A device for generating bi-photons, comprising: a nonlinear crystal element comprising a first face, a second face parallel to the first face, and a first axis perpendicular to the first and second faces; a crystal holder configured to mechanically support the crystal and provide a thermal conduit between the crystal and the crystal holder; a crystal holder mount (302, 303) configured to mechanically support the crystal holder a pump focusing optic configured to produce a converging pump beam from the linearly polarized output of a polarization-maintaining optical fiber and provide translational adjustment of the position of the produced pump beam relative to the first axis, the pump beam traversing a first optical path to a first face of the nonlinear crystal element along the first axis, the nonlinear crystal element providing down-conversion of a subset of photons in the pump beam, resulting in a beam of down-converted bi-photons and a beam of non-down-converted photons along the first axis; characterized in that a thermal element operably connected to the crystal holder to heat and/or cool the crystal holder and the nonlinear crystal element; a thermal sensor operably connected to the crystal holder and configured to measure the temperature of the crystal holder and/or the nonlinear crystal element; a thermal control circuit operably connected to the thermal sensor and the thermal element and configured to control the temperature of the crystal holder to within +/- 0.1 degrees Celsius; the crystal holder mount (302, 303) configured to thermally isolate the crystal holder from the rest of the device components; an optical element comprising a dichroic reflector configured to receive photons exiting a second face of the nonlinear crystal element along the first axis and to direct down-converted bi-photons to a third optical path along the first axis, and to reflect non-down-converted photons to a fourth optical path along a third axis; and a bi-photon focusing optic configured to collect the down-converted photons into an optical fiber and provide adjustment of the optical fiber's optical axis relative to the first axis. a nonlinear crystal element comprising a first face, a second face parallel to the first face, and a first axis perpendicular to the first and second faces; a crystal holder configured to mechanically support the crystal and provide a thermal conduit between the crystal and the crystal holder; a crystal holder mount (302, 303) configured to mechanically support the crystal holder a pump focusing optic configured to produce a converging pump beam from the linearly polarized output of a polarization-maintaining optical fiber and provide translational adjustment of the position of the produced pump beam relative to the first axis, the pump beam traversing a first optical path to a first face of the nonlinear crystal element along the first axis, the nonlinear crystal element providing down-conversion of a subset of photons in the pump beam, resulting in a beam of down-converted bi-photons and a beam of non-down-converted photons along the first axis; characterized in that a thermal element operably connected to the crystal holder to heat and/or cool the crystal holder and the nonlinear crystal element; a thermal sensor operably connected to the crystal holder and configured to measure the temperature of the crystal holder and/or the nonlinear crystal element; a thermal control circuit operably connected to the thermal sensor and the thermal element and configured to control the temperature of the crystal holder to within +/- 0.1 degrees Celsius; the crystal holder mount (302, 303) configured to thermally isolate the crystal holder from the rest of the device components; an optical element comprising a dichroic reflector configured to receive photons exiting a second face of the nonlinear crystal element along the first axis and to direct down-converted bi-photons to a third optical path along the first axis, and to reflect non-down-converted photons to a fourth optical path along a third axis; and a bi-photon focusing optic configured to collect the down-converted photons into an optical fiber and provide adjustment of the optical fiber's optical axis relative to the first axis.