1. Field of the Invention
The present invention relates to methods and devices for generating the correlated photon pairs needed for information communication techniques that exploit the quantum mechanical correlation of photons. More particularly, the invention relates to methods and devices that enable quantum correlated photon pair generation to be indirectly monitored and controlled according to the monitoring results.
2. Description of the Related Art
In recent years quantum cryptography, quantum computing, and other quantum information and communication technologies using quantum mechanical physical phenomena have been attracting attention. Information communication systems that exploit the quantum non-locality of photon pairs are starting to come into practical use. An essential element of such systems is a reliable source of quantum correlated photon pairs, often referred to as entangled photon pairs.
One method that has been used to generate quantum correlated photon pairs is spontaneous parametric fluorescence in second-order or third-order nonlinear optical media, as illustrated in FIG. 1. In spontaneous parametric fluorescence, input of excitation light (pump light) of wavelength λp, wavenumber kp, and angular oscillation frequency ωp to a second-order or third-order nonlinear optical medium 10 produces output of signal photons of wavelength λs, wavenumber ks, and angular oscillation frequency ωs and idler photons of wavelength λi, wavenumber ki, and angular oscillation frequency ωi. The signal and idler photons are always generated in pairs.
When a second-order nonlinear optical medium is used, the wavenumbers and angular oscillation frequencies of the excitation light, signal photons, and idler photons satisfy the following relations (1) and (2), which are equivalent to the laws of conservation of momentum and energy, respectively.kp=ks+ki+K  (1)ωp=ωS+ωi  (2)
Spontaneous parametric fluorescence in a second-order nonlinear optical medium is also known as spontaneous parametric down-conversion (SPDC).
Spontaneous parametric fluorescence in a third-order nonlinear optical medium is also known as spontaneous four-wave mixing (SFWM). The wavenumbers and angular oscillation frequencies of the photons satisfy the following relations (3) and (4).2kp=ks+ki+K  (3)2ωp=ωs+ωi  (4)
The quantity K in equations (1) and (3) is a parameter corresponding to the period of the periodically modulated structure of the nonlinear optical medium. Nonlinear optical media with periodically modulated structures are frequently used nowadays to produce more efficient nonlinear optical effects by quasi-phase matching, as in the cases described below in which a lithium niobate (LiNbO3) crystal is used as the nonlinear optical medium.
Aside from the wavenumber and angular oscillation frequency relations given above, the signal photons and idler photons are also correlated by polarization. A correlated or entangled photon pair including a signal photon and an idler photon is properly referred to as a quantum correlated photon pair or simply as a correlated photon pair. The latter term will be used below.
A quantum correlated photon pair generating device is a device for generating correlated photon pairs. The following are some classical methods of obtaining correlated photon pairs.
In U.S. Pat. No. 7,211,812 (Japanese Patent Application Publication No. 2003-228091, now Japanese Patent No. 4098530), Takeuchi describes a quantum entangled photon pair generating device using β-BaB2O4 (BBO) crystals as second-order nonlinear optical media. Two BBO crystals are aligned in series with a half-wave plate centered between them. Input of linearly-polarized excitation light (pump light) with a wavelength of 351.1 nm produces spontaneous parametric down conversion in the BBO crystals, generating quantum correlated photon pairs with a wavelength equal to twice the wavelength of the excitation light (equal to 702.2 nm). The two photons in each pair are referred to as the signal photon and the idler photon. When the intensity of the excitation light is sufficiently weak and the probability of the occurrence of spontaneous parametric down conversion in both BBO crystals simultaneously is negligible, the device outputs a signal photon beam and an idler photon beam in which each photon in each beam could been generated in either of the two BBO crystals. The state of a correlated photon pair generated by this device is a superposition of two states: one state in which the two photons were generated in one of the BBO crystals, and another state in which the two photons were generated in the other BBO crystal.
The half-wave plate in this device rotates the polarization of the photons generated in the first BBO crystal by 90°, so photon pairs generated in different BBO crystals are polarized in mutually orthogonal planes. The signal and idler photons in each pair are said to be polarization entangled in that both give the same result when their polarization is measured in the same way.
Many other systems using similar structures to generate quantum entangled photon pairs with wavelengths in the 700-nm to 800-nm band have been reported. Generating entangled photon pairs with wavelengths in the 1550-nm band, which is the minimum absorption loss wavelength band of optical fibers, would be very useful in anticipation of long-haul quantum information communication systems.
In Japanese Patent Application Publication No. 2005-258232, Inoue describes a 1550-nm quantum entangled photon pair generating device using periodically poled lithium niobate (PPLN) waveguides as second-order nonlinear optical media. The device has a fiber loop structure incorporating two PPLN waveguides and a polarizing beam splitter (PBS). The two PPLN waveguides are placed so that their optical axes are mutually orthogonal. A femtosecond excitation light pulse with a wavelength of 775 nm and 45° plane polarization is input through the PBS, which splits it into photons having equal probabilities of being aligned in polarization with the axis of each PPLN waveguide. Like the BBO crystals described above, when the intensity of the excitation light is sufficiently weak, the PPLN waveguides generate quantum correlated photon pairs by spontaneous parametric down conversion, but the signal and idler photons have wavelengths of 1550 nm.
A 1550-nm wavelength quantum entangled photon pair generating device using a PBS and a polarization maintaining optical fiber loop with a single PPLN element has been described by Lim et al. in Stable source for high quality telecom-band polarization-entangled photon pairs based on a single, pulse-pumped, short PPLN waveguide (Optic Express, Vol. 16, No. 17, pp. 12460 to 12468, 2008). The polarization maintaining optical fiber loop also includes a fusion splice with a 90° twist. The PPLN waveguide generates quantum correlated photon pairs including signal photons with a wavelength of 1542 nm and idler photons with a wavelength of 1562 nm by spontaneous parametric down conversion. When the intensity of the excitation light is sufficiently weak, the state of each quantum correlated photon pair output from the PBS is a superposition of a state produced by clockwise travel around the loop and an orthogonally polarized state produced by counterclockwise travel.
There are also many reports of devices that generate quantum entangled photon pairs by spontaneous four-wave mixing, using third-order nonlinear optical media instead of the second-order nonlinear optical media employed in the devices described above. Zero-dispersion optical fiber, photonic crystal optical fiber, and more recently silicon wire optical waveguides have been used as the third-order nonlinear optical media.
To configure a practical system, its component devices and subsystems must be able to operate in a stable manner and maintain a specified state for an extended period of time. For example, the light source used in such a system must be capable of maintaining stable output power over an extended period of time.
Even when used under constant conditions, however, actual devices and systems undergo aging changes. It would therefore be desirable to detect whether or not the device or system has deviated from the specified state and use the deviation as feedback to restore the specified state.
The output of the semiconductor lasers and other light sources used in current optical communication systems is stabilized by the following method. Part of the light output from the light source is branched to a device that monitors its intensity, and if the intensity deviates from the specified value, the driving current is adjusted to restore the specified intensity. Alternatively, the light exiting one end of a semiconductor laser is used as output light and the light exiting the other end is monitored to perform a similar adjustment of the driving current.
A quantum correlated photon pair generating device used in a quantum information communication system must be able to generate correlated photon pairs at a stable rate over an extended period of time, and there is a need for a method of verifying that such stability is maintained.
The applications envisioned in the quantum information communication field, however, are predicated on the states of individual particles, that is, individual photons. In the quantum encryption field, for example, if a signal value were represented by multiple photon pairs per signal, it might be possible to eavesdrop by stealing some of the photons, seriously compromising the security of the encryption scheme. A quantum correlated photon pair generating device used in a quantum information communication system therefore ideally produces only one correlated photon pair at a time per signal channel.
The methods of stabilizing the output of the light sources used in existing optical communication systems are inapplicable to this type of ideal quantum correlated photon pair generating device, for the following reasons.
A first problem is that since in a quantum information communication system there is only one photon pair per signal, it is not possible to split off part of the photon pair for monitoring purposes. A further problem is that the act of monitoring, that is, measurement, changes the quantum state of the measured photons, and in quantum mechanics it is in principle impossible to copy the quantum state, so in quantum cryptography it would be impossible to deliver the correct information to the receiving party. In short, although there is a need to ensure that the quantum correlated photon pair generating device is maintaining stable operation in a specified state such as, for example, a state in which the continued stable production of single correlated photon pairs is maintained, no method that ensures this has been reported so far.
What is needed, accordingly, is an indirect method of monitoring the state, or more specifically the mean rate or expected value, of the generation of correlated photon pairs by a correlated photon pair generating device, and a method of controlling the correlated photon pair generation process based on such monitoring.