##### Project 1 – Developing high fidelity Read-out of a Multi-Qubit system

*Supervisors: Professor M. Y. Simmons; Dr. M. House, Dr. J. Keizer and Mr P. Gregory-Singh*

**Aim**: To design and test a hardware qubit control platform with a novel algorithm to generate and receive multiplexed Radiofrequency (RF) signals used to perform dispersive readout of a multi-qubit device.

**Method**: The student will be involved in modifying the existing code for a Field Programmable Gate Array (FPGA) development platform and transceiver development card to generate a frequency multiplexed signal for a 10-qubit system. The signal will be received and processed using a bank of digital filters to output a single in-phase (I) and quadrature (Q) data point for each qubit frequency i.e. 10 qubits => 10 frequencies => 10 filters => 10 IQ datapoints.

**Outcomes:**

- The student will become familiar in high frequency control of qubit states, in particular becoming familiar with FPGA devices; the advantages and disadvantages of using them as compared to Configurable Programmable Logic Devices (CPLDs), Digital Signal Processors (DSPs), Microprocessors, Application Specific Integrated Circuits (ASICs) etc.
- The student will be able to determine the difference in how to read-out qubit states and compare two different types of semiconductor qubits: single spin qubits and more complex singlet-triplet qubits.
- The student will understand the need for cryogenic measurements.
- The student will generate spin read-out data for a multiplexed 10 qubit chip.

##### Project 2 – Optimising high speed single spin qubit read-out

*Supervisors: Professor M. Y. Simmons; Dr. M. House, Dr. J. Keizer and Mr P. Gregory-Singh*

**Aim**: To optimise the speed and efficiency of spin qubit read-out using a new hardware-software platform. Here the student will generate a digital feedback function capable of determining the optimal location of spin read-out and controlling the qubit at this point.

**Method**: The student will use existing hardware to output waveforms and acquire the simulated response signal of a sensitive spin detector. Control software will be written to adjust the output waveform so that a voltage ramp across the detection peak will be centred at the optimal location. A similar edge align function will also be written which can detect the edge of a charge transition and adjust the output waveforms accordingly.

**Outcomes:**

- The student will understand the physical process underlying the read-out of spin qubits.
- The student will become familiar with device stability and the reason for alignment.
- The student will be able to determine the difference in how to read-out qubit states and compare two different types of semiconductor qubits: single spin qubits and more complex singlet-triplet qubits.
- The student will generate two alignment functions (peakAlign and edgeAlign) which can be tested against existing qubit data.

##### Project 3 – Developing a Logical Qubit in Silicon

*Supervisors: Professor Michelle Y. Simmons and Dr. M.G. House *

Quantum computation is regarded as a promising alternative to conventional silicon electronics. Whilst quantum logic operations have been already demonstrated in numerous systems including superconducting circuits, photonic qubits, quantum dot qubits and electromagnetic ion traps, few of these systems have realized a logical, error corrected qubit. Donor-spin qubits in silicon represent one of the most promising qubit types since they have extremely long coherence times with very high fidelities. Silicon based quantum computers also have an additional advantage as they employ the well-developed processing technologies of the semiconductor IT industry.

In this project within the Centre of Excellence for Quantum Computation and Communication Technology you will work to design a logical qubit architecture in silicon This work will comprise multiple physical qubits atomically engineered for spin initialization, control and single shot spin read-out. Devices will be made in a full silicon CMOS cleanroom with the manipulation of individual donors performed using scanning probe lithography. The nanofabricated gates and microwave magnetic field allow for spin control and readout. Measurements are done at cryogenic temperatures to provide high spin coherence. These results build on the recently demonstrated coupled donor qubits [1] and represent the absolute latest benchmark in silicon based quantum computing [2]. Whilst ambitious the goal of achieving a logical qubit will maintain international leadership in this field.

[1] B. Weber et al., Nature Nanotechnology 9, 430 (2014).

[2] C. Hill et al., to appear in Science Advances (2015).

##### Project 4 – Engineering donor molecules for silicon quantum computing

*Supervisors: Professor Michelle Y. Simmons and Dr Joris Keizer*

Current proposals for donor based scalable quantum computing require a method of coherent transport of qubit states. One method for achieving coherent transport is to use a spin bus which consists of a 1D array of strongly exchanged coupled electrons. For donor-based spin buses, the exchange coupling J needs to be much greater than the hyperfine coupling to remove the influence of the nuclear spins on the spin bus. The required exchange coupling can be estimated to be ~10GHz which corresponds to a much smaller inter-donor distance than the system needed to perform a two qubit gate (~100 MHz).

We propose to study strongly coupled donor molecules for the purpose of scaling up to a spin bus. We will design donor molecules with varying inter-donor distances [1] and plan to perform the following experiments:

- Measurement of the spin relaxation times of the first electron bound to the donor molecule. In this system we expect the spin relaxation times to be much longer than for single donors [2].
- Measurement of the S-T splitting (J) of the 2 electron state on the donor molecule and characterisation of this splitting as a function of inter-donor distance. Readout of singlet-triplet states of the donor molecule and the measurement relaxation times.

The project will involve the design and fabrication of a donor based qubit device using scanning tunnelling microscopy to precisely place single phosphorus atoms and electrical measurement at cryogenic temperatures.

[1] B. Weber, T.H. Matthias Tan, S. Mahapatra, T.F. Watson, H. Ryu, R. Rahman, L.C.L. Hollenberg, G. Klimeck and M.Y. Simmons, “Spin blockade in coulomb confined silicon double quantum dots”, Nature Nanotechnology 9, 430 (2014).

[2] Y. Hsueh, H. Buch, Y. Tan, Y. Wang, L. C. L. Hollenberg, G. Klimeck, M. Y. Simmons and R. Rahman, “Spin-lattice relaxation times of single donors and donor clusters in silicon”, Physical Review Letters 113, 246406 (2014).

##### Project 5 – Spin control of a precision qubit in silicon

*Supervisors: Prof. Michelle Simmons and Dr. Sam Gorman*

The Centre of Excellence for Quantum Computation and Communication Technology has developed a complete technology for fabricating semiconductor devices at the atomic scale. We have recently refined this technology to the point where we can make transistors in which the active area of the device is a single phosphorus atom [1]. This ability puts us in a unique position to be able to advance the science and technology of quantum computation, which requires precise control over individual quantum states of matter. We are researching the use of the quantum spin state of an electron to serve as a quantum bit (or “qubit”) for a future quantum computer. So far we have demonstrated the abilities to place single phosphorus atoms into a silicon crystal with atomic precision and to measure the spin state of a single electron at the phosphorus atom site. We are now advancing our ability to control the spin state of the electron directly.

This project will analyse data that is currently being taken for manipulating the spin state of an electron using electron spin resonance (ESR) techniques. The goal is to demonstrate full control over the spin of a single electron, and to measure its quantum coherence lifetime. The student will have the opportunity to observe several valuable experimental techniques and technologies, such as semiconductor device design and fabrication, scanning tunneling microscopy (STM), microwave electronics (>20 GHz), and low temperature measurements (< 1 K.).

[1] Fuechsle, *et al*., “A single-atom transistor,” Nature Nanotechnology **7**, 242 (2012).