Detecting an Itinerant Optical Photon Twice without Destroying It

Nondestructive quantum measurements are central for quantum physics applications ranging from quantum sensing to quantum computing and quantum communication. Employing the toolbox of cavity quantum electrodynamics, we here concatenate two identical nondestructive photon detectors to repeatedly detect and track a single photon propagating through a 60 m long optical fiber. By demonstrating that the combined signal-to-noise ratio of the two detectors surpasses each single one by about 2 orders of magnitude, we experimentally verify a key practical benefit of cascaded nondemolition detectors compared to conventional absorbing devices.

Figure 1

(a) Our vision. A series of QND detectors, depicted as bulbs, are attached to an optical fiber. A propagating photon (red wiggly arrow) is subsequently detected several times, indicated by the illuminated bulbs. (b) Setup of the experiment. A weak coherent pulse $|\alpha ⟩$ is coupled into an optical fiber and consecutively interacts with the two QND detectors. The latter are composed of an atomic qubit each, depicted as a Bloch sphere (light blue). The north and south poles of the Bloch spheres correspond to $|{\uparrow }_z⟩$ and $|{\downarrow }_z⟩$, respectively. The states $|{\uparrow }_x⟩$ $(|{\downarrow }_x⟩)$ on the equator are indicated by the red (yellow) arrows. Two circulators direct the photons onto the QND detectors before they are directed to a Hanbury Brown–Twiss (HBT) setup of single-photon detectors (blue half disks). (c) Quantum circuit diagram. The photon reflections from the two detectors comprise two $Z$ gates.

Figure 2

Click probabilities $\mathrm{P}({\uparrow }_{z,\text{QND1}})$ (red triangles) and $\mathrm{P}({\uparrow }_{z,\text{QND2}})$ (blue squares) of the two QND detectors when probing them with a coherent state input. When conditioning on a click of the other detector respectively, we observe $\mathrm{P}({\uparrow }_{z,\text{QND1}}|{\uparrow }_{z,\text{QND2}})$ (black circles) and $\mathrm{P}({\uparrow }_{z,\text{QND2}}|{\uparrow }_{z,\text{QND1}})$ (green diamonds). The solid horizontal line shows the threshold of 50%. The solid black lines are based on a simulation model outlined in the Supplemental Material [33]. The theory is not a fit, but employs experimental parameters characterized in independent measurements. The error bars represent standard deviations from the mean.

Figure 3

Correlations between the QND detectors with conditioning on the absorbing-detector click downstream. The green diamonds and black circles, show the probability that QND1 or/and QND2 clicks, conditioned on the absorbing-detector click. The black data points relate to Fig. 1, where ideally all upstream detectors click once the photon has passed. For comparison, we also show $P({\uparrow }_{z,\text{QND}1/2}|\text{click})$ (red triangles and blue squares). Error bars represent standard deviations from the mean. The solid lines show the predictions based on our simulation model.

Figure 4

Probabilities $P({\uparrow }_{z,\mathrm{QND}2}|{\uparrow }_{z,\mathrm{QND}1}\wedge \text{click})$ (black circles) and $P({\uparrow }_{z,\mathrm{QND}2}|\text{click})$ (blue squares). The solid lines show theoretical predictions based on our model. The gray area highlights the difference between the two theoretical curves. Error bars represent standard deviations from the mean.