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Journal of Optics

Synthesis and characterization of pure, two-qubit states encoded in path and polarization

David R. A. Ruelas Paredes, C. Montenegro La Torre, Jean Paul Marrou, Yonny Yugra, Mariano Uria, E. Massoni, F. De Zela

Published Aug 1, 2021• 1 citation
Publisher

Abstract

Abstract We have designed and experimentally put to test a protocol to synthesize and characterize pure, two-qubit states encoded in polarization and momentum (path) of light beams. Because these degrees of freedom are common to both classical and quantum light, it was possible to implement our protocol with classical light, thereby offering an advantageous and versatile testing ground. The experimental setup is robust and does not require state-dependent scheme modifications. We exhibited the capabilities of our protocol by generating three types of two-qubit states: Bell states, states with variable phases, and states with variable amplitudes.

DOI
10.1088/2040-8986/ac1bbf

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Abstract We have designed and experimentally put to test a protocol to synthesize and characterize pure, two-qubit states encoded in polarization and momentum (path) of light beams. Because these degrees of freedom are common to both classical and quantum light, it was possible to implement our protocol with classical light, thereby offering an advantageous and versatile testing ground. The experimental setup is robust and does not require state-dependent scheme modifications. We exhibited the capabilities of our protocol by generating three types of two-qubit states: Bell states, states with variable phases, and states with variable amplitudes.
We present an experimental proof-of-principle for the generation and detection of pure two-qubit states that have been encoded in degrees of freedom that are common to both classical-light beams and single photons. Our protocol requires performing polarization tomography on a single qubit from a qubit pair. The degree of entanglement in the qubit pair is measured by concurrence, which can be directly extracted from intensity measurements-or photon counting-entering single-qubit polarization tomography.
Abstract Light is an irreplaceable means of communication among various quantum information processing and storage devices. Due to their different physical nature, some of these devices couple more strongly to discrete, and some to continuous degrees of freedom of a quantum optical wave. It is therefore desirable to develop a technological capability to interconvert quantum information encoded in these degrees of freedom. Here we generate and characterize an entangled state between a dual-rail (polarization-encoded) single-photon qubit and a qubit encoded as a superposition of opposite-amplitude coherent states. We furthermore demonstrate the application of this state as a resource for the interfacing of quantum information between these encodings. In particular, we show teleportation of a polarization qubit onto a freely propagating continuous-variable qubit.
We explore the concepts of coherence and entanglement as they apply to both the classical and quantum natures of light. In the classical domain, we take inspiration from the tools and concepts developed in foundational quantum mechanics and quantum information science to gain a better understanding of classical coherence theory of light with multiple degrees of freedom (DoFs). First, we use polarization and spatial parity DoFs to demonstrate the notion of classical entanglement, and show that Bell's measure can serve as a useful tool in distinguishing between classical optical coherence theory. Second, we establish a methodical yet versatile approach called 'optical coherency matrix tomography' for reconstructing the coherency matrix of an electromagnetic beam with multiple DoFs. This technique 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. Third, we report the first experimental measurements of the 4 x 4 coherency matrix 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. In the quantum domain, we use the modal structure of classical fields to develop qubits and structure Hilbert spaces for use in quantum information processing. Advancing to three-qubit logic gates is an important step towards the success of optical schemes for quantum computing. We experimentally implement a variety of two- and three- qubit, linear and deterministic, single-photon, controlled, quantum logic gates using polarization and spatial parity qubits. Lastly, we demonstrate the implementation of two-qubit single-photon logic using polarization and orbital angular momentum qubits.
Abstract Entanglement and polarization of pure, two-qubit systems are known to be constrained by the relationship C 2 + P 2 = 1 . Here, C stands for concurrence, a standard measure of entanglement, and P refers to the degree of polarization of either of the two, single-qubit subsystems. The above constraint may be understood as reflecting that a system cannot be in a pure state, if it is entangled with another one. In order to explore the connection between entanglement and polarization beyond two-qubit systems, we addressed states in which at least one of the parties has dimensionality greater than two. The case of pure, tripartite states offered itself as a timely candidate. We focussed on three qubits and studied the connection between one qubit’s polarization and its entanglement with the other two taken together. We derived new constraints, akin to the above one, and submitted them to experimental tests using single photons entangled in polarization. The latter provided two qubits, each one attached to one photon. The third qubit was chosen to be the path (momentum) of one of the photons. Our experimental results confirmed the validity of the new constraints. Our theoretical results hold also for classical light. Possible experimental tests could be done with so-called structured light beams.
Light is an irreplaceable means of communication among various quantum information processing and storage devices. Due to their different physical nature, some of these devices couple more strongly to discrete, and some to continuous degrees of freedom of a quantum optical wave. It is therefore desirable to develop a technological capability to interconvert quantum information encoded in these degrees of freedom. Here we generate and characterize an entangled state between a dual-rail (polarization-encoded) single-photon qubit and a qubit encoded as a superposition of opposite-amplitude coherent states. We furthermore demonstrate the application of this state as a resource for the interfacing of quantum information between these encodings. In particular, we show teleportation of a polarization qubit onto a freely propagating continuous-variable qubit.
Light beams offer many degrees of freedom to be explored in discrete and continuous domains. In addition to the possibility of entangling photons in these many degrees of freedom, it makes light a very useful and versatile tool for quantum information and quantum foundation purposes. In this thesis, we endorse its importance and versatility by presenting novel contributions that further explore both discrete and continuous degrees of freedom. It begins with two experiments that use classical light and explore its analogous behavior to quantum systems. The first one is a classical optics simulation of the dynamics of a relativistic quantum particle. The second work is related to the theory of mutually unbiased measurements that are effectively discrete but constructed from continuous variables systems. In the second part of the thesis, three works are presented that use the polarization and path discrete degrees of freedom. The first one is a redefinition of the quantum nonlocal correlation called steering in the multipartite scenario, based on an inconsistency in the previous definition, namely the creation of this correlation from scratch using operations that supposedly would not be able to do so. We call this exposure of quantum steering. Steering exposure is observed with entangled photons. The other two works are related to the experimental implementation of quantum channels of qubits, one of them is a particular channel for which we test for non-Markovianity using a operational measure called conditional past-future (CPF) correlation. The thesis finishes with a proposal for an experimental realization of any quantum channel of a single qubit, where the qubit is realized by the polarization of single photons. The functioning of all optical devices used is didactically explained.