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

Ultra-broadband homodyne-detection for parallel processing of quantum-information

Yaakov Shaked, Rafi Z. Vered, Yoad Michael, Leon Bello, M. Rosenbluh, Avi Pe’er

Published Jun 1, 2017• 0 citations
Publisher

Abstract

Quantum-optical states, such as single-photons or quadrature-squeezed light, used for processing quantum-information easily reach optical bandwidths, however homodyne-detection - a major quantum-information tool, is sub-GHz limited. Using optical parametric-amplification we demonstrate effectively bandwidth unlimited optical-homodyne measurement.

DOI
10.1109/cleoe-eqec.2017.8087355

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•Jan 31, 2024•0 citations
Quantum-optical states, such as single-photons or quadrature-squeezed light, used for processing quantum-information easily reach optical bandwidths, however homodyne-detection - a major quantum-information tool, is sub-GHz limited. Using optical parametric-amplification we demonstrate effectively bandwidth unlimited optical-homodyne measurement.
Homodyne measurement is a corner-stone of quantum optics. It measures the fundamental variables of quantum electrodynamics - the quadratures of light, which represent the cosine-wave and sine-wave components of an optical field and constitute the quantum optical analog of position and momentum. Yet, standard homodyne, which is used to measure the quadrature information, suffers from a severe bandwidth limitation: While the bandwidth of optical states can easily span many THz, standard homodyne detection is inherently limited to the electrically accessible, MHz to GHz range, leaving a dramatic gap between the relevant optical phenomena and the measurement capability. We demonstrate a fully parallel optical homodyne measurement across an arbitrary optical bandwidth, effectively lifting this bandwidth limitation completely. Using optical parametric amplification, which amplifies one quadrature while attenuating the other, we measure two-mode quadrature squeezing of 1.7dB below the vacuum level simultaneously across a bandwidth of 55THz, using just one local-oscillator - the pump. As opposed to standard homodyne, our measurement is highly robust to detection inefficiency, and was obtained with >50%>50\%>50% loss in the detection channel. This broadband parametric homodyne measurement opens a wide window for parallel processing of quantum information.
Homodyne measurement is a corner-stone method of quantum optics that measures the quadratures of light-the quantum optical analog of the canonical position and momentum. Standard homodyne, however, suffers from a severe bandwidth limitation: while the bandwidth of optical states can span many THz, standard homodyne is inherently limited to the electronically accessible MHz-to-GHz range, leaving a dramatic gap between relevant optical phenomena and the measurement capability. We demonstrate a fully parallel optical homodyne measurement across an arbitrary optical bandwidth, effectively lifting this bandwidth limitation completely. Using optical parametric amplification, which amplifies one quadrature while attenuating the other, we measure quadrature squeezing of 1.7 dB simultaneously across 55 THz, using the pump as the only local oscillator. As opposed to standard homodyne, our measurement is robust to detection inefficiency, and was obtained with >50% detection loss. Broadband parametric homodyne opens a wide window for parallel processing of quantum information.
Quantum information processing enables secure communication, quantum teleportation, and computation. However, current protocols are limited by the narrow electronic bandwidth of standard measurement devices (megahertz to gigahertz), vastly underusing the broad optical bandwidth (10 to 100 terahertz) of readily available quantum light sources. We introduce a general framework for frequency-multiplexing of quantum channels along with methods for efficient processing of quantum information in those channels across the full optical bandwidth. Using a broadband squeezed-light source, spectral manipulation, and parametric homodyne detection, we generate, process, and measure multiple quantum channels in parallel. We demonstrate this through multiplexed protocols of both continuous-variable quantum key distribution (CV-QKD) and quantum teleportation. We experimentally demonstrate a proof-of-principle realization of multiplexed CV-QKD over 23 independent spectral channels with eavesdropping detection in each channel. These techniques pave the way for massively parallel quantum processing, potentially boosting the throughput of quantum protocols by orders of magnitude.
Measurement-based optical quantum computers (MBOQCs) using continuous-variables (CVs) of light are a candidate for large-scale fault-tolerant quantum computing [1]. This form of processing has a high carrier frequency of light, so it can also perform ultra-fast quantum computing. To achieve ultra-fast MBOQCs, broadband squeezed light generation and ultra-fast realtime amplitude measurement are required. So far, over-4.5-dB squeezed light generation, which is required for preparing a two-dimensional cluster state, has been demonstrated in THz bandwidth by using noise intensity measurement with a narrow bandwidth [2], [3]. However, the realtime amplitude measurement frequency is limited to several GHz. Up to now, the highest frequency is still limited to 9 GHz with 0.1-dB squeezing even though specific detectors have been designed [4]. This is because there is a tradeoff relationship between the bandwidth and the quantum efficiency of homodyne detectors. As a result, we cannot use a fast balanced homodyne detector such as a detector for high-speed optical communications with a large optical loss, which collapses the quantum states of light. To achieve ultra-fast realtime amplitude measurement, we need a new measurement method to overcome the tradeoff limitations.
Protocols of quantum information processing are the foundation of quantum technology, allowing to share secrets at a distance for secure communication (quantum key distribution), to teleport quantum states, and to implement quantum computation. While various protocols have already been realized, and even commercialized, the throughput and processing speed of standard protocols is generally low, limited by the narrow electronic bandwidth of the measurement apparatus in the MHz-to-GHz range, which is orders-of-magnitude lower than the optical bandwidth of available quantum optical sources (10-100 THz). We present a general concept and methods to process quantum information in parallel over multiplexed frequency channels using parametric homodyne detection for measurement of all the channels simultaneously, thereby harnessing the optical bandwidth for quantum information in an efficient manner. We exemplify the concept through two basic protocols: Multiplexed Continuous-Variable Quantum Key Distribution (CV-QKD) and multiplexed continuous-variable quantum teleportation. We demonstrate the multiplexed CV-QKD protocol in a proof-of-principle experiment, where we successfully carry out QKD over 23 uncorrelated spectral channels, with capability to detect eavesdropping in any channel. These multiplexed methods (and similar) will enable to carry out quantum processing in parallel over hundreds of channels, potentially increasing the throughput of quantum protocols by orders of magnitude.