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arXiv:2607.27144

Parity-Based Time-Bin Encoding Enabling SWAP Between Polarization and Time-Bin Qubits

Adam Sultan, Connor Kupchak

quant-phSubmitted Jul 29, 2026• Announced Jul 30, 2026• 0 citations• Revised Aug 31, 2026 (v2)
v1 · Jul 29, 2026v2 · Aug 31, 2026v3 · Oct 15, 2026
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Abstract

Multi-degree-of-freedom photonic quantum processing requires routing between degree-of-freedom (DOF) qubit encodings on a single photon. A SWAP between polarization and time-bin qubits is Multi-degree-of-freedom photonic quantum processing requires routing between degree-of-freedom (DOF) qubit encodings on a single photon. A SWAP between polarization and time-bin qubits is an advantageous primitive for such architectures, however conventional early/late time-bin encoding does not support bidirectional logical time-bin flips from late to early which limits the ability to implement certain quantum operations. We introduce a parity-based time-bin encoding in which logical $\vert 0 \rangle_T$ and $\vert 1 \rangle_T$ correspond to even and odd multiples of a spacing $Δt$, so that a physical delay of $Δt$ implements $\vert 0 \rangle_T \leftrightarrow \vert 1 \rangle_T$. This encoding is the enabling ingredient that makes a polarization-controlled delay line implement $\mathrm{CNOT}_{P \rightarrow T}$ and aligns naturally with periodic refractive index modulation for $\mathrm{CNOT}_{T \rightarrow P}$. Composing three such CNOT operations sequentially results in a deterministic SWAP between polarization and time-bin degrees of freedom. We analyze field-based modulation polarization-rotation error probability and timing-resolution constraints set by both EOM drive electronics and photon detection.

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Multi-degree-of-freedom photonic quantum processing requires routing between degree-of-freedom (DOF) qubit encodings on a single photon. A SWAP between polarization and time-bin qubits is Multi-degree-of-freedom photonic quantum processing requires routing between degree-of-freedom (DOF) qubit encodings on a single photon. A SWAP between polarization and time-bin qubits is an advantageous primitive for such architectures, however conventional early/late time-bin encoding does not support bidirectional logical time-bin flips from late to early which limits the ability to implement certain quantum operations. We introduce a parity-based time-bin encoding in which logical ∣0⟩T\vert 0 \rangle_T∣0⟩T​ and ∣1⟩T\vert 1 \rangle_T∣1⟩T​ correspond to even and odd multiples of a spacing ΔtΔtΔt, so that a physical delay of ΔtΔtΔt implements ∣0⟩T↔∣1⟩T\vert 0 \rangle_T \leftrightarrow \vert 1 \rangle_T∣0⟩T​↔∣1⟩T​. This encoding is the enabling ingredient that makes a polarization-controlled delay line implement CNOTP→T\mathrm{CNOT}_{P \rightarrow T}CNOTP→T​ and aligns naturally with periodic refractive index modulation for CNOTT→P\mathrm{CNOT}_{T \rightarrow P}CNOTT→P​. Composing three such CNOT operations sequentially results in a deterministic SWAP between polarization and time-bin degrees of freedom. We analyze field-based modulation polarization-rotation error probability and timing-resolution constraints set by both EOM drive electronics and photon detection.
Quantum key distribution allows for a provably secure transmission of cryptographic keys over an optical channel. Encoded polarization states or time-bin degree of freedom have been used for successful demonstrations. However, photon losses in long fibers, slow single photon detectors, and detector dark counts significantly limit the overall bit rate. Improving key throughput and reducing the overhead of key reconciliation remain as major challenges. Methods which utilize multiple time bins allow for multiple key bits to be encoded in a single photon, thus increasing the fidelity of transmitted keys and decreasing the overhead of key reconciliation in real-world conditions. Previous implementations of these methods required that Alice and Bob share a time reference by sharing a dedicated classical channel used for synchronization. This work presents a technique that allows two parties to exchange time-bin encoded photons without the need for synchronized time references. Our technique uses a framing protocol which allows Alice to encode a time reference along with a key which is determined by Alice before transmission. Security can be achieved by monitoring the visibility of a pair of Franson interferometers, using decoy pulses and measuring the round trip time between Alice and Bob. The bit rate of this technique is limited only by the recovery time of the detector and the speed of the modulation electronics. We experimentally demonstrate a raw bit rate of 5Mb/s over an optical channel with 55dB of loss, which is competitive with current research. We also demonstrate absolute timing synchronization with an accuracy of 20ps.
Photonic time-bin qubits are well suited to transmission via optical fibers and waveguide circuits. The states take the form 1(2)(α|0⟩+eiϕβ|1⟩), with |0⟩ and |1⟩ referring to the early and late time bin, respectively. By controlling the phase of a laser driving a spin-flip Raman transition in a single-hole-charged InAs quantum dot, we demonstrate complete control over the phase, ϕ. We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence. Finally, we encode different qubits in different energies of the Raman scattered light, paving the way for wavelength-division multiplexing at the single-photon level.
Photonic time-bin qubits are well suited to transmission via optical fibers and waveguide circuits.The states take the form 1 ffiffiffiffi ffi ð2Þ p ðαj0iþe iϕ βj1iÞ, with j0i and j1i referring to the early and late time bin, respectively.By controlling the phase of a laser driving a spin-flip Raman transition in a single-holecharged InAs quantum dot, we demonstrate complete control over the phase, ϕ.We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence.Finally, we encode different qubits in different energies of the Raman scattered light, paving the way for wavelengthdivision multiplexing at the single-photon level.
The time-bin quantum state is known to be highly robust against decoherence effects in both fiber-optic and atmospheric channels, a unique feature that renders the time degree of freedom (DOF) more appropriate for quantum communication in these channels. In this paper, we present a scheme to deliver with high fidelity an arbitrary, unknown quantum state in polarization or spatial DOFs over a stochastic channel without need for compensation. The sender swaps the polarization or spatial quantum state for a time-bin state of the photon before signaling it over the random channel, and the receiver swaps the state back. Because the signaled photon is assumed to be in a single spatial or polarization mode, no modal-dependent channel effects perturb the time-bin state. We find that by migration to the time bin, the fidelity of the transferred state is boosted by a margin dependent on the time-bin period and the standard deviations of the statistical parameters of the channel.
The encoding of quantum information in photonic time-bin qubits is apt for long-distance quantum communication schemes. In practice, due to technical constraints such as detector response time, or the speed with which copolarized time-bins can be switched, other encodings, e.g., polarization, are often preferred for operations like state detection. Here, we present the conversion of qubits between polarization and time-bin encodings by using a method that is based on an ultrafast optical Kerr shutter and attain efficiencies of 97% and an average fidelity of $0.827\ifmmode\pm\else\textpm\fi{}0.003$ with shutter speeds near 1 ps. Our demonstration delineates an essential requirement for the development of hybrid and high-rate optical quantum networks.
Frequency-bin encoding is massively parallelizable and robust for optical fiber transmission. When coupled with an additional degree of freedom (DoF), the expansion of the Hilbert space allows for deterministic controlled operations between two DoFs within a single photon. Such capabilities, when combined with photonic hyperentanglement, are of great value for quantum communication protocols, including dense coding and single-copy entanglement distillation. In this talk, we present an all-fiber-coupled, ultrabroadband polarization–frequency hyperentangled source and conduct comprehensive quantum state tomography across multiple dense wavelength division multiplexing channels spanning the optical C+L-band (1530–1625 nm). In addition, we design and implement a high-fidelity controlled-NOT (cnot) operation between polarization and frequency DoFs by exploiting electro-optic phase modulation within a fiber Sagnac loop. Collectively, our hyperentangled source and two-qubit gate should unlock new opportunities for harnessing polarization–frequency resources in established telecommunication fiber networks for future quantum applications.