Underground quantum sensing set to erupt

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Susan Curtis explores the new breed of quantum gravimeters finding their way onto the slopes of Mount Etna and into tunnels deep under Birmingham

Preliminary testing of the quantum gravity gradiometer designed by Michael Holynski and colleagues at the University of Birmingham, which has now been shown to locate an underground tunnel with a positional accuracy of 20cm. Credit: Crown Copyright

Quantum technologies are most often associated with the promise of the future, with quantum computing and the quantum internet poised to transform our lives in the coming decades. But some quantum devices are already migrating from the research lab to market-ready solutions. Atomic clocks, for example, originally devised by researchers as a hyper-accurate time standard, are now being deployed in investment banks for more precise time-stamping of high-speed financial transactions, while quantum accelerometers have been shown to enable accurate navigation in the absence of a GPS signal.

The quantum behaviour of ultracold atoms has also been harnessed in a new breed of gravity sensors, such as the Absolute Quantum Gravimeter (AQG), developed by French start-up Muquans and now owned by iXblue – iXblue acquired Muquans in May 2021. Based on a Newton free-fall experiment in which a cloud of rubidium atoms cooled close to absolute zero is used as the test mass, iXblue’s system integrates all the experimental components into a single unit that is robust and reliable enough to be deployed in the field for geophysical monitoring, with one of the company's devices currently installed on the slopes of Mount Etna.

‘I sometimes hear the question about what will be the first real-life application of quantum technologies,’ said Bruno Desruelle, managing director of the quantum sensors division at iXblue and former CEO of Muquans. ‘Well, there are already some quantum instruments that are in service now, and we really believe that quantum technology offers a very interesting competitive advantage for gravity measurements.’

Gravimeters are typically used for geophysical monitoring and underground mapping, since small variations in g, the acceleration due to gravity, can offer important clues about the distribution of mass underground. Classical versions already measure gravity with high precision, but the most common devices provide relative rather than absolute measurements and also suffer from significant drifts that degrade the quality of the data. More elaborate instruments can address these technical limitations, but they are delicate to operate and have moving parts that quickly wear out. ‘Our quantum gravity sensor is easy to use and provides absolute gravity measurements with state-of-the-art sensitivity,’ said Desruelle. ‘Thanks to the absence of moving parts, it also allows continuous monitoring of gravity over long periods of time.’

Inside the AQG is a quantum experiment in which laser light is used to create an atomic version of an interferometer. Laser beams are first used to trap and cool rubidium atoms down to temperatures of 1µK, and switching the lasers off allows the supercooled atoms to fall towards the ground. Laser pulses tuned to exactly the right frequency create a quantum superposition of two atomic states – one with extra energy and momentum provided by the lasers and one without. Further laser pulses separate the atoms into two beams and recombine them at the output, where their wave-like behaviour allows them to interfere. Since gravity pulls the two spatially separated atomic beams by a slightly different amount, a phase change is generated at the output that can be used to determine g very precisely.

However, integrating such a sensitive and complex experiment into a compact and field-ready system has been a huge challenge for the team that made the iXblue device. A typical academic setup for quantum gravimetry requires multiple laser beams to create the magneto-optical trap, along with another laser-beam pair to create the matter-wave interferometer and yet more light sources to detect the atoms at the output. To build an industrial equivalent, the key innovation for Muquans, and now iXblue, has been to design a pyramidal reflector that reduces all of this optical complexity into a single laser beam. ‘Reflections from the inner sides of the reflector generate all the laser beams we need for the measurement sequence,’ explained Desruelle. ‘That has simplified the optical architecture and enabled us to create a cost-effective solution.’

The team behind the iXblue system has also perfected the laser technology used to manipulate the cloud of rubidium atoms, which requires laser light at 780nm. They took the early decision to exploit the fibre-based solutions already developed for the telecoms industry, which meant that they had to develop their own frequency-doubling architecture to deliver the optical performance needed to control the cold-atom experiment. Based on a seed laser and optical amplifier operating at 1,560nm, the laser system exploits a nonlinear crystal that can generate 780nm light with a high conversion efficiency. ‘It was challenging to achieve the performance we needed, but this laser architecture has allowed us to exploit industrial fibre-coupled components that avoid any risk of misalignment,’ explained Desruelle. ‘We now have a robust and reliable laser system that also fits into a standard 19-inch rack.’

Such practical optical solutions have enabled Muquans and iXblue to engineer a self-contained unit that can be operated continuously in such remote and exposed locations as the side of a volcano. Consisting of a sensor head containing the cold-atom experiment and a separate control module, the system measures gravity with a precision of better than 10-9, revealing small variations that are caused by density heterogeneities below the ground. ‘The gravity measurement provides information about the spatial and temporal variation of the underground mass distribution,’ said Desruelle. ‘For volcano monitoring, for example, gravity measurements can reveal the underground movement of magma, which can help to anticipate future eruptions.’

More generally, said Desruelle, the quantum gravimeter can be used to detect changes in any type of underground reservoir, which is useful in hydrology, for example, for measuring the depletion of water aquifers, and in the oil and gas industry. ‘We are not claiming that quantum gravimetry will replace all the sensors currently used for geophysical monitoring,’ said Desruelle. ‘But the technique can complement the information provided by seismic sensors and radar, and it offers a better solution than a classical gravimeter.’

The Absolute Quantum Gravimeter from iXblue has been capturing gravity measurements on the slopes of Mount Etna for more than a year. Credit: Muquans/iXblue

Researchers at the University of Birmingham have also exploited atom interferometry to build an instrument that allows civil engineers to detect subsurface structures such as tunnels, cavities and sinkholes. Their quantum gravity gradiometer measures the vertical gradient of g by incorporating two atom interferometers on top of each other. ‘We have one cloud of atoms at one height and another at another height, and we use the same laser beam to measure gravity at the same time,’ explained lead researcher Michael Holynski. ‘Subtracting the two measurements eliminates the effect of external vibrations, which means that the precision we can achieve is limited by the noise of the instrument.’

Removing the effects of vibrations allows underground surveys to be completed more quickly than with classical gravimeters, and reduces noise to make the instrument more sensitive to smaller changes in the underground mass. The gradiometer, which Holynski and his colleagues reported in Nature earlier this year, can detect a tunnel around 2m wide underneath a road surface – a typical scenario for running electric cables and other utilities between buildings – with a positional accuracy of 20cm.

The Birmingham team has designed an hourglass configuration for the two atom interferometers to make the instrument both robust and portable. Reflectors at either end of the sensor head enable each atomic cloud to be trapped and cooled with the same laser beam, while frequency-doubling of telecoms wavelengths ensures reliable operation out in the field. However, commented Holynski, using fibre-based solutions can create problems for controlling the polarisation of the laser beams. ‘We need a specific polarisation state when the laser light interacts with the atoms, and polarisation-maintaining fibres are not perfect,’ he explained. ‘The hourglass configuration helps to get round that problem, since we use a single fibre to deliver the light needed to cool each of the atom clouds. Any change in the intensity of the light has no net effect on the position of the atoms, since the force is the same in every direction.’

Holynski and his colleagues are now attempting to make the instrument robust enough to be used on a moving platform, such as a train or a ship, and fast enough to detect any underground anomalies that might be caused, for example, by the weakening of an embankment. Another key focus is to find ways to make the instrument smaller and more practical, with one important objective to reduce the number of lasers inside the system. ‘The existing instrument has a dedicated laser system for cooling, and separate lasers for controlling and measuring the two atom clouds,’ said Birmingham research fellow Jonathan Winch. ‘We now want to reduce that to a single laser beam by using electro-optics to switch between the frequencies that are needed for different parts of the experiment.’

Such electro-optical approaches can introduce unwanted frequencies into the system, but Winch has solved that problem by using a series of microwave components to select the frequencies for electro-optic modulation, followed by a Fabry-Perot grating to filter out any undesirable wavelengths. ‘We are also looking at different ways to reduce the size of the chamber,’ he commented. ‘We had a bit of flexibility and redundancy in the previous design, but now we know it works, we can focus on making it more compact.’

The Birmingham team now aims to produce a range of instruments for different applications. Larger units offering greater sensitivity might offer the best solution for accurately detecting small subsurface structures, while other use cases might tolerate a small loss in sensitivity for a more portable system. The researchers have just formed a company called Delta G to commercialise the technology, which will require more work to make the sensors easier to use. ‘We have worked in partnership with our colleagues in the civil engineering department here at Birmingham to develop the instrument, but a user still needs input from an expert in atomic physics,’ said Holynski. ‘Our aim is to make a device that could be used by a survey company, which would provide us with the feedback we need to design a useful product.’

Meanwhile, iXblue has been developing its own quantum gravity gradiometer, dubbed the Differential Quantum Gravimeter (DQG). Described in detail in Physical Review A, the prototype uses the same concepts as those deployed in the AQG to measure both g and its vertical gradient. ‘There have been some marginal improvements to the technology, but the basic concept is to place two quantum gravimeters on top of each other,’ said Desruelle. ‘The laser system is almost identical, and we are also using pyramidal reflectors in the sensor head.’

Measuring gravity and its vertical gradient at the same time provides complementary information about the underground mass distribution: the gravity gradient is more sensitive to small underground structures located close to the surface, while the gravity signal can detect larger masses further underground. For example, the gradient measurement has been shown to detect a subsurface cavity with a volume of 1m3 when buried 0.5m below the surface, but the same cavity is not seen on the gravity signal. ‘The DQG can deliver state-of-the-art performance as well as long-term reliability for industrial applications,’ said Desruelle. ‘We see a strong competitive advantage for civil engineering, particularly when compared to classical gravimeters, and we are already having discussions with our customers.’

While iXBlue plans to deliver its first commercial units towards the end of 2023, Desruelle and his colleagues are already working to develop a more compact version of the DQG that also delivers a better signal-to-noise ratio. Their efforts are likely to be buoyed by the additional capabilities offered by iXblue since it acquired Muquans last year. ‘iXblue provides us with access to many high-performance technologies, such as specialty optical fibres, advanced modulation solutions, and micro-optics assembly,’ said Desruelle. ‘We have plenty of ideas for the future, and their industrial know-how and international sales network will help us to ramp up our activities and speed up the development of our quantum technologies. I am confident we will see the result of this merger within the next few years.’

 

SPONSORED: To manage it, you have to measure it

Ambient air monitoring assesses the state of the atmosphere and provides information on air quality to regulators, scientists, industry and the general public. 

Monitoring data is used to assess trends in air quality and the impact of pollution generated by various activities, in addition to determining areas where air quality standards are not being met. 

The National Physics Lab (NPL) is the UK’s National Metrology Institute. Owned by the government’s Department of Business, Energy and Industrial Strategy (BEIS), NPL develops and maintains the county’s primary measurement standards. It is also part of the National Measurement System (NMS), which provides the UK with a national measurement infrastructure and delivers the UK Measurement Strategy on behalf of BEIS. 

Image credit: kamilpetran/shutterstock.com

Objective 

As part of its research to address the measurement issues associated with emissions to the atmosphere of pollutants and greenhouse gases, and the impact of these emissions in terms of ambient air quality, NPL was looking to provide a unique system that offered quick and accurate measurements of multiple airborne pollutants in the atmosphere. 

Any proposed solution had to take into account the following considerations:

  • The mobile nature of the laboratory, therefore equipment would have to be robust and portable;

  • The importance of getting results quickly and so any solution would have to be reliable; 

  • The footprint inside the laboratory is an essential consideration, and therefore any equipment had to be compact. 

Solution 

Photonic Solutions was selected to provide a custom-built tunable laser solution that would enable NPL to provide rapid and accurate measurements of airborne atmospheric pollutants. 

The NPL Differential Absorption Lidar (DIAL) is a sophisticated remote sensing system that functions as a mobile laboratory. This unique system measures multiple airborne pollutants that require a custom suite of laser solutions with wavelengths in the UV and mid-IR. 

Photonic Solutions needed to understand NPL’s exact requirements to propose a solution that facilitated the collection of real-time data on a plethora of airborne pollutants. Photonic Solutions installed two Quantel Q-smart pulsed lasers pumping two Sirah dye lasers. The mid-IR wavelengths are provided by a custom Cobra-Stretch dye laser pumped by a Q-Smart 1500, and the UV wavelengths are provided by a PrecisionScan dye laser pumped by a Q-Smart 850. The tunable wavelength from each system is transmitted across the measurement region, and a small portion of this light is scattered back by the gases, aerosols and particulates in the atmosphere. 

Photonic Solutions knowledge and understanding of the flexibilities and capabilities of this custom suite of lasers gave NPL the assurance that it would understand how the entire system would allow NPL to realise the specific requirements of collecting a plethora of airborne pollutants in real time. T

The robust and field-proven Q-smart pulsed laser is designed for reliability. It has high energy, a small footprint and a wavelength range of 1064, 532, 355 and 266nm, with 213nm on demand. The laser has excellent beam quality, plug and play harmonic generators with automatic phase-matching and a linewidth of 0.005 cm-1 with SLM option. Controlled by the Q-Touch pad, it is a high energy, cost-effective laser system. It operates using an intuitive touch screen interface. 

The Sirah PrecisionScan three stage dye laser is designed to be compact, and so occupy less table space, whilst the controls are now oriented to the top side, and the lid can be attached to the front and backside of the laser, making operation more flexible. The custom dye laser includes Piezo Wavelength Control. This allows for rapid (10 Hz) control of the laser wavelength. The system uses a piezoelectric actuator stage to detune the laser, generating the on/off resonance functionality required for DIAL LIDAR. PrecisionScan laser systems feature double wavelength pumping optics, which allow a change of pump wavelength between 532 and 355nm without touching pump optics. Replaceable dye cells allow fast and easy dye change. 

The Sirah Cobra-Stretch is a two-stage dye laser with space for an additional amplifier or frequency conversion unit. The custom Cobra-Stretch dye laser supplied here included a difference mixing stage to generate the seed infrared pulse for the integrated optical parametric amplifier (OPA). Both these units were integrated into a TANDEM-housing. The OPA generated high energy laser pulses in the infrared (1.5μm to 4.2μm). 

Results 

The solutions detailed above, combined with DIAL’s unique software, can visualise total site emissions for NPL through a series of multi-dimensional concentration plots that highlight critical emission points and concentrations. By combining concentration measurements with wind measurements, DIAL can provide emission quantification in kg/h with low uncertainties. 

Andrew Finlayson, higher research scientist at NPL explained: ‘Photonic Solutions has supplied NPL with the laser source components and detection equipment over several decades and several iterations of NPL DIAL systems. Photonics Solutions’ expertise and continued support has seen the DIAL used over four different continents and for a range of applications, including the measurement landfill methane emissions, methane, volatile organic compounds (VOC) and benzene emissions from oil and gas facilities and the liquid natural gas (LNG) supply chain, SO2 emissions from lignite-fuelled power stations and methane emissions from anaerobic digestion plants.’ 

Further information 

Learn more at www.photonicsolutions.co.uk

 

Canadian start-up Xanadu has built a series of small-scale quantum chips, available in 8- and 12-qubit versions, along with a 24-qubit processor, and a 40-qubit chip now at the fabrication stage. Credit: Xanadu

24 January 2022