Security systems and monitoring methods using quantum states

1. A security system comprising:

a source capable of producing an entangled state of a sensing mode and a reference mode;

a sensing path for the sensing mode, wherein the sensing path enters an area around an object being secured;

a reference path for the reference mode; and

a state measurement system capable of measuring a combination of the sensing mode after traversal of the sensing path and the reference mode after traversal of the reference path.

2. The system of claim 1 , wherein the source comprises:

a single-photon source; and

a beam splitter in a path of a beam from the single-photon source, wherein the sensing mode and the reference mode are output modes of the beam splitter.

3. The system of claim 2 , wherein the beam splitter comprises a multi-mode beam splitter that splits the beam from the single-photon source into multiple modes including the reference mode and multiple sensing modes having paths that enter the area around the object being secured.

4. The system of claim 1 , wherein the source comprises a source of entangled photon pairs, each of the entangled photon pairs including a photon in the reference mode and a photon in the sensing mode.

5. The system of claim 4 , wherein each of the entangled photon pairs is in a singlet Bell state.

6. The system of claim 1 , wherein the sensing path consists of a first portion in a secure area and a second portion that is accessible to an intruder, wherein the second portion is straight.

7. The system of claim 6 , wherein the second portion of the sensing path passes through the object.

8. The system of claim 1 , wherein the reference path has an optical path length that is equal to an optical path length of the sensing path.

9. The system of claim 1 , wherein the source, the reference path, and the state measurement system are within a secure area, and the sensing path extends outside of the secure area.

10. The system of claim 1 , wherein the state measurement system comprises a Mach-Zehnder interferometer.

11. The system of claim 1 , further comprising:

a first quantum gate in one of the sensing path and the reference path;

a first control circuit connected to select a first operation that the first quantum gate performs on one of the sensing mode and the reference mode

a second quantum gate in the sensing path; and

a second control circuit connected to select a second operation that the second quantum gate performs on the sensing mode.

12. The system of claim 11 , further comprising

a secure area containing the first control circuit, wherein the first control circuit uses a first copy of a one time pad in the secure area to select the first operation; and wherein

the second control circuit is outside the secure area and the second control circuit uses a second copy of the one time pad to select the second operation.

13. A method for monitoring an object, comprising:

producing an entangled state of a sensing mode and a reference mode;

directing the sensing mode along a sensing path that enters an area around the object; and

measuring quantum interference of the sensing mode with the reference mode to detect tampering with the sensing mode.

14. The method of claim 13 , further comprising:

transmitting information between a first system and a second system, wherein the second system is at the object;

performing in the first system a first operation on one of the sensing mode and the reference mode before measuring the quantum interference, wherein the information identifies the first operation; and

performing a second operation in the second system on the sensing mode, wherein the information identifies the second operation.

15. The method of claim 13 , wherein the entangled state is a single-photon state.