In written Japanese language, the words borrowed from foreign languages are expressed by one of two types of phonetic characters called katakana. For the word "quantum," we have a proper Japanese word "ryōshi." Therefore, it is quite rare to see katakana expression for "quantum."
In reading books written in English, however, I had kept the habit of reading "quantum" in my mind by the pronunciation close to the katakana expression "クォンタム (kwontamu)." One or two years ago, I found a different katakana expression, "クァンタム (kwantamu)," in an essay by Sin-itiro Tomonaga. Because Tomonaga was a Nobel-winning physicist, I thought that his expression should be accepted and became to pronounce "quantum" like "kwantamu."
In the morning of Sunday, February 21, I saw the katakana expression "kwontamu" in the title of a new novel reviewed in the Asahi Shimbun. The full title of the book was "クォンタム・ファミリーズ (Kwontamu Famirīzu)"  meaning "Quantum Families." Then I wondered which expression is closer to the pronunciation of "quantum," "kwontamu" or "kwantamu." I looked up a dictionary and learned belatedly that "kwontamu" is closer to British pronunciation, and "kwantamu" to American pronunciation. Thus, we should consider both the expressions to be valid. Tomonaga's studies abroad were at Leipzig and Princeton, making it reasonable for him to use "kwantamu."
- Hiroki Azuma, Kwontamu Famirīzu (Shinchōsha, 2009). The author uses the parallel world hypothesis in modern physics to depict the mixture of plural stories.