Daniel Giovannini

Higher-dimensional orbital-angular-momentum-based quantum key distribution with mutually unbiased bases

We present an experimental study of higher-dimensional quantum key distribution protocols based on mutually unbiased bases, implemented by means of photons carrying orbital angular momentum. We perform (d + 1) mutually unbiased measurements in a classically simulated prepare-and-measure scheme and on a pair of entangled photons for dimensions ranging from d = 2 to 5. In our analysis, we pay attention to the detection efficiency and photon pair creation probability. As security measures, we determine from experimental data the average error rate, the mutual information shared between the sender and receiver, and the secret key generation rate per photon. We demonstrate that increasing the dimension leads to an increased information capacity as well as higher key generation rates per photon. However, we find that the benefit of increasing the dimension is limited by practical implementation considerations, which in our case results in deleterious effects observed beyond a dimension of d = 4.

Spatially structured photons that travel in free space slower than the speed of light

Slowing down light with added structure We are taught that the speed of light in free space is one of the universal physical constants: c. Giovannini et al. now show that there are certain conditions under which such certainty can be broken (see the Perspective by Sambles). Adding spatial structure to an optical beam of single photons reduced the speed of light. The magnitude of the decrease depended on the complexity of the structure imprinted onto the photons. Science, this issue p. 857; see also p. 828 Introducing spatial structure to an optical beam reduces the speed of light. [Also see Perspective by Sambles] That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wave vectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam’s transverse spatial structure. Using time-correlated photon pairs, we show a reduction in the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several micrometers over a propagation distance of ~1 meter. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.

Coherent absorption of N00N states

We experimentally investigate two-photon N00N state coherent absorption in a multilayer graphene film and show that coherent loss can be used as a resource for quantum operations.

Comparing the information capacity of Laguerre–Gaussian and Hermite–Gaussian modal sets in a finite-aperture system

Using a spontaneous parametric down-conversion process to create entangled spatial states, we compare the information capacity associated with measurements in the Hermite-Gaussian and Laguerre-Gaussian modal basis in an optical system of finite aperture. We show that the cross-talk imposed by the aperture restriction degrades the information capacity. However, the Laguerre-Gaussian mode measurements show greater resilience to cross talk than the Hermite-Gaussian, suggesting that the Laguerre-Gaussian modal set may still offer real-world advantages over other modal sets.

Spiniform phase-encoded metagratings entangling arbitrary rational-order orbital angular momentum

Quantum entanglements between integer-order and fractional-order orbital angular momentums (OAMs) have been previously discussed. However, the entangled nature of arbitrary rational-order OAM has long been considered a myth due to the absence of an effective strategy for generating arbitrary rational-order OAM beams. Therefore, we report a single metadevice comprising a bilaterally symmetric grating with an aperture, creating optical beams with dynamically controllable OAM values that are continuously varying over a rational range. Due to its encoded spiniform phase, this novel metagrating enables the production of an average OAM that can be increased without a theoretical limit by embracing distributed singularities, which differs significantly from the classic method of stacking phase singularities using fork gratings. This new method makes it possible to probe the unexplored niche of quantum entanglement between arbitrarily defined OAMs in light, which could lead to the complex manipulation of microparticles, high-dimensional quantum entanglement and optical communication. We show that quantum coincidence based on rational-order OAM-superposition states could give rise to low cross-talks between two different states that have no significant overlap in their spiral spectra. Additionally, future applications in quantum communication and optical micromanipulation may be found.

Comparing the Information Capacity of Entangled Laguerre-Gaussian and Hermite-Gaussian Modal Sets in a Finite Aperture System

Within a parametric down-conversion process we compare information capacity associated with the entangled spatial modes as measured in the Hermitte-Gaussian and Lagueere-Gaussian modal basis in an optical system of finite aperture.

Reconstruction of high-dimensional states entangled in orbital angular momentum using mutually unbiased measurements

Efficient quantum state reconstruction of a high-dimensional multi-partite quantum system can be performed by considering mutually unbiased measurements of the individual parts. We illustrate this approach experimentally using a bipartite photonic system entangled in orbital angular momentum.

Measuring Orbital Angular Momentum Quantum States in High-Dimensional Mutually Unbiased Bases

We perform local measurements of entangled photon pairs in mutually unbiased bases of orbital angular momentum subspaces, which provide a platform for the implementations of a number of high-dimensional quantum protocols.