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Real-Time Imaging of Quantum Entanglement
Robert Fickler, Mario Krenn, Radek Lapkiewicz, Sven Ramelow, Anton Zeilinger
TL;DR
Coincidence imaging of photonic entanglement has been difficult with cameras requiring long exposures and many sparse images. This paper uses a triggered ICCD camera to image the partner photon as the other photon is measured, and reports real-time spatial-pattern changes and entanglement-witness violations.
Problem
Cameras with microsecond exposures require summing many sparse images, making coincidence imaging of entanglement unfeasible with those cameras.
Source: p. 2Method
A trigger detector measures one photon’s polarization while a triggered ICCD records its entangled partner’s spatial-mode image.
Source: p. 3Results
1.68±0.03 for LG±1 violated the separable-state inequality by more than 20 standard deviations; image extrema also shifted with trigger polarization.
Source: pp. 4, 6Takeaways & Limitations
The ICCD can determine spatial-mode structure and properties more efficiently from single intensity images, without masking.
Source: p. 6
Abstract
from arXiv · showhide
Quantum Entanglement is widely regarded as one of the most prominent features of quantum mechanics and quantum information science. Although, photonic entanglement is routinely studied in many experiments nowadays, its signature has been out of the grasp for real-time imaging. Here we show that modern technology, namely triggered intensified charge coupled device (ICCD) cameras are fast and sensitive enough to image in real-time the effect of the measurement of one photon on its entangled partner. To quantitatively verify the non-classicality of the measurements we determine the detected photon number and error margin from the registered intensity image within a certain region. Additionally, the use of the ICCD camera allows us to demonstrate the high flexibility of the setup in creating any desired spatial-mode entanglement, which suggests as well that visual imaging in quantum optics not only provides a better intuitive understanding of entanglement but will improve applications of quantum science.
Methods
The photon-pair source uses SPDC in a Sagnac-type configuration, and a spatial light modulator creates the desired spatial modes.
Methods
- Polarization-entangled photon pairs are generated by SPDC in a 15 mm type-II ppKTP crystal in a Sagnac-type configuration.A 405 nm continuous-wave laser pumps the crystal to produce 810 nm photon pairs.Source: Methods · p. 7
- Two 3 nm band-pass filters and single-mode fibers prepare the photons, while fiber polarization controllers undo polarization changes before the transfer setup.At full pump power, approximately 1.3 million pairs per second can be detected.Source: Methods · p. 7
- A phase-modulating spatial light modulator creates the desired spatial modes in the transfer setup.The SLM has 1920 × 1080 resolution and 8 µm pixels.Source: Methods · p. 8
Supplementary Material
The supplementary material describes photon-number calibration and error estimation, then outlines the witness calculation used to assess entanglement.
Evaluation of the photon number
- Around 5,800 single-photon events calibrate the ICCD signal per photon, enabling photon-number estimates from intensity within a selected image region.Pixel readout noise is subtracted, and connected pixels above five standard deviations are summed as one photon event.Source: Supplementary Material · p. 10
- A fitted log-normal distribution of single-photon signals drives 50,000 Monte Carlo simulations per photon number to estimate its error margin.The simulated standard deviation is used as a lookup table for photon-number uncertainty.Source: Supplementary Material · p. 10, Figure 1 · p. 11
Calculation of the entanglement witness
- The entanglement witness sums two visibilities and can be rewritten using four projections onto trigger polarizations and angular positions.The angular position is linked to the phase between positive and negative OAM modes.Source: Supplementary Material · p. 11
- The angular phase relation makes it possible to distinguish equally weighted superpositions of Laguerre–Gauss states using the ICCD camera.The supplementary material relates the phase between opposite OAM values to angular position.Source: Supplementary Material · p. 11
- For the stated separable-state benchmark, the witness has a maximum value of 1; a visibility sum above 1 indicates a non-separable state.Because the witness is linear, the separable-state bound also applies to mixed states.Source: Supplementary Material · p. 12