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IEEE Transactions on Nuclear Science

Feedback Circuit Module in Frequency-Nondegenerate Entanglement Swapping Experiment

Ziru Sang, Xiao Qiang Jiang, Tianming Zhao, Han Zhang, Ge Jin

Published Aug 22, 2013• 2 citations
Publisher

Abstract

In this paper, a dedicated feedback circuit module is designed for an experiment of entanglement swapping with frequency-nondegenerate photons. This experiment, aiming to entangle two frequency-nondegerate photons without prior interaction, is an advanced strategy for quantum communication. The key point of the experiment is to actively compensate a random phase shift originating from the frequency difference. The feedback circuit module has functions of measuring the time intervals, converting the time information to control level codes, and outputting control levels to an electro-optic modulator (EOM). A precision TDC is implemented in a field programmable gate array (FPGA) with 40 ps per LSB ( σRMS is less than 30 ps). A programmable look-up table is used as computing unit. The propagation delay time of the whole circuit module is less than 110 ns.

DOI
10.1109/tns.2013.2274598

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In this paper, a dedicated feedback circuit module is designed for an experiment of entanglement swapping with frequency-nondegenerate photons. This experiment, aiming to entangle two frequency-nondegerate photons without prior interaction, is an advanced strategy for quantum communication. The key point of the experiment is to actively compensate a random phase shift originating from the frequency difference. The feedback circuit module has functions of measuring the time intervals, converting the time information to control level codes, and outputting control levels to an electro-optic modulator (EOM). A precision TDC is implemented in a field programmable gate array (FPGA) with 40 ps per LSB ( σRMS is less than 30 ps). A programmable look-up table is used as computing unit. The propagation delay time of the whole circuit module is less than 110 ns.
In this paper, a circuit module developed for the experiment of the frequency-nondegenerate photons entanglement swapping is reported. The module transfers the random time interval into level from +1 to - 1 voltage to drive an electro-optic modulator (EOM). This experiment, aiming to entangle the frequency-nondegerate photons that never interacted, is an advanced strategy for quantum communication. The key point of the experiment is to realize the time interval real-time measurement, computation and feedback control. A precision TDC is built in a field programmable gate array (FPGA) with 40ps per LSB (a RMS resolution about 20ps). A programmable look-up table is used as computing unit and the delay time of the module is less than 110ns.
We introduce our recent photonic quantum information processing experiments using the high-speed electro-optic modulator (EOM) based on a lithium niobate waveguides. First, we demonstrated the frequency shifter for single photons by using an optical single sideband (OSSB) modulator, which is based on the nested configuration of four phase modulators (PMs). By properly setting the RF modulation signal and the bias voltage to each PM, we can suppress unwanted sidebands to realize a frequency shifter that can tune the frequency of single photons with accuracy of an RF synthesizer. Using the OSSB modulator, we eliminated the frequency distinguishability between two single photons whose frequencies were different by 25 GHz, and as a result observed a Hong-Ou- Mandel interference with a visibility exceeding 90%. We also proposed and demonstrated a two-qubit controlled logic gate for time-bin qubits using a two-input, two-output optical switch. The switch enables us to individually perform different functions for two temporal bases as a time-dependent beam splitter, so that we can realize controlled-phase (C-Phase) gate for a time-bin qubit. We showed that the states of two separable single photons were entangled as a result of the C-Phase gate operation. We performed the quantum state tomography to obtain density matrices of the output states from the C-Phase gate for some specific input states, and confirmed that the averaged state fidelity to the pure states was 85%. These results indicate that the EOMs will be a useful tool for realizing advanced photonic quantum communication systems.
Feedback is a main component of many algorithms for quantum computing and communication. A key requirement for any quantum feedback scheme is that the $l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}y$ of the feedback loop (i.e.i.e.i.e. the time between beginning to measure a state and the end of feedback action on the state) must be significantly shorter than the coherence time of the system. In this work a superconducting qubit is initialized in its ground state by active feedback, using a field-programmable gate array (FPGA) with very short latency. This in-depth discussion of the FPGA-based processing unit provides a useful reference for future development of feedback electronics for quantum systems.
Basic in quantum communication and computing -- Quantum information -- Quantum communication -- Quantum protocols -- Experimental electronic requirements -- Photon detection -- Photon detection mechanisms -- Signal conversion and standardization -- The input circuit -- Information processor and applications : the FPGA -- Spartan-3 FPGA characteristics -- Other characteristics of ZestSC2 card -- Software used -- Built applications -- Outputs and control -- Electro-optic modulators -- Electric signal amplification -- The output circuit -- Results and analysis -- Input circuit -- Event frequency measurement -- Coincidence detection -- Time stamping -- Periodic signal generator -- Output circuit.
Control of quantum signals is very robust design technique and very important in quantum signal processing (QSP). It is not easy in experimentation platforms. Field Programmable Gate Array (FPGA) is using to control very various range of devices in quantum fields through PCs. FPGA needs to run modules of components to communicate and interfacing with the PC, through decode perform commands to direct control of digital hardware’s. If programmer has a real-time control of the FPGA via USB, it can be possible to evaluate design parameters changes in real-time, without reprogramming the FPGA. That makes the proposed design platforms easier for researchers. This paper discusses experiment of control quantum signals by FPGA to control coincidence logic for Quantum Entangled Pair (QEP), that able to measure polarization correlations relationships between photons of the QEP. This FPGA helps to determine detection events at different detectors which can be attributed to a single photon pair. Also to determine the correlation time between two different beams frequencies is extremely accurate according to the delay between the beams, which is very short.
Time-to-Digital Converters (TDCs) are a crucial tool in a wide array of fields, in particular for quantum communication, where time taggers performance can severely affect the quality of the entire application. Nowadays, FPGA-based TDCs present a viable alternative to ASIC ones, once the non-linear behavior due to the intrinsic nature of the device is properly mitigated. To compensate for said nonlinearities, a calibration procedure is required, which should be maintained throughout its runtime. Here we present the design and the demonstration of a TDC that is FPGA-based showing a residual FWHM jitter of 27 ps, that is scalable for multichannel operation. The target application in Quantum Key Distribution (QKD) is discussed with a calibration method based on the exploitation of single-photon detection that does not require stopping the data acquisition or using any estimation methods, thus increasing accuracy and removing data loss. The calibration was tested in a relevant environment, investigating the behavior of the device between 5 °C and 80 °C. Moreover, our design is capable of continuously streaming up to 12 Mevents/s for up to ~1 week without the TDC overflowing making it ready for a real-life scenario deployment.