A quantum positioning system (QPS) is proposed that can provide a user with all four of his space-time coordinates. The user must carry a corner cube reflector, a good clock, and have a two-way classical channel of communication with the origin of the reference frame. Four pairs of entangled photons (biphotons) are sent through four interferometers: three interferometers are used to determine the user's spatial position, and an additional interferometer is used to synchronize the user's clock to coordinate time in the reference frame. The spatial positioning part of the QPS is similar to a classical time-of-arrival (TOA) system, however, a classical TOA system (such as GPS) must have synchronized clocks that keep coordinate time and therefore the clocks must have long-term stability, whereas in the QPS only a photon coincidence counter is needed and the clocks need only have short-term stability. Several scenarios are considered for a QPS: one is a terrestrial system and another is a space-based-system composed of low-Earth orbit (LEO) satellites. Calculations indicate that for a space-based system, neglecting atmospheric effects, a position accuracy below the 1 cm-level is possible for much of the region near the Earth. The QPS may be used as a primary system to define a global 4-dimensional reference frame.
A quantum positioning system (QPS) is proposed that can provide a user with all four of his space-time coordinates. The user must carry a corner cube reflector, a good clock, and have a two-way classical channel of communication with the origin of the reference frame. Four pairs of entangled photons (biphotons) are sent through four interferometers: three interferometers are used to determine the user's spatial position, and an additional interferometer is used to synchronize the user's clock to coordinate time in the reference frame. The spatial positioning part of the QPS is similar to a classical time-of-arrival (TOA) system, however, a classical TOA system (such as GPS) must have synchronized clocks that keep coordinate time and therefore the clocks must have long-term stability, whereas in the QPS only a photon coincidence counter is needed and the clocks need only have short-term stability. Several scenarios are considered for a QPS: one is a terrestrial system and another is a space-based-system composed of low-Earth orbit (LEO) satellites. Calculations indicate that for a space-based system, neglecting atmospheric effects, a position accuracy below the 1 cm-level is possible for much of the region near the Earth. The QPS may be used as a primary system to define a global 4-dimensional reference frame.
The Global Positioning System (GPS) provides uninterrupted position and timing data across the globe with a precision of up to 40 ns, but is insufficient for advanced applications such as quantum communication, and is susceptible to jamming and spoofing attacks. This article introduces a protocol for synchronizing clocks using a constellation of satellites that relies on the exchange of entangled photons. Simulation shows that 50 low-Earth-orbit satellites bearing off-the-shelf atomic clocks can distribute time globally with a precision that is 2 to 4 orders of magnitude higher than that of GPS. Additionally, the use of entangled photons provides an extra layer of quantum security.
A wide variety of positioning and ranging procedures are based on repeatedly sending electromagnetic pulses through space and measuring their time of arrival. This paper shows that quantum entanglement and squeezing can be employed to overcome the classical power/bandwidth limits on these procedures, enhancing their accuracy by a factor of the square root of the number of pulses employed times the number of photons per pulse. Frequency entangled pulses could be used to construct quantum positioning systems (QPS), to perform clock synchronization, or to do ranging (quantum radar): all of these techniques exhibit a similar enhancement compared with analogous protocols that use classical light.
The quantum positioning system (QPS) takes advantage of the entanglement property of the quantum system itself, and it is possible to breakthrough the limitation of the positioning accuracy due to signal bandwidth and power limitation in classical radio-based global positioning system (GPS), and improve the positioning performance. In this paper, a comprehensive survey of the research status of QPS and its several key components are presented, including the quantum entanglement states preparation, the quantum satellite technique, the optical signal acquisition, tracking and pointing (ATP) system, and the quantum positioning and clock synchronization technique. We also discuss the current challenges and future research directions at the end of the paper.
Accurate timekeeping is essential for scientific and technological advancements, particularly in areas of communication, networking, navigation, and high precision measurements. While many methods of time resolution are already established using classical resources, they fail to combine high precision outcomes with large-scale implementations. Additionally, numerous quantum networking architectures using satellite-assisted methods have demonstrated quantum communication on scales exceeding ground-based methods. For these reasons, we propose the use of a time synchronization method by which pairs of highly time-correlated photons are exchanged between clocks on satellites and clocks on Earth, al- lowing users to reconstruct the time offsets between their clocks. This is known as Quantum Clock Synchronization (QCS). We analyze global time synchronization, via numerical simulation, using a network of satellites equipped with quantum resources, such as entangled photon sources (SPDC) and single photon detectors. We find that for satellites in low-earth orbits, a configuration of 50 evenly-space satellites provides constant visibility and sub-nanosecond time synchronization to any location on Earth. Due to practical constraints, clocks available for use on satellites are often of lesser quality than those available for ground use. However, by allowing the satellites in our network to synchronize among themselves, we overcome this limitation, forming a highly precise “master clock” capable of global, high precision time synchronization. We then examine the amount of quantum security provided by polarization-entangled photon sources as a potential way to protect an experimental implementation of QCS from attacks such as signal spoofing. To do this, we identify regions on Earth where Bell violations between exchanged entangled photons can ensure quantum security at various confidence vii levels. Combined with similar insights gained from determining the network scale of our QCS protocol at the single-link level, we demonstrate the potential for highly precise, global, and potentially quantum-secure clock synchronization. Finally, we modify our QCS protocol to support frequency, or tick-rate, offsets in addition to constant time offsets. We motivate this by presenting some relativistic effects that can introduce frequency differences between clocks in space and on Earth. Provided that relative velocities between clocks are corrected for, we identify constant and variable clock defects through numerical simulations.
This article will explore the design and implementation of quantum networks in space integrated with quantum networks on Earth. We propose a three-layer approach, involving GEO and LEO satellites integrated with terrestrial ground stations. We first analyze the channel conditions between the three layers, and then highlight the key role of LEO satellites in the integrated space-terrestrial system – namely the source of entanglement distribution between specified terrestrial stations via direct downlink quantum-optical channels. The GEO satellites in the considered system are used primarily as “coordination” stations, managing and directing the LEO satellites regarding the positioning and timing of entanglement distribution. Complexity, in the form of entanglement distillation and quantum-state correction, is concentrated at the terrestrial stations, and teleportation is used as the primary quantum channel in the LEO uplinks and the inter-terrestrial channels. Although our designs are futuristic in that they assume limited quantum memory at the transceivers, we also discuss some near-term uses of our network in which no quantum memory is available.
Position, navigation, and time (PNT) distributed by the global navigation satellite system (GNSS) have become one of the most relied upon commodities of our civilization. GNSS relies on clocks on satellite, the timing signal of which is send to Earth. On ground both, the resulting position and the timing signal are used in various different cases on ground. We are reliant on the quality of the timing signal. Quantum technologies offer novel methods of probing atoms or molecules to generate timing signals of higher quality. Additionally, optical cavities or resonators can provide precise timing signals. The main challenge involved with optical resonators is the long term stability in compatibility with the SWaP requirements of space. There are several works to prolong the stability of these systems and improve the quality of the received signal. Regardless of the quality of the GNSS signal, it is subject to manipulation, such as jamming and spoofing. Additionally, areas of absence of GNSS exist, which need to be breached. In these cases (classical) inertial measurement units can be deployed. However, the inherent drift of these systems allow GNSS-free navigation for short periods.