Holger F. Hofmann, Ortwin Hess, Guenter Mahler
We show that the influence of quantum fluctuations in the electromagnetic field vacuum on a two level atom can be measured and consequently compensated by balanced homodyne detection and a coherent feedback field. This compensation suppresses the decoherence associated with spontaneous emission for a specific state of the atomic system allowing complete control of the coherent state of the system.
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We experimentally stabilise the polarisation drift between the arms of an 8 km-long fibre-optical Mach-Zehnder interferometer, while simultaneously compensating the phase fluctuations. The single photons are wavelength-multiplexed with three classical channels, which are used as feedback for the control systems. Two of these channels are used for the active polarisation control systems, while the other is used to phase-lock the interferometer. We demonstrate long-term stabilisation of the single-photon visibility when polarisation control is used.
The Hamiltonian control of n qubits requires precision control of both the strength and timing of interactions. Compensation pulses relax the precision requirements by reducing unknown but systematic errors. Using composite pulse techniques designed for single qubits, we show that systematic errors for n qubit systems can be corrected to arbitrary accuracy given either two non-commuting control Hamiltonians with identical systematic errors or one error-free control Hamiltonian. We also examine composite pulses in the context of quantum computers controlled by two-qubit interactions. For quantu…
Software compensation of trigger-synchronous control-frame errors in qubits and qudits
针对交流电源线同步干扰引起的量子控制误差，提出一种通过软件更新控制序列进行补偿的方法，显著提高了量子门保真度和量子多能级算法的成功率。
A transducer of single photons between microwave and optical frequencies can be used to realize quantum communication over optical fiber links between distant superconducting quantum computers. A promising scalable approach to constructing such a transducer is to use ensembles of quantum emitters interacting simultaneously with electromagnetic fields at optical and microwave frequencies. However, inhomogeneous broadening in the transition frequencies of the emitters can be detrimental to this collective action. In this article, we utilise a gradient-based optimization strategy to design the te…
In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40*pi rad/s are effectively compensated fo…
Precision and resource scaling of real-time flux distortion compensation for superconducting quantum control
本文提出频域反演与时域拟合的实时磁通失真补偿方法，采用IIR/FIR混合滤波器在FPGA上实现1.2GSa/s处理，达到99.57%保真度，资源扩展近似对数增长，为动态超导量子电路提供可扩展硬件基础。
Recent advances in quantum information processing with trapped ions have demonstrated the need for new ion trap architectures capable of holding and manipulating chains of many (>10) ions. Here we present the design and detailed characterization of a new linear trap, microfabricated with scalable complementary metal-oxide-semiconductor (CMOS) techniques, that is well-suited to this challenge. Forty-four individually controlled DC electrodes provide the many degrees of freedom required to construct anharmonic potential wells, shuttle ions, merge and split ion chains, precisely tune secular mode…
Fast and Automated Optical Polarization Compensation of Fiber Unitaries
针对光纤传输导致的光子偏振变化限制量子通信的问题，提出一种基于旋转四分之一波片与液晶高速控制的快速自动化偏振补偿方法，直接求解补偿参数，在平均不到六次迭代内实现超过99%的保真度，微调后可达99.5%以上。
Recording the moving MEGs of a person in which a person's head could move freely as we record the brain's magnetic field is a hot topic in recent years. Traditionally, atomic magnetometers are utilized for moving MEGs recording and a large compensation coil system is utilized for background magnetic field compensation. Here we described a new potential candidate: an optically pumped atomic co-magnetometer(OPACM) for moving MEGs recording. In the OPACM, hyper-polarized nuclear spins could produce a magnetic field which will shield the background fluctuation low frequency magnetic field noise wh…
Experimental Evaluation of Passive Polarization Compensation Techniques for Fiber-Distributed Polarization-Entangled Photons
该研究评估自由空间QHQ波片与光纤FPC两种被动偏振补偿技术，通过逐路径优化实现光纤偏振纠缠光子补偿，恢复高可见度、高保真度纠缠，为短距离量子通信链路提供系统框架。
With the growing efforts in isolating solid-state qubits from external decoherence sources, the origins of noise inherent to the material start to play a relevant role. One representative example are charged impurities in the device material or substrate, which typically produce telegraph noise and can hence be modelled as bistable fluctuators. In order to demonstrate the possibility of the active suppression of the disturbance from a {\em single} fluctuator, we theoretically implement an elementary bang-bang control protocol. We numerically simulate the random walk of the qubit state on the B…
Experimental imperfections induce phase and population errors in quantum systems. We present a method to compensate unitary errors affecting also the population of the qubit states. This is achieved through the interaction of the target qubit with an additional control qubit. We show that our approach works well for single-photon and two-photon excitation schemes. In the first case, we study two reduced models (i) a two-level system in which the interaction corresponds to an effective level shift and (ii) a three-level one describing two qubits in the Bell triplet subspace. In the second case,…
Quantum key distribution (QKD) networks constitute promising solutions for secure communication. Beyond conventional point-to-point QKD, we developed 1xN QKD network systems with a sub-ns resolution optical path length compensation scheme. With a practical plug-and-play QKD architecture and compact timing control modules based on a field programmable gate array (FPGA), we achieved long term stable operation of a 1x64 QKD network system. Using this architecture, 64 users can simultaneously share secret keys with one server, without using complex software algorithms and expensive hardware. We de…
Polarization Tracking and Active Compensation Using Classical Headers in Quantum Wrapper Networking
本文提出利用经典头信息中的非正交偏振参考，通过机械波片和可变相位 retarder 实现对48公里光纤链路中偏振各向异性变化的主动补偿，验证了在环境波动和数据包切换中大变化下的补偿效果。
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