Our ability to fabricate in diamond has undergone a transformation that is perhaps not fully appreciated outside of the specialist community. Diamond is another material well suited to hosting qubits.
Point defects in a diamond lattice act in many ways like isolated atoms and can be used to form qubits. Nitrogen-vacancy colour centres are widely studied, but also other variants such as Silicon-vacancy colour centres also show promise.
Pioneering work with NV diamonds has often focussed on their remarkable near-term applications in quantum sensing and quantum communications. However the ability to form an expanded register of high fidelity qubits based on nuclear spins around a single defect is widening potential applications. Intriguingly this technology even holds out a realistic promise of operating at room temperature and ambient pressure.
For a detailed recent review of diamond based quantum processors see .
- 12C has no nuclear spin, so it forms a magnetically neutral substrate to host sensitive spin qubits, promoting long qubit lifetimes.
- The high thermal conductivity of the diamond lattice also suppresses loss of qubit coherence due to thermal interactions. This leads to useful performance even at room temperature; though fidelity improves with cryogenic cooling.
- Electron and nuclear spins associated with a single defect offer a connected series of hybrid registers. Nucleus/electron gates within a single defect have been demonstrated with fidelity of 99% even at room temperature .
- Electron spins offer fast control and high fidelity readout.
- Nuclear spins provide additional long lifetime ‘memory’ qubits: 1Q 75s, 2Q entanglement 10s .
- Coherent long-distance photonic interconnects are well established .
- Chemical vapour deposition is revolutionising our ability to grow high quality diamond.
- Can be fabricated as nanoparticles or on the tip of a scanning probe. Mounting on a CMOS chip has recently been demonstrated .
- Working with silicon impurities can enable improved precision fabrication.
- Naturally occurring 13C impurities form an additional resource in the neighbourhood of a defect. A 10Q register based on a single defect coupled to additional nearby impurities has been demonstrated with 2Q gate fidelity of 99% at 4K .
- Strong opportunities in quantum communications, including MDI-QKD and quantum repeaters and the Quantum Internet in general .
- Strong opportunities in quantum sensing, e.g. sensing magnetic field.
- Non-toxic host material and room temperature operation unlocks potential applications in bio-sensing .
- High thermal conductivity offers future promise for dissipating heat in control electronics mounted on the diamond substrate.
- 2Q gates between defect centres are currently lower fidelity, c.88% . This is a challenge for scaling on the model employed by other technologies.
- Laser systems are required for control and readout.
- Precision fabrication of Nitrogen impurities is difficult, and can damage the optical properties of the surrounding lattice, compromising the fidelity of laser control.
- Carbon does not enjoy the same established advanced nanofabrication base as silicon.
- Challenges fabricating ‘identical’ defects may limit the efficiency of gate performance.
Key Players and Approaches
The potential for room temperature operation is a tantalising feature. If the right NISQ algorithm and application can be found, a 50-100 diamond qubit device could be a uniquely deployable quantum co-processor. Quantum Brilliance is one startup well positioned for such opportunities.
Whatever technology becomes dominant for large scale FTQC, the unique properties of diamond could anyway make it a strong contender for special purpose components in the wider quantum ecosystem, especially in memory, communications and sensing applications. Element Six is a notable commercial champion of this technology.