Physicists from MIT have introduced a technique that halves the impact of quantum noise in optical atomic clocks, thereby practically doubling their sensitivity to the "ticks" of ytterbium atoms. This method, called global phase spectroscopy, for the first time utilizes the global phase induced by laser light on entangled atoms—a quantity many considered incidental—to stabilize the local oscillator and produce a much cleaner measurement signal. The paper was published on October 8, 2025, and builds on earlier demonstrations of entanglement and "time reversal" in atomic clocks, but this time at an optical, rather than microwave, frequency.
Why Quantum Noise is the Greatest Enemy of Perfect Time
In an ideal world, an atom is a perfect oscillator: its transition between two energy states "ticks" with a frequency that does not falter even under the pressure of millennia. In practice, measuring these ticks is limited by the standard quantum limit (SQL)—stochastic fluctuations in the state projection of each individual particle, which in sum create measurable noise. In optical lattice clocks with 171Yb, where the system contains hundreds of thousands of atoms, the SQL decreases more slowly than desired because increasing the number of atoms also introduces new systematic density shifts. Consequently, techniques that suppress projection noise—such as quantum entanglement and spin squeezing—have become crucial in the race for better clocks.
From Cesium to Ytterbium: A Leap from Microwaves to Optics
While today's international time standard is still based on cesium and microwave transitions, the new generation of clocks works with atoms that oscillate up to 100 trillion times per second. Ytterbium is a favorite in this regard: its optical transition frequency allows for a time resolution that is orders of magnitude finer than that of microwaves, while lattice confinement keeps the atoms "in place," thus reducing the Doppler effect. NIST's ytterbium clocks have already demonstrated stability and a total fractional uncertainty on the order of 10−18, and the reproducibility between two independent clocks surpasses what was until recently considered the limit of laboratory metrology.
A New Idea: Quantum-Enhanced Global Phase
The central discovery of the MIT team is that the interaction of a laser field with entangled atoms leaves a global phase trace even when the system returns to its initial energy state. This "memory" imprint is not a meaningless side effect but a carrier of information about detuning—the difference between the laser's frequency and the atomic transition frequency. By measuring and quantum-enhancing this global phase, it is possible to squeeze quantum noise out of the critical measurement, thereby making the clock more sensitive to tiny frequency differences. The article reports a directly measured metrological gain of several decibels above the SQL and a further improvement in sensitivity to laser noise, which practically means a twofold finer resolution of the "ticks" under the same time conditions.
From the Lab to the Field: Why a Stable Laser is Crucial
Regardless of the quality of the atomic "reference," optical clocks are in practice limited by the stability of the local oscillator—an ultrastable laser that interrogates the transition. When the laser "breathes," all the benefits of a high-frequency standard are erased. This is precisely why the new technique, which extracts additional information from the atom-light interaction to correct the laser, represents a double gain: the clock simultaneously defends itself against quantum noise and learns to make its "heart" beat more calmly.
A Brief History of the Idea: From Entanglement to "Time Reversal" Protocols
Between 2020 and 2022, the MIT team demonstrated that entangling large ensembles of atoms redistributes measurement uncertainties so that the clock can better "see" the average tick. They then introduced a time reversal approach: after generating complex entangled states, the evolution is, in a way, "rewound" to amplify the useful signal in the meantime and read it out more reliably. All of this was first demonstrated at lower (microwave) frequencies, and today's work is the first step that transfers the same principles to the optical transition of ytterbium with a measurable gain.
What the Experiment Looks Like: Cavity QED, a Lattice, and a "Feedback" Laser
An ensemble of ytterbium atoms is cooled to microkelvin temperatures and trapped in an optical lattice so that all atoms "see" the same laser field. The system is placed in an optical cavity (two curved mirrors) where the probe is reflected multiple times, achieving a strong collective coupling of light and matter. In the quantum non-demolition measurement regime, the cavity allows information about detuning to be extracted from the global phase (which arises after excitation and de-excitation back to the initial state). The clock feeds this information back to its own laser through a feedback loop, thereby practically "synchronizing" the oscillator with the atoms.
How Much is "Twice as Good" in Real Clocks
"Twice as precise" in the context of an optical clock means that the system can distinguish a frequency difference that is twice as small in the same integration time. If, for example, the standard deviation of fractional instability was 1×10−16 at 1 second, switching to global phase and quantum enhancement can reduce this number to approximately 7×10−17 with the same number of atoms and the same interrogation length. In integrations from a minute to an hour, the improvement translates into reaching the 10−18 domain faster, which opens the door to new applications where time becomes a substitute for an altimeter, a seismograph, or a dark matter detector.
Transportable Optical Clocks: A Step Closer to the Real World
In the past year, it has been demonstrated that a ytterbium optical clock can be packed, shipped via commercial delivery over a distance of about 3,000 km, and restarted as an independent frequency standard at another location. Although this is a system that still requires strict conditions, it is a clear signal that optical clocks are moving out of clean laboratories and into the realm of "in-the-field" comparisons. The new MIT method directly addresses the biggest problem of these platforms—oscillator stability after transport and in variable conditions—thus expanding the horizon of applications from metrology to geoscience and security.
Geodesy and the "Clock Altimeter"
Optical clocks are so sensitive that they can detect the difference in gravitational potential between two locations separated by just a few centimeters of height difference. This means they can serve as gravimetric sensors and new-generation "altimeters," useful for monitoring water resources, land uplift, volcanic activity, and gravitational anomalies that precede earthquakes. Recent laboratory experiments with miniature clocks and differential comparisons have confirmed that the gravitational redshift can be measured on small scales in conditions closer to the real world, which opens a direct path towards clocks at geodetic points, hydroelectric power plants, or geological faults.
Astrophysics and Fundamental Physics: Dark Matter, Dark Energy, and Symmetries
The stability and accuracy of optical clocks make them superior tools for testing fundamental theories. A sufficiently long and precise record of frequency drifts between two clocks of different elements can, for example, search for variations in fundamental constants or the passage of a compact dark matter object through the solar system. In the future, clocks in space—on satellites with good optical links to the ground—could simultaneously test general relativity and provide a global reference for time and height, a completely new infrastructure for science and industry.
What This Technique Brings to Industry and Everyday Life
More precise time means more reliable financial transactions and data center synchronization, finer timestamping in telecommunications, fewer errors in navigation systems, and more efficient radio networks. For global GNSS systems, more stable reference oscillators reduce position determination errors, and in 5G/6G networks, tighter phase synchronization increases bandwidth and reduces latency. In the energy sector, precise time improves the synchronous stability of grids and facilitates the integration of distributed resources. These are all areas where a transportable optical clock that can be "taken on-site" changes the rules of the game.
Comparison with Other Approaches and What Still Needs to be Solved
Other research teams have demonstrated spin squeezing, quantum non-demolition measurements, and direct comparisons of two "entangled" clock ensembles as ways to surpass the SQL. In parallel, engineering techniques are also being developed: better vacuum chambers, passive/active vibration damping for ultrastable cavities, and laser topologies that reduce excited modes and thermal shifts. MIT's global phase spectroscopy fits into this picture as a solution that "captures" information we have previously discarded—increasing the gain from existing components without necessarily complicating the setup. The remaining challenges include scaling without unwanted density-dependent interactions, the robustness of feedback loops in the field, and interoperability with frequency combs for time distribution.
Who Benefits Most from This Innovation Today
For metrology institutes and laboratories that already have optical lattice clocks based on Yb or Sr, the new method can be implemented as an upgrade to the measurement scheme and stabilization logic—with minimal changes to the atomic package. For geodetic and geophysical teams in the process of transitioning from "portable" microwave standards to optical ones, the added stability of the local oscillator is crucial for operating outside of controlled chambers. For the optical communication and time synchronization industry, improved oscillators mean a more solid reference for fiber-optic networks and faster locking to a frequency standard.
Key Figures and What They Mean in Practice
- 2× higher resolution – the clock can distinguish frequency shifts that are twice as small in the same integration time; this translates to a metrological gain of several decibels above the SQL, along with better resistance to laser noise.
- 10−18 domain – today's Yb lattice clocks already achieve total fractional uncertainties on the order of 10−18, and reaching this domain faster means practical field campaigns can be conducted in days, not weeks.
- 3,000 km field transport – the commercial shipping of a transportable Yb clock and its recommissioning at a new location have been demonstrated, confirming that the logistics are feasible.
How to Get to Application: The Necessary Ecosystem
A portable optical clock is not just an atomic package and a laser. It requires flexible frequency combs, shock-resistant reference optical cavities, stable opto-mechanical platforms, and reliable optical or microwave signal distribution. A successful field campaign also depends on a network of trust: agreements for intercomparison with other institutes, infrastructure for optical links, and software that analyzes stability in real-time and pairs the data with geophysical measurements. The new stabilization technique via global phase fits in here as the "glue" that keeps all subsystems in phase coherence.
The Bigger Picture: From Clocks to New Standards
As the community builds consensus that it is time to redefine the second based on an optical standard, technologies that accelerate the path to low uncertainties under real-world conditions become strategically important. More stable and sensitive clocks are not an end in themselves: they are a fundamental resource for quantum sensors, networks, and communications, and a testing ground for physics beyond the reach of today's accelerators and telescopes. In this sense, harnessing the global phase is not just a clever idea, but an example of how "invisible" quantum quantities can become the measurement currency of future technology.
Additional Resources for Readers Who Want to Go Deeper
For an overview of the state of Yb lattice clocks and their records in stability and uncertainty, it is worthwhile to study the concise technical pages of national metrology institutes. To understand the quantum techniques for surpassing the SQL, review articles on entanglement and time reversal protocols are useful. For applications in geodesy and fundamental tests—from laboratory measurements of gravitational redshift to concepts of clocks in space—extensive reviews and white papers detailing experimental and engineering requirements are available.
Related Reading and Useful Links