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Quantum Computing, Communication, and Simulation

Serrodyne transceiver for entangled quantum and coherent co-transmission on a single frequency channel

Jan Heine, Philip Rübeling, Robert Johanning, Michael Kues

Published Mar 19, 2025• 0 citations
Publisher

Abstract

Fiber optic networks are essential for managing global data traffic, and future telecommunications will require integrating quantum information for applications like quantum key distribution. Encoding quantum information in the frequency domain offers a scalable solution to quantum information transmission. Although the integration of frequency-entangled photons with coherent information transmission is possible through spectral multiplexing, more resource-efficient methods are necessary. We present and experimentally validate a transceiver concept that transmits coherent and frequency-entangled photons within a single frequency channel using the serrodyne technique via electro-optic phase modulation. This approach exploits distinct photon dynamics to enable temporal multiplexing and signal separation. We demonstrate preserved entanglement through the serrodyne transceiver despite coherent light, showing potential for efficient bandwidth use in hybrid quantum-coherent networks.

DOI
10.1117/12.3042168

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Fiber optic networks are essential for managing global data traffic, and future telecommunications will require integrating quantum information for applications like quantum key distribution. Encoding quantum information in the frequency domain offers a scalable solution to quantum information transmission. Although the integration of frequency-entangled photons with coherent information transmission is possible through spectral multiplexing, more resource-efficient methods are necessary. We present and experimentally validate a transceiver concept that transmits coherent and frequency-entangled photons within a single frequency channel using the serrodyne technique via electro-optic phase modulation. This approach exploits distinct photon dynamics to enable temporal multiplexing and signal separation. We demonstrate preserved entanglement through the serrodyne transceiver despite coherent light, showing potential for efficient bandwidth use in hybrid quantum-coherent networks.
Fiber-optical networks are well established to accommodate global data traffic via coherent information transmission. The next generation of telecommunications will require the integration of quantum information into fiber-optic networks, e.g., for quantum key distribution. A promising and scalable route to enable quantum networking is encoding quantum information into the frequency of photons. While the cointegration of frequency-entangled photons with coherent information transmission is achieved via spectral multiplexing, more resource-efficient approaches are required. In this work, we introduce and experimentally demonstrate a transceiver concept that enables the transmission of coherent and frequency-entangled photons over a single-frequency channel. Our concept leverages the serrodyne technique via electro-optic phase modulation leading to very different dynamics for entangled and coherent photons. This enables temporal multiplexing of the respective signals. We demonstrate the preservation of entanglement over the channel in the presence of coherent light. Our approach reveals a strong potential for efficient bandwidth use in hybrid networks.
Spectral-temporal manipulation of optical pulses has enabled numerous developments within a broad range of research topics, ranging from fundamental science to practical applications. Within quantum optics spectral-temporal degree of freedom of light offers a promising platform for integrated photonic quantum information processing. An important challenge in experimentally realizing spectral-temporal manipulation of quantum states of light is the need for highly efficient manipulation tools. In this context the intrinsically deterministic electro-optic methods show great promise for quantum applications. We experimentally demonstrate application of electro-optic platform for spectral-temporal manipulation of ultrashort pulsed quantum light. Using techniques analogous to serrodyne frequency shifting we show active spectral translation of few-picosecond single photon pulses by up to 0.5 THz. By employing an approach based on an electro-optic time lens we demonstrate up to 6-fold spectral compression of heralded single photon pulses with efficiency that enables us to significantly increase single photon flux through a narrow bandpass filter. We realize the required temporal phase manipulation by driving a lithium niobate waveguided electrooptic modulator with 33 dBm sinusoidal RF field at the frequency of either 10 GHz or 40 GHz. We use a phase lock loop to temporally lock the RF field to the 80 MHz repetition rate of approximately 1 ps long optical pulses. Heralded single photon wavepackets are generated by means of spontaneous parametric down-conversion in potassium dihydrogen phosphate (KDP) crystal, which enables preparation of spectrally pure single photon wavepackets without the need for spectral filtering. Spectral shifting is achieved by locking single-photon pulses to the linear slope of sinusoidal 40 GHz RF phase modulation. We verify the spectral shift by performing spectrally resolved heralded single photon counting, using frequency-to-time conversion by means of a highly dispersive chirped fiber Bragg grating. We verify the non-classicality of spectrally shifted single photons by measuring high-visibility Hong-Ou-Mandel interference using a reference single photon pulse. Spectral compression is based on the time lens principle, which requires locking optical pulses to approximately quadratic region of sinusoidal phase modulation. We utilize both 10 GHz and 40 GHz RF driving frequencies. Bandwidth compression is achieved by chirping the single photon pulse using an appropriate length of single-mode fibre and subsequently subjecting it to the action of the time lens. We verify spectral compression directly using the aforementioned spectrally-resolved heralded single photon counting method. We achieve 3-fold spectral compression of 2 nm bandwidth single photon pulses using 40 GHZ modulation frequency, and 6-fold spectral compression of 0.9 nm bandwidth single photon pulses using 10 GHz modulation frequency. Overall transmission of our set-up exceeding 30% enables practical usability of our spectral compression method which we demonstrate experimentally by showing an increased photon flux through a narrowband filter. Our results present an important contribution towards implementing quantum information processing in the spectral-temporal degree of freedom of a photon. In the context of quantum networks they present an enabling tool towards efficient photonic interfacing of different quantum information processing platforms.
Creating and controlling well-defined single-photon states is important for many quantum enhanced technologies. Information can be encoded in any degree of freedom associated with a single-photon field excitation, for example in the polarization or transverse spatial mode structure. The ability to encode multiple qubits in these states is desirable for high data transmission rates and increased information processing capacity. The spectral-temporal domain offers a large Hilbert space for encoding information, well suited to integrated optical architectures owing to the low cross-talk between channels. Indeed, time-frequency encoding is an integral component of existing information technologies infrastructure. Thus, complete, coherent control of the spectral-temporal mode structure of light is essential to advancing optical quantum technologies. Arbitrary control of single-photon states has been demonstrated in the polarization and spatial-momentum degrees of freedom however, is yet to be established in the spectral-temporal domain. Recently it was shown that spectral-temporal pulse shaping is experimentally feasible using nonlinear optical methods. However, such techniques can introduce noise photons, are challenging to implement deterministically and require specially prepared auxiliary pump pulses. Furthermore, they are typically only possible at relatively low repetition rates to avoid damage of nonlinear optical materials. Here, we demonstrate deterministic pulse shaping of single-photon wave packets by introducing a time-varying phase. Applying linear or quadratic phase to the wave packet introduces a spectral shift or spectral broadening, respectively. Achieving significant spectral manipulation requires temporal-phase modulation on the order of π. The phase must also vary on the same time scale as the optical pulses, approximately 1 ps in duration here. This relatively rapid phase variation is achieved using a fast electro-optic phase modulator. The modulator is driven by a time-varying voltage that is synchronized with a pulse train of single-photon wave packets for accurate temporal phase control. In this thesis, we experimentally demonstrate control of the spectral-temporal state of single-photon wave packets using this method. Heralded single-photon wave packets are generated by spontaneous parametric down conversion pumped by a frequency-doubled Titanium-Sapphire laser oscillator with 80 MHz repetition rate. To achieve significant modulation of single-photon pulses a 10 V peak-to-peak radio-frequency signal of 40 GHz drives the phase modulator, which requires approximately 3.6 V to achieve a π phase shift. The signal is synchronized with the oscillator pulse train using custom designed electronics. Details of these electronics are presented, resulting in a robust phase and amplitude controllable 40 GHz voltage source, phase locked to the single-photon pulse train. To demonstrate the utility of this pulse-shaping method we experimentally show spectral shearing of almost 1 nm for single-photon wave packets with 830 nm central wavelength and 1 nm bandwidth. Measurement of the second-order intensity correlation before and after the wave-packet manipulation remains constant within uncertainty of the measurement, showing that the quantum nature of the single-photon source is not deteriorated by the modulator. Preservation of the wave-packet coherence is verified by two-photon interference between spectrally sheared and non-modulated single-photons.
Processing and distributing quantum information using photons through fibre-optic or free-space links are essential for building future quantum networks. The scalability needed for such networks can be achieved by employing photonic quantum states that are multiplexed into time and/or frequency, and light-matter interfaces that are able to store and process such states with large time-bandwidth product and multimode capacities. Despite important progress in developing such devices, the demonstration of these capabilities using non-classical light remains challenging. Here, employing the atomic frequency comb quantum memory protocol in a cryogenically cooled erbium-doped optical fibre, we report the quantum storage of heralded single photons at a telecom-wavelength (1.53 μm) with a time-bandwidth product approaching 800. Furthermore, we demonstrate frequency-multimode storage and memory-based spectral-temporal photon manipulation. Notably, our demonstrations rely on fully integrated quantum technologies operating at telecommunication wavelengths. With improved storage efficiency, our light-matter interface may become a useful tool in future quantum networks.
Frequency conversion of nonclassical light enables robust encoding of quantum information based upon spectral multiplexing that is particularly well-suited to integrated-optics platforms. Here we present an intrinsically deterministic linear-optics approach to spectral shearing of quantum light pulses and show it preserves the wave-packet coherence and quantum nature of light. The technique is based upon an electro-optic Doppler shift to implement frequency shear of heralded single-photon wave packets by ±200 GHz, which can be scaled to an arbitrary shift. These results demonstrate a reconfigurable method to controlling the spectral-temporal mode structure of quantum light that could achieve unitary operation.