Kumel H. Kagalwala

Optical coherency matrix tomography

The coherence of an optical beam having multiple degrees of freedom (DoFs) is described by a coherency matrix G spanning these DoFs. This optical coherency matrix has not been measured in its entirety to date—even in the simplest case of two binary DoFs where G is a 4 × 4 matrix. We establish a methodical yet versatile approach—optical coherency matrix tomography—for reconstructing G that exploits the analogy between this problem in classical optics and that of tomographically reconstructing the density matrix associated with multipartite quantum states in quantum information science. Here G is reconstructed from a minimal set of linearly independent measurements, each a cascade of projective measurements for each DoF. We report the first experimental measurements of the 4 × 4 coherency matrix G associated with an electromagnetic beam in which polarization and a spatial DoF are relevant, ranging from the traditional two-point Young’s double slit to spatial parity and orbital angular momentum modes.

Two-point optical coherency matrix tomography

The two-point coherence of an electromagnetic field is represented completely by a 4×4 coherency matrix G that encodes the joint polarization-spatial-field correlations. Here, we describe a systematic sequence of cascaded spatial and polarization projective measurements that are sufficient to tomographically reconstruct G--a task that, to the best of our knowledge, has not yet been realized. Our approach benefits from the correspondence between this reconstruction problem in classical optics and that of quantum state tomography for two-photon states in quantum optics. Identifying G uniquely determines all the measurable correlation characteristics of the field and, thus, lifts ambiguities that arise from reliance on traditional scalar descriptors, especially when the fields degrees of freedom are correlated or classically entangled.

Single-photon three-qubit quantum logic using spatial light modulators

The information-carrying capacity of a single photon can be vastly expanded by exploiting its multiple degrees of freedom: spatial, temporal, and polarization. Although multiple qubits can be encoded per photon, to date only two-qubit single-photon quantum operations have been realized. Here, we report an experimental demonstration of three-qubit single-photon, linear, deterministic quantum gates that exploit photon polarization and the two-dimensional spatial-parity-symmetry of the transverse single-photon field. These gates are implemented using a polarization-sensitive spatial light modulator that provides a robust, non-interferometric, versatile platform for implementing controlled unitary gates. Polarization here represents the control qubit for either separable or entangling unitary operations on the two spatial-parity target qubits. Such gates help generate maximally entangled three-qubit Greenberger–Horne–Zeilinger and W states, which is confirmed by tomographical reconstruction of single-photon density matrices. This strategy provides access to a wide range of three-qubit states and operations for use in few-qubit quantum information processing protocols.Photons are essential for quantum information processing, but to date only two-qubit single-photon operations have been realized. Here the authors demonstrate experimentally a three-qubit single-photon linear deterministic quantum gate by exploiting polarization along with spatial-parity symmetry.

Photonic Three-Qubit CNOT Gates Using Spatial Light Modulators

We experimentally demonstrate linear and deterministic, single-photon three-qubit controlled-NOT (CNOT) gates implemented by a polarization-sensitive spatial light modulator. The polarization qubit acts as the control, whereas the photon spatial-parity along x and y directions are the target qubits.

Classical Entanglement in Electromagnetic Beams

We study classical entanglement for an optical beam with two binary degrees of freedom: polarization and spatial parity; and demonstrate experimentally a measurement of Bell’s inequality and show its usefulness as a quantitative tool in classical optical coherence theory.

Implementing an optical CNOT using spatial parity qubits

We describe an optical implementation of a CNOT gate in which the control qubit is the polarization of a single photon and the target qubit is the spatial parity of the same photon. The gate is implemented with a polarizationsensitive spatial light modulator. We characterize the operation of the gate using quantum process tomography and the spatial parity is analyzed with a modified Mach-Zehnder interferometer. We also demonstrate the CNOT-gate operation with arbitrary rotation of the target qubit and discuss the possibility of implementing multi-qubit CNOT gates using the same approach.