Neutral atoms can be confined in 2D arrays and 3D lattice structures by laser tweezers. Because of this natural ability to mimic systems of interest, they have long been considered a leading platform for analogue quantum simulation. However recent progress in demonstrating long-range 2Q gates across such structures have drawn attention to their potential as a platform for gate-model quantum computation.
Highly focussed lasers are used as ‘optical tweezers’ to confine neutral atoms in a lattice. This offers a very natural base for use as an analogue quantum simulator. Gate-style operations can also be performed using lasers. Multi qubit gates are performed by pushing atoms into highly excited Rydberg states which allows them to interact widely across the lattice.
While they share many of the advantages and challenges of trapped ions, they promise a significantly higher density of qubits in a single trap. Grassroots players can also expect to benefit from opportunities in quantum sensing and timing.
For a detailed recent review of neutral atom quantum processing see .
- Potential to build qubit atoms into multidimensional arrays.
- 1Q gates with fidelity 99.8% have been demonstrated in large 2D arrays , and with fidelity 99.6% in 3D arrays .
- 2Q gates have been demonstrated between Rydberg states with fidelity of 97.4% .
- Gate speeds of c. 1μs are quick compared to conventional trapped ion gates.
- Extreme cryogenic cooling is not required; laser cooling is used instead.
- Atoms form fundamentally identical qubits; hyperfine states offer long qubit lifetimes > 10s.
- Lattice structure provides a strong potential fit with many quantum simulation problems. A 51-atom simulator has already been used to probe computationally difficult quantum many-body dynamics .
- Simple opportunities to improve fidelity, inc. better laser cooling, better vacuum systems and higher power lasers .
- Gates based on Rydberg excited states offer the prospect of long-range 2Q gates between ‘distant’ qubits and of native multi-qubit gates. This could greatly enhance practical device connectivity for NISQ applications .
- Academic groups have demonstrated arrays of >100 atoms. Scaling to thousands of qubits with a small scale mm footprint seems a realistic goal for the next few years.
- Photonic interconnects promise ease of connection with the Quantum Internet.
- Strong related opportunities in quantum sensing and timing.
- Qubits can also be encoded in states connected by optical frequency transitions. The latter may provide a means to develop entangled atomic clocks, transforming performance.
- Gate speeds of c. 1μs are slow compared to qubit technologies such as superconducting, silicon spin or photonic qubits.
- Though the underlying qubit states can have long life, Rydberg excited states have relatively short lifetimes c. 100μs; potentially impacting fidelity.
- Scaling-up the required laser systems is a significant unresolved engineering challenge.
- Ultra high vacuum is required.
- Architectures for scaling this approach to very large qubit numbers have not yet been elaborated.
- Analogue quantum simulation use cases may ultimately be replicated by a general purpose FTQC.
Key Players and Approaches
Notable commercial players/startups: ColdQuanta, QuEra, Pasqal and Atom Computing.
ColdQuanta are using arrays of Cs atoms; Pasqual and QuEra arrays of Rb atoms, and Atom Computing arrays of Sr atoms.
Perhaps the biggest challenge for neutral atoms has been the lack of a good PR operation outside of the academic community. It’s arguable that Harvard’s 51-atom quantum simulator demonstrated its own version of analogue quantum supremacy as far back as 2017 . With champions such as Bo Ewald, the new CEO of ColdQuanta, and active engagement in US and European governmental programmes we can expect this to change.