Quantum sensing is an overall term that encompasses techniques and methods that use quantum mechanical phenomena to make precise measurements of physical quantities. Quantum mechanical states and effects such as quantum superposition, quantum interference, and quantum coherence are used to improve measurement accuracy beyond the limits of classical sensors.
In quantum sensing, solid-state or even photonic quantum sources are being developed to enable quantum-based measurements. Remarkable progress has been made in this field in recent years.
Remark:
The article on quantum sensor applications for our products is intended to give visitors to our website who are interested in such topics a little insight. We ourselves are primarily concerned with the data acquisition requirements of our customers and are not active in the telecommunications industry or in fiber optic technology. The following content is therefore not to be understood as a scientific treatise, but also reflects subjective impressions.
How it works - the basic principle:
The building blocks for quantum mechanical operations and applications are called quantum dots. These are nanoscale regions (e.g., in a semiconductor material, a metal, or in organic molecules) that exhibit quantum mechanical properties due to their tiny size and special structure. The energy of the charge carriers in a quantum dot no longer takes on continuous values, but discrete values. The spectrum of a quantum dot ensemble can be represented as a Gaussian curve since different size classes of quantum dots emit at slightly different wavelengths. Quantum dots and the combination of several quantum dots (so-called quantum dot ensembles) can be produced in various processes depending on the application.
The figure shows an idealized (pyramid-shaped) free-standing quantum dot of InAs/GaAs with a wetting layer. Similar quantum dots exhibit high symmetry and stability and are used, for example, to generate polarization entanglement in experiments on the interaction of photons from different sources. Image source: see below.
Photonic quantum sensing
For instance, the emission or absorption of photons can occur in a quantum dot, which is regarded as a quantum mechanical transition. The detection of such quantum mechanical transitions is often done using single photon detection (TCSPC). This enables the detection of emitted or absorbed photons, and in this way makes the quantum-based properties and applications of the quantum dot useful in practice.
In photonic quantum sensing, Bose-Einstein condensates, different degrees of freedom of an electromagnetic field, or vibrational modes of solids are used to exploit the characteristic transformation between two quantum states. The characteristic optical states of light show quantum mechanical properties like squeezing "squeezed states" or two-mode entanglement. Therefore, they are extremely sensitive to physical changes, which can be detected by interferometric measurements. By using these quantum states, higher measurement accuracy can be achieved.
Non-photonic quantum sensing
Non-photonic quantum sensing refers to the use of quantum systems that are not based on photons (i.e., light measurements) to make high-precision measurements. Various quantum states and effects play a crucial role in this process:
Spin qubits, for example, are based on the spin of electrons or atomic nuclei and can thus be regarded as tiny magnetic needles.
trapped ions (charged atoms) are retained and controlled using electric and magnetic fields and serve as highly precise quantum sensors for various quantities such as electric fields, time, and frequencies.
Superconducting flux qubits use the Josephson quantum effect and are extremely sensitive to changes in magnetic flux.
Nanoparticles, such as diamonds with nitrogen defect centers (NV centers), can serve as high-precision sensors for magnetic and electric fields.
Note: At the end of this article, we go into more detail about the operation and use of up-to-date sensors.
Because the data analyzed in quantum sensing is collected at the atomic level, it offers tremendous potential in terms of measurement accuracy. Quantum sensors, which exploit quantum phenomena such as superposition and entanglement to measure physical quantities with the greatest possible precision, have become increasingly sophisticated and practical in recent years. For example, measurement setups with nitrogen defect centers in diamonds, superconducting nanowire single-photon detectors, and cryogenic systems including the corresponding components are becoming more and more standard in research and development laboratories.
For instance, quantum gradiometers have already been presented, which work with an atomic interferometer on the basis of quantum.
Also, other environmental parameters, such as the exact analysis of geomagnetic fields or even the atmospheric composition become measurable for the first time in this accuracy.
The ever-improving quantum sensing technology has been increasingly integrated with other quantum technologies. For example, the combination of quantum sensing and QKD enables efficient error correction and noise-resistant measurements.
Practical applications can already be found today in the following application areas:
Microscopy and medical imaging
Navigation, positioning systems, and gyroscopes
communication technology
electric and magnetic rock sensors
geophysical exploration, prospecting of minerals, and mineral resources
Research on new quantum sensor technology is currently working on the continuous improvement of quantum coherence so that the possibilities of quantum physics can be used for powerful new technologies. This comes into play, especially in the study of molecular biological processes. These processes typically exhibit only limited coherence because they are extremely susceptible to interactions with the environment.
Currently, the following quantum detectors have become established:
(Follow the links for more information about each sensor.)
View A of Nitrogen-vacancy Center: the blue atoms represent Carbon atoms, red atom represents Nitrogen atom substituting for a Carbon atom, and yellow atom represents a lattice vacancy. Image source: see below.
An ion trap in an ultra-high vacuum vessel. In the center of the picture, a small bright dot is visible – a single trapped 88Sr+ ion. (Overall 1st in the EPSRC 2018 Science Photography Competition; crop slightly changed here.)
In cooperation with the Physikalisch-Technische Bundesanstalt and the Leibnitz Universität Hannover, the Institute for Experimental Quantum Metrology is developing a new and particularly compact linear ion trap, which can be integrated into a compact vacuum chamber. The aim is to use multi-ion spectroscopy in a commercial optical clock. Image source: see below.
Photon-based detectors include e.g. SPADS, PMTs, or APDs. The figure shows an InGaAs APD receiver from Exelitas. This hybrid receiver features an InGaAs avalanche photodiode (InGaAs APD) and a preamplifier. The hybrid concept with an amplifier and photodetector in the same hermetically sealed TO-8 package enables low noise detection and reduces parasitic capacitance. Image source: see below.
Schematic design of a novel room-temperature photodetector using semimetallic bismuth nanowire arrays in conjunction with graphene. The I(V) between the bismuth base and drain is linear and the resistance is 90 ohms. Due to the generation and transfer of photocarrier pairs at the interface, the photocurrent generated by the built-in interface field is robust without affecting the detection spectrum. Image source: see below.
The figure shows the pairwise tunneling of electron pairs through a barrier in a SQUID. These are based on cryogenics and have been used for years in SQUID magnetometers to measure minimal magnetic fields. Image source: see below.
The figure shows a Quantum Hall Effect chip from the manufacturer graphensic with a chip size of 3.5mm*3.5mm, on a 350 μm substrate (semi-insulating SiC with single-layer graphene), which can accommodate up to 9 Hall bars. The graphene allows the quantum Hall effect to be observed at relatively low magnetic fields and high temperatures when properly tuned, here at T ≤ 4K, B ≥ 5T. Image source: see below.
The figure shows the design concept and geometry of the Triad quantum accelerometer (QuAT). The hybrid 3D architecture combines cold atoms and classical accelerometers to achieve a data rate of 1 kHz and an extremely low bias (∼ 5 μg) for the measurement of the three acceleration components. Here, the acceleration components are measured in three mutually orthogonal directions along the wave vectors (kx, ky, and kz) that are perpendicular to the surface of the respective mirror. Image source: see below.
Requirements for TDCs and ADCs used in quantum sensing
Especially in quantum optics, ultrafast spectroscopy, and QKD applications, the requirements for measurements in the time domain are currently increasing. Here you can learn more about the most important of these requirements.
High time resolution: Quantum sensing requires high time resolution to accurately detect and discriminate events that occur on short time scales. TDCs and ADCs must have a fast response time to match the dynamics of the quantum system in question.
Low noise: quantum systems are very sensitive to external noise sources. Therefore, TDCs or ADCs used in quantum sensing applications should have minimal inherent noise so as not to compromise measurement accuracy.
Quantum-limited sensitivity: Ideally, the TDC or ADC should operate close to the quantum limit, i.e., it should not introduce additional uncertainty beyond the fundamental limits imposed by quantum mechanics.
Quantum compatibility: in some quantum sensor setups, the TDC or ADC may interact directly with the quantum system. It is important that the transducer be compatible with the measured quantum state to minimize interference.
Error correction and calibration: Quantum measurements are susceptible to various sources of error. TDCs or ADCs should offer, in addition to the internal calibration features, the possibility to use error correction techniques on the software side to always ensure reliable and precise measurements.
Conclusion:
The latest developments in the field of quantum sensing and the integration of quantum technologies into novel measurement techniques are based in particular on high resolution in the time domain. They thus pave the way to new, highly sensitive, and highly precise quantum measurements. Consequently, to fully exploit the potential of quantum sensing, TDCs, and ADCs must meet stringent requirements to enable accurate, low-noise, and quantum-compatible measurements.
An outlook on future gravity mapping used with a spatial resolution of 0.5 m in a region at an uncertainty level of 20 E. Expected signal magnitudes for a range of applications are given.
Image Sources:
idealized free-standing quantum dot, creator: Alexander Kleinsorge, image rights: public domain, image has been taken from Wikimedia Commons and reformatted to webP.
View of a nitrogen-vacancy center: via Wikimedia Commons, image rights: public domain, image has been taken from Wikimedia Commons and reformatted to webP.
pairwise tunneling of electron pairs in a SQUID: by courtesy of Physikalisch-Technische Bundesanstalt, image has been taken from the article "Quantum magnetic-field sensors" and reformatted to webP.
quantum Hall effect sensor: by courtesy of Amer Ali from the manufacturer graphensic and reformatted to webP.
future gravity mapping: image rights: public domain, image has been taken from Wikimedia Commons and reformatted to webP, creator is Stray et al. 2022 Nature DOI: 10.1038/s41586-021-04315-3
Remark:
The article on quantum sensor applications for our products is intended to give visitors to our website who are interested in such topics a little insight. We ourselves are primarily concerned with the data acquisition requirements of our customers and are not active in the telecommunications industry or in fiber optic technology. The following content is therefore not to be understood as a scientific treatise, but also reflects subjective impressions.