The development of precise atomic clocks plays an increasingly important role in modern society. Shared timing information constitutes a key resource for navigation with a direct correspondence between timing accuracy and precision in applications such as the Global Positioning System.
Atomic clocks are one of the most accurate time devices available today that track time by measuring the resonance frequency of atoms. The ticking of time is measured through microwaves emitted by the electrons around those atoms jumping from a lower to higher orbit as they absorb and then lose energy from a laser. Most atomic clocks use atoms of the isotope caesium-133. Cesium atoms emit microwaves precisely 9,192,631,770 times per second.
Atomic clocks are extremely accurate because they are based on natural and universal atom vibrations. However, even the best atomic microwave clocks can still accumulate an error of about 1 nanosecond over a month due to stability of clocks. For the atomic clocks aboard GPS satellites, making frequent resynchronization efforts across thousands of miles can be logistically costly. That’s why computer scientists and physicists are exploring ways to improve the efficiency of and reduce the error rate for transferring time information.
Exploiting quantum effects can improve clock precision by several orders of magnitude, with an advance to TRL 6 feasible within 10 years. However, utilizing these better clocks will require major reductions in system size, weight, and power. The same report noted that quantum magnetometers, which enable quantum navigation, could be “an important part of achieving GPS-denied advantage,” including because quantum inertial sensing is not susceptible to jamming.
Many defense-critical applications require exceptionally precise time and frequency standards enabled only by atomic clocks. The U.S. Air Force Scientific Advisory Board noted in a recent report that quantum clocks and quantum sensors would merit further investment, since enhanced timing precision could enhance Air Force missions and capabilities, including SIGINT, counter-DRFM, electronic warfare (EW), and also more robust communications. Clock synchronization plays a crucial role in enhancing the bandwidth for communication satellites, and the new approach may help future satellites to be more efficient — and therefore process data more quickly — as quantum communication technology matures over the next few decades.
A Quantum Way to Synchronize Atomic Clocks
Researchers from Singapore and japan suggest in a new paper in the journal Proceedings Of The Royal Society A, to use quantum communication technology to transfer time information directly between satellites.
“The synchronization of clocks has always been done with classical communication, but more recently there have been papers that suggest quantum communication can do this better,” said David Leibrandt, a quantum physicist from the National Institute of Standards and Technology in Boulder, Colorado, not involved in the project.
Atomic clocks keep time by keeping tabs on the quantum information contained in atoms inside the clock. By using quantum communication techniques, that information can be directly transferred between clocks without having to be translated into numbers first, thereby eliminating the fundamental uncertainties associated with measuring and converting quantum data into classical information. While the transfer of quantum information by quantum communication also carries error, it is likely to be much lower than the errors associated with the old technique, according to Giulio Chiribella, a computer scientist from the University of Oxford who worked with colleagues from Hong Kong and Japan on the recent paper on the subject.
They did the math to figure out “what is the minimum amount of [quantum] memory as a function of the precision we want,” said Chiribella. The team’s new paper laid out the theoretical groundwork for what can be gained if clock networks use the new technique, and the trade-off between resources and performance, reports Yuen Yiu, Staff Writer in insidescience.
Global atomic clock network through quantum entanglement
A small team of physicists from US and Denmark have outlined the idea of networking the atomic clocks located all around the world through quantum entanglement. They proposed that such a clock would allow all countries to agree on a precise measurement of time, while also creating a massive quantum sensor for probing cosmic mysteries.
The current global standard Coordinated Universal Time (UTC) is generated by International Bureau of Weights and Measures in Paris, France, by one month signal average of clocks of Timekeeping institutes around the world each having their own caesium clocks. Eric Kessler at Harvard University and his colleagues think that quantum entanglement could provide a real-time solution. When quantum objects such as atoms are entangled, measuring one has a direct and predictable effect on the other.
If you were to entangle atomic clocks around the world and on orbiting satellites, it would help them to tick in unison, says Kessler. The team calculated that a such a global quantum clock network would be about 100 times more precise than any individual clock due to reduced measurement noise. It would also be more secure, as the laws of quantum mechanics would immediately alert you to any attempts at eavesdropping.
Physicists in the US and Serbia have created an entangled quantum state of nearly 3000 ultra cold atoms using just one photon, thirty times the previous record of only 100. The precision of modern atomic clocks is “proportional to the square root of the number of atoms”, that means the accuracy of clock has been enhanced more than five times.
Light is shone into one side of the cavity and allowed to bounce back and forth between the mirrors. Some of the light will eventually escape through the opposite side of the cavity, where it is captured by a detector. A magnetic field is applied to the atoms, which causes them to align their spins along the length of the cavity. However, the probabilistic nature of quantum mechanics means that the spins are not all aligned and their directions will fluctuate about the magnetic field.
The researchers solved this challenge by fireing an extremely weak polarized laser pulse into the cavity. Occasionally, just one photon in the pulse will bounce back and forth in the cavity and interact with nearly all of the atomic spins. This succession of interactions is what entangles the atoms.
The quantum network is a worthy goal, says Ruxandra Bondarescu at the University of Zurich, Switzerland, because it could double as a sensor for conducting fundamental physics experiments. A highly sensitive global clock could be used to measure minute variations in Earth’s gravitational field, or to hunt for ripples in space-time known as gravitational waves, which would fractionally shift the clock’s tick. But entanglement is a very delicate state, so it may be a while before such a large quantum network could come online.
Chinese Researchers develop a new satellite-based quantum-secure time transfer (QSTT) protocol
In a new study, scientists at the University of Science and Technology of China used a similar principle to exploit quantum signals (i.e., single photons) as carriers for what is known as time transfer. They have come up with a new satellite-based quantum-secure time transfer (QSTT) protocol that could enable more secure communications between different satellites or other technology in space. Feihu Xu, one of the researchers who carried out the study, said, “Thanks to the quantum non-cloning theorem we used, any attempt to intercept the single-photon will inevitably disturb the quantum state, which can be checked via post-processing. This allowed us to attain a quantum-secure time transfer scheme.”
This new protocol has been demonstrated by applying it to the Micius quantum satellite. The time precision it achieved is remarkable than that of T2L2, a state-of-the-art technique to achieve time transfer that was applied on the Jason-2 satellite, which is based on the use of intense classical laser pulses. Xu said, “We performed a satellite-to-ground time synchronization using single-photon-level signals and achieved a quantum bit error rate of less than 1%, a time data rate of 9 kHz and a time-transfer precision of 30 ps.”
Scientists also demonstrated the feasibility of achieving satellite-based high-precision time transfer with single photons; their work also opens up new exciting possibilities for future research. Xu said, “Our work introduces new perspectives for the physics field to exploit quantum technology to attain greater security and higher accuracy for time-frequency transfer, clock synchronization and quantum networks of clocks. We now plan to construct a satellite-based global-scale quantum network to test fundamental physics and to provide practical applications, such as distributing secret keys, synchronizing clocks, and so forth.”
A quantum network of clocks
On the one hand, capabilities to maintain phase coherent optical links spanning the entire visible spectrum and over macroscopic distances have been demonstrated, with the capability of delivering the most stable optical oscillator from one color or location to another . On the other hand, quantum communications and entanglement techniques are enabling distant quantum objects to be connected in a quantum network , that can enable novel, extraordinary capabilities.
Combining these two technological frontiers, REsearchers from Harvard and Colorado universities have shown that a distributed network composed of quantum-limited clocks separated by large distances – as appropriate, e.g., for the satellite-based clocks possibly operated by different nations – can be operated as an ultimate “world clock”, where all members combine their individual resources in a quantum coherent way to achieve greater clock stability and distribute this international time scale in real time for all.
The distributed achitecture allows each participant of the network to profit from a stability of the local clock signal that is enhanced by a factor proportional to the total number of parties (as compared to an independent operation of the individual clocks) without losing sovereignty or compromising security. This cooperative gain strongly incentivizes joining the collaborative network while retaining robustness against disruptions of communication channels by allowing the parties to fall back to individual clock operation.
Furthermore, enabled through the use of quantum communication techniques, such a network can be made secure, such that only parties contributing to its operation may enjoy the benefit of an ultra-precise clock signal. Besides serving as a real-time clock for the international time scale, the proposed quantum network also represents a large-scale quantum sensor that can be used to probe the fundamental laws of physics, including relativity and connections between space-time and quantum physics.
Each clock cycle consists from three stages: preparation of the clock atom state (initialization), interrogation by the LOs (measurement) and correction of the laser frequency according to the measurement outcome (feedback). In the initialization stage of each clock cycle, entangled states spanning across the nodes at different geographical positions of the network are prepared. In addition to locally operating the individual clocks, the different nodes (i.e., satellites) employ network-wide entangled states to interrogate their respective local oscillators (LOs). The acquired information is sent to a particular node serving as a center where it is used to stabilize a center of mass mode of the different LOs. In the final step of entangling, all nodes (including the center) extend the entanglement to all of their remaining clock qubits.This yields an ultra-precise clock signal accessible to all network members. This guarantees an optimal use of the global resources, achieving an ultra-precise clock signal limited only by the fundamental bounds of quantum metrology and, in addition, guaranteeing secure distribution of the clock signal.
References and Resources also include
References and Resources also include