Quantum computers have been heralded as a novel paradigm for the solution of today's intractable problems, whereas the core principles of quantum computation are superposition, entanglement and interference, three fundamental properties of quantum mechanics [1]. A quantum computer generally comprises a quantum processor, made of an array of quantum bits or qubits, and a classical controller, which is used to control and read out the qubits. Quantum algorithms are generally mapped onto a circuit of quantum gates that operate on multiple qubits. Unlike conventional digital bits, qubits can take a coherent state ranging from |0〉 to |1〉 on a continuous sphere, known as the Bloch Sphere and they are implemented based on several mechanisms. While many solid-state implementations of qubits exist, an exhaustive description of available technologies is beyond the scope of this paper [2] [3].@en

LC VCOs with low phase noise (PN) and an octave frequency-tuning range (FTR) are required for multistandard communication devices, software-defined radios, and wireline data links. A viable popular approach is to exploit multicore mode-switching VCOs for two reasons: (1) their PN improves linearly by in-phase coupling of N identical VCOs; (2) the resonant-mode switching enhances the VCO FTR without degrading the tank quality factor (Q) as no RF current ideally flows through lossy mode-selection switches. However, it is still challenging for dual-mode VCOs to achieve a competitive FoM while covering an octave FTR at oscillation frequencies (F_OSC) above 6GHz [1]. To enhance the number of oscillation modes to 3, [2] added a center-loop inductor (L_C) to a transformer, as shown in Fig. 9.2.1. However, a large FTR gap is measured, since the transformer windings should be strongly coupled to accommodate L_C, The authors of [3] and [4] realized a triple- and quad-mode operation, respectively, by coupling two individual transformer-based resonators (see Fig. 9.2.1). Apart from the large area penalty, the former needs an extra third winding (L_T) in each transformer that degrades the tank Q, while the latter used large, fixed coupling capacitors (C_M) that load the tank in two of the resonant modes, thus limiting the VCO FTR.

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