  We present a garbling scheme for quantum circuits, thus achieving a
decomposable randomized encoding scheme for quantum computation. Specifically,
we show how to compute an encoding of a given quantum circuit and quantum
input, from which it is possible to derive the output of the computation and
nothing else. In the classical setting, garbled circuits (and randomized
encodings in general) are a versatile cryptographic tool with many applications
such as secure multiparty computation, delegated computation, depth-reduction
of cryptographic primitives, complexity lower-bounds, and more. However, a
quantum analogue for garbling general circuits was not known prior to this
work. We hope that our quantum randomized encoding scheme can similarly be
useful for applications in quantum computing and cryptography.
  To illustrate the usefulness of quantum randomized encoding, we use it to
design a conceptually-simple zero-knowledge (ZK) proof system for the
complexity class $\mathbf{QMA}$. Our protocol has the so-called $\Sigma$ format
with a single-bit challenge, and allows the inputs to be delayed to the last
round. The only previously-known ZK $\Sigma$-protocol for $\mathbf{QMA}$ is due
to Broadbent and Grilo (FOCS 2020), which does not have the aforementioned
properties.
