Coherence time

For an <a href="/facts/Electromagnetic_wave/Ynx7ujCW">electromagnetic wave</a>, the coherence time is the <a href="/facts/Time/HceJKUGB">time</a> over which a propagating wave (especially a <a href="/facts/Laser/jk6PTvvT">laser</a> or <a href="/facts/Maser/yqdMpoDv">maser</a> beam) may be considered <a href="/facts/Coherence_(physics)/NF1Rgdoa">coherent</a>, meaning that its <a href="/facts/Phase_(waves)/cpGtuVll">phase</a> is, on average, predictable.
In <a href="/facts/Telecommunications/vxoj1cxR">long-distance transmission systems</a>, the coherence time may be reduced by <a href="/facts/Wave_propagation/PXhWIt7k">propagation</a> factors such as <a href="/facts/Dispersion_(optics)/cUdHnYXz">dispersion</a>, <a href="/facts/Scattering/nMou7lEH">scattering</a>, and <a href="/facts/Diffraction/Mk80xXHT">diffraction</a>.
The coherence time, usually designated τ, is calculated by dividing the <a href="/facts/Coherence_length/A845Dc1z">coherence length</a> by the <a href="/facts/Phase_velocity/m43q3X33">phase velocity</a> of <a href="/facts/Light/jeofpMn4">light</a> in a medium; approximately given by

τ
        =
        
          
            1
            
              Δ
              ν
            
          
        
        ≈
        
          
            
              λ
              
                2
              
            
            
              c
              
              Δ
              λ
            
          
        
      
    
    {\displaystyle \tau ={\frac {1}{\Delta \nu }}\approx {\frac {\lambda ^{2}}{c\,\Delta \lambda }}}

where λ is the central <a href="/facts/Wavelength/kcJU0ymz">wavelength</a> of the source, Δν and Δλ is the <a href="/facts/Spectral_width/cPPAkubY">spectral width</a> of the source in units of frequency and wavelength respectively, and c is the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a> in vacuum.
A single mode <a href="/facts/Fiber_laser/JrNSrAyY">fiber laser</a> has a linewidth of a few kHz, corresponding to a coherence time of a few hundred microseconds. Hydrogen masers have linewidth around 1 Hz, corresponding to a coherence time of about one second. Their coherence length approximately corresponds to the distance from the Earth to the Moon.
As of 2022, research groups worldwide have demonstrated <a href="/facts/Superconductivity/pqQAlSzJ">superconducting</a> <a href="/facts/Qubit/WveeEubw">qubits</a> with coherence times up to several 100 <a href="/facts/Microsecond/zdW1DEb2">μs</a>.

Coherence time open-in-new

Coherence time