Happy National Poetry Month! The United States welcomes word and whim in April, and Quantum limits continues its tradition of celebration. As a Cambridge, Massachusetts resident and a quantum information scientist, I have trouble avoiding the poem “Paul Revere’s Ride.”

Henry Wadsworth Longfellow wrote the poem as well as others in the American canon in the 19th century. Longfellow taught at Harvard, Cambridge, and lived in a national historic site a few blocks from the university. Across the street from the house, a bust of the poet in Longfellow Park looks down as if lost in thought. Longfellow wrote one of his most famous poems about an event that took place a short drive from – and arguably part of – Cambridge.

Longfellow Park

The event took place “on April 18 in [Seventeen] Seventy-five ”, as the narrator of“ Paul Revere’s Ride ”reports. Revere was a Boston silversmith and an advocate for the independence of the American colonies from Great Britain. Revolutionaries expected British troops to leave Boston sometime in the spring. The British planned to confiscate the revolutionaries’ weapons in the nearby town of Concord and imprison the revolutionary leaders in Lexington. The troops left Boston on the night of April 18th.

When the sexton Robert Newman learned of their movements, he sent a signal from Boston’s Old North Church to Charlestown. Revere and Doctor William Dawes rode out of Charlestown to warn people in and around Lexington. A series of artificial hoof prints left on a sidewalk a few minutes from Longfellow house marks part of Dawes’ walk through Cambridge. The first horsemen brought more horsemen on their toes, stirring up colonial militias that opposed the advance of the troops. The battles of Lexington and Concord followed, sparking the War of Independence.

Longfellow took liberties with the facts he claimed to be telling them. But “Paul Revere’s Ride” has blown the dust off the history books for generations of school children. The reader shares Revere’s nervous excitement as he wriggles and waits for Newman’s signal:

Now he patted his horse’s side, 
Now gazed on the landscape far and near, 
Then impetuous stamped the earth, 
And turned and tightened his saddle-girth;
But mostly he watched with eager search 
The belfry-tower of the old North Church.

The moment the signal arrives, that excitement is bursting at the seams and Revere jumps astride his horse. The reader comes to gallop by with the silversmith in the night, the poem Clip-clop-clip-clop Rhythm reminiscent of a horse’s hooves on cobblestones.

The author in front of Longfellow House on April 18th in… Twenty Twenty.

Paul Revere’s Ride not only enlivens the story, but also offers a lesson in information theory. As he makes plans, Revere instructs Newman:

He said to his friend, “If the British march
By land or sea from the town to-night,
Hang a lantern aloft in the belfry-arch
Of the North-Church-tower, as a signal light.

Then comes one of the most famous lines in the poem: “One by land and two by sea.” The British could have left Boston on foot or by boat, and Newman had to tell which one. He gave one of two options and included one littleor a basic unit of information. Newman thus illustrates a cornerstone of information theory: the coding of a bit of information – an abstraction – in a physical system that can be in one of two possible states – a light shining from one or two lanterns.

Benjamin Schumacher and Michael Westmoreland point in their textbook on quantum information to the information-theoretical interpretation of Newman’s actions. I used your textbook on my first quantum information class as a senior in college. Before reading the book, I never felt I could explain what information is or how it can be quantified. Information is an abstraction and a big idea, like awareness, life and piety. However, Schumacher and Westmoreland have shown that most readers already understand the basics of information theory. Some readers even learned the basics while memorizing a poem in elementary school. So I take off my hat – or, since we have been talking about the 18th century, my mob cap – the authors.

Reading poetry enriches us more than we think. So read a poem this April. You can find Longfellow’s poem here or go wherever you want.


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