High fidelity, scalable spin readout using double quantum dots

Lead Research Organisation: University College London
Department Name: Physics and Astronomy


This project is focused on high fidelity, scalable readout of silicon-based double quantum dots. From all the many qubit implementations we have chosen spin qubits double quantum dots since they have a small pitch, long coherence times and requires a lower magnetic field to be operated than the spin in single quantum dots.
The objectives of this project are:

Improving the sensitivity in reflectometry measurements used for qubit readout.
Including low noise amplifiers in the reflectometry circuit such as JPA or TWPA.
Multiplexing (frequency multiplexing or cross-bar scheme) the readout in an integrated way.
Applying the above points to double quantum dots both nanowires and planar structures.
Understanding the limits of fidelity error mechanisms.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512400/1 01/10/2017 31/03/2022
1937067 Studentship EP/R512400/1 25/09/2017 30/09/2021 Virginia Natalia Ciriano Tejel
Description Key results: Readout of the spin state of one electron confined in a quantum dot using a nearby quantum dot as a sensor using gate-based reflectometry in silicon nanowires. The measured spin relaxation times were up to 8s.

In the same way that bits form the building blocks of classical computers, qubits will form the building blocks of quantum information. There are many proposals of how to create a qubit (ion traps, NV centres, superconductors, photons, spin). Here, I focused on the spin degree of freedom of an electron trapped in a silicon structure. Spin qubits in silicon combine a mature technology with quantum properties such as long coherence times and small spin to orbit coupling. We use field-effect transistors (FETs) based on complementary metal-oxide-semiconductor made of a silicon nanowire. Its design, similar to a FinFET transistor, makes its fabrication process compatible for the silicon foundry industry.

The electrons get trapped in the corners of the nanowire, creating two quantum dots in parallel, one in each corner. The ends of the nanowire connect with electronic reservoirs made of highly doped silicon. The number of electrons within the dots is controlled by two metal electrostatic gates, one over each corner, that regulate the dots electrochemical potential independently. When a dot electronic state becomes available, electrons can tunnel into the dot from the reservoir.

The key finding of this project was reading out the spin state of one electron, using the other quantum dot as a sensor using gate-based reflectometry. Although the spin of a single electron is too small to be measured directly, the spin state can be made dependent on the dot electron occupation. Once the spin degeneracy is lifted by applying a magnetic field, the reservoirs Fermi energy is placed between the spin up and down dot electrochemical potential. This way, only a spin-up electron can tunnel-out from the dot, and subsequently tunnel back to the spin-down state. On the other hand, a spin-down electron would remain in the dot. This spin-to-charge mechanism was developed by Elzerman et.al. in 2004.

Spin-dependent tunnelling in the so-called "qubit dot" was detected by looking at the so-called "sensor dot" electrochemical potential. The dots are capacitively coupled, which means that an electron leaving the qubit dot produces a shift in the sensor dot electrochemical potential. The sensor potential is monitored by gate-based reflectometry, a dispersive charge readout method that uses the same gate that controls the dot for charge readout. This readout method improves the scalability prospects since it reduces the number of cables inserted in a small dilution fridge. Moreover, I have also measured the spin relaxation time, which can be up to 8s.

Additional key results: single-shot readout of the spin with the method described above.

Single-shot readout is the first step to develop a quantum computing since it allows you to measure the qubit state without average. We have managed to achieve single-shot but improving different aspects of our setup such us the resonator matching and Q factor used for gate-based reflectometry. We are planning to add a Josephson Parameter Amplifier to increase the fidelity of the readout to hopefully achieve a fidelity above the quantum fault-tolerance.
Exploitation Route We are planning on publishing the new results when the measurements are completed.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Other

URL https://arxiv.org/abs/2005.07764
Description Telefónica Scholarship Award 2019 from the British Spanish Society
Amount £5,000 (GBP)
Organisation Telefonica S.A 
Sector Private
Country Spain
Start 06/2019 
End 06/2020
Description Industrial PhD in partnership with Hitachi Cambridge 
Organisation Hitachi Cambridge Laboratory
Country United Kingdom 
Sector Private 
PI Contribution We are planning on publishing an article collaboratively.
Collaborator Contribution I have an industrial scholarship, which means that my second supervisor, Fernando González-Zalba works on Hitachi Cambridge laboratories. We chat every two weeks nad he's been helping me with my project in general.
Impact I have an industrial scholarship, which means that my second supervisor, Fernando González-Zalba works on Hitachi Cambridge laboratories. We chat every two weeks nad he's been helping me with my project in general.
Start Year 2018