Enhancing coherence in molecular spin qubits via atomic clock transitions

Abstract

Quantum computing is an emerging area within the information sciences revolving around the concept of quantum bits (qubits). A major obstacle is the extreme fragility of these qubits due to interactions with their environment that destroy their quantumness. This phenomenon, known as decoherence, is of fundamental interest. There are many competing candidates for qubits, including superconducting circuits, quantum optical cavities, ultracold atoms and spin qubits, and each has its strengths and weaknesses. When dealing with spin qubits, the strongest source of decoherence is the magnetic dipolar interaction. To minimize it, spins are typically diluted in a diamagnetic matrix. For example, this dilution can be taken to the extreme of a single phosphorus atom in silicon, whereas in molecular matrices a typical ratio is one magnetic molecule per 10,000 matrix molecules. However, there is a fundamental contradiction between reducing decoherence by dilution and allowing quantum operations via the interaction between spin qubits. To resolve this contradiction, the design and engineering of quantum hardware can benefit from a 'bottom-up' approach whereby the electronic structure of magnetic molecules is chemically tailored to give the desired physical behaviour. Here we present a way of enhancing coherence in solid-state molecular spin qubits without resorting to extreme dilution. It is based on the design of molecular structures with crystal field ground states possessing large tunnelling gaps that give rise to optimal operating points, or atomic clock transitions, at which the quantum spin dynamics become protected against dipolar decoherence. This approach is illustrated with a holmium molecular nanomagnet in which long coherence times (up to 8.4 microseconds at 5 kelvin) are obtained at unusually high concentrations. This finding opens new avenues for quantum computing based on molecular spin qubits.

Coherent manipulation of three-qubit states in a molecular single-ion magnet

“…This strategy has recently led to spin coherence times [3,8,9] comparable to those reported for other solid-state spin qubit systems, such as NV centres in diamond and P donors in silicon [10,11]. An alternative is to identify the qubit states with clock transitions, which are relatively insensitive to magnetic field fluctuations [12]. However, even if the qubit-qubit interactions are minimized these strategies offer no route to scalability.…”

“…Lanthanide ions are promising candidates to realize these systems since the multiplet associated with the angular momentum J, given by Hund's rules, defines (2J + 1) states. Its practical implementation is not straightforward, though, because the level splitting induced by the crystal field around the lanthanide is often so large that only the two lowest lying electronic levels are experimentally accessible [12,16,17]. Nuclear spin states might still allow the definition of several qubits in lanthanides with a nonzero I [16,18].…”

Sensitive spin detection using an on-chip SQUID-waveguide resonator

“…Typical implementations of these qubits are molecule-based magnets [3][4][5][6] , nitrogen-vacancy (NV) centers in diamond 7 and quantum spins in crystals [8][9][10][11] . These spin-based qubits are designed such that the spins are well separated in the crystal, leading to an increased decoherence time due to weak spin dipolar interactions.…”

Anisotropic optical trapping as a manifestation of the complex electronic structure of ultracold lanthanide atoms: The example of holmium

“…Like other lanthanides, the complex electronic structure of holmium induces a large magnetic dipole moment (9 µ B ) that makes it an interesting candidate to investigate the anisotropic interactions between atoms [39,40]. Recently the holmium single magnetic atom and holmium molecular nanomagnet was also presented as a competing candidate for the realization of quantum bits [41,42].…”