An official website of the United States government
Comparisons of high-accuracy optical atomic clocks \cite{Ludlow2015} are essential for precision tests of fundamental physics \cite{Safronova2018}, relativistic geodesy \cite{McGrew2018, Grotti2018, Delva2019}, and the anticipated redefinition of the SI second \cite{Riehle2018}. The scientific reach of these applications is restricted by the statistical precision of interspecies comparison measurements. The instability of individual clocks is limited by the finite coherence time of the optical local oscillator (OLO), which bounds the maximum atomic interrogation time. In this letter, we experimentally demonstrate differential spectroscopy \cite{Hume2016}, a comparison protocol that enables interrogating beyond the OLO coherence time. By phase-coherently linking a zero-dead-time (ZDT) \cite{Schioppo2017} Yb optical lattice clock with an Al+ single-ion clock via an optical frequency comb and performing synchronised Ramsey spectroscopy, we show an improvement in comparison instability relative to our previous result \cite{network2020frequency} of nearly an order of magnitude. To our knowledge, this result represents the most stable interspecies clock comparison to date.
more »
« less
Application of quantum-limited optical time transfer to space-based optical clock comparisons and coherent networks
With the demonstration of quantum-limited optical time transfer capable of tolerating the losses associated with long ground-to-space links, two future applications of free-space time transfer have emerged: intercontinental clock comparisons for time dissemination and coherence transfer for future distributed sensing in the mm-wave region. In this paper, we estimated the projected performance of these two applications using quantum-limited optical time transfer and assuming existing low-size, low-weight, and low-power hardware. In both cases, we limit the discussion to the simplest case of a single geosynchronous satellite linked to either one or two ground stations. One important consideration for such future space-based operations is the choice of reference oscillator onboard the satellite. We find that with a modestly performing optical reference oscillator and low-power fiber-based frequency combs, quantum-limited time transfer could support intercontinental clock comparisons through a common-view node in geostationary orbit with a modified Allan deviation at the 10−16 level at 10-s averaging time, limited primarily by residual turbulence piston noise. In the second application of coherence transfer from ground-to-geosynchronous orbit, we find the system should support high short-term coherence with ∼10 millirad phase noise on a 300 GHz carrier at essentially unlimited integration times.
more »
« less
- PAR ID:
- 10594245
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Photonics
- Volume:
- 9
- Issue:
- 1
- ISSN:
- 2378-0967
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We describe a general design for a compact frequency comb-based optical time transfer and ranging node with a volume of 14L, a mass of 10 kg, and a power consumption of 46 W. We assess the residual noise from the comb-based system by making both ranging and time transfer measurements using these compact nodes over a 4.4 km free-space testbed. We demonstrate that this node design has the potential to support sub-femtosecond clock comparisons and sub-micron range measurements at averaging intervals of 1 s with a mean received power of 20 nW. This is more than sufficient to support future space-based distributed coherent sensing at observing frequencies beyond 1 THz.more » « less
-
We demonstrate the transfer of a cesium frequency standard steered to UTC(NIST) over 20 km of dark telecom optical fiber. Our dissemination scheme uses an active stabilization technique with a phase-locked voltage-controlled oscillator. Out-of-loop characterization of the optical fiber link performance is done with dual-fiber and single-fiber transfer schemes. We observe a fractional frequency instability of 1.5 × 10−12and 2 × 10−15at averaging intervals of 1 s and 105s, respectively, for the link. Both schemes are sufficient to transfer the cesium clock reference without degrading the signal, with nearly an order of magnitude lower fractional frequency instability than the cesium clocks over all time scales. The simplicity of the two-fiber technique may be useful in future long-distance applications where higher stability requirements are not paramount, as it avoids technical complications involved with the single-fiber scheme.more » « less
-
Abstract As more global satellite-derived precipitation products become available, it is imperative to evaluate them more carefully for providing guidance as to how well precipitation space-time features are captured for use in hydrologic modeling, climate studies and other applications. Here we propose a space-time Fourier spectral analysis and define a suite of metrics which evaluate the spatial organization of storm systems, the propagation speed and direction of precipitation features, and the space-time scales at which a satellite product reproduces the variability of a reference “ground-truth” product (“effective resolution”). We demonstrate how the methodology relates to our physical intuition using the case study of a storm system with rich space-time structure. We then evaluate five high-resolution multi-satellite products (CMORPH, GSMaP, IMERG-early, IMERG-final and PERSIANN-CCS) over a period of two years over the southeastern US. All five satellite products show generally consistent space-time power spectral density when compared to a reference ground gauge-radar dataset (GV-MRMS), revealing agreement in terms of average morphology and dynamics of precipitation systems. However, a deficit of spectral power at wavelengths shorter than 200 km and periods shorter than 4 h reveals that all satellite products are excessively “smooth”. The products also show low levels of spectral coherence with the gauge-radar reference at these fine scales, revealing discrepancies in capturing the location and timing of precipitation features. From the space-time spectral coherence, the IMERG-final product shows superior ability in resolving the space-time dynamics of precipitation down to 200 km and 4 h scales compared to the other products.more » « less
-
Abstract Recent advances in optical atomic clocks and optical time transfer have enabled new possibilities in precision metrology for both tests of fundamental physics and timing applications. Here we describe a space mission concept that would place a state-of-the-art optical atomic clock in an eccentric orbit around Earth. A high stability laser link would connect the relative time, range, and velocity of the orbiting spacecraft to earthbound stations. The primary goal for this mission would be to test the gravitational redshift, a classical test of general relativity, with a sensitivity 30 000 times beyond current limits. Additional science objectives include other tests of relativity, enhanced searches for dark matter and drifts in fundamental constants, and establishing a high accuracy international time/geodesic reference.more » « less
