A method for a verifying system to verify a location of a target apparatus, the method comprising: transmitting a plurality of signals from two or more of a plurality of communication devices (610, 612, 614) of the verifying system to the target apparatus, each signal of the plurality of signals comprising information; sending quantum information to the target apparatus via a quantum communication channel, wherein a plurality of bits is encoded in the quantum information and the encoding of the plurality of bits is based on an output of a function of the information associated with the plurality of signals, wherein the function is a composition of functions containing, as a first component, a linear function and, as a second component, a non-linear function, so that the image of the linear function is the argument of the non-linear function; receiving a response related to the plurality of bits at two or more of the plurality of communication devices; analysing (222, 228) the response, the plurality of bits encoded in the quantum information, and a time between the transmitting of the plurality of signals and the receiving of the response; and determining from the analysing whether or not the response originated from the location of the target apparatus, wherein the plurality of signals are transmitted such that each signal arrives at the location of the target apparatus within a time frame, wherein the time frame is an instant in time, wherein the transmitting of the plurality of signals from the two or more communication devices (610, 612, 614) of the verifying system to the target apparatus comprises, at each of the two or more communication devices, transmitting a signal of the plurality of signals to the target apparatus, and wherein the information associated with the signal is distinct from information associated with a signal of the plurality of signals transmitted from another communication device of the two or more communication devices, wherein the non-linear function in the composition is cryptographic hash function and, optionally wherein the linear function in the composition is a bitwise XOR of its arguments. transmitting a plurality of signals from two or more of a plurality of communication devices (610, 612, 614) of the verifying system to the target apparatus, each signal of the plurality of signals comprising information; sending quantum information to the target apparatus via a quantum communication channel, wherein a plurality of bits is encoded in the quantum information and the encoding of the plurality of bits is based on an output of a function of the information associated with the plurality of signals, wherein the function is a composition of functions containing, as a first component, a linear function and, as a second component, a non-linear function, so that the image of the linear function is the argument of the non-linear function; receiving a response related to the plurality of bits at two or more of the plurality of communication devices; analysing (222, 228) the response, the plurality of bits encoded in the quantum information, and a time between the transmitting of the plurality of signals and the receiving of the response; and determining from the analysing whether or not the response originated from the location of the target apparatus, wherein the plurality of signals are transmitted such that each signal arrives at the location of the target apparatus within a time frame, wherein the time frame is an instant in time, wherein the transmitting of the plurality of signals from the two or more communication devices (610, 612, 614) of the verifying system to the target apparatus comprises, at each of the two or more communication devices, transmitting a signal of the plurality of signals to the target apparatus, and wherein the information associated with the signal is distinct from information associated with a signal of the plurality of signals transmitted from another communication device of the two or more communication devices, wherein the non-linear function in the composition is cryptographic hash function and, optionally wherein the linear function in the composition is a bitwise XOR of its arguments.