To quantify quantum optical coherence requires both the particle- and wave-natures of light. For an ideal laser, it can be thought of as the number of photons emitted into the beam with the same phase. This number, C, for an ideal laser was thought to be on the order of the square of the photon number in the laser itself, O(μ2). Here, assuming nothing about the laser operation except that its inputs are incoherent, but making some assumptions on the ideality of the laser beam, we find the ultimate (Heisenberg) limit: C = O(μ4). Moreover, we find a laser model that can achieve this scaling, and show that, in principle, it could be realised with familiar physical couplings.
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