Molecules as qubits, qudits and quantum gates

Quantum coherence is a fundamental property of electron spins that could be exploited in quantum computers for the processing of information. Within quantum calculation protocols, information is encoded in two-level quantum objects (Qubits) and processed by the operations of logical quantum gates (or Qugates). Examples of qubits include electronic spins of magnetic molecules, which present the advantage that they can be easily manipulated by external electromagnetic fields, via electron paramagnetic resonance (EPR). Creation of multi-level systems (so-called Qudits, where d is the dimension of the Hilbert space) is also possible in magnetic molecules carrying both electronic and magnetic nuclear spins due to cooperative hyperfine interactions. Such qudits can simultaneously access a multitude of states reducing the number of iterations in quantum-computation algorithms. The implementation of the Grover's algorithm in a single molecular unit was experimentally probed in a multi-level molecular nanomagnet thanks to hyperfine transitions combined with the shielded nature of nuclear states that limits decoherence. This chapter gives an overview of the latest advances in the design and testing of molecular electron spin systems with Quantum Information Processing (QIP) attributes, while highlighting the tremendous progress made in electron spin manipulations driven by pulse EPR spectroscopy.