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Abstract
Quantum optical coherence can be quantified only by accounting for both the particle and wavenature of light. For an ideal laser beam ^{1–3}, the coherence can be thought of roughly as the number of photons emitted consecutively into the beam with the same phase. This number, ℭ, can be much larger than the number of photons in the laser itself, μ. The limit for an ideal laser was thought to be of order μ^{2} (refs. ^{4,5}). Here, assuming only that a laser produces a beam with properties close to those of an ideal laser beam and that it has no external sources of coherence, we derive an upper bound on ℭ, which is of order μ^{4}. Moreover, using the matrix product states method ^{6}, we find a model that achieves this scaling and show that it could, in principle, be realized using circuit quantum electrodynamics ^{7}. Thus, ℭ of order μ^{2} is only a standard quantum limit; the ultimate quantum limit—or Heisenberg limit—is quadratically better.
Original language  English 

Pages (fromto)  179183 
Number of pages  12 
Journal  Nature Physics 
Volume  17 
Issue number  2 
DOIs  
Publication status  Published  Feb 2021 


Quantum algorithms for computational physics
Berry, D., Brennen, G., Childs, A., Pachos, J. K. & AspuruGuzik, A.
1/01/16 → 20/09/19
Project: Research