles).

Figure 2

(a) Mounting of the cryostat and a top view of the separated platform, consisting of the main plate (gray) that the cryocooler sits on and the split plate (light cyan) that supports the vacuum chamber (dark cyan). The split plate is mechanically decoupled from the main plate and is directly attached to the active vibration isolation (AVI) table. (b) Vibration noise on top of the AVI table when the cryostat is on (black line) and off (red line). (c) Frequency noise of the beat between the silicon cavity and the 40 cm ULE cavity (gray line), and the vibration-induced frequency noise (green line), calculated from the measured vibration noise in the region of good spectral coherence above 10 Hz.

Figure 3

Instability of the 6 cm silicon cavity (circles), 40 cm ULE cavity (squares), and 25 cm ULE cavity (triangles), derived from a three-cornered-hat measurement, whose setup is shown in the inset. Between 0.06 and 100 s, the silicon cavity has an instability around $1\times {10}^{-16}$.

Figure 4

(a) Beat note at 771 nm (green circles), fit to a Lorentzian (red line). (b) Histogram of the beat note linewidths for 75 measurements (blue) and simulations (red) at 771 nm. (c) Computed laser linewidth of the silicon cavity at 1542 nm. The median of the distribution is 17 mHz.