Quantum engineering for optical clocks

Today’s most accurate clocks mark the passage of time using transitions of electrons between two energy states in an atom. Such clock transitions are typically induced by the oscillating light wave of a laser that has long-term and ultrahigh frequency stability. In the case of atomic clocks called optical-lattice clocks, ensembles of atoms that have two outer electrons are trapped in a lattice-like pattern of superimposed optical waves, known as a standing wave. The frequency of the optical-lattice light is chosen to be ‘magic’, which means that the light has minimal impact on the frequency of the electronic transitions used for timekeeping. In two papers in Nature, Pedrozo-Peñafiel et al.1 and Young et al.2 demonstrate how concepts in modern quantum technology could aid the development of next-generation optical-lattice clocks.

In the past decade, optical-lattice clocks have undergone improvement at a tremendous rate. However, at least three challenges remain. First, the magic-frequency trap still has a slight effect on the electronic-transition frequency. Second, the clock stability is degraded by clock dead times — periods in which the atoms are being prepared for use, rather than used for timekeeping. And third, quantum fluctuations pose a limit to clock stability. This limit can be surpassed only by the production of a quantum phenomenon called squeezing.

Pedrozo-Peñafiel and colleagues transferred squeezing in an ensemble of ytterbium-171 atoms from a transition between two sublevels of the lowest-energy electronic state of the atoms to the clock transition (Fig. 1a). How does this squeezing help the clock’s performance? Typically, atomic clocks use atoms that are simultaneously in the lower and upper electronic states of the clock transition — a situation known as a quantum superposition. For optical-lattice clocks, many such atoms are prepared in parallel.

Figure 1

Figure 1 | Improvements to optical-lattice clocks. a, Precise timekeepers called optical-lattice clocks fine-tune the frequency of laser light so that it can drive transitions of electrons between a lower energy state and an upper energy state in an atom (the clock transition); the fine-tuned light waves thus act as a precise ‘pendulum’ for the clock. Pedrozo-Peñafiel et al.1 demonstrated that a quantum phenomenon known as squeezing can be transferred from a transition between two sublevels of the lower state to the clock transition. Such squeezing can improve the stability of optical-lattice clocks. b, Young et al.2 made a 16 × 20 array of laser traps called tweezers, and trapped about 150 neutral atoms for clock measurements. They found that this set-up enables a highly stable clock operation.

If the lower and upper states contribute equally to each superposition, the atomic clock ticks at the correct rate. In a clock measurement, each superposition is randomly and individually reduced to either the lower or upper state. These random outcomes add up to the quantum fluctuations mentioned earlier, and degrade the clock stability. But if a squeezed state is used in the clock, the atoms no longer act individually, and the quantum fluctuations are suppressed.

Pedrozo-Peñafiel et al. achieved such suppression in a realistic clock measurement sequence, albeit with a short, millisecond-long probe time for the squeezed state. For high-performance clock operation, this interrogation time would need to be extended by a factor of 1,000, to about one second — a long time for such a fragile system to exist. However, the authors found that their squeezed state could persist for nearly that long.

These measurements indicate that quantum correlations can be combined with the second-long interrogation times that are accessible in optical clocks. But before these clocks can benefit from the demonstrated squeezing, a further technical challenge must be overcome. Often, clock stability is limited by the combination of tiny frequency fluctuations in the ultrastable laser that excites the superposition state and of the dead times for atom preparation.

Young et al. devised an approach to mitigate this challenge. They added an ensemble of about 150 neutral clock atoms to a 16 × 20 array of magic-frequency laser traps known as tweezers (Fig. 1b). The array was loaded with the atoms extremely quickly by an optical potential (potential-energy profile) that was much stronger than that of the tweezer array. The main achievement of the work presented here is the engineering of the array, which enables a high-performance clock operation.

The authors minimized dead time by combining such high performance with long interrogation times of more than 20 seconds. They then made use of the fact that the tweezers in the system can be read out individually to carry out simultaneous clock measurements on two sub-ensembles in the array. They observed a relative stability of the clocks operating on the sub-ensembles that is close to the current record for optical-lattice clocks35.

The geometry of the tweezer array can be chosen at will. For instance, increased distances between tweezers suppress undesirable hopping of atoms between the tweezers and frequency shifts called Doppler shifts. Furthermore, the clock can be operated with much lower intensities of the trapping lasers than when the distances are not increased. Therefore, Young and colleagues’ approach provides an alternative to the use of traps that have high potential-energy barriers between tweezers to prevent hopping, or of gravity-assisted hopping inhibition in optical-lattice clocks6. Moreover, the concept opens a path for a new type of neutral-atom clock based on individually controlled atoms.

Although Young et al. have achieved excellent clock stability, the characterization of the frequency uncertainty in the clock transition is the next step to realizing a fully operational optical-tweezer clock. For example, the method used to form the tweezer array causes frequency shifts across the array that must be controlled.

These two studies impressively demonstrate how quantum-technology developments and precision metrology benefit each other. For optical clocks, sophisticated tools and platforms are now at hand. And in turn, squeezed ensembles of individually detectable clock atoms constitute exciting systems for further applications in the fields of quantum simulation and quantum information.

Nature 588, 397-398 (2020)

doi: https://doi.org/10.1038/d41586-020-03510-y


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