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A Quantum RAM Using Superconducting Circuits and Donors in Silicon

Final Report Summary - QURAM (A Quantum RAM Using Superconducting Circuits and Donors in Silicon)

Project Background:
Electronic circuits made from superconducting materials have become forerunners amongst solid-state implementations for a quantum computer. Small quantum processors based on this technology have already been developed in commercial laboratories, for example at Google and IBM. The attraction of superconducting based architectures resides in the ease with which they can be fabricated, and their basic computational elements – the qubits – can be readily controlled experimentally through strong interactions with other circuit elements. But these same interactions with the surroundings cause superconducting circuit qubits to suffer from short coherence lifetimes, which describes the duration over which a qubit can retain its quantum phase information before it is corrupted by the environment.

Another promising candidate for a solid-state qubit is the spin of a donor in silicon. A single electron is a spin one-half particle (S = 1/2), providing a well defined two-level system. A donor possesses one additional electron to those shared with the surrounding silicon lattice. The excess electron or the nuclear spin of a single donor can act as a qubit and has been shown to have remarkable coherence lifetimes – seconds for the electron and hours for the nucleus. These long lifetimes can be attributed to the fact that donor spins in silicon are extremely well isolated from their environment. Whilst isolation is essential for the long coherence times reported, it also makes interacting with them a difficult task.

It is becoming increasingly apparent in the field of quantum information processing that no single system currently under study possesses the complete tool-set required to construct a large-scale quantum computer. Hybrid quantum systems is a growing field of research where two or more different physical quantum systems are combined. By exploiting their individual strengths, the hybrid system obtains properties which no single system possesses.

Project Objective:
Whilst superconducting qubits can be coupled together using demonstrated methods, they are not suitable memory elements for longer term storage of quantum states. Donor spins on the other hand are extremely well suited for storing quantum information and could act as a quantum random access memory (quRAM). This leads to the overarching goal of this project: to create a long-lived memory element for a superconducting-based quantum computer by investigating hybrid systems of superconducting circuits and donor spins in silicon.

Summary of Results:
This project was a collaborative effort between researchers at UCL (the UK) and the Commission for Atomic Energy and Alternative Energies (CEA) in Saclay (France). Through the course of the project, a number of important experimental demonstrations were made towards the goal of a hybrid quantum memory. A device was designed, modelled and characterised that consisted of bismuth (209Bi) donor atoms implanted into a silicon chip, on top of which a high quality factor superconducting resonator was patterned. By cooling the device down to milliKelvin temperatures and by utilising a noiseless Josephson parametric microwave amplifier (JPA), electron spin resonance (ESR) was performed on the 209Bi atoms at the quantum limit (i.e. where the noise is set by vacuum quantum fluctuations of the microwave field). This unprecedented sensitivity in ESR enabled the study of small spin ensembles in close proximity to a superconducting micro-circuit, providing valuable insight into how a hybrid memory should be constructed. For example, it was found that the spin resonance transitions were sensitive to strain in the silicon generated by the superconducting surface circuitry, and methods of minimising such strain (e.g. through deeper donor implants) should be adopted. Furthermore, it was found that the coherence time of the bismuth donors was not reduced by the presence of the superconducting resonator, an important result for a quantum memory. These results and the techniques developed in this experiment are also applicable to other fields, such as biology and chemistry, as it enables very sensitive spectroscopy study of small samples. As a result, the work was published in the high-impact journal Nature Nanotechnology in 2015 [1].

Another important experiment performed in this project demonstrated the enhanced spontaneous emission of photons from the donor spins into the superconducting resonator. For the first time the regime where spontaneous emission of photons constituted the dominant spin energy relaxation mechanism (i.e. the Purcell regime) was reached. The relaxation rate was shown to increase by three orders of magnitude when the spins were tuned to the cavity resonance, showing that energy relaxation can be engineered and controlled on-demand. The results provide a novel way to initialise spin systems into their ground state, an essential technique for resetting a quantum memory at low temperature. This was considered a landmark discovery in the community and was published in the academic journal Nature in 2016 [2].

The outcomes of this project have made substantial progress towards the demonstration of a hybrid quantum memory – a device that would allow for the development of more advanced superconducting quantum processors. The results have shown that bismuth donors in silicon are an excellent spin system on which to base a memory. The techniques developed as a result of this project will find application in other fields (such as chemistry, biology and materials science) with additional societal benefits delivered to those which come through the quantum computing application of this work.

IF = 5-year Impact Factor (ISI 2014 Journal Citations Report)

[1] A. Bienfait, J. J. Pla, Y. Kubo, M. Stern, X. Zhou, C.C. Lo, C.D. Weis, T. Schenkel, M.L.W.
Thewalt, D. Vion, D. Esteve, B. Julsgaard, K. Moelmer, J.J.L. Morton, and P. Bertet,
“Reaching the quantum limit of sensitivity in electron-spin resonance”, Nature Nanotechnology (2015), Advance online publication doi:10.1038/nnano.2015.282.
[IF = 37.601]

[2] A. Bienfait, J. J. Pla, Y. Kubo, X. Zhou, M. Stern, C. C. Lo, C. D. Weis, T. Schenkel, D. Vion, D. Esteve, J. J. L. Morton, and P. Bertet, “Controlling spin relaxation with a cavity”, Nature (2016), Advance online publication doi:10.1038/nature16944.
[IF = 41.296]