Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Optical atomic clocks, due to their unprecedented stability and uncertainty, are already being used to test physical theories and herald a revision of the International System of Units. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations. Here, we report the first field measurement campaign with a transportable $^{87}$Sr optical lattice clock. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a $^{171}$Yb lattice clock also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.

/pdf/geodesy-and-metrology-with-a-transportable-optical-clock-4e0rdi6kb6.pdf

Geodesy and metrology with a transportable optical clock

Fermionic Quantum Magnetism Optical lattices loaded with cold atoms have been used successfully as quantum simulators of condensed matter systems; however, in the case of fermionic quantum magnetism, achieving low enough temperatures has been a major obstacle. Greif et al. (p. 1307, published online 23 May; see the Perspective by Porto) selectively tuned the exchange interactions in an optical lattice of fermions, forcing a redistribution of entropy such that in the low-entropy subsystem the effective temperature was sufficiently low enough to lead to magnetic correlations. A redistribution of entropy in an optical lattice loaded with atoms leads to magnetic correlations. [Also see Perspective by Porto] Quantum magnetism originates from the exchange coupling between quantum mechanical spins. Here, we report on the observation of nearest-neighbor magnetic correlations emerging in the many-body state of a thermalized Fermi gas in an optical lattice. The key to obtaining short-range magnetic order is a local redistribution of entropy, which allows temperatures below the exchange energy for a subset of lattice bonds. When loading a repulsively interacting gas into either dimerized or anisotropic simple cubic configurations of a tunable-geometry lattice, we observe an excess of singlets as compared with triplets consisting of two opposite spins. For the anisotropic lattice, the transverse spin correlator reveals antiferromagnetic correlations along one spatial axis. Our work facilitates addressing open problems in quantum magnetism through the use of quantum simulation.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005819.pdf

Short-range Quantum Magnetism of Ultracold Fermions in an Optical Lattice

The pursuit of ever more precise measures of time and frequency motivates redefinition of the second in terms of an optical atomic transition. To ensure continuity with the current definition, based on the microwave hyperfine transition in Cs133, it is necessary to measure the absolute frequency of candidate optical standards relative to primary cesium references. Armed with independent measurements, a stringent test of optical clocks can be made by comparing ratios of absolute frequency measurements against optical frequency ratios measured via direct optical comparison. Here we measure the S01→P03 transition of Yb171 using satellite time and frequency transfer to compare the clock frequency to an international collection of national primary and secondary frequency standards. Our measurements consist of 79 runs spanning eight months, yielding the absolute frequency to be 518 295 836 590 863.71(11) Hz and corresponding to a fractional uncertainty of 2.1×10−16. This absolute frequency measurement, the most accurate reported for any transition, allows us to close the Cs-Yb-Sr-Cs frequency measurement loop at an uncertainty <3×10−16, limited for the first time by the current realization of the second in the International System of Units (SI). Doing so represents a key step towards an optical definition of the SI second, as well as future optical time scales and applications. Furthermore, these high accuracy measurements distributed over eight months are analyzed to tighten the constraints on variation of the electron-to-proton mass ratio, μ=m
e
/m
p
. Taken together with past Yb and Sr absolute frequency measurements, we infer new bounds on the coupling coefficient to gravitational potential of k
μ
=(−1.9±9.4)×10−7 and a drift with respect to time of μ˙μ=(5.3±6.5)×10−17/yr.

Towards the optical second: verifying optical clocks at the SI limit

We describe the use of fiber Brillouin amplification (FBA) for the coherent transmission of optical frequencies over a 480 km long optical fiber link. FBA uses the transmission fiber itself for efficient, bi-directional coherent amplification of weak signals with pump powers around 30 mW. In a test setup we measured the gain and the achievable signal-to-noise ratio (SNR) of FBA and compared it to that of the widely used uni-directional Erbium doped fiber amplifiers (EDFA) and to our recently built bi-directional EDFA. We measured also the phase noise introduced by the FBA and used a new and simple technique to stabilize the frequency of the FBA pump laser. We then transferred a stabilized laser frequency over a wide area network with a total fiber length of 480 km using only one intermediate FBA station. After compensating the noise induced by the fiber, the frequency is delivered to the user end with an uncertainty below 2 × 10−18 and an instability σy(τ) = 2 × 10−14 /(τ/s).

Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber

We measured the absolute frequency of the optical clock transition 1S0(F = 1/2)–3P0(F = 1/2) of 171Yb atoms confined in a one-dimensional optical lattice and it was determined to be 518 295 836 590 863.5(8.1) Hz. The frequency was measured against Terrestrial Time (TT; the SI second on the geoid) using an optical frequency comb of which the frequency was phase-locked to an H-maser as a flywheel oscillator traceable to TT. The magic wavelength was also measured as 394 798.48(79) GHz. The results are in good agreement with two previous measurements of other institutes within the specified uncertainty of this work.

https://iopscience.iop.org/article/10.1088/0026-1394/50/2/119/pdf

Absolute frequency measurement of 1S0(F = 1/2)–3P0(F = 1/2) transition of 171Yb atoms in a one-dimensional optical lattice at KRISS

The weighted mean is widely used in combining data sets of experimental measurements with a weight proportional to the value of the data number divided by the sample variance in a conventional method. However, this standard procedure is not appropriate for obtaining the weighted mean frequency of a phase-stabilized signal with white phase noise, since the data are autocorrelated. The autocorrelation is obtained in the case of white phase noise and a new weighting method is proposed. Using this, the uncertainty associated with the weighted mean frequency of a phase-stabilized signal with white phase noise is given. The effect of counter dead-time is also discussed.

The uncertainty associated with the weighted mean frequency of a phase-stabilized signal with white phase noise

The linewidth of a distributed-feedback (DFB)diode laser at 1156 nm, of which free-running linewidth was 3 MHz, was reduced to 15 kHz using an all-fiber interferometer with 5-m-long path imbalance. Optical power loss and bandwidth limitation were negligible with this short optical fiber patch cord. This result was achieved without acoustic and vibration isolations, and the frequency lock could be maintained over weeks. In addition to its simplicity, compactness, robustness, and cost-effectiveness, this technique can be applied at any wavelength owing to the availability of DFBdiode lasers and fiber-optic components.

Linewidth reduction of a distributed-feedback diode laser using an all-fiber interferometer with short path imbalance.

Cold atomic beam from a two-dimensional magneto-optical trap with two-color pushing laser beams

The current status of an Yb optical lattice clock at the Korea Research Institute of Standards and Science (KRISS) is reported. The systematic uncertainty of the Yb clock in the first accuracy evaluation was 1.5 × 10−14 [Park et al., Metrologia 50, 119 (2013)]. The uncertainty was dominated by the large uncertainties in the lattice ac Stark shift and the collisional shift, which were mainly limited by the large linewidth and jitter of the clock laser. Recently, a highly stable clock laser at 578 nm was developed with a short-term linewidth of 3.5 Hz and a frequency jitter of about 25 Hz at 1 s and 10 s measurement times, respectively. The long-term frequency drift showed only a linear dependence on time, confirming that the temperature of the super-cavity was maintained a zero coefficient of thermal expansion. The frequency of the lattice laser at 759 nm was phase locked to the optical frequency comb and could be stabilized at the “absolute” frequency of the “magic wavelength”, to within a 1-MHz uncertainty. This improvement greatly reduced the fractional uncertainty due to the lattice ac Stark shift down to 2 × 10−16. The systematic uncertainty of the clock is currently 5.3 × 10−15 and is dominated by the collisional frequency shift.

Jongchul Mun

Papers

Absolute frequency measurement of 1S0(F = 1/2)–3P0(F = 1/2) transition of 171Yb atoms in a one-dimensional optical lattice at KRISS

The uncertainty associated with the weighted mean frequency of a phase-stabilized signal with white phase noise

Linewidth reduction of a distributed-feedback diode laser using an all-fiber interferometer with short path imbalance.

Cold atomic beam from a two-dimensional magneto-optical trap with two-color pushing laser beams

An Yb Optical Lattice Clock: Current Status at KRISS