Optimal photons for quantum-information processing

Photonic quantum-information processing schemes, such as linear optics quantum computing, and other experiments relying on single-photon interference, inherently require complete photon indistinguishability to enable the desired photonic interactions to take place. Mode-mismatch is the dominant cause of photon distinguishability in optical circuits. Here we study the effects of photon wave-packet shape on tolerance against the effects of mode mismatch in linear optical circuits, and show that Gaussian distributed photons with large bandwidth are optimal. The result is general and holds for arbitrary linear optical circuits, including ones which allow for postselection and classical feed forward. Our findings indicate that some single photon sources, frequently cited for their potential application to quantum-information processing, may in fact be suboptimal for such applications.

Figure 1

Hong-Ou-Mandel dip for Gaussian (solid), Lorentzian (long dash), and double-sided Lorentzian (short dash) wave functions, where $\Delta x^2=1$.

Figure 2

An arbitrary linear optics network consisting of $n$ single photon inputs, all characterized by the wave function $\psi \left(x\right)$, and $m$ vacuum inputs. The detectors following some of the outputs facilitate postselection.

Figure 3

${\tau }_{k,l}$ represents the displacement introduced into the photon wave packet when it travels from the $k\text{th}$ input to the $l\text{th}$ output.

Figure 4

Conditional production of filtered photons through cavity parametric down-conversion. The down-converter produces an entangled pair of photons into two distinct spatial modes. One of the modes is filtered and a photodetector conditions upon the detection of a single photon following the filter. When postselection succeeds, a photon will be present in the other path, whose wave function will reflect that of the detected filtered photon.