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A method and apparatus for detecting non-linear tags. In one advantageous embodiment a system for detecting non-linear tags has a transmitter and a receiver. The transmitter transmits a plurality of electromagnetic signals having a first frequency and a second frequency. The receiver monitors for an

A method and apparatus for detecting non-linear tags. In one advantageous embodiment a system for detecting non-linear tags has a transmitter and a receiver. The transmitter transmits a plurality of electromagnetic signals having a first frequency and a second frequency. The receiver monitors for an electromagnetic signal having a frequency equal to a difference between the first frequency and a second frequency that is generated by a non-linear tag in response to receiving the plurality of electromagnetic signals, wherein the non-linear tag is detected when the electromagnetic signal is detected by the receiver.

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1. A system for detecting non-linear radio frequency identification tags, the system comprising: a transmitter, wherein the transmitter transmits a plurality of electromagnetic signals having a first frequency and a second frequency; anda receiver, wherein the receiver monitors for an electromagneti

1. A system for detecting non-linear radio frequency identification tags, the system comprising: a transmitter, wherein the transmitter transmits a plurality of electromagnetic signals having a first frequency and a second frequency; anda receiver, wherein the receiver monitors for an electromagnetic signal having a third frequency equal to a difference between the first frequency and the second frequency, wherein the electromagnetic signal is generated by a non-linear tag in response to the non-linear tag receiving the plurality of electromagnetic signals, wherein the non-linear tag comprises an antenna, a logic control circuit, a memory, a power supply, and a sensor, and wherein the non-linear tag is detected when the electromagnetic signal at the third frequency is detected by the receiver. 2. The system of claim 1, wherein the transmitter comprises: a first collimated millimeter wave source transmits first signals in the plurality of electromagnetic signals having the first frequency, wherein the first signals are collimated; anda second collimated millimeter wave source that transmits second signals in the plurality of electromagnetic signals having the second frequency, wherein the second signals are collimated. 3. The system of claim 2, wherein the first signals and the second signals converge to form a zone where the first signals and the second signals overlap. 4. The system of claim 2, wherein the first signals and the second signals are combined into a single beam. 5. The system of claim 1, wherein the plurality of electromagnetic signals having the first frequency and the second frequency are continuous wave radio frequency signals. 6. The system of claim 1, wherein the third frequency is pre-selected to identify the non-linear tag. 7. A non-linear tag comprising: a non-linear element, wherein the non-linear element receives electromagnetic signals, and wherein the electromagnetic signals comprise a first frequency and a second frequency; anda logic circuit, wherein the logic circuit causes the non-linear element to emit information in a response signal having a third carrier frequency equal to a difference between the first frequency and the second frequency in response to the non-linear element receiving the electromagnetic signals. 8. The non-linear tag of claim 7 further comprising: a storage device connected to the logic circuit, wherein information in the response signal is stored in the storage device. 9. The non-linear tag of claim 7, wherein the logic circuit modulates the response signal to include the information. 10. The non-linear tag of claim 7 further comprising: a power source. 11. The non-linear tag of claim 7 further comprising: a sensor connected to the logic circuit, wherein the sensor generates information usable to modulate the response signal. 12. The non-linear tag of claim 11, wherein the non-linear tag acquires the information from an object associated with the non-linear tag. 13. The non-linear tag of claim 7, wherein the non-linear element is part of an antenna structure containing a plurality of non linear elements. 14. The non-linear tag of claim 7, wherein the non-linear element is selected from one of a diode and a varactor. 15. A method for detecting tags, the method comprising: transmitting a plurality of electromagnetic signals having a first frequency and a second frequency; andmonitoring for an electromagnetic signal generated by a non-linear tag, wherein the electromagnetic signal comprises a third carrier frequency equal to a difference between the first frequency and the second frequency, and wherein the non-linear tag generates the electromagnetic signal in response to receiving the plurality of electromagnetic signals, wherein the non-linear tag comprises an antenna, a logic control circuit, a memory, a power supply, and a sensor; anddetecting a tag having a non-linear element when the electromagnetic signal is detected. 16. The method of claim 15, wherein the transmitting step comprises: transmitting a first beam having the first frequency from a first electromagnetic wave source; andtransmitting a second beam having the second frequency from a second electromagnetic wave source. 17. The method of claim 16, wherein the first beam and the second beam intersect to form a zone comprising a volume where the first beam and the second beam intersect. 18. The method of claim 16, wherein the first beam and the second beam are combined into a single beam. 19. The method of claim 16, wherein the first electromagnetic wave source is a first collimated millimeter wave source and the second electromagnetic wave source is a second collimated millimeter wave source. 20. The method of claim 15, wherein the electromagnetic signal includes at least one of an identifier for an object associated with the tag and information acquired by the tag.