Quantum noise cancellation in asymmetric speed meters with balanced homodyne readout
Sagnac speed meter (SSM) topology is known as an alternative technique to reduce quantum back-action in gravitational-wave interferometers. However, any potential imbalance of the main beamsplitter was shown to reduce the quantum noise superiority of speed meter at low frequencies, caused due to increased laser noise coupling to the detection port. In this paper, we show that implementing balanced homodyne readout scheme and for a particular choice of the local oscillator (LO) delivery port, the excess laser noise contribution to quantum noise limited sensitivity (QNLS) is partly compensated and the speed meter sensitivity can outperform state-of-the-art position meters. This can be achieved by picking the local oscillator from interferometer reflection (\textit{co-moving} LO) or the main beamsplitter anti-reflective coating surface (BSAR LO). We also show that this relaxes the relative intensity noise (RIN) requirement of the input laser. For example, for a beam splitter imbalance of $0.1 \%$ in Glasgow speed meter proof of concept experiment, the RIN requirement at frequency of 100Hz decreases from $4\times 10^{-10}/\sqrt{\rm Hz}$ to $4\times 10^{-7}/\sqrt{\rm Hz}$, moving the RIN requirement from a not practical achievable value to one which is routinely achieved with moderate effort.