Thesis
Quantum coherence of ions in a microfabricated trap
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2022
- Thesis identifier
- T16449
- Person Identifier (Local)
- 201653103
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- This thesis reports on experiments towards improving the quantum coherence of trapped ion qubits, such that coherent control of the qubit can be achieved. This is essential for the applications that ion traps have in the fields of quantum information processing, quantum metrology and for use in atomic clocks. 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 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 less than what 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. One requirement of implementing fault-tolerant quantum information processing is that the Rabi frequency of the laser-ion interaction must have a fractional stability between 10−2 and 10−4. Additionally to coherently control the state of a qubit the gate operations must be performed at timescales that are much faster the coherence time of the qubit. To achieve both of these the construction of an optical frequency tuner for the qubit laser is presented. This subsystem utilises a direct digital synthesis RF source to apply fast detuning operations to the qubit laser over a ±25 MHz range, whilst simultaneously stabilising the laser power to the extent that its Allan deviation reaches the more stringent level for fault tolerant quantum gates over the 10 − 700 s time frame, which correspond to the approximate durations of spectroscopy experiments. To ensure that the tuner and downstream subsystems do not add excessive frequency noise onto the laser a beat measurement is made against the laser at the source. All noise and sidebands are observed to be < −36.5 dBc and the linewidth of the beatnote is < 0.21 Hz. Fluctuations in the magnetic field that the ion is exposed to will induce modulations in the Zeeman shift of the 2S1/2(mj = −1/2)−2D5/2(mj = −5/2) qubit transition that is used here, which in turn causes decoherence in the qubit. Due to this transition having Δmj = 2 the qubit transition will have a higher sensitivity to any fluctuations in the magnetic field, however the geometry of the apparatus results in this Zeeman component giving the highest laser-ion coupling. The apparatus used for the work in this thesis features a magnetic shield and a set of coils for nullifying any remaining field and applying a bias field. The ability of this setup to stabilise the field the ion experiences is investigated. Whilst investigating the stability of the magnetic field the effects of nearby devices on the magnetic field are considered. The shield is shown to attenuate the external field by a factor of 1065×, and the magnetic field is stabilised enough that the Allan deviation of the qubit transition frequency is < 10 Hz for 1 s − 7000 s timescales and < 1 Hz for 20 s − 350 s. Investigation into the magnetic field stability also revealed the presence of ground loops from the source of the trap DC potentials and the piezo electric transducers that are used to steer the qubit laser beam pointing. Finally, spectroscopy experiments on a single ion and two-ion string are demonstrated. The motional frequencies of the single ion and two-ion string are measured and are found to agree with the calculated frequencies. The state initialisation into the S1/2(mj = −1/2) Zeeman component is measured to be ≥ 99.6 %. A raster scan of the beam pointing is used to ensure that the laser intensity maximum of the laser is incident on the ion. Coherent control of a trapped ion qubit is demonstrated with the generation of Rabi oscillations and Ramsey fringes. By fitting Bloch equations to the results of these experiments the unknown experiment parameters such as the Rabi frequency and mean vibrational number are able to be determined. The ability to initialise the ion into the motional ground state via sideband cooling is shown. Utilising amplitude shaped pulses to minimise off-resonant excitation has been demonstrated by applying pulses whose amplitude follows a Blackman profile in time. This pulse shape has reduced Fourier components that are far from resonance, which manifests as the suppression of spectral side-lobes. The data fitting routines used iterate on previous work to reduce the computational overhead and improve computation times. The experimental procedures to perform and optimise the Mølmer Sørensen entanglement gate are presented, as is discussion on how the results of these experiments should be interpreted. However, demonstration of this routine remains the next step in the experiment’s investigation.
- Advisor / supervisor
- Riis, Erling
- Sinclair, Alastair
- Resource Type
- DOI
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