<span class="mw-page-title-main">Qubit</span>

Qubit

<p>In <a href="/en/Quantum_computing" title="Quantum computing" class="wl">quantum computing</a>, a <b>qubit</b> (<span class="rt-commentedText nowrap"><span class="IPA nopopups noexcerpt"><a href="/en/Help:IPA/English" title="Help:IPA/English" class="wl">/<span style="border-bottom:1px dotted"><span>ˈ</span><span>k</span><span>juː</span><span>b</span><span>ɪ</span><span>t</span></span>/</a></span></span>) or <b>quantum bit</b> is a basic unit of <a href="/en/Quantum_information" title="Quantum information" class="wl">quantum information</a>—the quantum version of the classic binary <a href="/en/Bit" title="Bit" class="wl">bit</a> physically realized with a two-state device. A qubit is a <a href="/en/Two-state_quantum_system" title="Two-state quantum system" class="wl">two-state (or two-level) quantum-mechanical system</a>, one of the simplest quantum systems displaying the peculiarity of quantum mechanics.  Examples include the <a href="/en/Spin_(physics)" title="Spin (physics)" class="wl">spin</a> of the <a href="/en/Electron" title="Electron" class="wl">electron</a> in which the two levels can be taken as spin up and spin down; or the <a href="/en/Photon_polarization" title="Photon polarization" class="wl">polarization</a> of a single <a href="/en/Photon" title="Photon" class="wl">photon</a> in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent <a href="/en/Quantum_superposition" title="Quantum superposition" class="wl">superposition</a> of both states simultaneously, a property that is fundamental to <a href="/en/Quantum_mechanics" title="Quantum mechanics" class="wl">quantum mechanics</a> and <a href="/en/Quantum_computing" title="Quantum computing" class="wl">quantum computing</a>.</p>

<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none" about="#mwt1">Basic unit of quantum information</div>
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<a href="/en/Unit_of_measurement" title="Unit of measurement" class="wl">Units</a> of<br><a href="/en/Units_of_information" title="Units of information" class="wl">information</a></td></tr><tr><th class="sidebar-heading">
<a href="/en/Information_theory" title="Information theory" class="wl">Information-theoretic</a></th></tr><tr><td class="sidebar-content">
<ul><li><a href="/en/Shannon_(unit)" title="Shannon (unit)" class="wl">shannon</a> (<a href="/en/Binary_number" title="Binary number" class="wl">base 2</a>)</li>
<li><a href="/en/Nat_(unit)" title="Nat (unit)" class="wl">nat</a> (<a href="/en/Natural_logarithm" title="Natural logarithm" class="wl">base e</a>)</li>
<li><a href="/en/Hartley_(unit)" title="Hartley (unit)" class="wl">hartley</a> (<a href="/en/Decimal" title="Decimal" class="wl">base 10</a>)</li></ul></td>
</tr><tr><th class="sidebar-heading">
Data storage</th></tr><tr><td class="sidebar-content">
<ul><li><a href="/en/Bit" title="Bit" class="wl">bit</a> (<a href="/en/Binary_number" title="Binary number" class="wl">binary</a>)</li>
<li>trit (<a href="/en/Ternary_numeral_system" title="Ternary numeral system" class="wl">ternary</a>)</li>
<li><a href="/en/Hartley_(unit)" title="Hartley (unit)" class="wl">dit</a> (<a href="/en/Decimal" title="Decimal" class="wl">decimal</a>)</li></ul></td>
</tr><tr><th class="sidebar-heading">
<a href="/en/Quantum_information" title="Quantum information" class="wl">Quantum information</a></th></tr><tr><td class="sidebar-content">
<ul><li><a href="/en/Qubit" title="Qubit" class="wl">qubit</a> (binary)</li>
<li><a href="/en/Qutrit" title="Qutrit" class="wl">qutrit</a> (ternary)</li>
<li><a href="/en/Qudit" title="Qudit" class="mw-redirect wl">qudit</a> (<i>d</i>-dimensional)</li></ul></td>
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In quantum computing, a qubit or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics.  Examples include the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superposition of both states simultaneously, a property that is fundamental to quantum mechanics and quantum computing. 