NV diamonds

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 .

SWOT Analysis

Strengths

  • 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 .

Opportunities

  • 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.

Weaknesses

  • 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.

Threats

  • 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.

References

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M. K. Bhaskar et al., “Experimental demonstration of memory-enhanced quantum communication,” Nature, vol. 580, no. 7801, pp. 60–64, 2020 [Online]. Available: http://arxiv.org/abs/1909.01323. [Accessed: 04-May-2020]
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F. Dolde et al., “High fidelity spin entanglement using optimal control,” Nat Commun, vol. 5, no. 1, p. 3371, 2014 [Online]. Available: http://arxiv.org/abs/1309.4430. [Accessed: 16-Jul-2020]
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D. Kim, M. I. Ibrahim, C. Foy, M. E. Trusheim, R. Han, and D. R. Englund, “CMOS-Integrated Diamond Nitrogen-Vacancy Quantum Sensor,” Nat Electron, vol. 2, no. 7, pp. 284–289, 2019 [Online]. Available: http://arxiv.org/abs/1810.01056. [Accessed: 06-Jul-2020]
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Y. Chen, S. Stearn, S. Vella, A. Horsley, and M. W. Doherty, “Optimisation of diamond quantum processors,” arXiv:2002.00545 [quant-ph], Feb. 2020 [Online]. Available: http://arxiv.org/abs/2002.00545. [Accessed: 16-Jul-2020]
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A. Tchebotareva et al., “Entanglement between a diamond spin qubit and a photonic time-bin qubit at telecom wavelength,” Phys. Rev. Lett., vol. 123, no. 6, p. 063601, Aug. 2019 [Online]. Available: http://arxiv.org/abs/1905.08676. [Accessed: 15-Jul-2020]
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C. E. Bradley et al., “A Ten-Qubit Solid-State Spin Register with Quantum Memory up to One Minute,” Phys. Rev. X, vol. 9, no. 3, p. 031045, Sep. 2019 [Online]. Available: https://link.aps.org/doi/10.1103/PhysRevX.9.031045. [Accessed: 27-Apr-2020]
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X. Rong et al., “Experimental fault-tolerant universal quantum gates with solid-state spins under ambient conditions,” Nat Commun, vol. 6, no. 1, p. 8748, 2015 [Online]. Available: http://arxiv.org/abs/1506.08627. [Accessed: 14-Jul-2020]

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David Shaw

About the Author

David Shaw has worked extensively in consulting, market analysis & advisory businesses across a wide range of sectors including Technology, Healthcare, Energy and Financial Services. He has held a number of senior executive roles in public and private companies. David studied Physics at Balliol College, Oxford and has a PhD in Particle Physics from UCL. He is a member of the Institute of Physics. Follow David on Twitter and LinkedIn

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