CSU and Hefei Silicon Zhen realize high-security quantum key distribution on-chip modulator

Recently, the Center of Excellence for Innovation in Quantum Information and Frontiers of Quantum Science and Technology, Chinese Academy of Sciences (CAS) and the Center for Micro and Nano Research and Fabrication, University of Science and Technology of China (USTC), in collaboration with Hefei Silicon Zhen Chip Technology Co., Ltd. designed and prepared an on-chip phase modulator with constant transmittance to address the modulation-related loss problem and insufficient modulation depth of modulators in existing integrated QKD systems.

On October 13, 2022, the research results were published in the journal Optics Express under the title of "Transmittance-constant phase modulator for chip-based quantum key distribution".

01Integrated QKD systems have security vulnerabilities: High-speed phase modulation is urgently needed

Quantum key distribution (QKD) provides a feasible method to achieve unconditional secure key sharing between legitimate communicators (often referred to as Alice and Bob) based on the basic principles of quantum mechanics. To promote its practicality, integrated QKD, an emerging technology, has been shown in recent years to reduce the cost and size of QKD systems. In integrated QKD systems, high-speed phase modulation is a crucial function, as it is used not only as a phase modulator (PM) but also as a core component of intensity modulators and polarization modulators: it involves almost all on-chip high-speed operations of quantum state preparation, including pulse generation, phase encoding or randomization, decoy state preparation, polarization encoding, and base point selection.

However, due to the imperfections of real devices, there are always some potential security vulnerabilities in QKD systems, and this is also true for integrated systems. In some chip-based polarization encoders, the polarization-dependent loss (PDL) is basically caused by the phase-dependent loss of the EOPM. Thus, the presence of phase-correlation loss leads to a correlation between the phase, intensity and polarization of the modulated quantum state.

In QKD systems, more phase points (more than two) are needed for encoding or phase randomization in addition to the uncorrelated modulation of the signal states. However, none of the existing solutions can meet all the requirements for quantum state preparation in chip-based QKD systems while eliminating the phase-dependent losses.

02Transmittance constant phase modulator: enhancing the practical security of QKD

In this experiment, Guangcan Guo's team proposed a transmittance constant phase modulator (TIPM) to solve the above combined problems.

(a) TIPM can eliminate the correlation between phase, intensity and polarization of quantum states, and thus can be used to solve the practical security problem of on-chip QKD systems due to phase-dependent losses.

(b) At the same time, TIPM breaks the limitation of reaching only two phase points in previous schemes and can even achieve 2𝜋 continuous modulation, allowing for more phase readiness and perfect phase randomization in on-chip QKD systems, which can be applied to more QKD protocols, such as continuously variable QKD.

TIPM has considerable fabrication tolerances and its design method TIPM can be easily extended to other EOPM and material platforms.

In summary, the proposed TIPM improves the practical safety and performance of chip-based QKD systems.

The basic design idea of TIPM can be described as follows: for electro-optical phase modulators (EOPM) with phase loss, the phase and intensity of the modulation result are correlated, and another typical structure that also connects phase to intensity is the Mach-Zehnder interferometer (MZI). If an appropriate structure is constructed so that the intensity of the phase-dependent EOPM and MZI can cancel each other out, then phase modulation with constant transmittance can be achieved.

Schematic diagram of TIPM. (a) Design structure of the TIPM and (b) picture of the mold. There is a thermal optical phase modulator (TOPM) and a carrier depletion phase modulator (CDPM) on both arms. Using the same device on both arms, the optical field can be well balanced. the TOPM can be considered as an ideal PM that can modulate the phase without phase loss, while the CDPM causes a change in transmittance. A grating coupler (GC) is used to couple the input and output optical signals.

Simulation results of TIPM.

Verify the TIPM modulation settings. LD: 1550 nm laser diode; SMF BS: single-mode fiber beam splitter with 50:50 BSR; PMF BS: polarization-preserving fiber beam splitter with 50:50 BSR; PC: polarization controller; VOA: variable optical attenuator; TDL: tunable optical delay line; PM: fiber phase modulator; PBS: polarization beam splitter. PD: optical power detector. The black and blue lines correspond to single-mode fiber and polarization-maintaining fiber, respectively.

The red asterisks (or lines) and blue triangles are the simulated and experimental results of the TIPM for (a) {0, 𝜋} and (b) {0, 𝜋/2, 𝜋, 3𝜋/2} discrete phase modulation, respectively, and (c) [0, 2 𝜋] continuous phase modulation. All data are normalized to their respective 0-phase points.

In the presence of phase loss, using different phase modulators (PMs), the secret key rate is a function of the communication distance. The blue line indicates the TIPM with phase loss eliminated. the yellow, purple, and red lines indicate the three plasma dispersion effect-based PMs on the SOI platform: the CDPM1 for CUMEC, the CDPM2 for IMEC, and the CIPM for IME. the green line indicates the Franz-Keldysh effect PM for the indium phosphide platform.

03Real-world implications: safer, high-performance, chip-based QKD systems

In this work, the experimental team proposes and demonstrates a TIPM to remove the correlation between phase, polarization and intensity of quantum states, which has the benefit of enhancing the practical security of chip-based QKD systems.

Based on this, the TIPM increases the modulation depth achievable by existing EOPMs, thus enabling multi-point discrete (more than two points) or even [0,2𝜋] continuous phase modulation, which was not possible in previous solutions. This perfectly meets the requirements of real QKD systems and greatly broadens the application scenario of EOPM with insufficient modulation depth. In addition, TIPM has considerable manufacturing tolerances and its implementation does not require the design of special components, which are sufficient in a standard process design kit (PDK). the design approach of TIPM can be easily extended to other types of EOPMs, such as CIPM and FKE-based PMs, to solve the same problem. In addition, its ability to modulate both intensity and phase promotes the TIPM as an all-in-one device that can be used directly as a simple transmitter for some QKD protocols.

In conclusion, TIPM can greatly facilitate the creation of a safer and higher performance chip-based QKD system.

Link to the paper：

https://opg.optica.org/oe/fulltext.cfm?uri=oe-30-22-39911&id=509921