Prospects for a Millihertz-Linewidth Laser

Abstract

We propose a new light source based on having alkaline-earth atoms in an optical lattice collectively emit photons on an ultra-narrow clock transition into the mode of a high Q-resonator. The resultant optical radiation has an extremely narrow linewidth in the mHz range, even smaller than that of the clock transition itself due to collective effects. A power level of order 10 −12 W is possible, sufficient for phase-locking a slave optical local oscillator. Realizing this light source has the potential to improve the stability of the best clocks by two orders of magnitude.PACS numbers: 42.50. Nn, 06.30.6v, 37.10.Jk, 37.30.+i, 46.62.Eh Time and frequencies are the quantities that we can measure with the highest accuracy by far. From this fact derives the importance of clocks and frequency standards for many applications in technology and fundamental science. Some applications directly relying on atomic clocks are GPS, synchronization of data and communication networks, precise measurements of the gravitational potential of the earth, radio astronomy, tests of theories of gravity, and tests of the fundamental laws of physics.With the advent of octave spanning optical frequency combs [1,2] it has become feasible to use atomic transitions in the optical domain to build atomic clocks. Optical clocks based on ions [3] and ultracold neutral atoms confined in optical lattices [4] have recently demonstrated a precision of about 1 part in 10 15 at one second and a total fractional uncertainty of 10 −16 [4] or below [3], surpassing the primary cesium microwave standards [5,6].The state-of-the-art optical atomic clocks do not achieve the full stability that is in principle afforded by the atomic transitions on which they are founded. These transitions could have natural line-Qs of order 10 18 , exceeding the fractional stability of the clocks by a factor of ∼ 100. The main obstacle that prevents us from reaping the full benefit of the ultra-narrow clock transitions is the linewidth of the lasers used to interrogate these transitions. These lasers are stabilized against carefully designed passive high-Q cavities and achieve linewidths < 1 Hz, making them the most stable coherent sources of radiation. It is mainly the thermal noise of the reference cavity mirrors that prevent a further linewidth reduction [7] and substantially reducing this noise is hard [8].An elegant solution to these problems would be to directly extract light emitted from the ultra-narrow clock transition [9]. That light could then be used as an optical phase reference, circumventing the need for an ultra stable reference cavity. Unfortunately, the fluorescence light emitted on a clock transition is too weak for practical applications. For instance, for 10 6 fully inverted 87 Sr atoms the power of the spontaneously emitted light is of the order of 10 −16 W.The key observation that motivates this work is that if we could coerce the ensemble of atoms to emit the energy stored in them collectively rather than individually, the resulting power of order 10 −...

Nonlinear spectroscopy of Sr atoms in an optical cavity for laser stabilization

“…Recent studies have proposed a new scheme for laser stabilization to a mK thermal sample of atoms placed in a low finesse optical cavity [16][17][18][19]. By placing the atoms inside the optical cavity atom-light interactions are enhanced by order of the finesse of the cavity and non-linear effects are brought into play, which enhances the spectral sensitivity of the detected photo current used for stabilization.…”

Active optical clock based on four-level quantum system

“…Considering the accuracy requirement, lasing of a three-level conventional laser is not so suitable for an active optical frequency standards because the light shift due to the high intensity pumping laser cannot be tolerable. Thus it needs special design of pumping scheme for active optical clock with three-level configuration [2,9] . Four-level quantum system active optical frequency standards not only possesses the advantages of active clock such as narrow linewidth and high stability and accuracy, but also overcomes the limitation on stability and accuracy due to light shift.…”

“…The potential quantumlimited linewidth of active optical clock is narrower than mHz, and it is possible to reach this unprecedented linewidth since the thermal noise of cavity mode can be reduced dramatically with the mechanism of active optical clock. It has been recognized that active optical clock has the potential to improve the stability of the best atomic clocks by about 2 orders of magnitude [9,10,15].Until now, the major experimental schemes of active optical clock are based on trapped quantum system and atomic beam quantum system. To the latter, the residual Doppler shift will be the main limitation, thus the final accuracy and stability of two-level quantum system are limited by second-order Doppler shift of thermal atomic beam.…”

“…Since the proposal of active optical clock [1][2][3], a number of neutral atoms with two-level, three-level and four-level at thermal beam, laser cooling and trapping configurations have been investigated recently [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The potential quantumlimited linewidth of active optical clock is narrower than mHz, and it is possible to reach this unprecedented linewidth since the thermal noise of cavity mode can be reduced dramatically with the mechanism of active optical clock.…”

Frequency dependence of coherently amplified two-photon emission from hydrogen molecules

“…Coherent amplification of rare processes is one of the uses of the coherence among ensembles of particles. For example, a superradiant laser using an optical clock transition has been proposed as a way to improve the stability of today's best clocks [5]. Coherent amplification can also be used to observe extremely rare deexcitation processes in atoms or molecules that emit neutrinos and a photon, by which the important parameters of neutrinos, such as their absolute masses, mass type, and CP -violating phases, can be determined [6].…”