Project Description
Decoherence
The benchmark at the building block level is that the loss in fidelity has to be small during an elementary one- or two-qubit gate operation.
An estimate of this loss for the magnetic qubits can be made from the reciprocal of the quality factor (Q) for coherent quantum oscillations.
The decoherence phenomena which are mostly responsible for damping of spin quantum coherence in magnetic clusters are the couplings of the net spin S to
- the crystalline lattice,
- nuclei, and
- electromagnetic fields.
That is, when addressing decoherence we should pay attention to the study of thedecoherence time of magnetic molecules and magnetic particles
- embedded in different chemical environments an
- in the presence of small and large external electromagnetic fields.
There will be two main objectives:
- the study of the effect of both chemical purification and nuclear spins on the decoherence time, and
- the correlation of the values of the decoherence time to the interactionbetween the qubits.
That is, we will work with molecules/particles forming different networks and theinteraction between clusters will be characterised by using magnetic force microscopy.
The different samples of molecular clusters and ferri- and antiferromagnetic particles will differ in
- the impurity concentration,
- the strength of the interaction between qubits, and
- the nature of the substrate used to deposite the qubits.
The figure of merit Q for these samples will be derived from resonant experiments in the frequency range between MHz and 100 GHz.
Taking into account preliminary results performed by us, we may anticipate that the value of Q, defined as the product of the coherence resonance frequency and the decoherence time, is in the range 600 to 6000.
If this is confirmed, it would turn out that the Q values of magnetic qubits would becompetitive with those reported for ion traps, quantum dots and NMR while also, uniquely, providing scalability.
The experimental work on decoherence will be supported by theory and simulations. Quantitative understanding of the relevant decoherence process will help to identify and construct the highest magnetic qubit.
We anticipate that the most important contributions are the interactions of the electronic spin with
- the nuclear spin,
- free non superconducting electrons, and
- the phonon bath.
Nuclear spin always destroys the resonance at zero field and must be eliminated from magnetic qubits by isotopic purification.
Similarly the presence of free nonsuperconducting electrons in the sample willdecohere tunneling through the spin scattering of electrons, so strongly insulating materials should be chosen for magnetic qubits.
Due to the phonon decoherence contribution, the perfection of the lattice should be given serious thoughts.
A big effort should also be made to shield the magnetic qubit from unwanted magnetic fields during the process of quantum computation.
