diff --git a/Advanced/Introduction to Quantum/src/parts/07 superdense.tex b/Advanced/Introduction to Quantum/src/parts/07 superdense.tex index 558b466..0361fcd 100644 --- a/Advanced/Introduction to Quantum/src/parts/07 superdense.tex +++ b/Advanced/Introduction to Quantum/src/parts/07 superdense.tex @@ -163,7 +163,7 @@ to Bob by only sending one qubit? - \draw[wire, double] + \draw[wire] ($([shift={(4,0)}] c)!0.5!([shift={(5,0)}] c)$) -- ($([shift={(4,0)}] d)!0.5!([shift={(5,0)}] d)$) ; @@ -179,7 +179,6 @@ to Bob by only sending one qubit? \qubox{c}{6.3}{c}{8}{measure} \qubox{d}{6.3}{d}{8}{measure} - \end{tikzpicture} \end{center} \end{solution} diff --git a/Advanced/Introduction to Quantum/src/parts/08 teleport.tex b/Advanced/Introduction to Quantum/src/parts/08 teleport.tex index 0533738..fd487e0 100644 --- a/Advanced/Introduction to Quantum/src/parts/08 teleport.tex +++ b/Advanced/Introduction to Quantum/src/parts/08 teleport.tex @@ -119,8 +119,60 @@ What should Bob do so that $\ket{\Phi^+_B}$ takes the state $\ket{\psi}$ had ini \end{tikzpicture} \end{center} + + Note how similar this is to the superdense coding circuit. \end{solution} \vfill \pagebreak +\problem{} +With an informal proof, show that it is not possible to use superdense coding to send +more than two classical bits through an entangled two-qubit quantum state. + +\begin{solution} + If superdense coding was any more efficient, we could repeatedly apply superdense coding and quantum teleporation, + to compress an arbitrary number of bits into two \say{seed} bits. + + \linehack{} + + \textbf{Even worse, this would allow faster-than-light communication:} \par + + Because the seed message is only 4 bits, Alice has decent odds of just + guessing it. She'll guess wrong and trash the message the majority of the + time but, by using an error correcting code, she can tell whether or not + the guess was correct or she trashed the message. And by repeating the protocol + enough times, we can increase the odds of the message being received arbitrarily + close to certainty. + + \note[Note]{ + I'm implicitly assuming that if Alice uses the wrong seed, she gets a totally random message---or + at least a message that isn't guaranteed to follow the error correction scheme better than chance would. + The alternative, where Alice receives noise that's uncorrelated with the message and yet somehow satisfies + arbitrary error correction schemes, is waaay too magical for me to even consider. + } + + \vspace{2mm} + + Suppose Alice and Bob perform the iterated-ultradense-encode-and-guess process 100 times. + That gives a failure rate of $(\nicefrac{15}{16})^{100} \approx 0.5\%$. + Sure it's a hundred times more work than just sending the 4 bits, and less likely to succeed to boot, + but the new protocol \textit{doesn't require any bits to be physically transmitted}. + There's no signalling delay! + + \vspace{2mm} + + In fact, Alice could even perform the decoding process \textit{before} Bob did the encoding. + But we're already so far into \say{everything is clearly broken} territory that creating time travel paradoxes is overkill. + + + \vspace{5mm} + + + From \url{https://algassert.com/2016/05/29/ultra-dense-coding-allows-ftl.html} +\end{solution} + + +\vfill +\pagebreak + diff --git a/Advanced/Introduction to Quantum/src/week 1.tex b/Advanced/Introduction to Quantum/src/week 1.tex index 9fa8953..30cdafd 100755 --- a/Advanced/Introduction to Quantum/src/week 1.tex +++ b/Advanced/Introduction to Quantum/src/week 1.tex @@ -20,19 +20,17 @@ singlenumbering ]{../../../resources/ormc_handout} \usepackage{../../../resources/macros} - -\def\ket#1{\left|#1\right\rangle} -\def\bra#1{\left\langle#1\right|} - \usepackage{units} \input{tikzset} - \uptitlel{Advanced 2} \uptitler{Winter 2022} \title{Intro to Quantum Computing I} \subtitle{Prepared by \githref{Mark} on \today{}} +\def\ket#1{\left|#1\right\rangle} +\def\bra#1{\left\langle#1\right|} + % TODO: spend more time on probabalistic bits. % This could even be its own handout, especially diff --git a/Advanced/Introduction to Quantum/src/week 2.tex b/Advanced/Introduction to Quantum/src/week 2.tex index ce0863a..c259b10 100755 --- a/Advanced/Introduction to Quantum/src/week 2.tex +++ b/Advanced/Introduction to Quantum/src/week 2.tex @@ -21,13 +21,11 @@ shortwarning ]{../../../resources/ormc_handout} \usepackage{../../../resources/macros} - -\def\ket#1{\left|#1\right\rangle} -\def\bra#1{\left\langle#1\right|} - \usepackage{units} \input{tikzset} +\def\ket#1{\left|#1\right\rangle} +\def\bra#1{\left\langle#1\right|} \uptitlel{Advanced 2} \uptitler{Winter 2022} @@ -35,6 +33,13 @@ \subtitle{Prepared by \githref{Mark} on \today{}} +% Hack to keep numbering from previous handout +\makeatletter +\setcounter{\@problemcounter}{34} +\setcounter{section_counter}{6} +\makeatother + + \begin{document} \maketitle