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<h2 class="hd hd-2 unit-title">Quantum teleportation</h2>
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Quantum teleportation
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This is the problem on a slightly modified version of the quantum teleportation protocol where we use a different Bell state than what was used in the lecture. </p>
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<div class="caption"><b>Figure 1</b>: <span>Bell measurement</span></div>
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<li>
<p>
Recall the four Bell states </p>
<table id="a0000000010" class="eqnarray" cellspacing="0" cellpadding="7" width="100%" style="table-layout:auto">
<tr id="a0000000011">
<td style="width:40%; border:none">&#160;</td>
<td style="vertical-align:middle; text-align:right; border:none">
[mathjaxinline]\displaystyle |{\Phi ^+}\rangle[/mathjaxinline]
</td>
<td style="vertical-align:middle; text-align:center; border:none">
[mathjaxinline]\displaystyle =[/mathjaxinline]
</td>
<td style="vertical-align:middle; text-align:left; border:none">
[mathjaxinline]\displaystyle \frac{1}{\sqrt{2}}(|{00}\rangle +|{11}\rangle ),\ \ \ |{\Phi ^-}\rangle =\frac{1}{\sqrt{2}}(|{00}\rangle -|{11}\rangle ),[/mathjaxinline]
</td>
<td style="width:40%; border:none">&#160;</td>
<td class="eqnnum" style="width:20%; border:none;text-align:right">(1.6)</td>
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<tr id="a0000000012">
<td style="width:40%; border:none">&#160;</td>
<td style="vertical-align:middle; text-align:right; border:none">
[mathjaxinline]\displaystyle |{\Psi ^+}\rangle[/mathjaxinline]
</td>
<td style="vertical-align:middle; text-align:center; border:none">
[mathjaxinline]\displaystyle =[/mathjaxinline]
</td>
<td style="vertical-align:middle; text-align:left; border:none">
[mathjaxinline]\displaystyle \frac{1}{\sqrt{2}}(|{01}\rangle +|{10}\rangle ),\ \ \ |{\Psi ^-}\rangle =\frac{1}{\sqrt{2}}(|{01}\rangle -|{10}\rangle )[/mathjaxinline]
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<td style="width:40%; border:none">&#160;</td>
<td class="eqnnum" style="width:20%; border:none;text-align:right">(1.7)</td>
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<p>
form an orthonormal basis for two-qubit states. </p>
<p>
The circuit in the above figure is called Bell measurement because it allows for the measurement with [mathjaxinline]\{ |{\Phi ^+}\rangle , |{\Phi ^-}\rangle , |{\Psi ^+}\rangle , |{\Psi ^-}\rangle \}[/mathjaxinline] basis. What is the probability of getting the measurement result 01 for the state [mathjaxinline]|{\psi }\rangle =a|{\Phi ^+}\rangle +b|{\Phi ^-}\rangle +c|{\Psi ^+}\rangle +d|{\Psi ^-}\rangle[/mathjaxinline]? Note that measurement result 01 means that you obtain result 0 for the upper measurement and result 1 for the lower measurement in the figure. Input <tt class="ttfamily">abs(a)</tt> for [mathjaxinline]|a|[/mathjaxinline] etc. </p>
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<div class="caption"><b>Figure 2</b>: <span>Teleportation</span></div>
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</li>
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<p>
The above figure shows the quantum teleportation protocol where [mathjaxinline]A[/mathjaxinline] has a quantum state [mathjaxinline]|{\psi }\rangle =a|{0}\rangle +b|{1}\rangle[/mathjaxinline] and [mathjaxinline]A[/mathjaxinline] and [mathjaxinline]B[/mathjaxinline] share the entangled state [mathjaxinline]|{\Psi ^-}\rangle[/mathjaxinline]. We take [mathjaxinline]a,b[/mathjaxinline] as real numbers. [mathjaxinline]A[/mathjaxinline]'s goal is to send [mathjaxinline]|{\psi }\rangle[/mathjaxinline] to [mathjaxinline]B[/mathjaxinline] using only local operations and classical communications. Find the state of [mathjaxinline]B[/mathjaxinline] right after the Bell measurement when the measurement result for the Bell measurement is 00. Take the global phase so that the coefficients are real. Remember to include [mathjaxinline]*[/mathjaxinline] for the multiplication; input <tt class="ttfamily">a*|0&gt;</tt> for [mathjaxinline]a|{0}\rangle[/mathjaxinline]. </p>
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Find [mathjaxinline]U[/mathjaxinline] that [mathjaxinline]B[/mathjaxinline] needs to apply so that [mathjaxinline]B[/mathjaxinline] obtains [mathjaxinline]|{\psi }\rangle[/mathjaxinline] up to global phases for the following measurement results. </p>
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<h3 class="hd hd-3 problem-header" id="problem_Superdense_Coding__1_of_3_-problem-title" aria-describedby="block-v1:MITx+8.370.2x+1T2018+type@problem+block@problem_Superdense_Coding__1_of_3_-problem-progress" tabindex="-1">
Superdense Coding (1 of 3)
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<p>
Superdense coding is the protocol where [mathjaxinline]A[/mathjaxinline] can send 2 bits of information by only sending 1 qubit to [mathjaxinline]B[/mathjaxinline] given a preshared entanglement among [mathjaxinline]A[/mathjaxinline] and [mathjaxinline]B[/mathjaxinline]. We will see how this protocol works. In this problem, we consider a slightly modified protocol from the one presented in the lecture where we use a different Bell state. </p>
<p>
Let [mathjaxinline]A[/mathjaxinline] and [mathjaxinline]B[/mathjaxinline] share [mathjaxinline]|{\Psi ^-}\rangle[/mathjaxinline]. What is the state when [mathjaxinline]A[/mathjaxinline] applies [mathjaxinline]I,X,Z,Y[/mathjaxinline]. Answer in the ket notation with [mathjaxinline]\{ |{00}\rangle ,|{01}\rangle ,|{10}\rangle ,|{11}\rangle \}[/mathjaxinline] basis. Take the global phase so that the coefficients are real. </p>
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[mathjaxinline]I[/mathjaxinline]<br/><div class="wrapper-problem-response" tabindex="-1" aria-label="Question 1" role="group"><div id="inputtype_problem_Superdense_Coding__1_of_3__2_1" class="text-input-dynamath capa_inputtype textline">
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[mathjaxinline]X[/mathjaxinline]<br/><div class="wrapper-problem-response" tabindex="-1" aria-label="Question 2" role="group"><div id="inputtype_problem_Superdense_Coding__1_of_3__3_1" class="text-input-dynamath capa_inputtype textline">
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[mathjaxinline]Z[/mathjaxinline]<br/><div class="wrapper-problem-response" tabindex="-1" aria-label="Question 3" role="group"><div id="inputtype_problem_Superdense_Coding__1_of_3__4_1" class="text-input-dynamath capa_inputtype textline">
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[mathjaxinline]Y[/mathjaxinline]<br/><div class="wrapper-problem-response" tabindex="-1" aria-label="Question 4" role="group"><div id="inputtype_problem_Superdense_Coding__1_of_3__5_1" class="text-input-dynamath capa_inputtype textline">
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<h2 class="hd hd-2 unit-title">Superdense Coding (2 of 3)</h2>
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Superdense Coding (2 of 3)
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<p>
Suppose [mathjaxinline]A[/mathjaxinline] would like to send 2 bits [mathjaxinline]xy[/mathjaxinline] by the superdense coding protocol. [mathjaxinline]A[/mathjaxinline] applies some unitaries to her part of the Bell pair, sends her qubit to [mathjaxinline]B[/mathjaxinline], and [mathjaxinline]B[/mathjaxinline] makes the Bell measurement. Suppose [mathjaxinline]B[/mathjaxinline] carries out the Bell measurement that is described in the previous teleportation problem to obtain the 2 bits of information. Complete the following quantum circuit for [mathjaxinline]A[/mathjaxinline] where [mathjaxinline]A[/mathjaxinline] encodes the 2 bits of information so that [mathjaxinline]B[/mathjaxinline] obtains the same 2 bits that [mathjaxinline]A[/mathjaxinline] would like to send. Note that the single wire corresponds to the one of the qubits of the preshared entangled state which is obtained by [mathjaxinline]A[/mathjaxinline], and double wires refer to the classical control, i.e. if [mathjaxinline]x[/mathjaxinline] is 1, it triggers the unitary in the box below etc. </p>
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String.prototype.trim = function()
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get_search_module();
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$("#site-search-box").keypress(function(event) {
if (event.which == 13) {
event.preventDefault();
go_to_search();
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});
// $('#site-search-box').bind("enterKey", go_to_search);
var get = function(x){
return eval(x);
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return {'course_static_url': course_static_url,
'jump_to_url': jump_to_url,
'go_to_search': go_to_search,
'get_search_module': get_search_module,
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'get': get,
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<h2 class="hd hd-2 unit-title">Superdense Coding (3 of 3)</h2>
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Superdense Coding (3 of 3)
</h3>
<div class="problem-progress" id="block-v1:MITx+8.370.2x+1T2018+type@problem+block@problem_Superdense_Coding__3_of_3_-problem-progress"></div>
<div class="problem">
<div>
<p>
Suppose [mathjaxinline]A[/mathjaxinline] and [mathjaxinline]B[/mathjaxinline] share [mathjaxinline]|{\psi }\rangle =\frac{1}{\sqrt{2}}\left(\sqrt{1-\epsilon }|{01}\rangle -\sqrt{1+\epsilon }|{10}\rangle \right)[/mathjaxinline] instead of the perfect [mathjaxinline]|{\Psi ^-}\rangle[/mathjaxinline]. Suppose they follow the same protocol as above. Find the probability that [mathjaxinline]A[/mathjaxinline] fails to send her 2 bits of information to [mathjaxinline]B[/mathjaxinline] up to the leading order of [mathjaxinline]\epsilon[/mathjaxinline]. Input <tt class="ttfamily">E</tt> for [mathjaxinline]\epsilon[/mathjaxinline]. </p>
<p>
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<div class="unanswered ">
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<span class="trailing_text" id="trailing_text_problem_Superdense_Coding__3_of_3__2_1"/>
<span class="status unanswered" id="status_problem_Superdense_Coding__3_of_3__2_1" data-tooltip="Not yet answered.">
<span class="sr">unanswered</span><span class="status-icon" aria-hidden="true"/>
</span>
<p id="answer_problem_Superdense_Coding__3_of_3__2_1" class="answer"/>
<div id="display_problem_Superdense_Coding__3_of_3__2_1" class="equation">`{::}`</div>
<textarea style="display:none" id="input_problem_Superdense_Coding__3_of_3__2_1_dynamath" name="input_problem_Superdense_Coding__3_of_3__2_1_dynamath"/>
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<p>
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<span id="solution_problem_Superdense_Coding__3_of_3__solution_1"/>
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var add_site_search = function(){
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if (typeof String.prototype.startsWith != 'function') {
// see below for better implementation!
String.prototype.startsWith = function (str){
return this.indexOf(str) === 0;
};
}
if(typeof(String.prototype.trim) === "undefined")
{
String.prototype.trim = function()
{
return String(this).replace(/^\s+|\s+$/g, '');
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var lb = String.fromCharCode(60);
var rb = String.fromCharCode(62);
var amp = String.fromCharCode(38);
var rlb = rb + lb;
var mke = function(x){ return lb + x + rb; }
var search_module_url = "";
var get_search_module_ficus = function(){
var cid = $('div.xblock').data('course-id');
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var course_root_link = $('span.nav-item-course').find('a').attr('href');
if (course_root_link){
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console.log("cannot determine search module url");
}
var get_search_module = function(){
// find search this module link
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return get_search_module_ficus();
}
$('div.course-index').find('nav').find('a').each(function(){
if ($(this).text().trim().startsWith("Search this course")){
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console.log("search_module_url = ", search_module_url);
}
});
}
var go_to_search = function(){
get_search_module();
var sterm = $('#site-search-box').val();
// new_url = jump_to_url + "/Search_this_module/?q=" + sterm;
new_url = search_module_url + "?q=" + sterm;
console.log("sterm = ", sterm, " ; going to ", new_url);
window.location.href = new_url;
}
if (!$('#site-search-box').length){
$("nav.courseware").find("ol").append(lb + "section style='float:right'" + rlb + "input size='20'"
+ " id='site-search-box'"
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}
$("#site-search-box").keypress(function(event) {
if (event.which == 13) {
event.preventDefault();
go_to_search();
}
});
// $('#site-search-box').bind("enterKey", go_to_search);
var get = function(x){
return eval(x);
}
return {'course_static_url': course_static_url,
'jump_to_url': jump_to_url,
'go_to_search': go_to_search,
'get_search_module': get_search_module,
'get_search_module_ficus': get_search_module_ficus,
'get': get,
}
}
var the_site_search = add_site_search();
var add_fix_transcript = function(){
if ($('div.wrap-instructor-info').length==0){
return;
}
$('div.xblock-student_view-video').each(function(key, vblock_e){
var vblock = $(vblock_e);
var vuid = vblock.data('usage-id').split('@');
var vid;
if (vuid.length==1){
vuid = vblock.data('usage-id').split(';_')
vid = vuid[5];
}else{
vid = vuid[2];
}
var mfnpre = vid.split("_video",1)[0];
var mfnid = mfnpre; // no periods
mfnpre = mfnpre.replace('8_370', '8.370'); // periods in gh filename
var lb = String.fromCharCode(60);
var rb = String.fromCharCode(62);
var mke = function(x){ return lb + x + rb; }
var ftid = "fix_transcript_" + mfnid;
if (!$('#' + ftid).length){
var html = lb + "span id='" + ftid + "' style='float:right'" + rb + lb + "a href='#'" + rb;
html += "contribute transcript fix" + mke("/a") + mke("/span");
console.log("html = ", html);
vblock.after(html)
}
$('#' + ftid).click(function(){
var cst = $('ol.subtitles').find('li.current');
var cindex = Number(cst.data('index'));
var gurl;
if (mfnpre.endsWith('_cq_sol')){
gurl = "https://github.com/mitocw/content-mit-8370x-cq-sol-subtitles/blob/master/";
}else{
gurl = "https://github.com/mitocw/content-mit-8370x-subtitles/blob/master/";
}
gurl += mfnpre + ".txt#L" + String(cindex + 10 + 1);
console.log("going to ", gurl);
window.open(gurl, "MITx 8.370x subtitle source");
});
});
}
try{
add_fix_transcript();
}
catch(err){
console.log(err);
}
try{
var rb = String.fromCharCode(62);
setTimeout(function(){ $('.math' + rb + 'span').css("border-left-color","transparent"); }, 3000);
setTimeout(function(){ $('.math' + rb + 'span').css("border-left-color","transparent"); }, 8000);
}
catch(err){
console.log(err);
}
</script></span>
</div>
</div>
</div>
</div>
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