View source for Ion trap

{{Short description|Device for trapping charged particles}}
{{About|gas phase ions|ions in cells|ion trapping}}
{{Multiple issues|
{{Technical|date=February 2020}}
{{Context|date=February 2020}}
}}

[[File:Linear Paul ion trap.jpg|thumb|right|upright=1.35|An ion trap, used for precision measurements of radium ions, inside a vacuum chamber. View ports surrounding the chamber allow laser light to be directed into the trap.]] 

An '''ion trap''' is a combination of [[electric field|electric]] and/or [[magnetic field]]s used to capture charged particles — known as [[ion]]s — often in a system isolated from an external environment. Atomic and molecular ion traps have a number of applications in physics and chemistry such as precision [[mass spectrometry]], improved atomic frequency standards, and [[quantum computing]].<ref>{{cite journal |title=Quantum computing with trapped ions |author1=H. Häffner |author2=C. F. Roos |author3=R. Blatt  |journal=Physics Reports |volume=469 |issue=4 |pages=155–203 |year=2008 |doi=10.1016/j.physrep.2008.09.003 |arxiv=0809.4368 |s2cid=15918021 |bibcode=2008PhR...469..155H}}</ref> In comparison to neutral atom traps, ion traps have deeper trapping potentials (up to several [[electronvolt]]s) that do not depend on the internal electronic structure of a trapped ion. This makes ion traps more suitable for the study of light interactions with single atomic systems. The two most popular types of ion traps are the [[Penning trap]], which forms a potential via a combination of static electric and magnetic fields, and the [[Paul trap]] which forms a potential via a combination of static and oscillating electric fields.<ref>{{cite journal |title=Quantum dynamics of single trapped ions |author1=D. Leibfried |author2=R. Blatt |author3=C. Monroe |author4= D. Wineland |journal=Reviews of Modern Physics |volume=75 |issue=1 |pages=281–324 |year=2003 |doi=10.1103/RevModPhys.75.281|bibcode=2003RvMP...75..281L }}</ref>

Penning traps can be used for precise magnetic measurements in spectroscopy. Studies of quantum state manipulation most often use the Paul trap. This may lead to a [[trapped ion quantum computer]]<ref>{{cite journal |title=Entangled states of trapped atomic ions |author1=R. Blatt |author2=D. J. Wineland |journal=Nature |volume=453 |pages=1008–1014 |year=2008 |url=http://tf.nist.gov/general/pdf/2284.pdf |doi=10.1038/nature07125 |pmid=18563151 |issue=7198 |bibcode=2008Natur.453.1008B |s2cid=316118}}</ref> and has already been used to create the world's most accurate [[atomic clocks]].<ref>{{cite journal |title=Frequency Ratio of Al+ and Hg+ Single-Ion Optical Clocks; Metrology at the 17th Decimal Place |author1=T. Rosenband |author2=D. B. Hume |author3=P. O. Schmidt |author4=C. W. Chou |author5=A. Brusch |author6=L. Lorini |author7=W. H. Oskay |author8=R. E. Drullinger |author9=T. M. Fortier |author10=J. E. Stalnaker |author11=S. A. Diddams |author12=W. C. Swann |author13=N. R. Newbury |author14=W. M. Itano |author15=D. J. Wineland |author16=J. C. Bergquist |journal=Science |volume=319 |pages=1808–1812 |year=2008 |doi=10.1126/science.1154622 |url=http://tf.nist.gov/general/pdf/2280.pdf |pmid=18323415 |issue=5871 |bibcode=2008Sci...319.1808R |s2cid=206511320}}
</ref><ref>{{cite journal |title=Al+ Quantum-Logic Clock with a Systematic Uncertainty below 10^-18 |author1=S. M. Brewer |author2=J.-S. Chen |author3=A. M. Hankin |author4=E. R. Clements |author5=C. W. Chou |author6=D. J. Wineland |author7=D. B. Hume |author8=D. R. Leibrandt |journal=Phys. Rev. Lett. |volume=123 |pages=033201 |year=2019 |doi=10.1103/PhysRevLett.123.033201 |issue=3 |pmid=31386450 |arxiv=1902.07694 |bibcode=2019PhRvL.123c3201B |s2cid=119075546}}
</ref> [[Electron gun]]s (a device emitting high-speed electrons, used in [[Cathode-ray tube|CRT]]s) can use an ion trap to prevent degradation of the [[cathode]] by positive ions.

==History==

The physical principles of ion traps were first explored by [[Frans Michel Penning|F. M. Penning]] (1894–1953), who observed that electrons released by the cathode of an [[Hot-filament ionization gauge|ionization vacuum gauge]] follow a long cycloidal path to the anode in the presence of a sufficiently strong magnetic field.<ref>{{cite journal |title=Die glimmentladung bei niedrigem druck zwischen koaxialen zylindern in einem axialen magnetfeld|author1=F.M. Penning |journal=Physica |volume=3 |pages=873 |year=1936 |doi=10.1016/S0031-8914(36)80313-9 |issue=9}}</ref> 
A scheme for confining charged particles in three dimensions without the use of magnetic fields was developed by [[Wolfgang Paul|W. Paul]] based on his work with [[quadrupole mass analyzer|quadrupole mass spectrometers]].

Ion traps were used in [[television receiver]]s prior to the introduction of aluminized [[Cathode-ray tube|CRT]] faces around 1958, to protect the phosphor screen from ions.<ref>{{cite web |last=Hartson |first=Ted |year=2004 |url=http://www.ieeeghn.org/wiki/images/b/bd/Hartson.pdf |title=How the World Changed Television |access-date=2008-10-13}}</ref> The ion trap must be delicately adjusted for maximum brightness.<ref>{{US patent|2569517|Magnet for cathode-ray tube ion traps}}</ref><ref>{{US patent| 2727171 |Ion Trap for a Cathode Ray Tube}}</ref>

==Theory==

[[File:Paul trap with motion arrows oriented correctly.png|thumb|right| Trapped ion with axes of motion. The ion is shown with the radial confining electrodes of a linear [[Paul trap]]. Axial motion (red arrow) is parallel to the radial electrodes and radial motion takes place in the plane given by the green arrows. In a Paul trap, axial motion is confined by a static field and radial motion by the oscillating field. In a Penning trap, axial motion is confined by the static electric field and radial motion is confined by the static magnetic field.]]

Any charged particle, such as an [[ion]], feels a force from an electric or magnetic field. Ion traps work by using this force to confine ions in a small, isolated volume of space so that they can be studied or manipulated. Although any static (constant in time) electromagnetic field produces a force on an ion, it is not possible to confine an ion using only a static electric field. This is a consequence of [[Earnshaw's theorem]]. However, physicists have various ways of working around this theorem by using combinations of static magnetic and electric fields (as in a [[Penning trap]]) or by an oscillating electric field and a static electric field([[Paul trap]]). Ion motion and confinement in the trap is generally divided into axial and radial components, which are typically addressed separately by different fields. In both Paul and Penning traps, axial ion motion is confined by a static electric field. Paul traps use an oscillating electric field to confine the ion radially and Penning traps generate radial confinement with a static magnetic field.

===Paul Trap===

A Paul trap that uses an oscillating [[quadrupole]] field to trap ions radially and a static potential to confine ions axially. The quadrupole field is realized by four parallel electrodes laying in the <math>z</math>-axis positioned at the corners of a square in the <math>xy</math>-plane. Electrodes diagonally opposite each other are connected and an a.c. voltage <math>V = V_0\cos(\Omega t)</math> is applied. Using [[Maxwell's equations]], the electric field produced by this potential is electric field <math>\mathbf{E} = \mathbf{E}_0\sin(\Omega t)</math>. Applying [[Newton's second law]] to an ion of charge <math>e</math> and mass <math>M</math>  in this a.c. electric field, we can find the force on the ion using <math>\mathbf{F} = e\mathbf{E}</math>. We wind up with

:<math> M\mathbf{\ddot{r}} = e\mathbf{E}_0\sin(\Omega t) \!</math> .

Assuming that the ion has zero initial velocity, two successive integrations give the velocity and displacement as

:<math> \mathbf{\dot{r}} = \frac{e\mathbf{E}_0}{M\Omega}\cos(\Omega t) \!</math> ,

:<math> \mathbf{r} = \mathbf{r}_0 - \frac{e\mathbf{E}_0}{M\Omega^2}\sin(\Omega t) \!</math> ,

where <math>\mathbf{r}_0</math> is a constant of integration. Thus, the ion oscillates with angular frequency <math>\Omega</math> and amplitude proportional to the electric field strength and is confined radially.

Working specifically with a linear Paul trap, we can write more specific equations of motion. Along the <math>z</math>-axis, an analysis of the radial symmetry yields a potential<ref>{{cite book | last = Foot | first = Christopher | title = Atomic Physics | publisher = Oxford University Press | location = New York | date = 2005 | page = 259 | isbn = 0198506961}}</ref>

:<math> \phi = \alpha + \beta(x^2 - y^2) \!</math> .

The constants <math>\alpha</math> and <math>\beta</math> are determined by boundary conditions on the electrodes and <math>\phi</math> satisfies [[Laplace's equation]] <math>\nabla^2\phi = 0</math>. Assuming the length of the electrodes <math>r</math> is much greater than their separation <math>r_0</math>, it can be shown that

:<math> \phi = \phi_0 + \frac{V_0}{2r_0^2}\cos(\Omega t)(x^2 - y^2) \!</math> .

Since the electric field is given by the gradient of the potential, we get that

:<math> \mathbf{E} = -\frac{V_0}{r_0^2}\cos(\Omega t)(x\mathbf{\hat{e}}_x - y\mathbf{\hat{e}}_y) \!</math> .

Defining <math>\tau = \Omega t/2</math>, the equations of motion in the <math>xy</math>-plane are a simplified form of the [[Mathieu equation]],

:<math> \frac{d^2x_i}{d\tau^2} = -\frac{4eV_0}{Mr_0^2\Omega^2}\cos(2\tau)x_i \!</math> .

===Penning Trap===

[[Image:PenningTrapTrajectory.pdf|thumb|right|The radial trajectory of an ion in a Penning trap; the ratio of cyclotron frequency to magnetron frequency is <math>\omega_c/\omega_m = 10/1</math>. ]]

A standard configuration for a [[Penning trap]] consists of a ring electrode and two end caps. A static voltage differential between the ring and end caps confines ions along the axial direction (between end caps). However, as expected from [[Earnshaw's theorem]], the static electric potential is not sufficient to trap an ion in all three dimensions. To provide the radial confinement, a strong axial magnetic field is applied.

For a uniform electric field <math>\mathbf{E} = E\mathbf{\hat{e}}_x</math>, the force <math>\mathbf{F} = e\mathbf{E}</math> accelerates a positively charged ion along the <math>x</math>-axis. For a uniform magnetic field <math>\mathbf{B}= B\mathbf{\hat{e}}_z</math>, the [[Lorentz]] force causes the ion to move in circular motion with [[Cyclotron motion#Cyclotron resonance|cyclotron frequency]]

:<math> \omega_c = \frac{eB}{M} \!</math> .

Assuming an ion with zero initial velocity placed in a region with <math>\mathbf{E} = E\mathbf{\hat{e}}_x</math> and <math>\mathbf{B}= B\mathbf{\hat{e}}_z</math>, the equations of motion are

:<math> x = \frac{E}{\omega_c B}(1-\cos(\omega_c t)) \!</math> ,
:<math> y = -\frac{E}{\omega_c B}(\omega_c t-\sin(\omega_c t)) \!</math> ,
:<math> z = 0 \!</math> .

The resulting motion is a combination of oscillatory motion around the <math>z</math>-axis with frequency <math>\omega_c</math> and a drift velocity in the <math>y</math>-direction. The drift velocity is perpendicular to the direction of the electric field.

For the radial electric field produced by the electrodes in a Penning trap, the drift velocity will precess around the axial direction with some frequency <math>\omega_m</math>, called the magnetron frequency. An ion will also have a third characteristic frequency <math>\omega_z</math> between the two end cap electrodes. The frequencies usually have widely different values with <math>\omega_z \ll \omega_m< \ll \omega_c</math>.<ref>{{cite book | last = Foot | first = Christopher | title = Atomic Physics | publisher = Oxford University Press | location = New York | date = 2005 | page = 273 | isbn = 0198506961}}</ref>

==Ion trap mass spectrometers==
[[File:Dual linear ion trap.JPG|thumb|right|A linear ion trap component of a mass spectrometer]]
An ion trap [[mass spectrometer]] may incorporate a Penning trap ([[Fourier-transform ion cyclotron resonance]]),<ref>{{cite journal | title = High-accuracy mass spectrometry with stored ions | author = Blaum, Klaus | journal = Physics Reports | volume = 425 | issue = 1 | pages = 1–78 | year = 2006 | doi = 10.1016/j.physrep.2005.10.011 |bibcode = 2006PhR...425....1B }}
</ref> [[Paul trap]]<ref>{{cite journal | title = Linear ion traps in mass spectrometry | journal = Mass Spectrometry Reviews | author = Douglas, DJ |author2=Frank, AJ |author3=Mao, DM | volume = 24 | issue = 1 | pages = 1–29 | year = 2005 | doi = 10.1002/mas.20004 | pmid = 15389865 | bibcode = 2005MSRv...24....1D }}
</ref> or the [[Kingdon trap]].<ref name=":0">{{cite journal | author=Kingdon KH | title=A Method for the Neutralization of Electron Space Charge by Positive Ionization at Very Low Gas Pressures| journal=[[Physical Review]] | volume=21 | pages=408–418 | doi=10.1103/PhysRev.21.408 |year = 1923 | issue=4  |bibcode = 1923PhRv...21..408K }}</ref> The [[Orbitrap]], introduced in 2005, is based on the Kingdon trap.<ref>{{cite journal | title = The Orbitrap: a new mass spectrometer
| journal = Journal of Mass Spectrometry | year = 2005 | volume = 40 | issue = 4 | pages = 430–443 | last = Hu | first = QZ |author2=Noll, RJ |author3=Li, HY |author4=Makarov, A |author5=Hardman, M |author6= Cooks, RG  | doi = 10.1002/jms.856 | pmid = 15838939 |bibcode=2005JMSp...40..430H }}</ref> Other types of mass spectrometers may also use a linear quadrupole ion trap as a selective mass filter.

===Penning ion trap===
[[File:IonSpec FT-ICR (Fourier transform Ion cyclotron resonance) Mass spectrometer.jpg|thumb|left|FTICR mass spectrometer – an example of a Penning trap instrument]] 
A [[Penning trap]] stores charged particles using a strong homogeneous axial [[magnetic field]] to confine particles radially and a quadrupole [[electric field]] to confine the particles axially.<ref>{{cite journal |last1=Brown|first1=L.S.|last2=Gabrielse|first2=G.|title=Geonium theory: Physics of a single electron or ion in a Penning trap|journal=Reviews of Modern Physics|volume=58|issue=1|pages=233–311|year=1986 |url=http://gabrielse.physics.harvard.edu/gabrielse/papers/1986/Review.pdf |bibcode=1986RvMP...58..233B|doi=10.1103/RevModPhys.58.233|access-date=2014-08-09 |archive-url=https://web.archive.org/web/20170313212604/http://gabrielse.physics.harvard.edu/gabrielse/papers/1986/Review.pdf|archive-date=2017-03-13|url-status=dead}}</ref> Penning traps are well suited for measurements of the properties of [[ions]] and stable charged [[subatomic particle]]s. Precision studies of the electron magnetic moment by Dehmelt and others are an important topic in modern physics.

Penning traps can be used in [[quantum computation]] and [[quantum information processing]]<ref>Häffner, Hartmut, Christian F. Roos, and Rainer Blatt. "Quantum computing with trapped ions." Physics Reports 469.4 (2008): 155–203.</ref> and are used at [[CERN]] to store antimatter. Penning traps form the basis of [[Fourier-transform ion cyclotron resonance mass spectrometry]] for determining the [[mass-to-charge ratio]] of [[ions]].<ref name="primer">{{cite journal | url=https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9768511 | pmid=9768511 | date=1998 | last1=Marshall | first1=A. G. | last2=Hendrickson | first2=C. L. | last3=Jackson | first3=G. S. | title=Fourier transform ion cyclotron resonance mass spectrometry: A primer | journal=Mass Spectrometry Reviews | volume=17 | issue=1 | pages=1–35 | doi=10.1002/(SICI)1098-2787(1998)17:1<1::AID-MAS1>3.0.CO;2-K }}</ref>

The Penning Trap was invented by [[Frans Michel Penning]] and [[Hans Georg Dehmelt]], who built the first trap in the 1950s.<ref>{{cite web|title=Hans G. Dehmelt – Biographical|publisher=Nobel Prize |year=1989 |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1989/dehmelt-bio.html|access-date=June 1, 2014}}</ref>

===Paul ion trap===
[[Image:Bruker HCT-schematicJune2008.PNG|thumb|right|Schematic diagram of ion trap mass spectrometer with an electrospray ionization (ESI) source and Paul ion trap.]]
A Paul trap is a type of [[quadrupole ion trap]] that uses static [[direct current]] (DC) and [[radio frequency]] (RF) oscillating [[electric field]]s to trap ions. Paul traps are commonly used as components of a [[mass spectrometer]]. The invention of the 3D quadrupole ion trap itself is attributed to [[Wolfgang Paul]] who shared the [[Nobel Prize in Physics]] in 1989 for this work.<ref>Paul W., Steinwedel H. (1953). "Ein neues Massenspektrometer ohne Magnetfeld". RZeitschrift für Naturforschung A '''8''' (7): 448–450</ref><ref>{{patent|DE|944900|"Verfahren zur Trennung bzw. zum getrennten Nachweis von Ionen verschiedener spezifischer Ladung", W. Paul and H. Steinwedel, filed on December 24, 1953, priority December 23, 1953}}</ref> The trap consists of two [[hyperbola|hyperbolic]] metal electrodes with their foci facing each other and a hyperbolic ring electrode halfway between the other two electrodes. [[Ion]]s are trapped in the space between these three electrodes by the oscillating and static electric fields.

===Kingdon trap and orbitrap===
[[File:Orbitrap mass analyzer - partial cross-section.JPG|thumb|left|Partial cross-section of Orbitrap mass analyzer – an example of a Kingdon trap.]]
A Kingdon trap consists of a thin central wire, an outer cylindrical electrode and isolated end cap electrodes at both ends. A static applied voltage results in a radial logarithmic potential between the electrodes.<ref name=":0" /> In a Kingdon trap there is no potential minimum to store the ions; however, they are stored with a finite angular momentum about the central wire and the applied electric field in the device allows for the stability of the ion trajectories.<ref name='Major:2005'>{{cite book |last=Major |first=Fouad G. |title=Charged particle traps: physics and techniques of charged particle field |publisher=[[Springer (publisher)|Springer]] |year=2005 |isbn=3-540-22043-7}}</ref> In 1981, Knight introduced a modified outer electrode that included an axial quadrupole term that confines the ions on the trap axis.<ref name='knight:221'>{{cite journal|title=Storage of ions from laser-produced plasmas|journal=[[Applied Physics Letters]]|year=1981|first=R. D. |last=Knight|volume=38|issue=4|pages=221–223|doi=10.1063/1.92315 |bibcode=1981ApPhL..38..221K}}</ref> The dynamic Kingdon trap has an additional AC voltage that uses strong defocusing to permanently store charged particles.<ref name='Blumel:R30'>{{cite journal |title=Dynamic Kingdon trap |year=1995 |first=R. |last=Blümel |pmid=9911663| journal=[[Physical Review A]]| volume=51| issue=1 |pages=R30–R33 |doi=10.1103/PhysRevA.51.R30 |bibcode=1995PhRvA..51...30B}}</ref> The dynamic Kingdon trap does not require the trapped ions to have angular momentum with respect to the filament.  An [[Orbitrap]] is a modified Kingdon trap that is used for [[mass spectrometry]]. Though the idea has been suggested and computer simulations performed<ref>{{cite journal |journal=[[International Journal of Mass Spectrometry and Ion Processes]] |title=A Fourier-transform time-of-flight mass spectrometer. A SIMION calculation approach |author=Oksman, Pentti |volume=141 |issue=1 |date=1995-01-10 |pages=67–76 |doi=10.1016/0168-1176(94)04086-M |bibcode=1995IJMSI.141...67O}}</ref> neither the Kingdon nor the Knight configurations were reported to produce mass spectra, as the simulations indicated mass resolving power would be problematic.

==Trapped ion quantum computer==
{{main|Trapped ion quantum computer}}
Some experimental work towards developing quantum computers use [[trapped ion quantum computer|trapped ion]]s. Units of [[quantum information]] called [[qubits]] are stored in stable electronic states of each ion, and [[quantum information]] can be processed and transferred through the collective quantized motion of the ions, interacting by the [[Coulomb force]].  [[Lasers]] are applied to induce [[Coupling (physics)|coupling]] between the qubit states (for single qubit operations) or between the internal qubit states and external motional states (for entanglement between qubits).

== See also ==
*[[Laser cooling]]
*[[Mass spectrometry]]
*[[Quantum jump]]

==References==
{{Reflist}}

== External links ==
* [http://www.vias.org/science_cartoons/ion_trap.html VIAS Science Cartoons] A cranky view of an ion trap...
* [http://www.mikomma.de/rotsattel/paulfalle.htm Paul trap]

{{Mass spectrometry}}

{{DEFAULTSORT:Ion Trap}}
[[Category:Mass spectrometry]]
[[Category:Ions]]
[[Category:Particle traps| ]]