Generation of entangled photon pairs on a nonlinear chip
James G. Titchener, Che Wen Wu, Alexander S. Solntsev, Dragomir N. Neshev, and Andrey A. Sukhorukov* Nonlinear Physics Centre, Research School of Physics and Engineering,
Australian National University Canberra, ACT 0200, Australia
* Andrey.Sukhorukov@anu.edu.au
Abstract— We propose a design for an integrated photonic circuit capable of generating photon pairs in any path entangled quantum state, and develop a robust method of pump filtering based on adiabatic coupling.
Keywords— photon pair, coupled waveguides, Bell state
I. INTRODUCTION
Precise control over the unusual properties of quantum systems would facilitate the development of many new technoligies, such as quantum information processing, communication and enhanced precision measurment. Since we interact with the world in a macroscopic classical way a key ingredient in any quantum technology will be a flexiable interface between classical and quantum information.
Entangled photons are an ideal medium for creating and manipulating quantum states due to the low noise and ease of transmission. A quibit encoded into a photon can be easily sent between different photonic elements along an optical fiber, in analogy the the transmission of classical bits along electrical wires [1]. Furthermore logic operations can be preformed on entangled photons by exploiting the nonlinerity inherent in qunatum measurement [1, 2]. Therefore a device that can flexibly map information from classical states of light onto the quantum state of entangled photon pairs would be essential for realising practical quantum technologies.
Integrated quantum photonic circuits have the potential to produce and control entangled photons far more efficiently than traditional bulk optics. Integrating optical elements onto a single chip reduces the systems’ contact with the enviroment, preserving the fidelity of the quantum entanglement [3]. Also integrated devices are compact and stable, so they can be combined to build complex quantum circuits that would be impossibly large using tratitional bulk optics. Hence the generation and control of entangled photons integrated within a photonic chip is an important scientific and technological milestone. Indeed, on-chip generation of nonclassical photon states has been actively explored [4-6].
An essential ingredient of a photon source is the ability to produce different states on demand. Reconfigurable on-chip creation of two-photon Bell states was demonstrated using thermally tunable phase shifters [3]. In this work, we
demonstrate that a nonlinear photonic chip can be designed for fast all-optically controlled generation of an arbitrary set of photon states, in which quantum wave functions are directly mapped from the classical amplitudes and phases of the input pump laser beams. This approach provides speed, simplicity and extended flexibility.
Fig. 1. Illustrative diagram showing the waveguide array. The pump lasers are shown in blue and the field of the down converted photon pairs is in red. Classical input lasers , and create a quantum state, , at the end of the array.
II. GENERATING AND MANIPULATING ENTANGLED PHOTON PAIRS
We show how reconfigurable quantum states of photon pairs can be created within an integrated photonic device. A sample device consists of an array of three coupled waveguides (WGA) which allow photons to couple between neighbouring waveguides, shown in figure 1. Entangled photon pairs (biphotons) can be generated within the WGA via spontaneous parametric down conversion (SPDC). This is achieved by using a material with quadratic nonlinearity for the WGA then driving the waveguides with a pump laser. Entangled pairs of signal and idler photons will be continuously generated at all points along each laser driven waveguide. Hence there is a continuous source of entangled photons integrated into the device.
Once generated the photon pairs will propagate down the WGA, coupling between neighbouring waveguides. This is a realisation of a quantum random walk, where photons walk every possiable path along the WGA, and exit in a superposition of many possible quantum states. Quantum
interference between different walkers can result in the seperation of signal and idler photons [7], producing so called anti-bunched states. These are states where two photons are entangled but have different spatial locations in the WGA, so measuring one photon in a particular waveguide implies the existance of another photon in a different location.
The quantum state of the entangled photon pairs at the end of the WGA can be manipulated by two techniques.
Firstly poling of the ferro-electric dipol moment in each waveguide can alter the generation of bi-photons when that waveguide is driven [8]. Secondly real time tuning of the quantum output can be achieved by altering the phase and magnitide of the classical lasers driving SPDC in each waveguide of the array. Driving multiple waveguides simultaneoulsy will produce a linear combination of the outputs weighted by the power of the driving laser in each waveguide.
III. PUMP FILTERING
We show that by employing adiabatic waveguide coupling, we can generate spatially entangled photon pairs though spontaneous parametric down-conversion (SPDC), while simultaneously providing spatial pump filtering and keeping photon-pair states pure. We estimate the performance of the pump filtering in our adiabatic couplers to be of the order of 72 dB [9]. The scheme of adiabatically coupled waveguiding structure is shown in Fig. 1(a). For particular coupling between the central waveguides, we can generate any two-photon Bell state based on spatial entanglement between the outer waveguides 1 and 6, respectively.
Fig. 2: Scheme of adiabatically coupled waveguiding structure: pump beams (green colour) are coupled to the central waveguides 3 and 4, the Bell states (red colour) are formed in the edge waveguides 1 and 6 that are separate from the pumped waveguides.
IV. CONCLUSIONS
We have proposed a practical integrated device capable of creating any path-entangled photon state.
Futhermore we show how to achieve real-time switching to any quantum state within an dimensional subspace of the total output space. We also develop a scheme for combining flexible Bell states generation and high-quality spatial pump filtering in an integrated photonic chip.
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