Title of Invention	METHOD FOR CONVERSION OF QUANTUM INFORMATION FROM ONE PHOTONIC REPRESENTATION TO ANOTHER PHOTONIC REPRESENTATION
Abstract	Systems and methods convert or transfer quantum information from one photonic representation or state to another. This permits conversion of quantum information from one encoding to another and to representations that are convenient, efficient, or required for desired manipulations.

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QUANTUM OPTICAL STATE CONVERTER BACKGROUND Photonic systems provide a variety of different physical representations for quantum information. One type of physical representation uses the polarization of photons to encode quantum information. In particular, each photon has a polarization state that can be expressed as a linear combination of two basis states associated with orthogonal polarizations (e.g., horizontal and vertical polarization states |H> and |V> or right and left circularly polarized states |R> and |L>). A quantum information processing system can use the two orthogonal polarization states of a photon as the basis state values |0> and |1> of a qubit. In some alternative physical representations, a qubit has basis state values |0> and |1> corresponding to orthogonal eigen states of orbital angular momentum of a photon, the presence or the absence of a photon in a single spatial channel or time bin, or to the alternative presence of a single photon in one or the other of two spatial channels or in one or the other of two time bins. More generally, quantum information is not limited to qubits. A qudit, for example, refers to quantum information represented using d discrete basis states, and a qunat refers to quantum information represented using a continuous range of basis states. One photonic representation of a qudit uses the alternative presence of a single photon in any one of d distinct spatial channels or of d distinct time bins. Alternatively, the Fock states (e.g., states |0> to |d-l>) containing definite numbers (e.g., 0 to d-1) of photons can be the basis states for encoding qudit having any desired number d of discrete values, and the orbital angular momentum of states containing one or more photons can similarly represent three or more discrete values of a qudit. Similarly, a qunat can be physically represented using photons having a continuous range of eigen values for an operator such as position or momentum or approximately represented using coherent or squeezed photon states |of> corresponding to a continuous quantum variable a Many other representations of quantum information such as qubits, qudits, and qunats are possible using photonic systems. The entanglement of photon states is another general form of quantum information that can be physically represented using many types of photon states. Quantum information processing generally manipulates quantum states to perform tasks such as calculations or communications, storage, or measurement of quantum information. For example, a system that manipulates quantum states can implement a variety of logical operations that are often associated with the quantum gates. However, an implementation of one quantum gate may be more efficient for one physical representation of quantum information, while another quantum gate is more difficult to implement for that physical representation. More specifically, a first quantum gate for a qubit, for example, may be most easily implemented if the qubit is represented with photons of one frequency, but a second quantum gate may be more efficiently implemented if the qubit is represented using photons having a different frequency. Specific implementations of systems that store or measure quantum information may similarly be most efficient or easiest to implement for a specific physical representation of the quantum information. Accordingly, the choice of a fixed physical representation (e.g., of the frequency and the encoding of photons used to represent the quantum information) may limit the availability or efficiency of a quantum information processing system. SUMMARY In accordance with an aspect of the invention, systems or methods convert or transfer quantum information from one photonic representation or state to another. This permits conversion of quantum information to representations that are convenient, efficient, or required for desired manipulations. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a block diagram of a state converter in accordance with an embodiment of the invention using CNOT gates for conversion of a qubit from one photonic representation to another photonic representation. Fig. IB is a block diagram of a state converter in accordance with an embodiment of the invention using phase gates and Hadamard transforms for conversion of a qubit from one photonic representation to another photonic representation. Fig. 2 is a block diagram of a state converter in accordance with an embodiment of the invention using electromagnetically induced transparency (EIT) to transfer quantum information from a photon having a first frequency to a photon having a second frequency. Fig. 3 is an energy level diagram for a matter system suitable for an EIT system in the state converter of Fig. 2. Figs. 4A and 4B are block diagrams of state converters in accordance with embodiments of the invention using teleportation for conversion of quantum information from one photonic representation to another. Figs. 5A and 5B illustrate converters in accordance with embodiments of the invention that convert qubits represented using dual-rail Fock states to qunats represented using coherent photon states. Figs. 6A and 6B show embodiments of the invention in which optical systems have an effective Hamiltonian that converts quantum information from one physical representation to another. Fig. 7 illustrates some conversions of physical encodings for quantum information. Use of the same reference symbols in different figures indicates similar or identical items. DETAILED DESCRIPTION In accordance with an aspect of the invention, a quantum information processing system can convert or transfer quantum information from one physical photonic representation to another. A general conversion transfers the quantum information such as a qubit, qudit, qunat, or state entanglement from a first encoded photon state to a second encoded photon state. The conversion can change the encoding and/or the properties of the underlying photons. In one specific embodiment, the information encoded using photons of a first frequency is transferred to an encoded state of one or more photons having a second frequency. As described further below, some alternative implementations of the state converters can be based on swap gates, teleporters, or an optical system that provides an effective Hamiltonian that transfers the quantum information from one state to another. Fig. 1A illustrates a converter 100 including a swap gate formed using a pair of controlled NOT (or CNOT) gates 110 and 120. The input to converter 100 is a product of a state |a representing a qubit for conversion and a known state |0>c as shown in Equation 1. The state |a being converted is a linear combination of basis states |0>a and |l>a in an "a-channel" that uses a first photonic representation of a qubit. State |0>c is a known state in a "c-channel" that provides a second photonic representation for a qubit. For example, basis states |0>a and |l>a can be orthogonal polarization states |H>a and |V>a of a photon having frequency o>a, while state |0>c is one of two orthogonal polarization states |H>C and |V>C of a photon having an angular frequency wc. Coefficients C0 and Q in Equation 1, which are generally complex values, indicate the quantum information that is encoded in state |^>>a and that converter 100 transfers to a state |0>c in the c-channel. State |0>a in the a-channel controls CNOTac gate 110, which changes the initial product state as indicated in Equation 2. The state in the c-channel then controls CNOTca gate 120 and changes the resulting product state from CNOTac gate 110 as indicated in Equation 3. As a result of the operation of CNOT gates 110 and 120, the quantum information reflected by probability amplitudes Co and C\ of state $>a are transferred to an output state \>c in the second channel. The state |0>a output from the a-channel is not required and can be discarded or reused elsewhere. Each CNOT gate 110 or 120 can be constructed from two Hadamard transform gates 112 and 116 or 122 and 126 and a controlled phase gate 114 or 124 as illustrated in Fig. 1 A. However, CNOT gates 110 and 120 can alternatively be constructed without containing separable elements 112,114,116, 122,124, and 126. Fig. IB illustrates a converter 130 that is a simplified version of converter 100 found by eliminating elements that are unnecessary to state conversion. Converter 130 consists of two Hadamard transformation gates 122 and 116 and two controlled phase gates 114 and 124. Converter 130 contains fewer elements when compared to converter 100 of Fig. 1A because converter 130 in performing a state conversion is not required to perform a full swap. In particular, converter 130 docs not require an output Hadamard transform 126 since the output state in the a-channel is simply a known photon state and may be discarded. Further, an input Hadamard transform 112 on the c-channel can be eliminated by directly preparing an input state for the c-channel of converter 130 that is the same as the output state from Hadamard transform 122 in converter 100. Accordingly, an input state for converter 130 of the form shown in Equation 4 provides a converted output state \ The physical implementations of CNOT gates 110 and 120 (or Hadamard transforms 122 and 116 and controlled phase gates 114 and 124) determine the conversions that converter 100 or 130 performs. In one specific embodiment, the a-channel states |0>a and |l>a correspond to states (e.g., orthogonal polarization states or alternative spatial channel states) of a photon having an angular frequency ua, and the c-channel states |0>c and |1>C correspond to states (e.g., orthogonal polarization states or alternative spatial channel states) of a photon having an angular frequency o>c, and converter 100 or 130 then operates as a frequency converter for an optical quantum information signal. Such frequency conversions are particularly useful because current optical fibers are generally designed for transmission of photons having wavelengths in the 1.3 to 1.5 micron range while shorter wavelengths (e.g., about 650 nm) have the advantage of better available optical elements, for example, for single photon detection or creation. Fig. 2 illustrates an example of an optical quantum state converter 200 in which the input quantum state |a is in a representation where states |0>a and |l>a are orthogonal linear polarization states |H>a and |V>a of a photon having an angular frequency coa and the output quantum state (0>c conveys the same quantum information as state (0>a but in a physical representation where basis states |0>c and |1>C are orthogonal linear polarization states |H>C and |V>C of a photon having angular frequency coc. State converter 200 of Fig. 2 includes two controlled phase gates 210 and 240 and two Hadamard gates 220 and 230. Controlled phase gates 210 and 240 in Fig. 2 are implemented using an electromagnetically induced transparency (EIT) system. Alternatively, other implementations of a controlled phase gate using devices such as linear optics with conditioning as described by Knill et al., "A Scheme for Efficient Quantum Computations with Linear Optics," Nature, Vol. 409, page 46 (2001) or a system using giant optical non-linearities with trap atoms such as described in "Generating Optical Nonlinearity using Trapped Atoms," Alexei Gilchrist, G.J. Milburn, W.J. Munro, and K. Nemoto, quant-ph/0305167 could be used. Controlled phase gate 210 has input optics (e.g., polarizing beamsplitters 212) that direct the polarization states corresponding to states |l>a and |1>C into a matter system 214 having energy levels capable of creating an interaction between photons of frequencies 6)a and wc. hi particular, matter system 214 is such that the simultaneous combination of a photon in state jl>a with frequency wa and a photon of angular frequency o>c in matter system 214 changes the phase of the state |l>a by a factor -1 (or e114). One way to create the desired change in the phase of state 11 >a uses a four-level matter system containing atoms or molecules having energy eigenstates 11>, |2>, |3>, and |4> with energy levels illustrated in Fig. 3. In matter system 214, state 13> is preferably a metastable state in that a conservation rule prohibits a single-photon transition between states |3> and 11>, and spontaneous emissions causing transitions from state |4> are preferably suppressed, for example, by a surrounding photonic bandgap crystal. Additionally, energy differences between states |l> and |2>, |2> and |3>, and |3> and |4> are respectively equal to h(a)a-va), h(ojc-vc), and h(cob-Vb), where detuning parameters va, vc, and Vb may be small. As described further in U.S. Patent Application No. 10/364,987, entitled "Quantum Information Processing using Electromagnetically Induced Transparency", a matter system having the energy levels of Fig. 3 and the properties just described will exhibit electromagnetically induced transparency and will change the phase of the input state when simultaneously exposed to photons of angular frequencies o>a, 6)b» and o?c. Controlled phase gate 210 includes a laser 216 that provides photons of frequency a>b. With proper selection of parameters such as detuning parameters va, Vb, and vc and the number of four-level atoms or molecules that interact with the photons, the simultaneous presence of photon of angular frequencies oja and wc induces EIT and the desired change in phase of the state I l>a 11>C- No change in the phase occurs for input states 10^> 10>c, 10>a 11 ^c> or 11 >a 10>c because of the absence of a photon of frequency coa or wc. Hadamard gates 220 and 230 can be implemented using linear optical elements that rotate the separated polarization states. After polarization rotation, beam combining optics (e.g., polarizing beamsplitters 218) recombine the separated polarization components. Controlled phase gate 240 is implemented in the same manner as controlled phase gate 210 and includes a four-level matter system 222. A laser 224 provides photons of wavelength 6>b> which permits matter system 222 to provide the desired phase change to the | l>a component of the a-channel photon. The operation of Hadamard gate 220 and controlled phase gate 240 unentangle the a-channel and c-channel photon state, allowing output of the desired c-channel gate 430 when the a-channel photon is in state | l>a, and signal Mc is asserted to enable NOT gate 420 when the c-channel photon is in state 11>C. As a result, the final output state |a is of the form C010>d+C1 11 >d as indicated by Table 1. The teleportation phenomenon is well known in the art, but in accordance with an aspect of the invention, teleportation is used to convert quantum information from one photonic representation to another. For example, for a frequency conversion, a-channel photons have a first angular frequency o)a, and c-channel photons have a second angular frequency Wd- The c-channel can include photons having a frequency o)c that can interact with the a-channel photons in CNOT gate 412. In one specific embodiment, c-channel photons and d-channel photons have the same frequency o>c (Fig, 4A), and a-channel photons have a different frequency coa. CNOT gate 412 can then be implemented using ETC in a 4-level matter system such as described above in regard to Figs. 2 and 3. Fig. 4B illustrates a converter 402 in accordance with an embodiment of the invention that converts quantum information from a physical representation using polarization basis states |H>a and |V>a to a physical representation using Fock states |0>d. The input qubit state being converted is thus of the form ColH>+Cj|V>. In an exemplary embodiment, the input Bell state is an entangled state that is a combination of a state 10>c 10>d with no photons in the c or d channel and a state 11 >c I l>d corresponding to one photon in each of the c-channel and the d-channel. The c-channel uses photons having an angular frequency CJC of Fig. 3, and the a-channel uses photons having an angular frequency ua of Fig. 3. Accordingly, a CNOT gate 510 can be implemented using a laser that irradiates a 4-level matter system with photons of angular frequency cob* The a-channel has polarization splitting optics that directs the polarization corresponding to state | V> into the matter system and routes photons with polarization corresponding to state |H> around the matter system. The c-channel using a physical representation using Fock states 10>c and 11>C as basis states does not require polarization optics. The d-channel can use any angular frequency a>d for which the input entangled photon state can be generated. More generally, a converter using quantum teleportation can be used to converter to any physical representation having basis states basis state |0>d and |l>a that can be entangled with the basis states 10>c and 11>C. For example, basis state 10>a and | l>d for the encoding in the d channel can be coherent states |ct> and |-ot>. Coherent states |a> and I -a> can be entangled with polarization eigen states | H>c and I V>c or photon number eigen states using known coherent state quantum computing techniques that employ conditional logic and linear optics. Alternatively, the entanglement of Fock states with coherent states can be achieved using a system having a Hamiltonian of the form given in Equation 7, where D(a) is the "displacement" operator that creates coherent state |a> from state |0>. As a result, the Hamiltonian converts an entangled state |0>c|0>d+|0>c|l>d to the entangled state 10>c I a>d+11 >c I ~a>d. The conversion from a Fock state representation of quantum information to a coherent state representation of the quantum information is particularly useful for converting discrete quantum information such as a qubit to a representation suitable for representation of continuous quantum information (i.e., a qunat). Fig. 5A illustrates a converter 500 in accordance with an embodiment of the invention that converts a qubit from a representation using dual-rail Fock states to a representation using coherent states that are suitable for qunats. Converter 500 can accordingly convert a qubit into a corresponding qunat. In the illustrated embodiment of converter 500, an input state |IN is of the form given in Equation 8 where states |0>IN and I 1>TN are the basis states of the input representation of the qubit. States 10>A and 11>A are Fock states respectively corresponding to 0 and 1 photon in a first spatial channel or rail A, and states | 0>B and 11 >B are Fock states respectively corresponding to 0 and 1 photon in a second spatial channel or rail B. Coefficients Co and Q, which in general are complex numbers, are the quantum information that the input qubit conveys. With a coherent state I a> input on a third spatial channel, the full input state at point X0 in converter 500 is given in Equation 9. Converter 500 includes a controlled phase gate 510, a 50-50 beam splitter 520, photon detectors 530 and 535, and output state correction elements 540 and 545. The B-rail photon state controls the controlled phase gate 510 that operates on the input coherent state |a>. Controlled phase gate 510, as illustrated in Fig. 5 A, can be implemented using an EIT matter system having respective energy level transitions that respectively correspond to energies of photons in the B-rail state and coherent state |a>, but other implementations of a controlled phase gate could alternatively be employed. As a result of the operation of control phase gate 510 on coherent state I a> only when a photon is present in the B-rail, the full photon state at point XI following controlled phase gate 510 is given in Equation 10, where x is a change in phase that controlled phase gate 510 causes. The 50-50 beam splitter 520 mixes the A-rail and B-rail states and outputs photons through output channels or rails C and D. The resulting state after beam splitter 520 can be expressed using states |0>c and I l>c that are Fock states respectively corresponding to 0 and 1 photon in rail C and states |0>D and |1>D that are Fock states respectively corresponding to 0 and l photon in rail D. Equation 11 indicates the photon state after the transformation of the state from point XI to point X2, which is after the operation of 50-50 beam splitter 520. The relative phases of terms in Equation 11 are those found when 50-50 beam splitter 520 is asymmetric. Other types of beam splitters such as a symmetric 50-50 beam splitter will provide similar results but may required phase correction that differs from the specific example described below. Photon detectors 530 and 535 determine whether the photon from the input state is in the C or D channel. Efficient single photon detectors suitable for this purpose are described, for example, in "A High Efficiency Quantum Non-demolition Single Photon Number Resolving Detector," WJ. Munro, R.G. Beausoleil, and T.P. Spiller, Quant-PH/0310066. As a result of the measurement, the state collapses to if a photon was measured in the C rail but not the D rail and collapses toif a photon was measured in the D rail but not the C rail. The desired form of the output state |0>OUT is where states |a> and |e'xa>, e.g., |a> and |-a> when xis equal to IT, are the basis states |0>OUT and I 1>OUT for qubit values within the qunat representation. State correction element 540 is a classically controlled Z gate that is activated when detector 530 detects a photon in the C rail. A Z gate for a coherent state can be implemented using the techniques presented in "Quantum Computation with Optical Coherent States," T.C. Ralph, A. Gilchrist, G.J. Milburn, W.J. Munro, and S. Glancy, Phys. Rev. A 68, 042319 (2003) or "Quantum Computation with Coherent State, Linear Interactions and Superposed Resources," T.C. Ralph, G.J. Milburn and W.J. Munro, Quant-PH/0110115. When a photon is measured in the D rail, the collapsed state has the desired form, and Z gate does nothing. When state results from detection of the photon in the C rail, Z gate 540 corrects the relative phase and the final output state IOUT is again in the desired state Further corrective optics 545 could be employed if necessary Fig. 5B shows a detector 502 in accordance with another embodiment of the invention for conversion of quantum information from a dual-rail Fock state representation to a representation using coherent photon states. A total input state in detector 502 is a product of the input state |0>TN and two coherent states Ioci>i and αofc>2. Input state |IN uses the dual rail representation described above to represent a qubit as indicated in Equation 12, and the full input state at point XI in detector 502 is of the form given in Equation 12. The photon state on the A rail controls controlled phase gate 515 that operates on coherent state I a,\>u and the photon state on the B rail controls controlled phase gate 510 that operates on coherent state |o&>2. Controlled phase gates 510 and 515 can be implemented using an EIT matter system as illustrated in Fig. 5B, but other implementations of controlled phase gates could alternatively be employed. As a result of the operation of control phase gates 510 and 515 on coherent states | a2>2 and \ot\>\ for the alternative presence of a photon in the A or B rail, the full photon state at point XI in Fig. 5B is given in Equation 13, where Xi and & are the change in phase that controlled phase gates 510 and 515 respectively cause. Equation 13: The 50-50 beam splitter 520 mixes the A-rail and B-rail states and outputs photons through C and D rails. The full photon state after 50-50 beam splitter 520 can be expressed 1 using states 10>c and I l>c that are Fock states respectively corresponding to 0 and 1 photon on rail C and states | 0>D and 11 >D that are Fock states respectively corresponding to 0 and 1 photon on rail D. Equation 14 indicates the transition of the full photon state from point XI to point X2, which is after the operation of 50-50 beam splitter 520. Detectors 530 and 535 measure the C and D rail states to determine whether the coherent state channels are in state or state State correction elements 540 and 545 can correct the phase of selected terms in the collapsed state to provide a representation of the original quantum information using entangled coherent states. More particularly, correction element 540 can perform an unconditional correction, which is not necessary in the illustrated embodiment of Figs. 5 A and 5B but may be convenient in other embodiments. For example, an embodiment using a symmetric beam splitter in place of the asymmetric beam splitter could use correction element 540 to correct both collapsed states. Correction 545 performs a conditional correction according to the collapsed state identified by the measurement. Converters 500 and 502 of Figs. 5 A and 5B input at least one known coherent state and transfer quantum information to a representation using coherent states as basis states. The same structures could similarly convert to representations using other types of photon states such as squeezed states. For such conversions, the type of a known state |S1> used must be such that operation of a controlled phase gate 510 or 515 converts the known input state |S1> into a CNOT output state |S2> that is at least approximately orthogonal to the known input state. More specifically, the overlap I I2 is approximately zero so that the known input state | Sl> and the CNOT output state i S2> are suitable for use as the basis states for the output representation. Generally, coherent states |ot> and \etxa> satisfy this condition even for relatively small values of x if a is large. In yet another embodiment of the invention, a state converter is implemented through construction of an optical system that provides an evolution operator that directly transforms the basis states for one representation of quantum information to the basis states of another quantum information. Equation 15, for example, shows a Hamiltonian operator H corresponding to an evolution operator that converts an input state |a of the form C0l0>a+C] I l>a to an output state $>b of the form C0|0>b+C| I l>b. In Equation 15, the coupling constant 6 determines the strength of the interaction and therefore the speed of the state conversion process. A physical system having a Hamiltonian operator H and an interaction time 6t that provides the desired evolution can be constructed from a combination of linear devices, quadratic devices, and a higher-order non-linear device that provide interactions between states in the a-channel and the c-channel. S. Lloyd and S.L. Braunstein, "Quantum Computing over Continuous Variables", P.R.L. 82, 1784 (1999) describes a general technique for generating an arbitrary Hamiltonian using a non-linear device with squeezing and displacements. This technique when applied to a photon state \> employs a series of optical elements having separate Hamiltonians Hi, H2, ..., HN- The optical elements cause the state \ to evolve as indicated in Equation 16 when 5t is the interaction time for each optical element and HEFF is the effective combined Hamiltonian. A system or sub-system using a first pair of optical elements having Hamiltonians A and B with a second pair of Hamiltonians -A and -B provides an effective Hamiltonian that is related to the commutator [A,B] as indicated in Equation 17. The commutator of a non-linear polynomial operator such as (X2+P2)2 with a linear operator such as X or P is generally a higher order polynomial operator (e.g., [(X2+P2)X] is equal to i(X2P+PX2+2P3)/2). Using this principle, an arbitrary polynomial Hamiltonian operator can be constructed using an appropriate combination of optical elements having linear and quadratic Hamiltonian operators with one type of optical element having a higher-order non-linear Hamiltonian operator. The specific elements used to provide the desired Hamiltonian H of Equation 15 depend on the basis states of the input and output physical representations. For the case of light fields generally, linear and quadratic elements include displacers, phase shifters, beam splitters, and squeezers. A higher order quadratic element can be implemented using a Kerr non-linearity such as created in an EIT system. Fig. 6A illustrates an embodiment of the invention in which as an optical system 600 uses beam splitters 610 and 630 and non-linear elements 620 and 640 to implement a Hamiltonian suitable for a converter. The input state of system 600 is a product of a state ColO>a+C1 I l>a carrying the quantum information, i.e., coefficients C0 and C1, and a known state | l>b. The information-carrying state ColO>a+ C1 I l>a is in a representation where the basis states |0>a and | l>a are Fock states respectively including zero and one photon in an a-channel. Known state I l>b is a Fock state corresponding to one photon in a b-channel. Beam splitters 610 and 630, which mix the a-channel and b-channel, have respective Hamiltonian operators A and -A, where the operator A given in Equation 18. In one specific embodiment beam splitters 610 and 630 are substantially identical but oriented so that light traverses beam splitter 620 in a direction opposite to the direction with which light traverses beam splitter 630. Non-linear elements 620 and 640, which can be EIT systems or any alternative elements that provide a Kerr non-linearity in the b-channel, have respective Hamiltonian operators B and -B, where the operator B given in Equation 19. As a result, the effective Hamiltonian of system 600 is given in Equation 20. In Equations 18, 19, and 20, operators a' and a are respectively creation and annihilation operators for the a-channel, and fcT and b are respectively creation and annihilation operators for the b-channel. Fig. 6B illustrates an alternative embodiment of the invention that achieves the Hamiltonian of Equation 20 using a different ordering of beam splitters 610 and 630 and nonlinear elements 620 and 640. In particular, beam splitters 610 and 630 can either precede respective non-linear elements 620 and 640 as shown in Fig. 6A or follow the respective nonlinear elements 620 and 640 as shown in Fig. 6B. Each element 610, 620, 630, and 640 preferably has the same interaction time t, so that the evolution of input state is indicated in Equation 21. Equation 22 is a simplification of Equation 21 arising because an annihilation operator a or b operating on the vacuum state is 0. Couplings d and x and interaction time t, control the values of constants ki and ki in Equation 22 and can be adjusted so that ki is zero and magnitude |k2| is one. With ki equal to zero, the output state is a factorable and includes a state |0>b in the b-channel that represents the quantum information (Co and CO in a physical representation where the basis states | l>b and 12>b are Fock states respectively corresponding to one and two photons in the b-channel. As described above, state converters in accordance with specific embodiments of the invention can perform a general conversion from a photon state using one physical representation for quantum information to a photon state using another physical representation of the quantum information. The general conversion can include conversion of physical encodings where the property of the photon state such as photon number, polarization, time bin, or orbital angular momentum that distinguishes the quantum values is converted to another physical encoding where a different physical property distinguishes quantum values as illustrated in Fig. 7. Additional or alternative conversion can transfer an initial physical type of underlying photon state, e.g., a state of photons of a particular frequency, to a photon state of a different type, e.g., a state of photons of a different frequency, while at the same time either changing or preserving the physical encoding, e.g., photon number, polarization, spatial channel, time bin, or orbital angular momentum, used to distinguish different quantum values. Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims. What is claimed is: 1. A system comprising: a channel for an input photon state that uses a first physical representation for quantum information; and a photo-interactive system (100) that interacts with the input photon state to generate an output photon state using a second physical representation for the quantum information. 2. The system of claim 1, wherein: the first physical representation uses a first property to encode the quantum information; and the second physical representation uses a second property to encode the quantum information, the second property differing from the first property, wherein each of the first property and the second property is selected from the group consisting of polarization, photon number, time bin, and angular momentum. 3. The system of claim 1 or 2, wherein the input photon state corresponds to photons having a first frequency and the output photon state corresponds to photons having a second frequency that differs from the first frequency. 4. The system of claim 1,2, or 3, wherein the photo-interactive system comprises a swap gate (100) or a teleporter (400). 5. The system of any preceding claim 1, 2, or 3, wherein the photo-interactive system comprises: a first controlled phase gate (114) into which a known photon state is input, wherein the input photon state controls action of the first controlled phase gate (114) on the known photon state; a first Hadamard gate (122) that operates on the input photon state; a second Hadamard gate (116) that operates on a state resulting from operation of the first controlled phase gate (114) on the known photon state; and a second controlled phase gate (124) in which a photon state output from the second Hadamard gate (116) controls operation of the second controlled phase gate(124) on a state output from the first Hadamard gate (122). 6. The system of claim 1, 2, or 3, wherein the photo-interactive system (600) provides a Hamiltonian operator that evolves basis states of the first physical representation 5 into basis states of the second physical representation. 7. The system of claim 1, 2, or 3, wherein: the first physical representation uses a first basis state corresponding to a photon being in a first channel and a second basis state corresponding to the photon being in a second channel; and the photo-interactive system comprises: a first controlled phase gate (510) into which a first known photon state is input, wherein a state of the first channel controls action of the first controlled phase gate (510) on the first known photon state; a beam splitter (520) positioned to receive photons from the first channel and the second channel, the beam splitter (520) mixing photon states on the first channel and the second channel to provide photon states on a third channel and a fourth channel; a detector system (530) that measures whether the photon associated with the first and second basis states of the first physical representation is in the third channel or the fourth channel; and a corrective system (540) under control of the detector system (530), wherein the corrective system (540) alters a state output from the first controlled phase gate (510) according to whether the photon is in the third channel or the fourth channel. 8. A method comprising: interacting an input state that uses a first photonic representation of quantum information with a known state in a second photonic representation; and using an output state that results from the interaction to represent the quantum information in the second photonic representation. 9. The method of claim 8, wherein the first photonic representation employs photons having a first frequency, and the second photonic representation employs photons having a second frequency that differs from the first frequency. 10. The method of claim 8 or 9, wherein: the first photonic representation uses a first property to encode the quantum information; and the second photonic representation uses a second property to encode the quantum information, the second property differing from the first property, wherein each of the first property and the second property is selected from the group consisting of polarization, photon number, time bin, and angular momentum.

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4026-CHENP-2006 OTHER PATENT DOCUMENT 2 29-08-2012.pdf

4026-CHENP-2006 POWER OF ATTORNEY 29-08-2012.pdf

4026-CHENP-2006 AMENDED CLAIMS. 08-04-2013.pdf

4026-CHENP-2006 AMENDED PAGES OF SPECIFICATION 08-04-2013.pdf

4026-CHENP-2006 CORRESPONDENCE OTHERS 15-03-2012.pdf

4026-CHENP-2006 EXAMINATION REPORT REPLY RECEIVED 08-04-2013.pdf

4026-CHENP-2006 FORM-1 08-04-2013.pdf

4026-CHENP-2006 POWER OF ATTORNEY 31-01-2013.pdf

4026-CHENP-2006 POWER OF ATTORNEY 15-03-2012.pdf

4026-chenp-2006-abstract.pdf

4026-chenp-2006-assignement.pdf

4026-chenp-2006-claims.pdf

4026-chenp-2006-correspondnece-others.pdf

4026-chenp-2006-description(complete).pdf

4026-chenp-2006-drawings.pdf

4026-chenp-2006-form 1.pdf

4026-chenp-2006-form 26.pdf

4026-chenp-2006-form 3.pdf

4026-chenp-2006-form 5.pdf