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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Removing Phase Ambiguity in Fiber-Based Interferometers for Coherent Time-Bin Operations

Piotr Roztocki, Mehedi Islam, Christian Reimer, Bennet Fischer, Stefania Sciara, Yanbing Young Zhang, David Moss, Michael Kues, Roberto Morandotti

Published Jun 1, 2019• 0 citations
Publisher

Abstract

Time is a practical and robust degree of freedom for the encoding of quantum information. Qubits encoded in so-called `time-bins', allowing a discrete superposition of two potential arrival times, have their entanglement preserved even over long propagation distances in standard fiber networks [1]. Time has also been used for the preparation of more complex quantum systems, such as hyper-entangled and cluster states [2]. These qualities put time-bin encoding at the center of applications ranging from quantum state preparation through to quantum communications and information processing. One of the hallmarks of the scheme is that a nonlinear element has to be pumped with phase-coherent double-pulses, which can be achieved by using unbalanced interferometers [1], making their phase stability the key determinant of both state generation and analysis accuracy. However, the phase ambiguity arising from the periodicity of feedback signals [3], as well as the difficulty of stabilizing on interference maxima/minima, leave progress to be made in fiberbased interferometry for time-bin encodings.

DOI
10.1109/cleoe-eqec.2019.8872195

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Time is a practical and robust degree of freedom for the encoding of quantum information. Qubits encoded in so-called `time-bins', allowing a discrete superposition of two potential arrival times, have their entanglement preserved even over long propagation distances in standard fiber networks [1]. Time has also been used for the preparation of more complex quantum systems, such as hyper-entangled and cluster states [2]. These qualities put time-bin encoding at the center of applications ranging from quantum state preparation through to quantum communications and information processing. One of the hallmarks of the scheme is that a nonlinear element has to be pumped with phase-coherent double-pulses, which can be achieved by using unbalanced interferometers [1], making their phase stability the key determinant of both state generation and analysis accuracy. However, the phase ambiguity arising from the periodicity of feedback signals [3], as well as the difficulty of stabilizing on interference maxima/minima, leave progress to be made in fiberbased interferometry for time-bin encodings.
We propose a scheme for the generation of hybrid states entangling a single-photon time-bin qubit with a coherent-state qubit encoded on phases. Compared to other reported solutions, time-bin encoding makes hybrid entanglement particularly well adapted to applications involving long-distance propagation in optical fibers. This makes our proposal a promising resource for future out-of-the-laboratory quantum communication. In this perspective, we analyze our scheme by taking into account realistic experimental resources and discuss the impact of their imperfections on the quality of the obtained hybrid state.
Discrete frequency modes, or bins, present a blend of opportunities and challenges for photonic quantum information processing. Frequency-bin-encoded photons are readily generated by integrated quantum light sources, naturally high-dimensional, stable in optical fiber, and massively parallelizable in a single spatial mode. Yet quantum operations on frequency-bin states require coherent and controllable multifrequency interference, making them significantly more challenging to manipulate than more traditional spatial degrees of freedom. In this mini-review, we describe recent developments that have transformed these challenges and propelled frequency bins forward. Focusing on sources, manipulation schemes, and detection approaches, we introduce the basics of frequency-bin encoding, summarize the state of the art, and speculate on the field’s next phases. Given the combined progress in integrated photonics, high-fidelity quantum gates, and proof-of-principle demonstrations, frequency-bin quantum information is poised to emerge from the lab and leave its mark on practical quantum information processing—particularly in networking where frequency bins offer unique tools for multiplexing, interconnects, and high-dimensional communications.
The temporal degree-of-freedom of light is incredibly powerful for modern quantum technologies, enabling large-scale quantum computing architectures and record key-rates in quantum key distribution. However, the generalized measurement of large and complex quantum superpositions of the time-of-arrival of a photon remains a unique experimental challenge. Conventional methods based on unbalanced Franson-type interferometers scale poorly with dimension, requiring multiple cascaded devices and active phase stabilization. In addition, these are limited by construction to a restricted set of phase-only superposition measurements. Here we show how the coupling of spatial and temporal information inside a single multi-mode fiber can be harnessed to program completely generalized measurements for high-dimensional superpositions of photonic time-bin. Using the multi-spectral transmission matrix of the fiber, we find special sets of spatial modes that experience distinct dispersive delays through the fiber. By exciting coherent superpositions of these spatial modes, we engineer the equivalent of large, unbalanced multi-mode interferometers inside the fiber and use them to perform high-quality measurements of arbitrary time-bin superpositions in up to dimension 11. The single fiber functions as a scalable, common-path interferometer for time-bin qudits that significantly eases the experimental overheads of standard approaches based on unbalanced Franson-type interferometers, serving as an essential tool for quantum technologies that harness the temporal properties of light.
Quantum photonics is increasingly making use of the energy-time degree of freedom (DoF) to encode information. Its inherently high-dimensionality offers higher information capacity and robustness against noise. Most frequently, the energy-time DoF is exploited in the form of time bins and frequency bins, with direct applications in both quantum information processing tasks and quantum communication protocols. More recently, it has been used also in its pulse modes (or temporal modes) form, sets of states overlapping in time and frequency but kept orthogonal considering their temporal and spectral phase, whose difficult manipulation and characterisation made it less appealing for experimental applications. The production of states encoded in pulse modes has been demonstrated through sum frequency generation in waveguides [1] and parametric down conversion in bulk crystals [2], as well as the demonstration of schemes for creating hyper-entanglement among energy-time DoF and other high-dimensional DoFs [3], [4]. However, their implementation is still an area of continuous development and evolution, given the complexity of manipulating and characterising them.
Temporal encodings of quantum information are prevalent in applications because of their suitability for long-distance quantum communication and their compatibility with optical fiber communication networks. Perhaps the simplest temporal encoding is time-bin encoding, i. e. in superpositions of two (or more) temporally separated optical pulses. Early attempts at generating time-bin entanglement from single quantum emitters was not able to avoid the problem of re-excitation or was converted probabilistically from polarization entangled photon pairs from a quantum dot. Direct generation requires a metastable level to carry the coherence and avoid double pair emission into the desired time bins. In order to use dark exciton states as metastable states we have worked on their efficient creation and coherent control in the presence of in-plane magnetic fields. Much of this is based on our recent work on advanced excitation schemes using chirped pulses. With chirped pulses we are now able to deterministically populate a dark exciton state and to transfer this population to the biexciton, which can then emit a photon pair.
The photonic temporal degree of freedom is one of the most promising platforms for quantum communication over fiber networks and free-space channels.In particular, time-bin states of photons are robust to environmental disturbances, support high-rate communication, and can be used in high-dimensional schemes.However, the detection of photonic time-bin states remains a challenging task, particularly for the case of photons that are in a superposition of different time bins.Here, we experimentally demonstrate the feasibility of picosecond time-bin states of light, known as ultrafast time bins, for applications in quantum communications.With the ability to measure time-bin superpositions with excellent phase stability, we enable the use of temporal states in efficient quantum key distribution protocols such as the BB84 protocol.