Abstract: In a general aspect, vapor cells are disclosed that include a dielectric body and an optical window. The dielectric body has a cavity and an exterior surface that defines an opening to the cavity. The optical window includes a substrate and first and second multilayer coatings. In some aspects, the substrate has first and second substrate surfaces on opposite sides of the substrate, and the first and second multilayer coatings are disposed on, respectively, the first and second substrate surfaces. The first multilayer coating defines a first window surface that is bonded to the exterior surface of the dielectric body and extends across the opening. The second multilayer coating defines a second window surface that faces an exterior of a vapor cell. The first and second multilayer coatings apply, respectively, first and second stresses to the substrate, with the second stress counteracting the first stress.

Abstract: In a general aspect, a quantum spectrum sensing network for detecting electromagnetic radiation in an environment is presented. In some implementations, a spectrum sensing network includes a computer system; and a plurality of portable systems spatially distributed over a geographic region. Each of the plurality of portable systems includes one or more vapor cell sensors each including a vapor and being configured to generate output optical signals based on interactions between input optical signals, the vapor and electromagnetic radiation; a portable control package including one or more processors configured to process the output optical signals; and a communication module configured to communicate, to the computer system, data based on the processed output optical signals.

Abstract: In a general aspect, a bonded interface in a vapor cell is formed using plasma-activated surfaces. In some aspects, manufacturing a vapor cell includes obtaining a dielectric body and an optical window. The dielectric body has a surface that defines an opening to a cavity in the dielectric body, and the cavity is configured to contain a vapor. The surface of the dielectric body and a surface of the optical window are contacted to form a seal around the opening to the cavity. The seal includes a metal oxynitride layer that is disposed along an interface between the surfaces of the dielectric body and the optical window. In certain cases, the seal is formed at a temperature no greater than 150° C.

Abstract: In a general aspect, antirelaxation coatings are disclosed for coating the interior surfaces of vapor cells. In certain aspects, a vapor cell includes a dielectric body having interior and exterior surfaces. The interior surface defines a cavity in the dielectric body, and the exterior surface defines an opening to the cavity. The vapor cell also includes an antirelaxation coating that is disposed on the interior surface of the dielectric body and includes an organosilane material. The vapor cell additionally includes a vapor or a source of vapor residing in the cavity as well as an optical window that covers the opening to the cavity. The optical window has a surface bonded to the exterior surface of the dielectric body to form a seal around the opening. The vapor or the source of vapor includes alkali metal atoms.

Abstract: In a general aspect, methods for manufacturing vapor cells are disclosed. In certain aspects, a method of manufacturing a vapor cell includes obtaining a dielectric body that has interior and exterior surfaces. The interior surface defines a cavity in the dielectric body, and the exterior surface defines an opening to the cavity. The method includes forming an antirelaxation coating on the interior surface of the dielectric body. The antirelaxation coating comprising an organosilane material. The method additionally includes disposing a vapor or a source of vapor in the cavity and obtaining an optical window that comprises a surface. The vapor or the source of vapor includes alkali metal atoms. The method also includes bonding the surface of the optical window to the exterior surface of the dielectric body to form a seal around the opening to the cavity.

Abstract: In a general aspect, gettering elements are disclosed for absorbing gas species from a cavity of a vapor cell. In certain aspects, a vapor cell includes a dielectric body having interior and exterior surfaces. The interior surface defines a cavity in the dielectric body, and the exterior surface defines an opening to the cavity. The dielectric body also includes a channel having a porous layer that is configured to absorb a gas species from the cavity. The vapor cell also includes a vapor or a source of vapor residing in the cavity as well as an optical window that covers the opening to the cavity. The optical window has a surface bonded to the exterior surface of the dielectric body to form a bonded interface of the vapor cell. The bonded interface includes a seal around the opening, and the vapor or the source of vapor includes alkali metal atoms.

Abstract: In a general aspect, a quantum spectrum sensing system is presented. In some implementations, a quantum spectrum sensing system includes a laser system, a vapor cell sensor, and an optical detector. The laser system includes first and second lasers configured to generate a first and second laser signals. The laser system also includes first and second comb generators configured to produce first and second frequency comb signals based on the first and second laser signals, respectively. The laser system includes a frequency separator that can select frequency components of the second frequency comb signal and a frequency shifter that can shift the selected frequency components toward Rydberg states of a vapor. The vapor cell sensor can receive input optical signals from the laser system and produce output optical signals based on interactions of the vapor with the input optical signals. The optical detector can detect the output optical signals.

Abstract: In a general aspect, a portable quantum spectrum sensing system for detecting electromagnetic radiation in an environment is presented. In some implementations, a portable system includes a vapor cell sensor and a portable control package. The vapor cell sensor includes a vapor and is configured to generate output optical signals based on interactions between input optical signals, the vapor and the electromagnetic radiation. The portable control package includes a laser system configured to generate laser signals and a photonic integrated circuit system configured to generate the input optical signals based on the laser signals from the laser system. The portable control package includes a system-on-chip that can communicate control signals to the laser system and the photonic integrated circuit system. The system-on-chip can also process the output optical signals to determine one or more properties of the electromagnetic radiation.

Abstract: In a general aspect, data from a vapor cell sensor system is used to characterize radio frequency sources. In some aspects, a vapor cell sensor system generates output signals by operating one or more vapor cells at an array of locations in a radiating near-field region of an RF field produced by an RF source. Based on the output signals, a signal processing system generates measurement data that represents measured intensities of the RF field in the radiating near-field region. Based on the measurement data, the signal processing system generates a model of the RF field produced by the RF source. The model includes modeled intensities of the RF field in the radiating near-field region and a far-field region of the RF field.

Abstract: In a general aspect, a quantum spectrum sensing system is presented. In some implementations, a system includes a laser system, a vapor cell sensor, an optical detector, and a signal processing system. The laser system includes a first laser that generates a first laser signal; a comb generator that produces a frequency comb signal based on the first laser signal; and a second laser generates a second laser signal. The vapor cell sensor includes a vapor and receives input optical signals based on the frequency comb signal and the second laser signal. The vapor cell sensor produces output optical signals based on interactions of the vapor with the input optical signals and electromagnetic radiation. The signal processing system processes the output optical signals to identifying signals in the electromagnetic radiation that are off resonance with a transition frequency between Rydberg states of the vapor.

Abstract: In a general aspect, vapor cells are used to characterize radio frequency sources. In some aspects, a system includes a vapor cell sensor system and a signal processing system. The vapor cell sensor system has one or more vapor cells that can be positioned at an array of locations. The vapor cell sensor system generates output signals based on operating the one or more vapor cells in a radiating near-field region of an RF field. The signal processing system is configured to generate measurement data based on the output signals; the measurement data represents measured intensities of the RF field in the radiating near-field region. The signal processing system is also configured to generate a model of the RF field based on the measurement data. The model includes modeled intensities of the RF field in the radiating near-field region and a far-field region of the RF field.

Abstract: In a general aspect, a quantum spectrum sensing system is presented. In some implementations, a system includes a laser system, a vapor cell sensor, an optical detector, and a signal processing system. The laser system includes a first laser that generates a first laser signal; a comb generator that produces a frequency comb signal based on the first laser signal; and a second laser generates a second laser signal. The vapor cell sensor includes a vapor and receives input optical signals based on the frequency comb signal and the second laser signal. The vapor cell sensor produces output optical signals based on interactions of the vapor with the input optical signals and electromagnetic radiation. The signal processing system processes the output optical signals to identifying signals in the electromagnetic radiation that are off resonance with a transition frequency between Rydberg states of the vapor.

Abstract: In a general aspect, data from a vapor cell sensor system is used to characterize radio frequency sources. In some aspects, a vapor cell sensor system generates output signals by operating one or more vapor cells at an array of locations in a radiating near-field region of an RF field produced by an RF source. Based on the output signals, a signal processing system generates measurement data that represents measured intensities of the RF field in the radiating near-field region. Based on the measurement data, the signal processing system generates a model of the RF field produced by the RF source. The model includes modeled intensities of the RF field in the radiating near-field region and a far-field region of the RF field.

Abstract: In a general aspect, vapor cells are used to characterize radio frequency sources. In some aspects, a system includes a vapor cell sensor system and a signal processing system. The vapor cell sensor system has one or more vapor cells that can be positioned at an array of locations. The vapor cell sensor system generates output signals based on operating the one or more vapor cells in a radiating near-field region of an RF field. The signal processing system is configured to generate measurement data based on the output signals; the measurement data represents measured intensities of the RF field in the radiating near-field region. The signal processing system is also configured to generate a model of the RF field based on the measurement data. The model includes modeled intensities of the RF field in the radiating near-field region and a far-field region of the RF field.

Abstract: In a general aspect, a radar method includes transmitting an outgoing series of RF pulses toward a target region. The radar method also includes receiving an incoming series of RF pulses that is based on the outgoing series of RF pulses after passing through the target region. The radar method additionally includes producing, by operation of a vapor cell system, output signals that are based on incoming series of RF pulses and the laser signals interacting with a vapor of the vapor cell system. The radar method also includes operations of a signal processing system, such as generating response data, determining a Doppler shift, and determining a velocity of an object in the target region. The response data includes transient response data that represents transient responses of a vapor in the vapor cell system to respective phase changes in the incoming series of RF pulses.

Abstract: In a general aspect, a radar method includes transmitting an outgoing series of RF pulses toward a target region. The radar method also includes receiving an incoming series of RF pulses that is based on the outgoing series of RF pulses after passing through the target region. The radar method additionally includes producing, by operation of a vapor cell system, output signals that are based on incoming series of RF pulses and the laser signals interacting with a vapor of the vapor cell system. The radar method also includes operations of a signal processing system, such as generating response data, determining a Doppler shift, and determining a velocity of an object in the target region. The response data includes transient response data that represents transient responses of a vapor in the vapor cell system to respective phase changes in the incoming series of RF pulses.

Abstract: In a general aspect, a quantum spectrum sensing system is presented. In some implementations, a quantum spectrum sensing system includes a laser system, a vapor cell sensor, and an optical detector. The laser system includes first and second lasers configured to generate a first and second laser signals. The laser system also includes first and second comb generators configured to produce first and second frequency comb signals based on the first and second laser signals, respectively. The laser system includes a frequency separator that can select frequency components of the second frequency comb signal and a frequency shifter that can shift the selected frequency components toward Rydberg states of a vapor. The vapor cell sensor can receive input optical signals from the laser system and produce output optical signals based on interactions of the vapor with the input optical signals. The optical detector can detect the output optical signals.

Abstract: In a general aspect, vapor cell sensors are used to perform over-the-air (OTA) testing of cellular base stations. In some implementations, a vapor cell sensor including a vapor having Rydberg states is tuned to a carrier frequency of RF radiation emitted from a cellular base station. A first set of optical signals is generated based on an interaction between the vapor and a synchronization signal transmitted from the cellular base station. The vapor cell sensor is tuned to a harmonic of the carrier frequency. A second set of optical signals is generated based on an interaction between the vapor and the RF radiation. The first and second sets of optical signals are converted to digital data. The digital data is processed to detect a condition of the cellular base station.

Abstract: In a general aspect, a method is described herein for detecting the phase properties of a radio frequency (RF) wave. The method includes generating an optical signal by interacting laser signals with a vapor of a vapor cell sensor. The optical signal is based on a transmission of one of the laser signals through the vapor. The method also includes altering an intensity of the optical signal by interacting a target RF electromagnetic field with a Rydberg electronic transition of the vapor. The target RF electromagnetic field includes a time series of RF pulses. The method additionally includes determining, by operation of a signal processing system, a magnitude of phase change in the time series of RF pulses. In some implementations, the method includes determining a phase of a target RF pulse in the times series of RF pulses.

Abstract: In a general aspect, a system is described herein for detecting the phase properties of a radio frequency (RF) wave. The system includes a vapor cell sensor that contains a vapor and is configured to generate an optical signal in response to laser signals that interact with the vapor. The vapor has a Rydberg electronic transition that interacts with a target RF electromagnetic field, and the optical signal is based on a transmission of one of the laser signals through the vapor. The system also includes an optical detection system and a signal processing system. The optical detection system is configured to generate a detector signal in response to receiving the optical signal, and the signal processing system is configured to receive the detector signal and perform operations that determine a phase change of the target RF electromagnetic field.