All of the measurements planned for the three Einstein Probe missions are technically challenging. Readiness must be evaluated before each competition. This will require an Einstein Probe technology development program. This program should be provided as early as possible to allow all of the promising approaches to each mission to be thoroughly vetted.

The Inflation Probe aims to detect signatures of gravitational waves (with wavelengths comparable to the size of the Universe) produced by quantum fluctuations of spacetime during inflation. It will do this by measuring the weak imprint they leave on the polarization of the cosmic microwave background.

Even for optimistic models, however, this weak polarization component is very difficult to detect. It is an order of magnitude weaker than the polarization components produced by quantum fluctuations in the inflation field. The sensitivity required is roughly 20-100 times that of the HFI focal plane detector on Planck. Achieving such a vast increase in sensitivity requires significant advances: e.g., large arrays of polarization-sensitive detectors with frequency multiplexing from 50-500 GHz. Other technical challenges include the need for cold optics at low cost and 100 mK detector operating temperatures with very stable temperature control.

The Black Hole Finder Probe will conduct a wide field survey of black holes. It is likely to operate at hard X-ray/soft gamma-ray energies, where radiation emitted from these objects can penetrate any surrounding veil of gas and dust.

Such a survey instrument would need to be sensitive over an energy range of about 10-600 keV, and to have angular resolution less than 5 arcmin. Since reflective optics provide very limited fields of view at these high energies, the telescope must use coded aperture imaging. To provide sufficient sensitivity, the detector plane must have an area of several square meters with millimeter-sized pixels to provide the required angular resolution. A CdZnTe detector array seems the most likely candidate, but there remain technical challenges. Other technology problems arise in the areas of mask fabrication and data acquisition at high trigger rates.

The Dark Energy Probe will be designed to perform measurements of the geometry of the Universe in the redshift range z = 0.7- 1.7, where the effects of dark energy are expected to leave their most prominent signature. A particularly promising approach (and the one emphasized in the NAS CPU report) is to obtain a large sample of Type 1a supernovae to redshifts of at least z = 1.5.

A mission capable of such observations requires a wide field of view telescope with about a 2-meter diameter mirror, diffraction limited down to 1 micron, and large arrays of optical and infrared imaging detectors. All of these elements require substantial technology development. The primary mirror must have much lower cost and mass-per-unit-area and be developed faster than the HST primary. The very large detector arrays are a serious challenge: they require of order a billion pixels. At optical wavelengths, silicon-based CCDs are the obvious candidates, but the requirements exceed the capabilities of current devices. At infrared wavelengths, the gap between requirements and current devices is even larger.