Quantum Talks

Find out about our latest quantum talks.

30 May 2023 - Iason Apostolatos (Ion Quantum Technology group): Designing Robust Two-qubit Gate Schemes via Quantum Control 'Tricks'.

Two-qubit gates are a much-coveted set of operations in the field of quantum computation. They are the critical ingredient which allows us to achieve entanglement, but they are also the hardest operations to execute. Performing two-qubit gates repeatedly with fidelities exceeding the fault-tolerant threshold (>99%) is a major challenge. This can be attributed to sensitivity to experimental noise, as well as miscalibrations of the gate’s many parameters. In our work, we identify two sources of noise that are detrimental to a gate’s performance: spin decoherence and motional decoherence. In this talk, we will be examining and summarising a set of quantum control techniques ('tricks') which can be applied in order to protect the two-qubit gate from the aforementioned sources of noise. This in turn leads to an improvement of the gate’s performance and an increase of its robustness to noise and miscalibration. However, introducing additional layers of protection and robustness to a two-qubit gate comes with an increased experimental overhead in the form of additional calibrations and additional costs to the available experimental hardware and software. This leads to a trade-off between the gate’s performance and its feasibility of being readily and quickly implemented. 

16 May 2023 - Vijay Singh (Ion Trap Cavity-QED and Molecular Physics group): Towards a Portable Single Ca+ ion Optical Clock.

Optical clocks are the most accurate measurement instruments to date. However, widespread use is being prevented by their large size, high cost and high technical complexity of operation. To overcome these obstacles, we are developing a compact, turn-key-operation portable optical clock based on trapped single Ca+ ions. The system is designed to fit in a 4-unit 19-inch module (50x52x16 cm), with a target weight under 20 kg and a target power consumption under 100 W. The targeted fractional frequency uncertainty of our system is ~10-16. The key for the miniaturisation of the system is optical fibre integration, which provides not only compactness but also robustness. A fibre-based laser system provides all the necessary frequencies to ionise and laser-cool a Ca+ ion. The ion trap is an endcap style trap. Light is delivered to the ion via optical fibres and aspheric lenses inside the vacuum chamber. Fluorescence from the ion is collected using multimode fibres embedded inside the trap electrodes, offering a collection efficiency similar or even superior to traditional high NA lens approaches. Combining this with the clean beam profiles offered by the delivery assemblies we can measure the presence of an ion with outstanding signal to background rations. The Ca+ quadrupole clock transition at 729 nm will be probed using a reference laser frequency stabilised to an ultra-stable optical cavity developed at NPL. Finally, on-board electronics controlling the various subsystems will run the system autonomously, making it a “black box” from the user’s perspective. 

2 May 2023 - Cencen Wei (Materials Physics group): Exotic Electronic Properties of 2D Mica Nanosheets Produced by Liquid Phase Exfoliation.

Phyllosilicate minerals such as mica family are generally considered as being spectrally inactive, electrically insulating, and chemically inert. Here, we demonstrate a method to obtain the aqueous suspensions of few-layer nanosheets by exfoliating the bulk mica in surfactant water solution to contradict the above. The quality of mica nanosheets was checked by employing the Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopies and electron diffraction. By using Raman spectroscopy, an interesting size and layer dependent property was observed. Through the UV-Visible spectroscopy, the high yield nanosheet suspensions of ~1 mg/mL was analysed and the bandgap narrowing from ~7 eV of the bulk to ~4 eV for in the single-layer sheets was obtained. Interestingly, the bandgap inversely scaled with nanosheet areal size, which was measured via Atomic Force Microscopy (AFM). This unusual relationship originated from semiconducting behaviour is due to quantum confinement effects. Moreover, modelling X-ray diffraction (XRD) spectra reveals that lattice relaxation caused the initial bandgap decrease. Finally, mica few-layer nanosheets were proved that they have impressive catalytic abilities in hydrogen production process owing to their broad range of isomorphically substituted ions. 

18 April 2023 - Daniel Nightingale (Quantum Systems and Devices group): Mobile Total Field Optically Pumped Magnetometers for Navigation.

Global navigation satellite systems (GNSS) are at the forefront of navigation and are ubiquitous in everyday life. However, GNSS has limitations for use cases where satellite reception is limited (such as underground navigation). Dependence on the upkeep of existing satellite infrastructure, as well as the prevalence of jamming and spoofing devices limit the reliability of GNSS for the localisation of critical hardware. One alternative solution that does not suffer from these drawbacks is magnetic field navigation. This technique uses maps of local magnetic field anomalies and map-matching algorithms to determine position. We present a total-field Mx optically pumped magnetometer (OPM) system capable of operating within Earth's magnetic field. Battery operated control electronics allow for the OPM setup to be mounted on vehicles for the mapping of local magnetic fields. The pairing of this system with a conventional GNSS system will enable the evaluation of map-matching algorithms. 

4 April 2023 - Pedro Taylor-Burdett (Ion Quantum Technology group): Cryogenic System for Characterization of Novel Quantum-Technologies. 

Impressive progress has been made in the field of quantum computing with trapped ions. In our group, we aim to build a ‘universal quantum computer’ capable of performing an arbitrary number of quantum algorithms, enabling problems to be solved that are classically intractable. However, progress towards this goal relies on the continual development of new quantum technologies, i.e. ‘the computer’s hardware’. When it comes to in-vacuum technologies, such as new ion-trap designs or hardware for quantum control, characterisation and design-development can be an arduous and time-consuming process. This is, in part, due to the requirement for cleanroom-conditions and vacuum system ‘baking’ every time the vacuum-system is opened to modify the set-up. Generally, this process means weeks of down-time. In this talk, I will be presenting the 4K cryogenic vacuum-system that has become the ‘testbed’ for many of our innovative technologies in the group. I will explain the advantages of using cryogenics and illustrate how we can achieve a 24h turn-over from one experimental set-up to the next.  

21 March 2023 - Ian Ford (Ion Trap Cavity-QED and Molecular Physics group): Cavity Assisted Ion-Photon Entanglement.

Trapped ions are a leading candidate in quantum networks. They benefit from long coherence times, and high fidelity state preparation and gate operations among other things. However the number of ions that can be well-controlled in any individual trap is very limited. To circumvent this, ions can be distributed among many smaller traps that are connected. Here the interaction of ions with single photons can be used to create these connections. In this way entanglement can be shared across traps allowing for more complex quantum algorithms. We couple calcium-40 ions trapped in a linear Paul trap to an optical cavity formed by two macroscopic mirrors to generate photons. By driving two separate Raman transitions simultaneously the ion state can be entangled to the polarisation state of the photon produced. By using a cavity to perform this ion-photon entanglement we aim to improve the rate of entanglement production that is possible.

7 March 2023 - Sathvik Ajay Iyengar (Rice University, Texas): Transfer-free in Situ Growth of Tunable Au-WSe2 Junctions.

Two-dimensional transition metal dichalcogenides (TMDs) remain a topic of immense interest. Specifically, given their low operational switching costs, they find many niche applications in new computing architectures (eg: neuromorphic computing, hybrid CMOS) with the added promise of continued miniaturization. However, challenges lie in Back End of Line (BEOL) integration temperature and time compliance with regards to current requirements for crystal growth. Additionally, deleterious and time-consuming transfer processes and multiple steps involved in channel/contact engineering can cripple device performance. Through a holistic approach, this work demonstrates kinetics-governed in-situ growth regimes (surface or edge growth from gold) of WSe2 and provides a mechanistic understanding of these regimes. As a proof-of-concept, we fabricate an in-situ device with flawless source-to-drain channel contacts, demonstrating a 2D semiconductor transistor via a “transfer-free” method within the 450-600 C 2h-time window requirement for Back End Of Line (BEOL) integration. We leverage directional edge growth to fabricate contacts with robust thickness-dependent junction tunability. Field effect transistor measurements reveal a low subthreshold swing of ~140 mV/decade, mobility of 107 ± 19 cm2V-1s-1, and robust ON/OFF ratios of ~106.

21 February 2023 - Leigh Thomas Page (Quantum Systems and Devices group): Automated Characterisation of Alkali Vapour-cells for Magnetometry.

The sensitivity of optically pumped magnetometers have matched and even surpassed that of superconducting quantum interference devices (SQUIDs), with a number of advantages including its lack of required cryogenic cooling, increased portability and reduced maintenance costs. This has brought about an increase of interest in the technology. Applications in magnetic imaging, such as medical magnetoencephalography (MEG) or current density imaging in electric vehicle batteries, require many sensor channels to create detailed maps of magnetic fields. One crucial component in the sensor head is the atomic vapour cell. Such cells can be manufactured at large scales using standard silicon microfabrication techniques. To analyse the quality of a large number of microfabricated cells, we developed a system consisting of an open source multipurpose 3-axis robot, mounted with a transmission spectroscopy setup, enabling the scanning of multiple cells in sequence via computer commands. The absorption lines in the scan can then be fit to linewidth broadening models, providing insight into the internal conditions of the cells. The automated quality analysis with our robotic system allows the calibration of manufacturing processes and selection of the vapour cells with desired parameters to be used in high-performance magnetometers.

7 February 2023 - Martin Siegele (Ion Quantum Technology group): Microfabricated Ion Trap Technology for Scalable Quantum Computing

A scalable system is fundamental for a large-scale quantum computer. For trapped-ion quantum computers, quantum charge-coupled devices (QCCD) are a promising approach. There are several challenges that need to be addressed. Shuttling ions between modules is one way to overcome device size limitations.  Another major challenge for quantum computers is the scalable simultaneous execution of quantum gates. One approach to address this in trapped ion quantum computers is the implementation of quantum gates based on static magnetic field gradients and global microwave fields. A scalable system will require local ion loading on modules which can be achieved with integrated atomic ovens. I will discuss how to build microfabricated ion traps that allow these features for the demonstration of key technologies for scalable quantum computers in the IQT group.

24 January 2023 - Amber Shepherd (Ion Trap Cavity-QED and Molecular Physics group): A Ti:Sapph Laser System for the State-selective Photoionisation of Nitrogen

We are investigating possible time variations in the proton-to-electron mass ratio, which are predicted by some extensions to The Standard Model. For this, we will use high precision spectroscopy to probe a vibrational transition in nitrogen ions. In order to load nitrogen ions into the ion trap, we use a REMPI scheme to ionise nitrogen state-selectively into the ground state for the spectroscopy transition. However, the dye lasers used for this have a broad linewidth compared to the REMPI transitions. Therefore, seeded Ti:Sapph lasers with far narrower linewidths are being set up to improve the ionisation efficiency. Currently, both lasers have been set-up and characterised. Following this, frequency conversions will be implemented to produce the desired UV wavelengths of 212 nm and 253 nm.

10 January 2023 - Sean Ogilvie (Materials Physics group): Solution-processed Nanosheet Networks for Environmental Pollutant Sensing

Printed electronic devices facilitate widespread low-cost integration of interconnected sensors for applications including environmental monitoring. Challenges remain however because promising active materials for gas sensing are either synthetic or require elevated operating temperatures, both affecting cost and power consumption. Van der Waals nanomaterials such as graphene and molybdenum disulfide (MoS2) can be prepared from inexpensive naturally-abundant bulk powders using solution processing techniques into nanosheet networks. Semiconducting MoS2 networks exhibit promising electronic properties, but charge transport and sensing response remain limited by inter-nanosheet junctions. Here, we study the electronic properties as a function of processing parameters including size and porosity and demonstrate design rules from enhanced conductivity. In turn, these optimised devices facilitate chemiresistive gas sensing where we demonstrate doping-sensitive and parts-per-billion sensitivity to important air pollutants such as nitrogen dioxide and ammonia, enabling applications in widespread sensor networks for public and environmental health.

29 November 2022 - Shobita Bhumbra (Quantum Systems and Devices group): BEC Magnetic Microscopy 

Cold atoms in the form of Bose-Einstein condensates (BECs) can be used to probe magnetic fields. This technique can be applied to indirectly measure electrical micro-currents within a sample. There are a wide range of applications for the sensing of micro-currents including:  Silver nanowires used in flexible touchscreen technology, flexible electronics and stem cell differentiation facilitated by conductive carbon nanotubes. This talk will cover the BEC microscope concept and the progression of its development.

15 November 2022 - Sahra Ahmed Kulmiya (Ion Quantum Technology group): Trapped Ion Transport and Quantum State Control on Surface Electrode Traps Evolution

Trapped ion qubits achieve excellent coherence times and gate fidelities, well beyond the threshold for fault-tolerant quantum error correction. One of the routes towards scalability is the coherent control and transport of ions between different zones on a microfabricated surface trap. Ion transport operations should be as fast as possible to speed up quantum computation but must also preserve the motional quantum state. Through simulation of trapping potentials and ion dynamics, we can observe the effects of a transport protocol on the ions’ motional state and explore the transition from adiabatic to non-adiabatic evolution. We explore the optimisation of transport protocols using experiment and simulation.

25 October 2022 - David Kay (Ion Trap Cavity-QED and Molecular Physics group): A Flexible Ion Photon Interface for Quantum Computation.

Optical cavities offer an avenue to scale up trapped ion-based quantum computers into a larger network. With an ion coupled to the cavity single photons can be efficiently generated, entanglement can be generated between the state of the ion and the emitted photon. To date, much work in this area generates entanglement between the ion state and the polarisation of the emitted photon. An alternative scheme, time bin entanglement, generates entanglement between the ion state and the time bin in which the photon was emitted. In this talk I will introduce time bin entanglement as our proposed scheme and how it compares to polarisation entanglement. I will discuss the process by which photons are generated in an ion-cavity system and how the indistinguishability of the photon is measured. I will then provide an overview of the system we are currently developing; this will include a look at the femtosecond laser written, selectively-etched electrode chips which feature multiple trapping regions with the ability to shuttle ions through the trap into the dedicated cavity region. Details of the cavity mirrors will be provided including how they are fabricated and how the cavity module is integrated into the overall system.

14 June 2022 - Foni Raphaël Lebrun (Ion Quantum Technology group): Interconnecting Ion Trap Microchip Modules using Qubit Transport Operations.

Recent efforts in trapped ion quantum computing have delivered impressive systems hosting up to 10s of qubits. However, to be able to solve broad and meaningful problems, practical quantum computers will require millions of such fully controlled qubits. To address this fundamental challenge, a quantum computer can be constructed following a modular approach. While this allows for far more modest qubit numbers per module, it also calls for the development of coherent links to distribute quantum information across a large-scale architecture. To realise fault-tolerant quantum computation on such a platform, it is then key that inter-module interactions are realised at rates commensurate with quantum gate speeds. A significant step towards this goal has been the probabilistic distribution of entanglement between ion trap modules using optical interfaces. However, technological limitations currently hamper this photonic interconnect approach from reaching the desired effective connection rates. In this talk, an alternative method based on the inter-module transport of trapped ion qubits will be presented. This method relies on the engineering an electric field interface at the modules' edges, which creates a continuous confining potential that spans multiple modules. I will discuss the experimental set-up that was constructed in the IQT group to investigate the feasibility of this technique. I will then discuss the realisation of quantum matter-links, i.e. inter-module qubit transfers, between two ion-trap microchips. To date, we have implemented such matter-links a rate of 2424 Hz, with a qubit transfer success probability in excess of 99.999993% and without measurable impact on the qubit state during transfer. Finally, I will detail our ongoing efforts to further increase the matter-link connection rates.

3 May 2022 - Scott Thomas (Ion Trap Cavity-QED and Molecular Physics group): Optical Sensing of RF Performance Limits in Microfabricated Ion Traps.

High RF potentials are applied to traps to give tight confinement and long storage times. Low-noise performance is essential for performing coherent control of the ion qubits with high fidelities. The ability for the trap to operate under these RF potentials can be compromised by the presence of electronic breakdown. Even the faintest amount of breakdown can severely diminish the trapping efficiency. An RF testbed that has been developed to characterise the performance of newly fabricated microtraps is presented. Should any breakdown occur during testing it is detected optically. Image processing routines enhance the sensitivity of the measurement such that the onset of surface flashover type breakdown can be detected at amplitudes up to 90 V than is possible with unprocessed images. A calibrated pickup measurement allows for the RF voltage amplitude on the trap to be determined without perturbing the resonant circuit that is used to apply the high voltages. These techniques will be used to improve the development of future devices. The principles demonstrated here also have applications beyond ion microtraps to other types of MEMS devices.