Coupling superconducting resonators to spins in silicon

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology


Spins in nuclear-spin-free solid state systems such as purified 28Si have seen extensive research as candidates for quantum information storage and processing, thanks to their long spin coherence lifetimes [1]. Strongly coupling such spins to a high Q superconducting resonator provides a route to develop microwave quantum memories, harnessing the long coherence times of spins with the fast control of superconducting circuits. Cavity-induced spin relaxation of bismuth donor spins (the Purcell effect), which may be used for resetting of the spin system, has been demonstrated up to 7 mT using Al resonators [2]. Certain donor spins such as bismuth donors can be tuned to so-called 'clock transitions', which, due to their insensitivity to magnetic field noise, can have electron spin coherence times as long as 3 seconds [3]. Achieving coupling to such transitions requires resonators which are both magnetic-field resilient, and frequency tuneable.
[1] A. M. Tyryshkin, S. Tojo, J. J. Morton et al. Electron spin coherence exceeding
seconds in high-purity silicon. Nature Materials, 11, 143 (2012). 1105.3772.
[2] A. Bienfait, J. J. Pla, Y. Kubo et al. Controlling spin relaxation with a cavity. Nature,
531, 74 (2016). 1508.06148.
[3] G. Wolfowicz, A. M. Tyryshkin, R. E. George et al. Atomic clock transitions in
silicon-based spin qubits. Nature Nanotechnology (2013). 1301.6567.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509577/1 01/10/2016 24/03/2022
1781365 Studentship EP/N509577/1 01/10/2016 30/06/2020 James O'Sullivan
Description We have developed the capability of detecting and manipulating donor electrons in silicon and/or yttrium orthosilicate using superconducting circuits with pulsed microwave electron spin resonance (ESR) techniques. This was not previously done in the group and has yielded numerous interesting results. We have pushed the limits of the magnetic field resilience of such superconducting devices, which have wide applications across physics, particularly in quantum information processing, but also for general ESR measurement techniques. These devices can operate at high field, have high quality factors (Q factors) ,over 100,000 and up to 2,000,000, have good frequency tuneability and can offer a platform for high sensitivity wide-band ESR. To date, 4 articles detailing resonator performance and tuning, coupling to donors in YSO and silicon with pulses and continuous wave (CW) techniques have been published, and more are in preparation. One of these articles detailed the linewidths of implanted Bi donors in Si, and demonstrated one of our initial core goals, to tune a superconducting resonator using global magnetic field reorientation, precisely to an optimal working point known as a clock transition, and perform pulsed ESR there. This was published in Phys Rev Applied.

In collaboration with CEA Saclay, we demonstrated a superconducting resonator coupled to an ensemble of Bi donors in silicon at a clock transition that resulted in spin coherence times exceeding 100ms - an enormous improvement over previous results. We used this to demonstrate a spin-echo quantum memory with weak pulses of microwave light mimicking a photon from a superconducting qubit. We stored 20 such pulses in the memory and show that they can be retrieved over 100ms later with their phase information in-tact. This work was published in PRL.

We have also successfully implemented complex shaped pulses and chirped pulses for manipulation of spins using these high Q resonators, and increased the sensitivity of our measurement techniques using state of the art parametric amplification to enhance incredibly weak signals from small quantities of implanted donors. This allowed us to develop a random access quantum memory protocol, in which multiple weak pulses of microwave light are sent into the hybrid resonator/spin ensemble system, then extracted on-demand in any desired order, with a maximum storage time over 2ms. This result is probably the most promising and advanced spin ensemble microwave quantum memory to date and overcomes many of the key issues of previous demonstrations (the key one being that none of those protocols could have been utilized as a real memory due to the nature of the protocol implemented which would cause massive errors even in a perfect system), making the useful application of such devices in a quantum processor within reach. Such a quantum memory has the capability to massively enhance qubit connectivity and reduce the number of qubits required for computation, owing to the dense multimode nature of the memory. Most quantum repeater schemes necessitate such a memory, and our protocol is entirely applicable to the optical regime, where such schemes are typically envisioned. An article detailing the work is in preparation and we expect this result to be high impact.

We have investigated the application of techniques learned from these superconducting quantum information oriented measurements and applied them to the wider field of electron spin resonance, which is of interest for a much broader audience in materials science, chemistry, biology and industry. We showed that by correct implementation of a low noise cryogenically cooled amplifier, the time to measure weak signals using otherwise standard, commercially available apparatus (the most commonly used apparatus in the field) can be reduced by a factor of 300x, with essentially no drawbacks and full compatibility with the original system. This result was published in the journal of magnetic resonance.

We have explored a new type of donor, 125Te, which has numerous desirable properties and is of great interest to the quantum memory and quantum dot communities. This donor has not been studied in such detail and never at the frequency range we investigated (close to zero magnetic field, which is highly desirable for integration with superconducting circuits). These types of donors (group VI) must be singly ionised in order to become useful for ESR. We showed that by implanting close to the silicon surface, naturally ocurring surface band-bending singly ionises the donors at certain depths, a method that has not been used before as far as we know. This removes the requirement for heavy co-doping of group III acceptors such as boron and has the potential to reach much higher activation levels. An article on this is in preparation and we intend to submit to a high impact journal.

Using more conventional ESR techniques, we also explored the effect of Floquet driving on ensembles of P donors in Si as a means of creating a discrete time crystalline state. This was published in NJP.

In summary, we have made great strides in the development of spin ensemble quantum memories and have applied techniques learned in this field to benefit the wider electron spin resonance community. We have explored new donors and new techniques relevant to the field and yielded numerous exciting results, many of which are already published and others which are in preparation.
Exploitation Route Direct application of this research as a microwave frequency quantum memory is the most exciting potential outcome of this research. If successful, superconducting qubits, which lead the way in terms of progress towards a useful quantum computer, could be equipped with hybrid spin memories to boost their coherence times by many orders of magnitude, which is currently one of the most severe limitations of superconducting qubits.

Understanding strain and surface decoherence effects on implanted donors is also of interest to the wider quantum technologies community, which may also be interested in coupling devices to donors for the purposes of developing spin qubits, microwave to optical transducers, etc. Superconducting resonators are a powerful tool for probing these surface effects.

Use of resonators we have developed as a general ESR measurement device could have widespread academic and non-academic uses. It may be extremely useful for applications requiring high sensitivity ESR, particularly when only small quantities of sample are available. Developing resonators that have a uniform magnetic field may make use of this technology, which has so far seen only limited application to general purpose ESR, an extremely attractive prospect.

Field resilient, frequency tunable high Q resonators are also of interest for the quantum information processing community for a variety of applications.
Sectors Aerospace, Defence and Marine,Chemicals,Digital/Communication/Information Technologies (including Software),Pharmaceuticals and Medical Biotechnology

Description Spin-out company has formed, with the support of with the UCLQ innovation fund and UCL business, to commercialize the development of a cryogenic preamplifier setup for ESR spectrometers. This is still in early stages but interest in ordering a device (currently being prototyped by a manufacturer) has been expressed by various academic groups. We expect these devices will attract a lot of attention, at first within the European ESR research community, but most likely this will spread worldwide with the adoption of the technology and may cross over to industrial research applications in time. This is a UK based company and thus the associated economic benefits will also benefit the UK.
First Year Of Impact 2020
Sector Other
Title Enhancement of commercial and home-built ESR spectrometer sensitivity by up to 300x 
Description We incorporated a cryogenic preamplifier into a commercial ESR spectrometer commonly used by physicists, chemists, biologists and materials scientists in research and industry. We managed to reduce measurement time of weak signals by up to 300x using this technique, with essentially no drawbacks. 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact Publication of the paper detailing the preamplifier and its improvements. Several other works, for example a study of a new donor, 125Te, are in writing, which use this technique. 
Title Pulsed ESR spectrometer 
Description Built a new ESR spectrometer, plus 4-8GHz bridge and software to run the experiment. This facilitates pulsed ESR experiments with a high degree of versatility for the purposes of measuring spins coupled to resonators in a dilution fridge in the 4-8GHz band. 
Type Of Material Improvements to research infrastructure 
Year Produced 2018 
Provided To Others? No  
Impact First measurements of ND doped YSO at millikelvin temperatures, using superconducting resonators, including T2 vs temperature measurements. Data is still being finalized but is to be written up into a paper. Coupling of resonators to a bismuth clock transition has been observed using this setup, however signal was weak and we are working on improving this before we move forward. 
Description Cambridge Stafford group 
Organisation University of Cambridge
Department Cavendish Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution We measure superconducting resonators for the quantum sensors group
Collaborator Contribution The quantum sensors group fabricates high Q superconducting resonators for us, for the purposes of coupling to spins
Impact First round of resonators have been designed, fabricated and measured with good yield and quality factors. We are moving to a doped substrate in order to perform coupling experiments
Start Year 2018
Description Pulsed ESR with improved spectrometer 
Organisation Imperial College London
Country United Kingdom 
Sector Academic/University 
PI Contribution Built a new ESR spectrometer for Imperial college collaborators.
Collaborator Contribution They will learn to use the apparatus provided and utilize it to do more advanced ESR experiments with higher sensitivity and greater spin control than was previously possible
Impact No results yet published from this apparatus
Start Year 2020
Description We sell cryogenic preamplifer inserts compatible with commercially available ESR apparatus for the purposes of reducing measurement time and enhancing sensitivity to weak signals. 
Year Established 2020 
Impact Company is still in early stages of manufacturing the first device and has not had any impact yet