S1. Assumptions about the Instrument and Target System

Table S1 displays assumptions about a facility with characteristics like the planned Keck Interferometer on Mauna Kea on which sensitivity estimates below are based.

Table S1: Instrument assumptions:

Aperture	d	10 m
Baseline	D	85 m
Center	0	10 µm
Fractional bandwidth	/0	0.3
Background temperature	T	273 K
Etendue	A	1 2
Emissivity		0.5
AO Strehl ratio at 0	S	0.984
Effective efficiency		0.1

The value for etendue in Table S1 assumes a single-mode filter as discussed in section S2, and the adaptive optics Strehl ratio assumes 200 nm rms wavefront errors. The effective efficiency includes adaptive optics system transmission, starlight relay transmission, diffraction losses, dewar internal losses, coupling losses into the single-mode filters, and detector quantum efficiency (assumed to be 0.75). A further efficiency factor of 0.5 is included to account for use of a nulling beam combiner for the exozodiacal signal.

Table S2 gives the signal and signal/noise ratios if the observation target is assumed to be the portion of an exozodiacal cloud located 1 AU from a sun-like star at a distance of 10 pc. The background flux 'B' and the flux from the central star 'S' in two apertures is 7 x 10¹⁰ and 1.5 x 10⁸ electrons s^-1, respectively.

Table S2: Fluxes and Signal/Noise Ratios without Nulling:

From a strictly photometric perspective, detection of the excess from a 1-zodi cloud is possible. From a practical perspective, two points need to be addressed: 1) the small relative size of the exozodiacal excess relative to the star (0.01% for the 1-zodi case), and 2) the small relative size of the exozodiacal excess relative to the mean IR background (2 x 10^-7 for the 1-zodi case). The latter point is probably the most challenging. An approach to problem 1) involves a nulling beam combiner between the two apertures to suppress the light from the central star, increasing the relative size of the exozodiacal excess. An approach to problem 2) adds additional nullers at the individual apertures. These nullers serve as source choppers to provide an accurate calibration of the background using only OPD (optical path differential) modulation.