Our quantum computing research aims at realizing a fully scalable quantum computer using rare earth ions in a crystal as qubits.

Two of the hyperfine levels in the ground state of Praseodymium (picture 1) or Europium are used as the |0> and |1> states of the qubit. Transitions between the states are done with an actively stabilized dye laser via an excited state.

              a)                                                               b)

Picture 1: a) The energy levels of Pr:YSO and the transitions used for qubit manipulation.
                  b) A qubit initialized to the |0>-state is characterized by three absorption peaks corresponding to the three hyperfine levels of the optically excited state. The qubit is in this case represented by billions of ions, all in the same quantum state.

To scale the quantum computer to several qubits it seems necessary to use a single ion for each qubit. Much effort is currently focused on detecting the state of a single dopant ion. Since qubits are required to have a long lifetime the idea is to use another dopant ion, with a shorter lifetime and a larger fluorescence yield, as a communicator ion that call tell the state of it's neighbouring qubits.

Picture 2: Bright blue fluorescence from a cerium doped crystal. Cerium has a high enough fluorescence yield to detect light emitted by a single ion.

Working with single ions poses new challenges and possibilities.  One challenge to overcome is the process of energy transfer which can be detrimental to the qubit-states during processing.

Looking at a single ion will give us direct insight into the microscopic environment in the crystal. It will offer us a new platform to learn from and better possibilities to develop scalable quantum computer hardware.

Do you want to know more?

A good overview can be found in the PhD-thesis of Yan Ying, which you can find here.

The experimental details can be found in our publications.