A major breakthrough! Oxford University creates world's first quantum network of entangled atomic clocks
Quantum networks have been used in quantum cryptography, quantum computing, and quantum theory verification, and they also have the potential to enhance quantum sensing by distributing entanglement between remote systems. Recently, a team at the University of Oxford in the UK used a photonic link to successfully "entangle" two 88Sr+ ions separated by 2 meters, demonstrating the world's first quantum network of entangled optical atomic clocks. The results show that quantum networks have now reached a sufficient level of maturity to be used in enhanced metrology.
01Beyond the Quantum Limit: Measuring Entangled Systems
Optical atomic clocks are our most accurate tools for measuring time and frequency. They enable precise frequency comparisons between atoms at different locations to detect temporal variations of fundamental constants, properties of dark matter, and for geological measurements. However, the accuracy of measurements on independent systems is constrained by the standard quantum limit (SQL). In contrast, measurements of entangled systems can go beyond the SQL to the ultimate accuracy allowed by quantum theory - the so-called Heisenberg limit - and will be able to detect phenomena on much shorter time scales and reveal previously undetectable signals by reducing the requirement for system stability.
In the standard approach, the optical atomic clock comparison requires measuring each atomic frequency relative to the laser. This laser is used to drive a narrow optical atomic transition, whose relative frequency is determined by observing changes in the atomic state, a measurement typically made using the Ramsey experiment.
Figure 1: a) Experimental setup. The network consists of two captured ion systems, Alice and Bob, separated by 2 m, each containing one 88Sr+ ion (not to scale). A photonic link was used in the experiment to generate remote entanglement; spontaneously emitted 422 nm photons were transmitted through an optical fiber to a Bell state analyzer where measurements were taken to project the ions into the entangled state. This entanglement was experimentally mapped to the S1/2 ↔ D5/2 optical clock transition using a common 674 nm laser with fiber optic noise cancellation (FNC) from the laser system to the optical bench. This laser is also used to detect clock transitions. Each trap has an independent acousto-optic modulator (AOM) for switching, frequency and phase control. A magnetic field of 0.5 mT represented by BA/BB provides a quantization axis in each trap.88Sr+ correlation energy levels are shown in the inset. b) Experimental pulse sequence. In a single experimental sequence, the team measures the ion states after performing Ramsey experiments on the entangled and non-entangled states. Ramsey experiments are performed simultaneously for each ion with a total detection time of TR. The process is repeated for the unentangled state after ion readout, cooling, and state preparation.
02First entanglement of two atomic clocks
The challenge in achieving this entanglement enhancement for remotely located atoms is that the entangled states need to be generated with high fidelity and at high speed (maximizing the measurement duty cycle). This has hampered previous experimental demonstrations. Therefore, the experimental team has proposed a "two-node captured ion quantum network" - the group can create the entangled states of two remotely located 88Sr+ ions with 0.960 fidelity in an average time of 8 ms, which is sufficient to realize the first fundamental network of entangled optical clocks. first fundamental network.
To compare 88Sr+ photoclock transition frequencies, the scientists compared three measurements.
Independent measurements for each atom.
Measurements on unentangled atoms.
Correlation measurements on entangled atoms.
Ultimately, the team described the enhanced effects obtained from entanglement and achieved the relevant proof of principle: entanglement reduced the uncertainty of the measurement by a factor of nearly √2, as predicted by the Heisenberg limit, thus halving the number of measurements needed to achieve a certain accuracy. More than that. In fact, today's optical clocks are typically limited by laser dissipation; in this regime, the experimental team found that using an entangled clock provides an even greater benefit: a fourfold reduction in the number of measurements compared to traditional correlation spectroscopy techniques.
Spectra with and without entanglement are shown. a), b) Ramsey experiments using a probe laser with frequency ωL for Alice's atom (blue) and Bob's atom (pink) were performed for both entangled and unentangled states with Ramsey durations from 0.1 ms to 20 ms. For c)-f), the single-ion (c) and two-ion parity (d) signals are plotted at Ramsey durations of 0.1 ms. The effect of imperfect entangled state generation and imperfect spin-echo pulses increases and decreases the contrast of the entangled states (green diamonds) compared to the single-ion scan (blue squares and pink triangles). The two-ion signal from the unentangled state (orange circles) has about half the contrast of the entangled state signal; similarly, the team plotted the single-ion (e) and two-ion parity (f) signals for a Ramsey duration of 17.5 ms. At this duration, the entanglement enhancement is still evident.
Characterization of entanglement enhancement. a) Single frequency uncertainty for single ion (δ∆-,s), correlated unentangled (δ∆-,u) and entangled (δ∆-,e) measurements, versus Ramsey time (TR). b) Entanglement enhancement versus Ramsey duration (TR) relative to single ion measurements (turquoise squares) and relative to measurements of two unentangled ions (olive circles). c) Entanglement enhancement versus Ramsey duration (TR). d) Entanglement enhancement versus Ramsey duration (TR). e) Entanglement enhancement versus Ramsey duration (TR). f) Entanglement enhancement versus Ramsey duration (TR). g) Entanglement enhancement versus Ramsey duration (TR). of the ions. The theoretical enhancement values for the single ion (blue dashed line) and unentangled state (orange dashed line) are 2 and 4, respectively.
03Reducing magnetic field fluctuations and creating larger atomic clock networks
In this experiment, the team demonstrated a frequency comparison of enhancements using two entangled quantum networks of captured ion atomic clocks; ultimately, showing that entangled atomic clocks can already provide practical enhancements for metrology.
However, the duration of the entanglement generation in this case was only 9 ms compared to the 500 ms detection duration used in state-of-the-art optical clocks. It is mainly the decoherence of the quantum bits due to magnetic field fluctuations that limits the duration of the team's use of probes (probes): this decoherence limits the absolute measurement accuracy to a fractional frequency uncertainty of 10-15, well below the level of the technology of the optical clock. In the future, it will be possible to significantly reduce magnetic field fluctuations by using superconducting solenoids, μ-metal shielding or more advanced dynamic decoupling schemes.
Further, a larger network of atomic clocks could further reduce the measurement uncertainty: the number of ions in the network could be increased by using additional local entanglement operations between ions at each node, or by increasing the number of nodes by using additional photonic links [2].
Reference links：
[1]https://www.nature.com/articles/s41586-022-05088-z
[2]https://www.physics.ox.ac.uk/research/group/ion-trap-quantum-computing/research-areas/quantum-networking
