A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s(-1).

Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s(-1) (ref. 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earth-like orbit, a precision of approximately 5 cm s(-1) is necessary. The combination of a laser frequency comb with a Fabry-Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration, with recent encouraging results. Here we report the fabrication of such a filtered laser comb with up to 40-GHz (approximately 1-A) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or 'astro-comb', is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s(-1) in astronomical radial velocity measurements.

Calibration of an astrophysical spectrograph below 1 m/s using a laser frequency comb.

We deployed two wavelength calibrators based on laser frequency combs ("astro-combs") at an astronomical telescope. One astro-comb operated over a 100 nm band in the deep red (∼ 800 nm) and a second operated over a 20 nm band in the blue (∼ 400 nm). We used these red and blue astro-combs to calibrate a high-resolution astrophysical spectrograph integrated with a 1.5 m telescope, and demonstrated calibration precision and stability sufficient to enable detection of changes in stellar radial velocity < 1 m/s.

Conjugate Fabry-Perot cavity pair for improved astro-comb accuracy.

We propose a new astro-comb mode-filtering scheme composed of two Fabry-Perot cavities (coined "conjugate Fabry-Perot cavity pair"). Simulations indicate that this new filtering scheme makes the accuracy of astro-comb spectral lines more robust against systematic errors induced by nonlinear processes associated with power-amplifying and spectral-broadening optical fibers.

Visible wavelength astro-comb.

We demonstrate a tunable laser frequency comb operating near 420 nm with mode spacing of 20-50 GHz, usable bandwidth of 15 nm and output power per line of ~20 nW. Using the TRES spectrograph at the Fred Lawrence Whipple Observatory, we characterize this system to an accuracy below 1m/s, suitable for calibrating high-resolution astrophysical spectrographs used, e.g., in exoplanet studies.

In-situ determination of astro-comb calibrator lines to better than 10 cm s(-1).

Improved wavelength calibrators for high-resolution astrophysical spectrographs will be essential for precision radial velocity (RV) detection of Earth-like exoplanets and direct observation of cosmological deceleration. The astro-comb is a combination of an octave-spanning femtosecond laser frequency comb and a Fabry-Pérot cavity used to achieve calibrator line spacings that can be resolved by an astrophysical spectrograph. Systematic spectral shifts associated with the cavity can be 0.1-1 MHz, corresponding to RV errors of 10-100 cm/s, due to the dispersive properties of the cavity mirrors over broad spectral widths. Although these systematic shifts are very stable, their correction is crucial to high accuracy astrophysical spectroscopy. Here, we demonstrate an in-situ technique to determine the systematic shifts of astro-comb lines due to finite Fabry-Pérot cavity dispersion. The technique is practical for implementation at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s.

Limits on anomalous spin-spin couplings between neutrons.

We report experimental limits on new spin-dependent macroscopic forces between neutrons. We measured the nuclear Zeeman frequencies of a 3He/129Xe maser while modulating the nuclear spin polarization of a nearby 3He ensemble in a separate glass cell. We place limits on the coupling strength of neutron spin-spin interactions mediated by light pseudoscalar particles like the axion [g(p)g(p)/(4pihc)] at the 3 x 10(-7) level for interaction ranges longer than about 40 cm. This limit is about 10(-5) the size of the magnetic dipole-dipole interaction between neutrons.