1. What is the proposed detection scheme in the CVQKD system and how does it achieve similar performance to heterodyne detection with only a single interferometer?
The proposed detection scheme in the CVQKD system is called time-division dual-quadrature homodyne detection (TDDQHD). It achieves similar performance to heterodyne detection by transmitting quantum states and LOs alternatively in a time-division manner with an equal interval. Bob performs homodyne detection twice for one quantum signal split by the balanced beam splitter (BS) of the asymmetric interferometer with unbalanced delays. The phase difference between the long and short arms of the asymmetric interferometer is kept constant through real-time control, allowing the previous two consecutive measurements to achieve coherent detections in the two quadratures between a quantum state and two LOs in the front and back in time with a 2 phase difference. After completing the transmission of all quantum states, Bob converts these measurements into Q and P quadrature values that have a linear correlation with Alice's through proper post-processing. This detection scheme with identical performance to heterodyne detection uses only a single interferometer, reducing complexity and cost compared to the traditional heterodyne detection scheme that requires two interferometers.
read more
2. What is time-division dual-quadrature detection?
Time-division dual-quadrature detection is a scheme that measures quantum states and LOs in a time-divided manner. It involves a phase-locking circuit to determine arbitrary rotation states. The measurement results without shot noise are equivalent to homodyne detection results for the rotation state of +- for quantum states that have passed the channel. Bob's Q and P quadrature values for secret key generation can be obtained through a reverse rotation transform. In our experiment, we executed this process for a 10^6 sized block of data. According to the GMCS protocol, data sets QA = qA1 are used as actual secret key information from the measured raw data.
read more
3. How does phase-lock circuit maintain stability?
The phase-lock circuit maintains stability by using preamble signals that are subject to change due to environmental factors such as temperature fluctuations. These preamble signals generate phase control pulses at a 50 ns interval preceding the key signals. The amplitude, angular frequency, and modulated phase of the -th control pulse are given by EQUATION. The voltage measured by the balance detector through the aFMI without phase control appears as a homodyne detection result between adjacent phase control pulses. By modulating D to have the same value for all according to Eq. (4), the detection results of the phase control pulses depend solely on (). The voltage readout of coherent detection is averaged from the control pulses through a low-pass filter and sampled through a self-developed sample&hold board. This technique guarantees the security of the phase stabilization technique in a CVQKD experiment and limits the phase error within tolerance.
read more
4. What is parameter estimation in CVQKD?
Parameter estimation in CVQKD involves estimating the excess noise of the system. In this paper, the True Local Oscillator (TLO) method is used, which directly transmits the Local Oscillator (LO) from the transmitter. However, this method is vulnerable to a security loophole caused by the transmission of a strong LO. To address this, the LLO method, using a separate LO in Bob, has been widely used. Real-time shot noise measurement can solve the vulnerability in the TLO transmission method. The TDDQHD system analyzed in this study includes real-time shot noise measurement. The process involves using data measured from each quadrature and satisfies the relationship between Alice and Bob's data. The parameters include the efficiency of the balanced detector, channel transmittance, variance of shot noise, excess noise in shot noise units, and electronic thermal noise of the detector. Maximum likelihood estimators for the normal linear model are obtained for the estimators t, s2, and s02. The secret key rate calculation considers an optimized attack by Eve, determining the maximum information Eve can acquire on Bob's key. The boundaries of each parameter are calculated using the normal distribution approximation for large sample sizes. A measurement of the total excess noise, including the finite-key effect and real-time shot noise measurement, was conducted for 24 hours, resulting in the obtained values represented in Fig. 6.
read more