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Laser spectroscopy of highly-charged ions
This work is covered on the HITRAP project page.
Quantum information processing studies with trapped ions
Motivation
Our key objectives are to contribute to the understanding and control of decoherence in trapped ion systems and to take steps towards scaling these systems to larger numbers of qubits. In the ion trap schemes suggested and tested to date a likely cause of decoherence is the presence of the large RF drive potential applied to the trap. The Penning ion trap has a significant difference from the conventional RF Paul trap and the RF linear trap, in that it is a static trap with no RF fields present. The absence of the RF drive may well lead to lower decoherence rates than in the RF traps. Decoherence rates in the Penning trap have not been investigated in detail before and we will perform such experiments, initially on single particles confined in a Penning trap. Scaling these systems to larger numbers of qubits requires the mastery of certain key operations such as coherent transport and separation of entangled ions in a distributed array of miniature traps. We are currently working towards the development of arrays of miniature Penning traps and will use these to demonstrate these operations.Method
In order to make detailed studies of decoherence processes, it is necessary to implement a cooling technique capable of putting the ions into the ground state of their motion in the trap. Such techniques have not yet been applied to single particles in Penning traps. We have performed Doppler cooling of Ca+ ions in our Penning trap using a suite of diode laser systems. We have developed a new, very narrow bandwidth Ti:sapphire laser system which we will use to address a weak forbidden transition in the ion. The ability to address such a narrow transition will allow us to perform a variation of laser cooling called sideband cooling. This technique should allow us to cool a trapped ion into the ground state of the ion's motion in our trap. Once this is achieved we will drive the forbidden transition coherently and observe Rabi oscillations on this transition. The quality of the Rabi oscillations and the rate at which their contrast degrades gives clear information about the decoherence rate.
Our institution also hosts a world-leading theoretical group in Quantum Optics. This group actively collaborates with us and with many other nodes within the consortium and has played a leading role in the theoretical development of the field of quantum information processing.
Aims of the project
There are various reasons why Penning traps would not initially seem to be the obvious choice for quantum information studies. Although ordered structures of ions do indeed form in a Penning trap, these structures rotate around the centre of the trap with the magnetron frequency which is usually of the order of tens or hundreds of kHz. One possible approach would be to decrease the axial confinement generating a short string of ions along the axis of the trap similar to the kind of string of ions that would be found in a linear RF trap. To scale this approach to strings longer than a few ions would require an unrealistically large magnetic field. Nonetheless, with a reasonably high magnetic field, few-ion strings could be investigated. On the other hand, it now seems unlikely that long strings of ions held in radiofrequency linear traps will form the basis of a quantum computer. Rather, groups are working towards distributed arrays of miniature rf traps each holding at most 2 ions at a time. This vision of a scalable ion trap quantum information processor can equally be applied to the Penning trap.
Another perceived problem with Penning traps is that the ions in a Penning trap are not as well localised in the radial plane as they are for a RF trap. This problem can be overcome to some extent by performing axialisation in which energy is pumped into the inherently unstable magnetron motion thus reducing the size of the orbit. We have performed axialisation on single Mg+ ions and have recently demonstrated this technique for small clouds of Ca+ ions.
Our immediate plans are to demonstrate axialisation using single Ca+ ions and then to perform sideband cooling in this system. We will then be in a position to measure Rabi oscillations in our system and thus characterise the inherent decoherence rate. In the mean time we are developing a linear array of miniature Penning traps which we will use to perform operations that are the building blocks of a distributed trapped ion processor.
We propose initial studies of Raman cooling techniques in calcium ions in Penning traps. Since three or four laser frequencies are required for such experiments, it is also sensible to use a system for which solid state lasers are available which can provide the high powers and narrow linewidths necessary. We are well-placed to perform this work, having previously gained much experience with Penning traps and in particular with single-ion work in the Penning trap.
Once we have established Raman cooling in calcium, we will apply these techniques to the study of decoherence in one- and two-ion systems in Penning traps. It is a natural extension of our current studies of the quantum Zeno effect in single Be ions in a Penning trap.
Our institution is fortunate in having a large theoretical group in the area of quantum information. In addition to our experimental work theoretical work will be carried, both in support of our own programme and in collaboration with other groups within the QUBITS consortium. Our theoretical focus for the immediate future is to investigate the use of entangled trapped atoms and ions, particularly within cavities as local information processors linked via fibre quantum channels. We will develop methods to entangle atoms through their dissipative interactions, and quantify the fidelity of gate operations as a measure of how well these basic quantum gate building blocks can be cascaded and scale in a quantum circuit. We will also develop new applications of few qubit entangled states (including applications to frequency standards).