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 1×10^{-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.

1.5 µm Lasers with Sub-10 mHz Linewidth.

We report on two ultrastable lasers each stabilized to independent silicon Fabry-Pérot cavities operated at 124 K. The fractional frequency instability of each laser is completely determined by the fundamental thermal Brownian noise of the mirror coatings with a flicker noise floor of 4×10^{-17} for integration times between 0.8 s and a few tens of seconds. We rigorously treat the notorious divergences encountered with the associated flicker frequency noise and derive methods to relate this noise to observable and practically relevant linewidths and coherence times. The individual laser linewidth obtained from the phase noise spectrum or the direct beat note between the two lasers can be as small as 5 mHz at 194 THz. From the measured phase evolution between the two laser fields we derive usable phase coherence times for different applications of 11 to 55 s.

Reduction of residual amplitude modulation to 1 × 10⁻6 for frequency modulation and laser stabilization.

Active control and cancellation of residual amplitude modulation (RAM) in phase modulation of an optical carrier is one of the key technologies for achieving the ultimate stability of a laser locked to an ultrastable optical cavity. Furthermore, such techniques are versatile tools in various frequency modulation-based spectroscopy applications. In this Letter we report a simple and robust approach to actively stabilize RAM in an optical phase modulation process. We employ a waveguide-based electro-optic modulator (EOM) to provide phase modulation and implement an active servo with both DC electric field and temperature feedback onto the EOM to cancel both the in-phase and quadrature components of the RAM. This technique allows RAM control on the parts-per-million level where RAM-induced frequency instability is comparable to or lower than the fundamental thermal noise limit of the best available optical cavities.

Atomic clocks with suppressed blackbody radiation shift.

We develop a concept of atomic clocks where the blackbody radiation shift and its fluctuations can be suppressed by 1-3 orders of magnitude independent of the environmental temperature. The suppression is based on the fact that in a system with two accessible clock transitions (with frequencies ν1 and ν2) which are exposed to the same thermal environment, there exists a "synthetic" frequency ν(syn) ∝ (ν1 - ε12ν2) largely immune to the blackbody radiation shift. For example, in the case of 171Yb+ it is possible to create a synthetic-frequency-based clock in which the fractional blackbody radiation shift can be suppressed to the level of 10(-18) in a broad interval near room temperature (300±15 K). We also propose a realization of our method with the use of an optical frequency comb generator stabilized to both frequencies ν1 and ν2, where the frequency ν(syn) is generated as one of the components of the comb spectrum.

Collisional losses, decoherence, and frequency shifts in optical lattice clocks with bosons.

We have quantified collisional losses, decoherence and the collision shift in a one-dimensional optical lattice clock on the highly forbidden transition (1)S(0)-(3)P(0) at 698 nm with bosonic (88)Sr. We were able to distinguish two loss channels: inelastic collisions between atoms in the upper and lower clock state and atoms in the upper clock state only. Based on the measured coefficients, we determine the operation parameters at which a 1D-lattice clock with (88)Sr shows no degradation due to collisions on the fractional uncertainty level of 10(-16).

Optical clock with ultracold neutral atoms.

We demonstrate how to realize an optical clock with neutral atoms that is competitive to the currently best single ion optical clocks in accuracy and superior in stability. Using ultracold atoms in a Ca optical frequency standard, we show how to reduce the relative uncertainty to below 10(-15). We observed atom interferences for stabilization of the laser to the clock transition with a visibility of 0.36, which is 70% of the ultimate limit achievable with atoms at rest. A novel scheme was applied to detect these atom interferences with the prospect to reach the quantum projection noise limit at an exceptional low instability of 4 x 10(-17) in 1 s.

Doppler cooling and trapping on forbidden transitions.

Ultracold atoms at temperatures close to the recoil limit have been achieved by extending Doppler cooling to forbidden transitions. A cloud of (40)Ca atoms has been cooled and trapped to a temperature as low as 6 microK by operating a magnetooptical trap on the spin-forbidden intercombination transition. Quenching the long-lived excited state with an additional laser enhanced the scattering rate by a factor of 15, while a high selectivity in velocity was preserved. With this method, more than 10% of precooled atoms from a standard magnetooptical trap have been transferred to the ultracold trap. Monte Carlo simulations of the cooling process are in good agreement with the experiments.

Photoassociation of cold Ca atoms

We present the first measurement of a photoassociative spectrum of an alkaline earth element near the dissociation limit. The observed spectrum of Ca2 formed from cold atoms shows the regular vibrational series with the characteristic spacing of the 1/R3 asymptotic potential. The interpretation is in principle simplified compared to previous measurements on alkali metals by the nondegenerate ground state and the missing hyperfine structure of 40Ca. As an example, we derive the natural decay rate of the excited atomic 4p 1P1 state from the positions of the observed vibrational and rotational resonances with reduced uncertainty compared to previous measurements.

Subkilohertz enhanced-power diode-laser spectrometer in the visible.

We report on a high-performance diode-laser spectrometer operating near 657 nm with narrow linewidth (<0.6 kHz) , enhanced power (as much as 40 mW), and low drift (<10 Hz/s) . The spectrometer comprised an extended-cavity diode-laser frequency stabilized to a high-finesse optical resonator and a broad-area antireflection coated laser diode as an amplifier with a single-lobe emission pattern of good spatial purity. The spectrometer was used to record time-domain optical Ramsey spectra of laser-cooled Ca atoms with a resolution of 0.6 kHz.

Simultaneous cw laser emission including a Raman line of a He-Ne laser at six wavelengths in the visible range.

Simultaneous cw laser emission has been observed in a He-Ne discharge at 611.8-, 629.3-, 632.8-, 635.1-, 640.1-, and 650.0-nm wavelengths. The output power and the mode spectra have been investigated for various operational conditions. Spontaneous mode locking of the different lines has been observed. The Raman transition (650.0 nm) pumped by the strong intracavity radiation at 632.8 nm has been investigated in detail and its relevance for a secondary multiwavelength standard is discussed.

Electron storage ring BESSY as a radiometric source of calculable spectral radiant power between 0.5 and 1000 nm.

The spectral radiant power of the electron storage ring BESSY was measured absolutely in the infrared and visible, and its angular distribution in the infrared, visible, and soft-x-ray ranges. The results prove BESSY to be a standard of calculable spectral radiant power, at least for wavelengths from 0.5 to 1000 nm.