April 13, 2024

Protecting Quantum Light in Space and Time

    Giuseppe Fumero

    • Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA (associate)
    • Department of Physics and Astronomy, West Virginia University, Morgantown, WV, USA

Physical 17, 53

A way to create single photons whose spatiotemporal shapes do not expand during propagation could limit information loss in future quantum photonic technologies.

J.Wang and others. [1] (Airy pattern); G. Fumero/National Institute of Standards and Technology and West Virginia University (noise surface)
Figure 1: Wang and colleagues produced single photons whose spatiotemporal profiles, known as Airy patterns, do not scatter during propagation. [1]. (Left) Such a pattern can be hidden by classical photon noise, represented here by a random surface. (Right) The pattern dominates this noise when only quantum correlated photons are selected using a technique called coincidence detection.

When enjoying the sight of a rainbow, the loss of information may not be the first thing that comes to mind. However, dispersion, the underlying process that causes different colors to travel at different speeds, also makes it difficult for scientists to control the propagation of light – a crucial capability for future quantum photonic technologies. As they move, short laser pulses tend to lengthen through scattering and wax and wane through diffraction. Together, these effects limit our ability to make light reach a target, although mitigation strategies have been developed for classical pulses and, recently, for quantum light. Now, Jianmin Wang of the South China University of Science and Technology and colleagues have discovered a quantum source of single photons that are immune to propagation during propagation, potentially protecting against the loss of information encoded in the photons’ spatiotemporal shapes. [1].

In 2007, physicists demonstrated beams of light, known as Airy beams, whose spatial profiles make them resistant to propagation. [2, 3]. These profiles consist of a pattern of bright and dark lobes surrounding a bright central component, with each feature propagating along a parabolic trajectory. Recently, scientists have created quantum Airy beams, which are technically challenging to realize [4, 5]. The goal of Wang and colleagues’ work was to extend this principle to the temporal domain, producing Airy quantum single photons that do not scatter in space and time. These quantum “bullets of light” could offer interesting possibilities for quantum technologies, just as their classical counterparts have done for applications in areas ranging from plasma physics to optical trapping. [3, 6]. Describing the spatiotemporal shapes of single photons may seem counterintuitive, but quantum mechanics works probabilistically: the Airy pattern arises after averaging the spatiotemporal distributions of many photons.

Wang and colleagues took advantage of a recently introduced method to generate entangled pairs of photons and simultaneously shape the temporal profile of each pair. [7]. The researchers induced a nonlinear optical process in a cloud of cooled rubidium atoms using a femtosecond laser beam whose spatial profile exhibited an Airy pattern along just one dimension. This optical process caused the atomic cloud to emit pairs of entangled photons – each comprising a “trigger” photon shortly followed by a “signal” photon – with a probability dependent on the intensity of the beam and modulated by the bright and dark lobes of the beam. The one-dimensional spatial Airy pattern of the beam was transformed into a temporal Airy pattern, which was encoded in the probability distribution for the time interval between the emission of the two photons from each pair. The researchers then spatially shaped the signal photons directly, so that these photons exhibited spatial Airy patterns in addition to the temporal.

Manipulating single photons is difficult because they have very low intensities and highly fragile quantum correlations. Wang and colleagues alleviated the first problem by realizing the space-time transfer of the femtosecond laser beam’s one-dimensional Airy pattern to the single photons – rather than directly shaping the photons’ spatial and temporal profiles after pair generation.

To verify the survival of quantum correlations, the researchers tested two quantum properties of photons. First, they used an interferometer to determine the so-called second-order autocorrelation of the signal photons, obtaining a value below one, which is considered a non-classical light signature. Second, they investigated the ability of these signal photons to be recovered from background noise using their temporal correlations with the trigger photons – a concept known as quantum illumination (Fig. 1). To do so, the researchers mixed the signal’s photons with a random pattern of classical light. They then measured the spatial distribution of all photons arriving at a camera, capturing images formed by photons at different distances from the atomic cloud. An observer without access to the trigger photons would not recognize any structure in these images except that of a random expanding cloud. However, by including only those photons that arrive in coincidence with the trigger photons, Airy’s spatiotemporal patterns and trajectories are revealed.

Researchers’ proof-of-principle demonstration of quantum space-time Airy photons opens up intriguing application scenarios. For example, if used in super-resolution microscopy, these photons would provide the high imaging depth and large field of view offered by Airy beams, while achieving lower-than-classical measurement uncertainties that are peculiar to detection. quantum. [8]. Another possibility is to increase the reach and information capacity of quantum communications, for example, in the context of quantum key distribution – a secure communication method in which cryptographic keys are shared between parties. Encoding these keys into the spatial and temporal profiles of the photons demonstrated would allow many keys to be transmitted simultaneously over a single communication channel, while also protecting their quantum states from environmentally induced decoherence over longer distances than would otherwise be possible. . [9].

More generally, Wang and colleagues’ work provides a path to full spatiotemporal control of single photons that is not necessarily restricted to Airy patterns. It will be interesting to see whether the researchers’ strategy can be generalized to other types of beamforming while maintaining the same efficiency, and how it can be fused with existing protocols for creating quantum states.


  1. J.Wang and others.“Spatiotemporal single-photon Airy bullets,” Physical. Rev. 132143601 (2024).
  2. GA Siviloglou and others.“Observation of Accelerated Aerial Beams,” Physical. Rev. 99213901 (2007).
  3. NK Efremidis and others.“Aerial Beams and Accelerated Waves: An Overview of Recent Advances,” Optics 6686 (2019).
  4. S.Maruca and others.“Airy quantum photons,” J. Physics. B: At. Mol. Opt. Physical. 51175501 (2018).
  5. O. Lib and Y. Bromberg, “Spatially entangled airborne photons,” Choose. Let’s go. 451399 (2020).
  6. A. Chong and others.“Airy-Bessel wave packets as versatile linear bullets of light,” Nat. Photonics 4103 (2010).
  7. L. Zhao and others.“Shaping the biphoton temporal waveform with spatial light modulation,” Physical. Rev. 115193601 (2015).
  8. SHOVEL. Moreau and others.“Images with quantum states of light,” Nat. Rev. 1367 (2019).
  9. M. G Raymer and IA Walmsley, “Temporal Modes in Quantum Optics: Then and Now,” Physical. Scr. 95064002 (2020).

About the author

Image by Giuseppe Fumero

Giuseppe Fumero is a research associate at the National Institute of Standards and Technology in Maryland and a postdoctoral fellow at West Virginia University. He is currently working on multidimensional spectroscopy and ultrafast dynamics in new semiconductors. His research activity covers experimental and theoretical aspects of light-matter interactions at the interface of nonlinear optics, photonics and femtochemistry. He earned his PhD at Sapienza University of Rome in 2019. He was then a postdoctoral fellow at Sapienza and a visiting scientist at the University of California, Irvine.

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