Mercury trapped ion frequency standard for ultra-stable reference applications

Patent metadata

Field	Value
Publication number	US-9766593-B2
Application number	US-201514626814-A
Country	US
Kind code	B2
Filing date	Feb 19, 2015
Priority date	Feb 19, 2014
Publication date	Sep 19, 2017
Grant date	Sep 19, 2017

Abstract

An atomic clock including an ion trap assembly, a C-field coil positioned for generating a first magnetic field in the interrogation region of the ion trap assembly, a compensation coil positioned for generating a second magnetic field in the interrogation region, wherein the combination of the first and second magnetic fields produces an ion number-dependent second order Zeeman shift (Zeeman shift) in the resonance frequency that is opposite in sign to an ion number-dependent second order Doppler shift (Doppler shift) in the resonance frequency, the C-field coil has a radius selected using data indicating how changes in the radius affect an ion-number-dependent shift in the resonance frequency, such that a difference in magnitude between the Doppler shift and the Zeeman shift is controlled or reduced, and the resonance frequency, including the adjustment by the Zeeman shift, is used to obtain the frequency standard.

First claim

What is claimed is:

1. A method for fabricating an apparatus used in an atomic clock to obtain a frequency standard, comprising:
positioning a C-field coil for generating a first magnetic field in the interrogation region;
positioning a compensation coil for generating a second magnetic field in the interrogation region, wherein:
a combination of the first and second magnetic fields produces an ion number-dependent second order Zeeman shift in the resonance frequency that is opposite in sign to an ion number-dependent second order Doppler shift in the resonance frequency;
the C-field coil has a radius selected using data indicating how changes in the radius affect an ion-number-dependent shift in the resonance frequency, providing the C-field coil with built-in compensation such that a variation of the number of the ions by 36% would result in the ion-number-dependent second order Doppler shift of at most 1.1(2) mHz when the second magnetic field is off;
the resonance frequency, including an adjustment by the second order Zeeman shift, is used to obtain the frequency standard; and
the combination of the first and second magnetic fields is selected taking into account decoherence due to coupling of Zeeman levels and motional side bands.

2. The method of claim 1 , wherein the radius is larger than a radius of the C-field coil in a LITS-9 standard, and the combination of the magnetic fields has a field strength of more than 60 milliGauss (mG).

3. The method of claim 2 , wherein the apparatus fits within a volume of 36 inches long by 18 inches wide by 18 inches high.

4. The method of claim 1 , wherein the ion trap assembly comprises a first trap connected to a second trap, the method further comprising:
placing the apparatus in a vacuum chamber, the vacuum chamber comprising:
a first window for inputting ultraviolet electromagnetic radiation to electromagnetically pump the ions into a ground state of the clock transition in the first trap,
a second window for inputting microwave radiation into the second trap to interrogate the resonance frequency, and
a third window for outputting fluorescence generated by the ions after excitation in the first trap by the ultraviolet electromagnetic radiation and after the interrogation in the second trap; and

baking out the vacuum chamber comprising the apparatus at a temperature of more than 200° C.

5. The method of claim 4 , wherein the temperature is more than 400° C.

6. The method of claim 4 , further comprising:
providing the vacuum chamber comprising a buffer gas and a getter that reduces pressure of unwanted background gases in the vacuum chamber; and
allowing the vacuum chamber to equilibrate for a time until methane gas evolution in the sealed chamber is no more than 6×10 −16 Torr/second; and wherein:
pressure in the vacuum chamber one day after sealing is no more than 1×10 −11 Torr, and
the ions' lifetime in the traps includes at least one day without replenishing the buffer gas.

7. The method of claim 6 , wherein the vacuum chamber has a housing consisting essentially of titanium.

8. The method of claim 6 , further comprising selecting the radius such that a shift in the resonance frequency due to the pressure is greater than the Doppler shift.

9. The method of claim 1 , further comprising providing a first current source for biasing the C-field coil and a second current source for biasing the compensation coil, wherein the current sources have a temperature coefficient of at most 500 parts per billion.

10. The method of claim 9 , further comprising providing Direct Current (DC) powered electronics that provide trapping voltages to the ion traps for trapping the ions in the ion trap assembly, drive or power the current sources, drive or power an optical source used to electromagnetically pump the ions into the ground state of clock transition, drive or power an electron emitter used to create the ions, and drive or power a local oscillator used to provide the electromagnetic radiation interrogating the resonance frequency.

11. The method of claim 10 , further comprising providing a controller including a field programmable gate array (FPGA), wherein the FPGA controls the DC powered electronics and a frequency of the electromagnetic radiation interrogating the resonance frequency provided by the local oscillator.

12. The method of claim 1 , wherein the ion trap assembly comprises a first trap connected to a second trap and the ions are transferred between the first trap and the second trap, the method further comprising:
positioning a first refractive optic for inputting ultraviolet electromagnetic radiation to electromagnetically pump the ions into a ground state of the clock transition in the first trap, and
positioning a second refractive optic for collecting the fluorescence generated by the ions after excitation in the first trap by the ultraviolet electromagnetic radiation and after the interrogation of the resonance frequency in the second trap.

13. The method of claim 12 , wherein the first and second refractive optics comprise first and second aspheric doublets, respectively, the method further comprising:
designing the first aspheric doublet to reduce unwanted scattering off of nearby surfaces and image the ultraviolet electromagnetic radiation, comprising diffuse mercury plasma discharge having a wavelength of 194 nm, onto an ion cloud comprising the ions in the first trap.

14. The method of claim 13 , wherein the C-field coil, the compensation coil, the ion trap assembly, and the refractive optics are dimensioned such that the apparatus fits within a surface area of 36 inches long by 18 inches wide by 18 inches high.

15. The method of claim 1 , further comprising the apparatus coupled to a local oscillator (LO) having a LO frequency, wherein: the resonance frequency and the LO frequency have an Allan Deviation of at most 4.5×10 −13 /T 1/2 and T is 10 5 seconds or less, and the ions are 199 Hg + ions.

16. The method of claim 15 , wherein the apparatus and local oscillator consume a power of less than 40 Watts.

17. An apparatus used in an atomic clock to obtain a frequency standard, comprising:
an ion trap assembly for trapping a number of ions participating in a clock transition and wherein a resonance frequency of the clock transition is interrogated in an interrogation region of the ion trap assembly using electromagnetic radiation;
a C-field coil positioned for generating a first magnetic field in the interrogation region;
a compensation coil positioned for generating a second magnetic field in the interrogation region, wherein:
a combination of the first and second magnetic fields produces an ion number-dependent second order Zeeman shift in the resonance frequency that is opposite in sign to an ion number-dependent second order Doppler shift in the resonance frequency;
the C-field coil has a radius selected using data indicating how changes in the radius affect an ion-number-dependent shift in the resonance frequency, such that a variation of the number of the ions by 36% would result in the ion-number-dependent second order Doppler shift of at most 1.1(2) mHz when the second magnetic field is off;
the resonance frequency, including an adjustment by the second order Zeeman shift, is used to obtain the frequency standard; and
the combination of the first and second magnetic fields is selected taking into account decoherence due to coupling of Zeeman levels and motional side bands.

18. An apparatus used in an atomic clock to obtain a frequency standard, comprising:

Assignees

• California Inst Of Techn

Inventors

• Burt Eric A
• Hamell Robert L
• Tucker Blake C
• Larsen Kameron
• Tjoelker Robert L

Classifications

• H03L7/26

  using energy levels of molecules, atoms, or subatomic particles as a frequency reference · CPC title

• Y10T29/49018

  with other electrical component · CPC title

• Y10T29/49016

  Antenna or wave energy "plumbing" making · CPC title

• G04F5/14 Primary

  using atomic clocks · CPC title

• H01S1/06

  Gaseous {, i.e. beam masers} · CPC title