I suggest you buy Money, Bank Credit, and Economic Cycles by Jesus Huerta de Soto for a full understanding of this subject.
To directly answer your question, let me quote the famous monetarist economists Milton Friedman and Anna J. Schwartz who concluded that between 1865 and 1879:
The price level fell to half its initial level in the course of less than fifteen years and, at the same time, economic growth proceeded at a rapid rate… Their coincidence casts serious doubts on the validity of the now widely held view that secular price deflation and rapid economic growth are incompatible (A Monetary History of the United States 1867-1960, p. 15 and statistical table on p. 30).
Price deflation is not a bad thing at all. It simply means that people have a lower time preference: they prefer to save and invest (use more money later) than spend (use less money now). When people save, they don’t horde just for the sake of hording. They save up money to spend later on more expensive, durable, capital goods (i.e. houses, vehicles, machinery, repairs to those capital goods, etc.). So what happens in a free market is that less spending on consumer goods (non durable goods like clothing, food, services, etc.) means price deflation in those areas. As prices go down, some business that produce nondurable consumer goods might go out of business or might fire workers to replace them with machinery. This increases the demand for capital goods, meaning more workers are needed in that sector. That coupled with the fact that average people are now saving more so that they can make use of capital goods and repair capital goods means even more jobs in the more industries that produce capital goods (i.e. the housing market). On top of all of that indirect investment in the form of certificates of deposit increase the loanable funds supply, driving interest rates down. And all the while direct investment occurs at a greater rate due to the change in time preference.
So, basically, price deflation, economic growth, and low unemployment are completely compatible. Your professor or teacher is probably mistaken by using the Phillips curve as a justification, which incorrectly states that higher inflation means less unemployment. Monetarists such as Friedman and Schwartz point towards deflation in the 30s as the primary reason why the Great Depression was so severe. The true reason why we see recessions and deflation occur simultaneously in modern times is due to central monetary expansion that blows up bubbles like the housing market. Monetary expansion through banks artificially increases demand temporarily, creating huge fiascos like tech, telecom, and housing bubbles (all of which are capital-intensive, not consumer-intensive industries).