Scientists and engineers are making progress each day within the domains of quantum technology to improve quantum computers. One such area of improvement is Photon Synchronization, which is a slow process for now due to the probabilistic nature of photon generation. The purpose of this article is to explore a new method developed by researchers to synchronize single photons, which results in a tenfold increase in photon pair coincidence and leads to over 1,000 detected synchronized photons per second.
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About Single-Photon Generation
Single photon generation is a concept where a single unentangled or uncorrelated photon is produced. A pair of photons are considered to be correlated if their interactions lead to a mathematical relationship between their physical properties, such as angular momentum or polarization. At the same time, entanglement suggests that each photon is dependent on the others and that it is impossible to distinguish between their distinct states.
These ideas suggest that when the state or characteristics of one photon change, the characteristics of the second photon also change. For instance, the wave function of the second photon will also collapse if a measurement has been done on the first photon and its wave function collapses. In other words, entanglement refers to a perfect correlation between the particles involved.
Conversely, an independently generated single photon, in its most basic sense, means one photon that is not connected or entangled to another photon even in the slightest.
Synchronization – Dancing with Photons
Picture synchronization as the conductor of an orchestra, ensuring that all the different instruments play in harmony. When we discuss temporal synchronization, it is comparable to two peers who always appear to be in the same location at the same time because they are perfectly aligned.
Synchronization in the context of photons refers to ensuring that their timings coincide, enhancing the probability of photon pairs arriving simultaneously. This synchronization is measured by the 'coincidence rate,' which essentially tells us how often two events occur simultaneously or very close in time. For single photons, it's all about detecting those synchronized pairs.
Quantum Memory – An Atomic Ensemble
Atomic quantum memory is a form of memory storage in which the information is stored in or retrieved from the state of atoms or atomic ensembles. Information is stored by the interaction of atoms with photons. When an atom or a group absorbs the photon, its properties get entangled into the state of those atoms. For retrieval, a controlled laser or electromagnetic field is applied, which causes the photon to be emitted by the atoms in its original state.
Synchronizing Photons at Room Temperature
In a recent article published in the American Physical Society’s Physics Journal, researchers were not only able to synchronize the photons using room temperature atomic quantum memory but also able to achieve 25% efficiency, which means how efficiently the memory can capture and preserve the quantum properties (polarization, state, etc.), of a photon.
These lab-produced photons also had an antibunching value of 0.023, which is the likelihood of a single photon being correlated to another single photon in its arrival time; in other words, this quantifies the probability of single photons being out of synchronization from each other due to quantum effects as classical photon beams, emitting from a regular light source, are deterministic in nature. Thus, antibunching is also a measure of the deviation of a single photon from a classical beam.
This resulted in a significant increase in the photon pair synchronization rate, with over 1000 synchronized photon pairs being detected in a second. This fact was verified by another paper published one month later on the preprint server arXiv.
Triggering Photon Excitation
Fibers were used to connect the photon sources and memory, and it was made sure that both of them had the same atomic scheme (energy levels and transitions of electrons within an electron cloud).
Two-photon sources were used and were excited almost at the same time to get a synchronized wave of the photons. The excitation was done using a triggering signal, which was emitted by a very precisely timed source such as a laser or electric signal. The synchronization of the photons was verified by Hong-Ou-Mandel interference measurement.
Hong-Ou-Mandel Interference Measurement
The Haong-Ou-Mandel effect is also known as the two-photon interference effect. This method was used to check the synchronization of the photons by splitting them into a beam and sending them to a detector. In theory, the probability of their simultaneous detection is greatly reduced if both of them are in synchronization due to destructive interference. This reduction in detection probability confirms the synchronization of the photons.
As per an article published in Reports on Progress in Physics, three decades ago researchers first observed two-photon interference, marking the commencement of a new era in quantum physics. The phenomenon of two-photon interference holds significant intrinsic interest due to its absence of a classical counterpart. Its applications span a wide range, encompassing precision measurement, state determination, quantum computation, and quantum communication.
In the realm of metrology, two-photon interference facilitates the achievement of femtosecond and even attosecond time resolution. Specifically within quantum research, it empowers the measurement of maximally entangled states through Bell state measurements or projection onto the Bell basis. Indeed, Bell state measurements play a pivotal role in the fundamental concepts of teleportation and entanglement swapping.
Applications of Synchronized Photons
Application areas for photon synchronization include quantum computing, quantum meteorology, and quantum communication. These steps can be used to produce pairs of photons with known quantum entanglement. Since the state of one photon affects the state of the other, these photons can be employed for instantaneous communication.
Synchronized photons can also be used to create quantum gates and quantum algorithms, which are the basic building blocks of quantum computers and are applied to create high-sensitivity sensors that can improve the quality of the meteorological data received exponentially.
The quantum technology field is expanding at a rapid rate, which has enthusiasts very excited. However, it's critical to understand that certain technological problems still need our attention and solutions.
For one, the efficiency of these systems is still very low for any practical applications. The author focused on improving the quantum communications network in an article published in Communications Physics.
The research team introduced an enhanced configuration for our previously documented fast ladder memory system. This upgraded setup consisted of improved efficiency and extended operational lifetime, while concurrently mitigating noise levels. The enhancements are achieved through the implementation of a more robust control field, a broader signal beam, reduced atomic density, and an increased optical depth. Notably, during testing with a 2 ns-long pulse, the system demonstrated an impressive 53% internal efficiency, along with a 35% end-to-end efficiency.
Other problems that arise are the low signal-to-noise ratio and the slow speed of photon generation. However, the scientific community is continuously working on all of these fronts, and we can hope to see improvements and practically applicable synchronized photon technology gradually.
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References and Further Reading
Davidson, O. et. al. (2023). F Single-Photon Synchronization with a Room-Temperature Atomic Quantum Memory. Phys. Rev. Lett.131(3). 033601. Available at: https://www.doi.org/ 10.1103/PhysRevLett.131.033601
Thomas, S. E., et al. (2022). "A Single-Photon-compatible Telecom-C-Band Quantum Memory in a Hot Atomic Gas." arXiv preprint arXiv:2211.04415. Available at:
Bouchard, Frédéric, et al. (2020). Two-photon interference: the Hong–Ou–Mandel effect. Reports on Progress in Physics. 84(1). 012402. Available at: https://www.doi.org/ 10.1088/1361-6633/abcd7a
Davidson, O. et al. (2023). Fast, noise-free atomic optical memory with 35-percent end-to-end efficiency. Commun Phys 6, 131. Available at: https://doi.org/10.1038/s42005-023-01247-4