  The recent discovery of gravitational waves (GW) by LIGO has impressively
launched the novel field of gravitational astronomy and it allowed us to
glimpse at exciting objects we could so far only speculate about. Further
sensitivity improvements at the low frequency end of the detection band of
future GW observatories rely on quantum non-demolition (QND) methods to
suppress fundamental quantum fluctuations of the light fields used to readout
the GW signal. Here we invent a novel concept of how to turn a conventional
Michelson interferometer into a QND speed meter interferometer with coherently
suppressed quantum back-action noise by using two orthogonal polarisations of
light and an optical circulator to couple them. We carry out a detailed
analysis of how imperfections and optical loss influence the achievable
sensitivity and find that the configuration proposed here would significantly
enhance the low frequency sensitivity and increase the observable event rate of
binary black hole coalescences in the range of $10^2-10^3 M_\odot$ by a factor
of up to $\sim300$.
