Macroscopic Single-Qubit Operation for Coherent Photons

TL;DR

This paper demonstrates a method to perform arbitrary single-qubit operations on macroscopic photon polarisation states using a device called the Poincaré rotator, enabling dynamic control over quantum states in a fibre-optic setup.

Contribution

It introduces the Poincaré rotator, a novel device that executes arbitrary $SU(2)$ operations on macroscopic photon polarisation qubits in a fibre-optic system.

Findings

Successfully realized over 100 distinguishable polarisation states.

Able to draw complex structures like Buckminsterfullerene and Earth's coastline on the Poincaré sphere.

Demonstrated dynamic control of polarisation states using the device.

Abstract

Polarisation is described by an $SU(2)$ wavefunction due to macroscopic coherence of photons emitted from a ubiquitous laser source, and thus, a laser pulse is expected to behave as a macroscopic quantum bit (qubit), i.e., a qubit realised by a macroscopic number of photons. Here, we show that an arbitrary single-qubit operation can be carried out for such a macroscopic qubit by employing optical modulators, together with standard optical plates, in a computer-controlled fibre-optic configuration. We named the device as a Poincar\'e rotator, which allows a dynamic control over a polarisation state by executing an arbitrary amount of rotations on the Poincar\'e sphere. The Poincar\'e rotator works as an arbitrary $SU(2)$ operator in a Lie group, by combining a $U(1)$ operation to change the phase and another $U(1)$ operation to change the amplitude of the wavefunction. We have realised various polarisation states, such as $4 \times 4=16$, $8 \times 8=64$, and $10 \times 10=100$ distinguishable states on the sphere. As a locus of the realised polarisation states on the sphere, we have successfully drawn the molecular structure of Buckminsterfullerene (C$_{60}$) and the coastline of the earth.