The Purcell effect, a common issue in qubit-resonator systems leading to fidelity loss is studied while its suppression is achieved using a novel qubit readout circuit design. Our approachutilizes a unique coupling architecture in which, the qubit first interacts with a filter resonator before linking to the readout resonator. This configuration enables precise control over the Purcell decay rate and ac Stark factor without impacting on measuring time. The mentioned factor is highly sensitive to the coupling strength between the readout resonator and the filter, meaning that the factor adjustment directly impacts the qubit state detection. A major advantage of this design is that tuning the resonator-filter coupling strength is relatively straightforward, offering flexibility in fine-tuning ac Stark factor.
This study unveils a comprehensive design strategy, intricately addressing the realization of transmon qubits, the design of Josephson parametric amplifiers, and the development ofan innovative fully integrated receiver dedicated to sensing ultra-low-level quantum signals. Quantum theory takes center stage, leveraging the Lindblad master and quantum Langevin equations to design the transmon qubit and Josephson parametric amplifier as open quantum systems. The mentioned quantum devices engineering integrated with the design of a fully integrated 45 nm CMOS system-on-chip receiver, weaves together a nuanced tapestry of quantum and classical elements. On one hand, for the transmon qubit and parametric amplifier operating at 10 mK, critical quantum metrics including entanglement, Stoke projector probabilities, and parametric amplifier gain are calculated. On the other hand, the resulting receiver is a symphony of high-performance elements, featuring a wide-band low-noise amplifier with a 0.8 dB noise figure and ~37 dB gains, a sweepable 5.0 GHz sinusoidal wave generator via the voltage-controlled oscillator, and a purpose-designed mixer achieving C-band to zero-IF conversion. Intermediate frequency amplifier, with a flat gain of around 26 dB, and their low-pass filters, generate a pure sinusoidal wave at zero-IF, ready for subsequent processing at room temperature. This design achieves an impressive balance, with low power consumption (~122 mW), a noise figure of ~0.9 dB, high gain (~130 dB), a wide bandwidth of 3.6 GHz, and compact dimensions (0.54*0.4 mm^2). The fully integrated receiver capability to read out at least 90 qubits positions this design for potential applications in quantum computing. Validation through post-simulations at room temperature underscores the promising and innovative nature of this design.
The present article mainly emphasizes the design of a low-noise amplifier that can be used for quantum applications. For this reason, the design circuit specifically concentrates onthe noise figure and its improvement to be used in quantum applications at which the noise added due to the circuit designed should be strongly limited. If the designed low noise amplifier could have quantum-associated applications, its noise temperature should be around 0.4 K, in which the designed circuit is comparable with the Josephson Junction amplifier. Although this task seems to be highly challenging, this work focuses on engineering the circuit, minimizing the mismatch and reflection coefficients in the circuit, and enhancing the circuit transconductance to improve the noise figure in the circuit as efficiently as possible. The results indicated the possibility of reaching the noise figure around 0.008 dB in the circuit operating at 10 K. Additionally, the circuit is analyzed via quantum mechanical analysis, through which some important quantities, such as noise figure, is theoretically derived. In fact, the derived relationship using quantum theory reveals that on which quantities the design should focus in order to optimize the noise figure. Thus, merging quantum theory and engineering the approach contributed to designing a highly efficient circuit for strongly minimizing the noise figure.
this study significantly emphasizes on the entanglement engineering using a transmon qubit. A transmon qubit is created with two superconducting islands coupled with two Josephson Junctionembedded into a transmission line. The transmon qubit energies are manipulated through its coupling to the transmission line. The key factor here is the coupling factor between transmission line and qubit by which the quantum features of the system such as transmon decay rate, energy dispersion, and related coherence time are controlled. To complete knowledge about the design, the system is quantum mechanically analyzed and the related Hamiltonian is derived. Accordingly, the dynamics equation of motions is derived and so the energy dispersion and the coupled system coherence time are investigated. The system engineering should be established in such a way that satisfies the energy dispersion and the coherence time. However, to analyze the entanglement between modes, it needs to calculate the number of photons of the transmission lines and the transmon qubit, and also the phase sensitive cross-correlation. The important section of this study emphasizes on engineering the coupling between the transmon qubit and transmission line to enhance the entanglement. The results show that around the Josephson Junction location where the more coupling is established the more entanglement between modes is created.