NOVELTY - An atomic clock comprises alkali vapor cell, vertical cavity surface emitting laser (VSCEL), photodiode, pair of radio frequency (RF) excitation coils, pair of Helmholtz coils, and pair of Zeeman coils. The alkali contains alkali atoms. The VSCEL is in optical communication with alkali vapor cell for pumping an optical signal through alkali atoms. The alkali atoms are excited into end resonance state. The photodiode is in optical communication with alkali vapor cell and detects optical signal. The VSCEL, alkali vapor cell, and photodiode are aligned along first axis.    USE - The atomic clock is used for a system requiring very accurate time base or frequency measurement, e.g. global positioning system satellites, cellular phone systems, scientific experiments, and military applications.    ADVANTAGE - The atomic clock provides high performance, compact and scalable, low power atomic time/frequency reference using direct RF interrogation on the end state transition. The atomic clock provides enhanced signal-to-noise, provides clock stability, and is capable of continual scaling in power and volume as the atomic vapor cell scales from 2 mm to 200 microns dimensions.    DETAILED DESCRIPTION - An atomic clock comprises alkali vapor cell, VSCEL, photodiode, pair of RF excitation coils, pair of Helmholtz coils, and pair of Zeeman coils. The alkali contains alkali atoms. The VSCEL is in optical communication with the alkali vapor cell for pumping an optical signal through the alkali atoms. The alkali atoms are excited into an end resonance state. The photodiode is in optical communication with the alkali vapor cell and configured to detect the optical signal. The VSCEL, the alkali vapor cell, and the photodiode are aligned along a first axis. The pair of RF excitation coils applies an RF-interrogation signal with wavelength greater than the dimensions of the alkali vapor cell to the alkali atoms along a second axis perpendicular to the first axis through the alkali vapor cell. Each RF excitation coil is proximal to and physically isolated from the alkali vapor cell. The pair of Helmholtz coils applies uniform direct current (DC) magnetic field along the first axis through the alkali vapor cell. Each bias coil is located external to and physically and thermally isolated from the alkali vapor cell. The pair of Zeeman coils applies Zeeman interrogation signal to the alkali atoms and oriented and configured to apply a time-varying magnetic field along the second axis through the alkali vapor cell. Each of the pair of Zeeman coils is located external to and physically and thermally isolated from the alkali vapor cell. Each of the RF-excitation coils comprises a substrate, an RF transmission line, and a loop. The atomic clock includes a pair of flex circuits, and a third flex circuit. The flex circuits include electrical traces embedded within the polyimide frame, silicon carbide layer, silicon dioxide, and bond pad. The atomic clock includes bobbin, ceramic spacer, package header, lower layer, argon-nitrogen gas mixture, feedback loop, and second feedback loop. An INDEPENDENT CLAIM is included for operating an atomic clock.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of the transition states of atomic clock vapor in the presence of magnetic fields.    0-0 Transition energy level (10)    Lower and Zeeman transition states (14)    Energy (16, 18, 20, 22)