Algorithmic Assertions - Craig Gidney's Computer Science BlogA blog about computer science, software engineering, quantum computing, mathematical puzzles, and so forth.
http://algassert.com
Efficient Controlled Phase Gradients<p>The phase gradient is a useful, but under-appreciated, quantum operation. When you apply a phase gradient to a register, each computational basis state $|k\rangle$ is phased by an amount proportional to $k$:</p> <p>$$\text{Grad}^t = \sum_{k=0}^{N-1} |k\rangle \langle k| \exp(i \tau t k / N)$$</p> <p>Phase gradients are the frequency-space equivalent...Wed, 24 May 2017 10:10:10 -0700
http://algassert.com/post/1708
http://algassert.com/post/1708Quirk v2.0 - Bowing to Convention<p>Last week I released version 2.0 of my <a href="https://github.com/Strilanc/Quirk">open-source</a> drag-and-drop <a href="/quirk">quantum circuit simulator Quirk</a>.</p> <p><a href="/quirk"><img style="max-width:100%;" src="/assets/2017-05-02-quirk-v2/empty-circuit-full-ui.png"/></a></p> <p>In this post, in addition to the breaking changes that prompted bumping the major version, I want to talk about events and improvements in the year since the <a href="/2016/05/22/quirk.html">previous...Tue, 02 May 2017 13:10:10 -0700
http://algassert.com/post/1707
http://algassert.com/post/1707Thinking of Operations as Controls<p>Controlled operations are a core part of quantum computation. Not <em>strictly</em> necessary, since any two-qubit gate tends to be sufficient for universal quantum computation, but certainly common.</p> <p>For example, the core of both Grover's and Shor's algorithms are defined by controlled operations. Shor's algorithm uses controlled multiplication operations as part...Sun, 09 Apr 2017 13:10:10 -0700
http://algassert.com/post/1706
http://algassert.com/post/1706Difficulty of Classical Control<p>Historically speaking, I think it's fair to say that quantum computing papers have often assumed that, when it comes to conditionally applying an operation, it's better for the conditionion to be classical. It makes intuitive sense, after all. Instead of having to do some complicated multi-qubit interaction, you just either...Sun, 02 Apr 2017 13:10:10 -0700
http://algassert.com/post/1705
http://algassert.com/post/1705Why care about circuit width?<p>I've been thinking about circuit constructions lately, and the question occurred to me: if circuit $A$ uses $O(N)$ gates and circuit $B$ uses $O(N^2)$ gates, but in parallel and with a better constant factor so that $B$'s depth is a bit better than $A$'s, would you ever use $A$ instead...Mon, 13 Mar 2017 10:10:10 -0700
http://algassert.com/post/1704
http://algassert.com/post/1704Bugs from the Future: Hadamard Coins and Implicit Measurement<p>Suppose it's fifty years from now, and everyone considers manipulating quantum information to be routine. You're new to the quantum programming profession, still trying stuff out, and you've come up with (well, rediscovered) a <em>clever idea</em> for generating random data.</p> <p>You call it the Hadamard coin. You'll use the fact...Sat, 04 Mar 2017 09:10:10 -0800
http://algassert.com/post/1703
http://algassert.com/post/1703[Meta] Joining Google's Quantum AI Team<p>I have really exciting news. This month I'm moving from New York to Santa Barbara, to become part of the <a href="https://research.google.com/pubs/QuantumAI.html">quantum ai team at Google</a>!</p> <p>If you've been following news or <a href="https://www.youtube.com/watch?v=kgMWommXxU8">talks</a> about quantum stuff at Google, then you already know why I'm excited. I don't know if...Wed, 01 Mar 2017 09:10:10 -0800
http://algassert.com/post/1702
http://algassert.com/post/1702Trouble Adding Constants into Qubit Registers<p>I had a bit of a surprise this week. Despite never publishing a paper, I've been cited in one! The preprint <a href="https://arxiv.org/abs/1611.07995">"Factoring using 2n+2 qubits with Toffoli based modular multiplication"</a>, by HÃ¤ner et al., cited <a href="http://cs.stackexchange.com/a/44292/535">this answer on cs.stackexchange</a>. The answer is me summarizing a few of the...Sun, 26 Feb 2017 09:10:10 -0800
http://algassert.com/post/1701
http://algassert.com/post/1701Quantum Bomb Detector Detectors<p>The <a href="https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester">Elitzur-Vaidman bomb tester</a> is one of the funnest quantum devices to teach people about. It uses the <a href="https://en.wikipedia.org/wiki/Quantum_Zeno_effect">quantum Zeno effect</a> to safely separate dud bombs from live bombs, even when the only way to "test" a bomb is by triggering it! The main caveat is that the...Sat, 11 Feb 2017 09:10:10 -0800
http://algassert.com/post/1700
http://algassert.com/post/1700Quantum Data Locking vs Recognizable Data<p>Quantum information theory differs from classical information theory in many interesting ways. The ability to perform "data locking", where the amount of classical information that you have access to can go up dramatically as you receive clarifying information, is one of those differences.</p> <p>The <a href="https://arxiv.org/abs/quant-ph/0303088">original quantum data locking paper,...Sun, 18 Dec 2016 09:10:10 -0800
http://algassert.com/post/1632
http://algassert.com/post/1632