Bruce Zweig
Jun29-04, 04:33 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>A number of posters have wondered why delayed choice quantum erasers\ncannot communicate backwards in time. I myself wondered about it\nafter reading popular reports about the phenomenon (especially Brian\nGreene\'s "The Fabric of the Cosmos"). To review, you first split a\nphoton beam into idlers and signals using a spontaneous parametric\ndown-converter. Then you run the beams through a double slit,\nrecording which-path information on the the idler. Finally, you\nobserve diffraction on the signal beam depending on whether or not the\nwhich-path information on the idler is erased. The erasure of the\nidler\'s information can occur after the signal is detected, and this\ncreates the temptation to contemplate backwards time communication\npossibilites.\n\nUnfortunately, it turns out that the signal photons cannot diffract on\ntheir own. Diffraction only occurs when a coincidence counter pairs\nup the signal photons with their original idler companions during\ndetection. There\'s a paper by Shimizu et. al.\n(http://arxiv.org/abs/quant-ph/0210142) in which the authors get\nnormal photons to diffract, and they get SDLC coincident pairs to\ndiffract, but they just can\'t get the darn signal photons by\nthemselves to diffract. A paper by Altschul & Altschul\n(http://arxiv.org/abs/quant-ph/0106113) which describes the erasure\nexperiment makes the statement "This process produces double-slit\ninterference if and only if it is subjected to coincidence counting\nwith an A+B idler beam." (The two slits are labeled \'A\' and \'B\').\n\nSo, although the erasure can occur after the signal photon registers\non its detector, the diffraction pattern cannot emerge until after the\nsignal photon measurements have been combined with the idler photon\nmeasurements in order to produce coincidence information, which can\nonly happen in a causal time frame.\n\nIf this were not the case, then things would get pretty complicated if\nthe idler photons were sent off on an infinitely long journey in which\nit might never be determined whether erasure occurred or not.\n\nIf anyone knows of a way to get diffraction out of the signal photons\nthemselves, I\'d love to hear about it. There\'s a thought experiment\nby R. Srikanth (http://arxiv.org/abs/quant-ph/0101022) that seems to\nbe after this possibility, but I don\'t think anyone has tested it.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>A number of posters have wondered why delayed choice quantum erasers
cannot communicate backwards in time. I myself wondered about it
after reading popular reports about the phenomenon (especially Brian
Greene's "The Fabric of the Cosmos"). To review, you first split a
photon beam into idlers and signals using a spontaneous parametric
down-converter. Then you run the beams through a double slit,
recording which-path information on the the idler. Finally, you
observe diffraction on the signal beam depending on whether or not the
which-path information on the idler is erased. The erasure of the
idler's information can occur after the signal is detected, and this
creates the temptation to contemplate backwards time communication
possibilites.
Unfortunately, it turns out that the signal photons cannot diffract on
their own. Diffraction only occurs when a coincidence counter pairs
up the signal photons with their original idler companions during
detection. There's a paper by Shimizu et. al.
(http://arxiv.org/abs/http://www.arxiv.org/abs/quant-ph/0210142) in which the authors get
normal photons to diffract, and they get SDLC coincident pairs to
diffract, but they just can't get the darn signal photons by
themselves to diffract. A paper by Altschul & Altschul
(http://arxiv.org/abs/http://www.arxiv.org/abs/quant-ph/0106113) which describes the erasure
experiment makes the statement "This process produces double-slit
interference if and only if it is subjected to coincidence counting
with an A+B idler beam." (The two slits are labeled 'A' and 'B').
So, although the erasure can occur after the signal photon registers
on its detector, the diffraction pattern cannot emerge until after the
signal photon measurements have been combined with the idler photon
measurements in order to produce coincidence information, which can
only happen in a causal time frame.
If this were not the case, then things would get pretty complicated if
the idler photons were sent off on an infinitely long journey in which
it might never be determined whether erasure occurred or not.
If anyone knows of a way to get diffraction out of the signal photons
themselves, I'd love to hear about it. There's a thought experiment
by R. Srikanth (http://arxiv.org/abs/http://www.arxiv.org/abs/quant-ph/0101022) that seems to
be after this possibility, but I don't think anyone has tested it.
cannot communicate backwards in time. I myself wondered about it
after reading popular reports about the phenomenon (especially Brian
Greene's "The Fabric of the Cosmos"). To review, you first split a
photon beam into idlers and signals using a spontaneous parametric
down-converter. Then you run the beams through a double slit,
recording which-path information on the the idler. Finally, you
observe diffraction on the signal beam depending on whether or not the
which-path information on the idler is erased. The erasure of the
idler's information can occur after the signal is detected, and this
creates the temptation to contemplate backwards time communication
possibilites.
Unfortunately, it turns out that the signal photons cannot diffract on
their own. Diffraction only occurs when a coincidence counter pairs
up the signal photons with their original idler companions during
detection. There's a paper by Shimizu et. al.
(http://arxiv.org/abs/http://www.arxiv.org/abs/quant-ph/0210142) in which the authors get
normal photons to diffract, and they get SDLC coincident pairs to
diffract, but they just can't get the darn signal photons by
themselves to diffract. A paper by Altschul & Altschul
(http://arxiv.org/abs/http://www.arxiv.org/abs/quant-ph/0106113) which describes the erasure
experiment makes the statement "This process produces double-slit
interference if and only if it is subjected to coincidence counting
with an A+B idler beam." (The two slits are labeled 'A' and 'B').
So, although the erasure can occur after the signal photon registers
on its detector, the diffraction pattern cannot emerge until after the
signal photon measurements have been combined with the idler photon
measurements in order to produce coincidence information, which can
only happen in a causal time frame.
If this were not the case, then things would get pretty complicated if
the idler photons were sent off on an infinitely long journey in which
it might never be determined whether erasure occurred or not.
If anyone knows of a way to get diffraction out of the signal photons
themselves, I'd love to hear about it. There's a thought experiment
by R. Srikanth (http://arxiv.org/abs/http://www.arxiv.org/abs/quant-ph/0101022) that seems to
be after this possibility, but I don't think anyone has tested it.