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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Impact of Laser Frequency Noise in Coherent Population Trapping with Cold Atoms

Gregory W. Hoth, Rachel Elvin, Michael W. Wright, Ben Lewis, Aidan S. Arnold, Paul F. Griffin, Erling Riis, Florian Gruet, C. Affolderbach, G. Mileti

Published Apr 1, 2019• 1 citation
Publisher

Abstract

Laser-cooled atoms and coherent population trapping (CPT) are promising tools for realizing a compact microwave frequency reference with excellent stability. To realize a high performance device, it is necessary to understand and minimize all sources of technical noise. Here, we investigate the role of laser frequency noise in cold-atom CPT with an apparatus based on the grating magneto-optical trap (GMOT). We compare the performance of our setup with an external cavity diode laser (ECDL) and a distributed feedback diode laser (DFB). With the DFB, laser frequency noise is one of the dominant noise sources in our system. With the ECDL, it is significantly reduced. We also report frequency stability measurements of our apparatus with a short-term Allan deviation σy(τ) = 3×10-11/√τ up to τ = 10 s.

DOI
10.1109/fcs.2019.8856060

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Laser-cooled atoms and coherent population trapping (CPT) are promising tools for realizing a compact microwave frequency reference with excellent stability. To realize a high performance device, it is necessary to understand and minimize all sources of technical noise. Here, we investigate the role of laser frequency noise in cold-atom CPT with an apparatus based on the grating magneto-optical trap (GMOT). We compare the performance of our setup with an external cavity diode laser (ECDL) and a distributed feedback diode laser (DFB). With the DFB, laser frequency noise is one of the dominant noise sources in our system. With the ECDL, it is significantly reduced. We also report frequency stability measurements of our apparatus with a short-term Allan deviation σy(τ) = 3×10-11/√τ up to τ = 10 s.
The combination of coherent population trapping (CPT) and laser cooled atoms is a promising platform for realizing the next generation of compact atomic frequency references. Towards this goal, we have developed an apparatus based on the grating magneto-optical trap (GMOT) and the high-contrast lin ⊥ lin CPT scheme in order to explore the performance that can be achieved. One important trade-off for cold-atom systems arises from the need to simultaneously maximize the number of cold atoms available for interrogation and the repetition rate of the system. This compromise can be mitigated by recapturing cold atoms from cycle to cycle. Here, we report a quantitative characterization of the cold atom number in the recapture regime for our system, which will enable us to optimize this trade-off. We also report recent measurements of the short-term frequency stability with a short-term Allan deviation of 3 × 10-11/τ up to an averaging time of τ = 10 s.
An atomic clock based on a compact source of cold atoms and coherent population trapping (CPT) is an encouraging goal for future low-volume atomic frequency references. Our experiment seeks to investigate the performance of such a system by applying CPT in a high-contrast lin⊥lin polarisation scheme to our 87Rb grating magneto optical trap (GMOT) apparatus. In this paper, we report on our progress of improving short- term stability of our cold-atom CPT apparatus. Our recent measurements have shown a short-term stability of 5 x 10-11/√τ, with the ability to average down for times τ>100s.
We present a coherent population trapping clock system based on laser-cooled 87Rb atoms. The clock consists of a frequency-stabilized CPT interrogation laser and a cooling laser as well as a compact magneto-optical trap, a high-performance microwave synthesizer, and a signal detection system. The resonance signal in the continuous wave regime exhibits an absorption contrast of ∼ 50%. In the Ramsey interrogation method, the linewidth of the central fringe is 31.25 Hz. The system achieves fractional frequency stability of 2.4 × 10 − 11 / τ , which goes down to 1.8 × 10−13 at 20000 s. The results validate that cold atom interrogation can improve the long-term frequency stability of coherent population trapping clocks and holds the potential for developing compact/miniature cold atoms clocks.
A compact cold-atom clock based on coherent population trapping (CPT) has been developed. The clock typically demonstrates a short-term fractional frequency stability of 4×10−11/√τ, limited by frequency noise on the interrogation lasers. The clock interrogates a sample of 2×105atoms under free-fall with typical cycle and Ramsey periods of 50 ms and 16 ms, respectively. The largest two systematic frequency shifts that can limit the clock's long-term stability are the light shift and the Doppler shift.
An all-optical microwave frequency standard based on coherent population trapping (CPT) in 85Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10-11 τ-1/2(1 s<τ<2000 s). The best stability of 6.4×10-13 is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.
Clocks based on cold atoms offer unbeatable accuracy and long-term stability, but their use in portable quantum technologies is hampered by a large physical footprint. Here, we use the compact optical layout of a grating magneto-optical trap (gMOT) for a precise frequency reference. The gMOT collects 10 7 87Rb atoms, which are subsequently cooled to 20 µK in optical molasses. We optically probe the microwave atomic ground-state splitting using lin┴lin polarised coherent population trapping and a Raman-Ramsey sequence. With ballistic drop distances of only 0.5 mm, the measured short-term fractional frequency stability is 2 × 10 −11/√τ.