Our Senior R&D Engineer Dr Mark Mackenzie and R&D Engineer Ben Szutor delivered an online Application Webinar covering laser system applications in Quantum Sensing, Holography, Flow Cytometry, Raman Microscopy, and Interferometric Lithography. This blog shares the webinar recording alongside transcribed text for each presentation slide.
Watch the video below for the full recording, or scroll down to navigate through the slides:
Dr Mark Mackenzie and Ben Szutor
Quantum Sensing & Metrology
When we talk about quantum technologies, we usually separate it into three different groups:
Quantum Computing Quantum computing is based on quantum entanglement - first proposed by Paul Benioff in the 1980s. It is an efficient way to do high-speed, high-efficiency parallel computing with a lot of potential to outperform classical methods. Lasers in these applications are involved in atom cooling or atom conditioning for the atoms that are used in the computers.
Quantum Sensing In quantum applications, sensors are based on doppler cooling and atom trapping - which is a process of confining the atoms keeping them very stable so their frequencies can be measured. Lasers are essential for these applications in processes such as atom cooling, trapping and inspection.
Quantum Cryptography Quantum cryptography is a very secure and relatively cheap method of communication. One of the most popular protocols is the ‘bb-84’ proposed by Bennett and Brassard. This protocol uses characteristics such as the ‘Non-Cloning Theorem’ – for example, when Person A sends a message to Person B, their information is reconciliated so an eavesdropper is always detected.
For these technologies, lasers can be useful for single photon emitters that use two atom or two photon absorption and require very stable output wavelengths.
Inside a quantum sensor
This next slide shows a very broad top-level diagram of a neutral atom clock - the core of atom sensors or quantum sensors. They have multiple stages of trapping:
Single atom beam
First stage doppler cooling
Clock transition and frequency correction
Starting from the left-hand side of the diagram shown on the slide, we see that there is an atomic beam generated which needs to be quite consistent in its flow. It is then cooled down in multiple stages - first it is doppler cooled to about micro-Kelvin temperature, then it is introduced into the Zeeman shifter, and finally confined in an optical lattice. In this optical lattice the atoms are inspected using the clock laser and this gives us a very good frequency reference.
What type of lasers do we need? What are the characteristics that are required for these applications?
Wavelength Lasers need to be at the exact wavelength of the transition for cooling atoms as well as trapping atoms. The wavelength is very important so we need to have a laser that can be tuned to that wavelength.
Power Level Good power levels are important. Intuitively, if we have a higher power we can assume that more atoms can be trapped.
Narrow Linewidth A narrow linewidth is essential. Referring to the graph in the slide below, if you assume that the atomic transition that we're interested in is the green area, then we try to make a laser that matches this green area, so all the power overlaps with the transition linewidth. As such, if our alignment is broader – which is the case of this diagram – then some of the grey area might not be used by the system.
Linewidth characteristics for quantum sensors
When we talk about linewidth, there are different types of linewidth characteristics that we can define and should be taken into account when introducing a laser to a quantum sensor:
1. Instantaneous or short-duration linewidth
Defined by Schawlow Townes limit, where the instantaneous line is inversely proportional to the output power and is proportional to the gain characteristics as well as the type of the laser. For example because of some inherent intensity noise in semiconductor lasers, we can assume that the DPSS laser usually has the lowest instantaneous linewidth.
2. External properties When we talk about longer time intervals such as a few hundred milliseconds then other factors such as external vibrations temperature fluctuations or pressure changes can also affect the linewidth.
3. Long-term wavelength stability
This is the absolute stability of the wavelength over a longer period such as 8 or 24 hours.
How does atom cooling and atom trapping work?
There are multiple stages of cooling atoms. Similarly to normal particles, we can use light to slow down an atom by interacting with its exact transition. Once the atoms are cooled down, they can be confined with an exact frequency – called the magic wavelength – that is used in optical lattice clocks. In this case we can make sure that no external intensity fluctuations will shift the frequency and we can maintain stability of the confined atoms. We can use the clock transition to inspect or measure the frequency of these atoms.
The current time or frequency reference we usually use is the ‘Caesium Standard’ which is at 9.19 gigahertz in the microwave range. This standard is used to define the second, one example is the strontium clock transition at 429 terahertz – the frequency of the forbidden transition from a singlet to triplet straight state. This is inherently very narrow linewidth, so in using this much higher frequency we can assume a better accuracy. As the ‘Allan instability’ shows, the instability is proportional to the linewidth. So the lower the linewidth, the lower the instability, therefore, the better the stability. We also have n which is the number of trapped atoms and tau which is the integration time so if we inspect the atoms for longer or if we have more atoms confined then we can also achieve better stability.
How is optical frequency measured?
We can confine these atoms and we have lasers to inspect them, but how do we measure the optical frequency? One challenge is that the Caesium Standard in the gigahertz range, which means that it can quite easily be measured using electronics. However, there are no electronics that can measure the terahertz frequency ranges. To get around this, a pulsed laser can be used.
So far, we've talked about very stable narrow linewidth continuous wave lasers, however, pulsed lasers are a bit different – they are pulsed and have a short duration, but a very wide frequency spectrum. So-called frequency combs can be created which act as an optical ruler and create discrete frequency lines separated by a discrete repetition frequency. This ruler can be used to measure the frequency or to down-convert the optical frequency to a measurable microwave frequency. Locking the frequency comb to the optical frequency of the sensor allows measurement of the repetition frequency which is usually in the gigahertz range. We can measure the repetition frequency by using another resonator and if we know the ratio between the optical frequency and the microwave frequency, then we can calculate the clock frequency – this is a very neat way of calculating high optical frequencies.
Why develop quantum sensors?
So, if we can all do this what can we actually use this for? One example which we have completed recently is the Gravity Pioneer project led by RSK and the Birmingham Quantum Hub. This project delivered a gravitometer using a quantum sensor. So as this example shows there are new ways of measuring things with quantum sensors. The gravity meter lets researchers see underground, but we can also use quantum sensors to see underwater – so there are there will be a lot of new opportunities to see beyond what we can see with current sensors. However, if you think about the better accuracy that can be achieved with quantum sensing, it can be estimated that with the introduction of quantum sensors we can increase time measurement accuracy by about a factor of 100 meaning that navigation systems such as GPS can also be 100 times more accurate and we could measure things down to millimetre precision instead of the current meter precision scales.
--- End of Laser System Applications in Quantum Technologies --
What is Holography?
Holography is a method of recording 3D images of objects and then later reproducing them. This can be achieved by taking a laser and splitting it into two parts – using a beam splitter – and then allowing one part to interact with the object while the other is unchanged. When these beams are recombined, there will be an interference pattern with the pattern itself depending on the phase changes induced by the object. Therefore, it will contain phase information. This information can be recorded in some light-sensitive media and stored. To later reproduce the image, you take the same laser source or the same frequency as used for the recording and illuminate the media. This effectively performs the same process in reverse, where we end up with a lifelike image of the object – it will have 3D properties and can be viewed from different angles. This has applications in a large number of fields.
For holographic applications, a single frequency laser helps avoid any issues with secondary images being formed. A high power, narrow linewidth laser will give a large acquisition area. Long coherence length will allow for greater acquisition times.
When working on colour or RGB images, specific wavelengths will create the correct colour balance.
Where are holograms used?
Holographic techniques can be applied in museums and in the arts. It can be used for things like taking images of one-of-a-kind artifacts and then reproducing them – allowing for wider distribution. In monochrome, it can be used for more artistic representation.
One of the most widespread and adopted applications of holography is in forgery prevention. This is what we have on all our bank notes and some documents where we have the these holograms which are very difficult to reproduce without the correct equipment.
Holography can be used in digital data storage, where data is saved in three dimensions. As the data is recorded sub-surface, holography drives potential for exceedingly high density of data storage as well as facilitating environmentally resistant, long term storage methods.
One of the biggest up-and-coming areas is virtual reality or augmented reality – where things like wearables or heads-up car displays have the potential to be a very large market.
What is Flow Cytometry?
Flow cytometry is a method which can be used for quickly and autonomously sorting a large population of small items such as biological cells. This is done by confining the cells to a small volume in a laminar flow and flowing them past a series of detectors and laser sources. From the laser, we can then measure the transmission, scatter, absorption, and fluorescence from the cell – giving a large range of information about the cell’s properties. Combining this with the exceedingly large range of fluorescent dyes and proteins available for measuring different properties of cells, this can also provide information about cell properties such as cell health, calcium levels, and protein levels – to name a few.
This enables fast analysis of very large quantities and allows for real-time sorting. If a cell is detected as either viable or non-viable, it can then be sorted into a different output channel, creating an enhanced population. This can be done electrostatically – where the charge is applied to move the cells. It can also be performed optically – where a powerful laser is used to manoeuvre the cell. These are all non-invasive techniques and do not damage the cells.
Which application areas use Flow Cytometry?
Flow cytometry has applications in many fields such as life sciences, microbiology, and food quality control. Flow cytometry has been widely adopted in the past, and what has been happening more recently in the research of this is a push to miniaturize flow cytometry systems. This has the benefits of making them smaller and cheaper, but also reduces sample volumes which are required. For example, instead of needing a full syringe of blood, you take a pin prick to perform the analysis – which is much easier for the patient. To enable this push to make cytometric systems smaller and more portable, you want small, robust, and cost-effective lasers.
What is Raman Microscopy?
Raman microscopy is a method of measuring the vibrational energy states of small items such as biological cells or small quantities of chemicals. Where traditionally when light is incident on a material, it will be absorbed and then re-emitted with scattering at the same wavelength – this is called Rayleigh Scattering. Instead, Raman microscopy use Stokes scattering. This is when light is incident on a sample and either gains or donates energy to the vibrational energy states of that material – giving information about it.
This is a very rare process compared to Rayleigh Scattering and therefore quite difficult to detect. It requires good filtering of the incident light and thus needs narrow linewidth lasers to enhance resolution and allow for large sample processing rates. Different samples require different wavelengths and can be in the UV, Visible, or Near-Infrared. This is quite a complex field, and you need knowledge of your sample to optimize wavelength selection. For example, if you have a biological sample and you illuminate it in the UV you may damage or vaporize it so you might want to use an IR beam. Conversely, with a crystal sample you may need the high signal from a UV source to facilitate inspection.
What are the applications for Raman Microscopy?
Applications for Raman Microscopy are in various fields within biology, pharmaceuticals, forensics, security, and semiconductors. It can be used for identifying materials, measuring the different levels of two chemicals within a sample, or determining the authenticity of an artifact in a museum.
A more advanced method of Raman Microscopy is Confocal Raman Microscopy – where you combine the abilities of confocal microscope which limits the acquisition area to a small 3D volume with the Raman technique. This can allow you to measure in three dimensions and allows for sub-surface measurements. For example, maybe you've modified a crystal and you want to measure how this has changed with depth.
What is Interferometric Lithography?
Interferometry is a highly accurate measurement technique that is useful for the inspection of mirrors, semiconductors, and even gravitational wave measurement.
To do this, a laser beam is split into two parts - one interacts with the sample, while the other remains unchanged. Then the beams are recombined, resulting in an interference pattern. This pattern will depend on the changes induced by the sample measured and information can be inferred from the resulting pattern.
One of the biggest applications of interferometric lithography is in semiconductor inspection. This is a field with very large amounts of throughput and measurement and thus requires a lot of laser systems.
This field requires high power laser systems in the UV to deliver the best resolution. Lasers also require very good stability and long-term reliability. This is difficult to achieve in the UV and is driving a lot of development in this sector. In laser companies, there will always be a demand to push this further to higher powers to shorter wavelengths and better reliability – so these developments have a lot of potential.
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